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Selective separation shaping: an additive manufacturing method for metals and ceramics

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Selective separation shaping: an additive manufacturing method for metals and ceramics

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Content
Selective Separation Shaping—An
additive manufacturing method for
metals and ceramics
By

Jing Zhang

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(Industrial and Systems Engineering)
August 2016
i

Acknowledgement
I would like to thank my advisor, Prof. Khoshnevis, for offering me the opportunity to
work on this great project of Selective Separation Shaping (SSS), which has a great
potential in the industry of additive manufacturing of ceramic and metallic parts. His
insight and insistence to solve the problem from an innovative way has inspired me to
seek profound solutions for challenges endured in the research.
I would also like to thank Prof. Yong Chen, for offering insight in the development of my
research.
I would also like to thank all the undergraduate and graduate assistants who have
worked together with me in the lab to bring this technology to its current stage. The
undergraduate and graduate assistants include: Christina Kneis, Zichen Xiao, Runxuan
Wei, Jerry Zhang, Sean Han, Brian Chantrupon, Taewoo Kim, etc.

ii

Abstract
SSS is a layer-based 3D printing approach that promises to deliver low cost 3D printing
of high quality parts for a variety of materials including ceramics, metals, etc. In the SSS
process a separator powder (S-powder) is selectively deposited into the base powder
(B-powder) material, which is the composition of the final part. The S-powder forms a
barrier that surrounds the part’s cross section in each layer. The green part is moved
into a furnace for sintering. After sintering, the S-powder is not fused to the B-powder
and is easily removed to reveal the finished part.
In the application fields, the challenges in making ceramic and metallic pieces at an
economic cost can be solved by this approach, and the high initial investments
associated with purchasing industrial additive manufacturing equipment can be
significantly cut down. The distinct separation based shaping process provides a larger
availability of materials. Additionally, in contrast with other methods, the process may be
used to manufacture large scale parts without compromising the resolution.
In the SSS process, the selection and deposition of the S-powder are the key factors
that determine the quality of SSS produced parts. The thesis presents a guideline to the
selection of the S-powder materials and powder sizes corresponding to the chosen B-
powders. Different methods for S-powder delivery are discussed and the most effective
method is identified for the experimental setup. The powder deposition process is
analyzed and experimental findings generate a stable deposition rate with the chosen
parameters. The produced samples demonstrated easy separation and high quality of
surface smoothness.
Keywords: Additive manufacturing, 3D Printing, Selective Separation Shaping, Selective
Separation Sintering, dry powder deposition

iii

Contents
Acknowledgement ....................................................................................................................................... i
Abstract ........................................................................................................................................................ ii
List of Figures ............................................................................................................................................ vi
List of Tables ............................................................................................................................................ viii
1. Introduction .......................................................................................................................................... 1
1.1. Introduction to Additive Manufacturing .................................................................................... 1
1.2. Introduction to Selective Separation Shaping (SSS) ............................................................ 2
1.2.1. The SSS Process ............................................................................................................... 3
1.2.2. How SSS works .................................................................................................................. 3
2. Fundamentals of Selective Separation Shaping ........................................................................... 4
2.1. Introduction to Selective Separation Shaping ........................................................................ 4
2.2. Preliminary results ...................................................................................................................... 8
2.3. Research focus ......................................................................................................................... 11
2.4. Research objectives ................................................................................................................. 11
2.5. Research contributions ............................................................................................................ 11
3. Background of AM of metallic and ceramic parts and the SSS methodology ........................ 13
3.1. Review of existing AM technologies for metallic parts ....................................................... 13
3.1.1. Powder bed fusion (PBF) ................................................................................................ 13
3.1.2. Binder jetting (BJ) ............................................................................................................. 16
3.1.3. Directed energy deposition (DED) ................................................................................. 17
3.1.4. Sheet lamination ............................................................................................................... 19
3.1.5. Material jetting ................................................................................................................... 20
3.1.6. Selective Inhibition Sintering (SIS) ................................................................................ 21
3.2. Review of existing AM technologies for ceramics ............................................................... 23
3.2.1. Powder bed fusion............................................................................................................ 23
3.2.2. Binder jetting ..................................................................................................................... 25
3.2.3. VAT photopolymerisation ................................................................................................ 26
3.2.4. Selective inhibition sintering ........................................................................................... 26
3.3. Critique of the existing approaches ....................................................................................... 27
3.3.1. High Cost ........................................................................................................................... 28
3.3.2. Limited available materials .............................................................................................. 29
3.3.3. High manufacturing cost.................................................................................................. 29
iv

3.3.4. Limited Build Volume ....................................................................................................... 30
3.3.5. Overall Comparison ......................................................................................................... 30
4. Research Methodology ................................................................................................................... 34
4.1. Research plan ........................................................................................................................... 35
4.2. Major stages of SSS research ................................................................................................ 36
4.3. Research methodology ........................................................................................................... 36
4.3.1. Analysis .............................................................................................................................. 36
4.3.2. Experimental verification ................................................................................................. 37
4.3.3. Future development ......................................................................................................... 37
5. Selection of S-powders materials .................................................................................................. 38
5.1. General analysis of S-powder candidates ............................................................................ 38
5.1.1. S-powder candidates-metal ............................................................................................ 39
5.1.2. S-powder candidates-ceramics ...................................................................................... 41
5.2. Candidates of S-powder materials with B-powders being metal ...................................... 42
5.3. Candidates of S-powder materials with B-powders being ceramics ................................ 45
5.4. S-powder selection for B-powders of super high melting temperature ............................ 46
5.5. Summary ................................................................................................................................... 47
6. Consideration of S-powder size and separation line width for effective separation .............. 49
6.1. Analysis on S-powder size selection ..................................................................................... 49
6.1.1. Minimum required size of S-powders in consideration of slipping into B-powders
regions using primitive cubic model .............................................................................................. 49
6.1.2. Maximum size required of S-powders in consideration of B-powders
interconnection ................................................................................................................................. 51
6.1.3. S-powder selection in consideration of parts surface quality .................................... 52
6.1.4. Exceptional dense B-powder by multi-modal powder mixtures ................................ 53
6.2. Thinnest separation wall thickness for effective part separation ...................................... 55
6.3. Summary ................................................................................................................................... 56
7. Properties of S-powder in a nozzle ................................................................................................ 57
7.1. Intrinsic flowability of powders ................................................................................................ 57
7.1.1. Mechanical interlocking ................................................................................................... 57
7.1.2. Cohesive force .................................................................................................................. 59
7.2. Arch formation and breaking .................................................................................................. 66
7.2.1. Identification of arch formed position ............................................................................ 66
7.2.2. Arch pattern building up time .......................................................................................... 68
v

7.3. Nozzle range for effective powder flow switch on/off .......................................................... 75
7.4. Potential applications ............................................................................................................... 80
8. Methodology of Vibration ................................................................................................................ 82
8.1. Plunger activated flow control ................................................................................................ 82
8.2. Magnetic sphere activated powder flow control................................................................... 85
8.3. Piezo disc driven ...................................................................................................................... 87
8.4. Advantages of piezo disc controlled powder flow ............................................................... 92
9. Powder deposition rate control ....................................................................................................... 93
9.1. DOE for determining the significant factors that affect volumetric flux in a confined
conduit .................................................................................................................................................... 93
9.1.1. Initial experiments to identify the factors of importance ............................................. 93
9.2. Study on the role of compaction ............................................................................................ 97
9.2.1. A buffer zone for stable compaction rate ...................................................................... 99
9.3. Powder contamination ........................................................................................................... 101
9.3.1. Powders clustered around contaminants ................................................................... 101
9.4. Identification of factors of importance ................................................................................. 104
9.4.1. Effects of frequency on powder deposition ................................................................ 104
9.4.2. Effects of wave shape on powder deposition............................................................. 105
9.5. Flow rate calculated as a function of printing speed, layer thickness and separation
wall thickness ...................................................................................................................................... 109
9.5.1. The maximum flow rate of powder under gravity ...................................................... 109
9.5.2. Maximum printing speed as a calculated result of flow rate .................................... 110
9.6. S-powder separation wall width ........................................................................................... 111
9.7. Experimental verification ....................................................................................................... 115
10. Future Study ................................................................................................................................ 118
10.1. Customization of deposition nozzles ............................................................................... 118
10.2. Application of active S-powder delivery .......................................................................... 118
10.3. Deposition of fine S-powders ............................................................................................ 118
10.4. Deposition of S-powder over large areas ....................................................................... 119
References .............................................................................................................................................. 120

vi

List of Figures
Figure 2-1 Stages of the SSS part printing process ............................................................................. 4
Figure 2-2 Sintering process for SSS...................................................................................................... 5
Figure 2-3 Microscopic illustration of the SSS principle ....................................................................... 6
Figure 2-4 The dry powder delivery system ........................................................................................... 7
Figure 2-5 Nozzle opening ........................................................................................................................ 8
Figure 2-6 Prototype and scheme of SSS machine(3
rd
generation)................................................... 9
Figure 2-7 Samples printed using SSS ................................................................................................. 10
Figure 3-1 Illustration of EBM process .................................................................................................. 14
Figure 3-2 Illustration of DMLS process of metallic parts .................................................................. 14
Figure 3-3 Acetabular cup (EBM) with SEM ........................................................................................ 15
Figure 3-4 Conventional design of the steel cast bracket .................................................................. 15
Figure 3-5 Binder jetting process ........................................................................................................... 16
Figure 3-6 Samples demonstrated by Exone ...................................................................................... 17
Figure 3-7 Wire based EBAM process illustration .............................................................................. 18
Figure 3-8 Powder based LENS process illustration .......................................................................... 18
Figure 3-9 Process of UAM .................................................................................................................... 19
Figure 3-10 Samples printed .................................................................................................................. 20
Figure 3-11 XJet illustration .................................................................................................................... 21
Figure 3-12 SIS process ........................................................................................................................ 22
Figure 3-13 Bronze samples produced ................................................................................................. 23
Figure 3-14 Selective laser melting of ceramics with preheating scheme ...................................... 25
Figure 3-15 Inhibition test on ceramics ................................................................................................. 27
Figure 3-16 Factors that affect wide adoption of additive manufacturing ........................................ 28
Figure 4-1 Scheme of SSS Printing Process on Top Level ............................................................... 35
Figure 5-1 Melting temperature of metals and oxides ....................................................................... 39
Figure 5-2 Printed pattern of different S-powder ................................................................................. 41
Figure 5-3 SEM image of H13 powder.................................................................................................. 41
Figure 5-4 A part built with S-powders half of H13, half of W90 ....................................................... 41
Figure 5-5 Sintering profile of bronze powder ...................................................................................... 42
Figure 5-6 Bronze sintering with alumina as S-powder ...................................................................... 43
Figure 5-7 Sintering test of alumina powder ........................................................................................ 44
Figure 5-8 316L steel sintering with tungsten as S-powder............................................................... 44
Figure 5-9 SSS printed interlocking tile units made of JSC-1A ......................................................... 46
Figure 5-10 Hydrochloric acid with tungsten powder and magnesium oxide ................................. 47
Figure 6-1 Illustration of simple cubic model ........................................................................................ 50
Figure 6-2 Cross section of tungsten powder deposited into powders ............................................ 51
Figure 6-3 B-powders in S-powders vacancies ................................................................................... 52
Figure 6-4 Surface roughness caused by S-powder patterns ........................................................... 53
Figure 6-5 Powder of different cohesion from large to small ............................................................. 54
Figure 6-6 Effective separation thickness test ..................................................................................... 56
Figure 7-1 Mechanical locking of powders ........................................................................................... 58
Figure 7-2 SEM image of W45 ............................................................................................................... 59
Figure 7-3 Scheme of capillary force .................................................................................................... 60
vii

Figure 7-4 SEM image of ceramic powders ......................................................................................... 62
Figure 7-5 SEM microscopic images of tungsten powder ................................................................. 63
Figure 7-6 Static repose angle v.s. Powder size and type ............................................................... 64
Figure 7-7 Lab built repose angle testing facility for fine powders ................................................... 65
Figure 7-8 Deposition nozzle in SSS .................................................................................................... 66
Figure 7-9 Flow rate with respect to the nozzle length ....................................................................... 67
Figure 7-10 Nozzle with a transparent glass conduit .......................................................................... 68
Figure 7-11 Deposition weight of W90 vs. Frequency ........................................................................ 70
Figure 7-12 Oscilloscopic image of the square wave ......................................................................... 71
Figure 7-13 Deposition weight of W25 vs. Frequency from 1Hz to 25.6kHzHz ............................ 73
Figure 7-14 The descend time for different frequencies .................................................................... 74
Figure 7-15 Maximum of the arch spread ............................................................................................ 76
Figure 7-16 Bridging illustration and forces on the powders ............................................................. 79
Figure 7-17 The flow rate of powders vs the inner diameter of nozzles. ......................................... 80
Figure 8-1 Plunder down and up under the magnetic force for powder switch of/on .................... 83
Figure 8-2 Flow Rate of W90 in 500um inner diameter nozzle ......................................................... 84
Figure 8-3 Caked powder by plunder compaction .............................................................................. 85
Figure 8-4 Powder switch on/off by rotation and dragging down of a magnetic sphere ............... 86
Figure 8-5 Magnetized nozzle tip clogged by steel powder .............................................................. 86
Figure 8-6 Agilent Precision Analyzer 4294A for resonant frequency ............................................. 88
Figure 8-7 Piezo vibration applied perpendicular to the nozzle ........................................................ 89
Figure 8-8 Piezo vibration applied longitudinally along the nozzle ................................................... 90
Figure 8-9 Piezo vibration applied diagonally along the nozzle ........................................................ 90
Figure 9-1 ANOVA analysis for factors of importance ........................................................................ 95
Figure 9-2 Plot of deposition rate against running order for all replicates ....................................... 97
Figure 9-3 Scheme of experimental setup for compaction study ..................................................... 98
Figure 9-4 Compaction rate of bronze and W-45 ................................................................................ 99
Figure 9-5 Scheme of secondary stage serving as a buffer zone for powder deposition ........... 100
Figure 9-6 Deposited powder weight for every 30 seconds with continuous deposition ........... 101
Figure 9-7 Development of clogging in the nozzle from contaminants .......................................... 102
Figure 9-8 Microscopic pictures 25 um sized powder absorbed to fiber surface. ........................ 103
Figure 9-9 Deposition of W25 with respect to the frequency........................................................... 105
Figure 9-10 Wave shapes for driving the piezo discs ...................................................................... 106
Figure 9-11 W25 deposition rate as a response of wave shape..................................................... 107
Figure 9-12 Amplified wave shapes as measured from the piezo disc. ........................................ 109
Figure 9-13 Parameters of printing process ....................................................................................... 111
Figure 9-14 Dynamic filling of the gap ................................................................................................ 112
Figure 9-15 Bronze samples printed with W25 at a layer thickness of 50 𝜇𝑚 .............................. 115
Figure 9-16 One layer of deposited S-powder pattern and final product for pliers ...................... 116
Figure 9-17 Steel samples printed with W90 ..................................................................................... 116
Figure 9-18 Steel samples printed with W90 of high surface quality ............................................ 117

viii

List of Tables
Table 3-1 Comparison of different approaches for 3D Printing of Metals ....................................... 31
Table 3-2 Comparison of different approaches for 3D Printing of Ceramics .................................. 32
Table 6-1 Lattice parameters of SCC and BCC models .................................................................... 50
Table 7-1 Comparison of C110 and W45 ............................................................................................. 58
Table 7-2 Material properties of powders ............................................................................................. 64
Table 7-3 Deposited W90 weight vs. frequency in 30 seconds ........................................................ 69
Table 7-4 Deposited W25 weight vs. frequency in 30 seconds ........................................................ 72
Table 7-5 Flow Rate of Powder vs Nozzle Gauge Based on 2 Minutes Test ................................. 77
Table 8-1 Parameters of the piezo disc ................................................................................................ 88
Table 8-2 Flow rate of powder vs. Vibration direction ........................................................................ 91
Table 9-1 Levels of the Full Factorial Design ...................................................................................... 93
Table 9-2 Flow rate experiments and order of each run .................................................................... 94
Table 9-3 Separation wall width as a response of insertion depth and printing speed ............... 114

1

1. Introduction
1.1. Introduction to Additive Manufacturing
According to ASTM Committee F42, Additive Manufacturing (AM) is defined as: a
process of joining materials to make objects from 3D model data, usually layer upon
layer, as opposed to subtractive manufacturing methodologies. AM is also commonly
referred to as additive fabrication, additive processes, additive techniques, additive layer
manufacturing, layer manufacturing, freeform fabrication and 3D printing (3D printing
and Additive Manufacturing will be used interchangeably throughout this paper).
Additive Manufacturing (AM) has been considered to be the new driving power for the
industry. In 2015, the global market size of additive manufacturing was worth $5.16
billion, and the AM market has been growing at a CAGR of 26.2% in the past 27 years
[1]. Further analysis expects such momentum to last far into the next decade. Countries
and regions such as the United States, China, Europe all bid on this technology to
reinvigorate the manufacturing economy by allocating funds for the supporting research
and commercialization of AM technologies.
The development of the AM field has been accelerating since the first invention of the
AM process SLA more than three decades ago. There are now seven categories
covering tens of AM processes. According to ASTM Committee F42 standards, the
seven listed categories are:
I. Binder jetting—an additive manufacturing process in which a liquid bonding
agent is selectively deposited to join powder materials.
II. Directed energy deposition—an additive manufacturing process in which focused
thermal energy is used to fuse materials by melting as they are being deposited.
III. Material extrusion—an additive manufacturing process in which a material is
selectively dispensed through a nozzle or orifice.
IV. Material jetting—an additive manufacturing process in which droplets of build
material are selectively deposited.
V. Powder bed fusion—an additive manufacturing process in which thermal energy
selectively fuses regions of a powder bed.
2

VI. Sheet lamination—an additive manufacturing process in which sheets of material
are bonded to form an object.
VII. Vat photopolymerization—an additive manufacturing process in which liquid
photopolymer in a vat is selectively cured by light-activated polymerization.
Because the focus of this research is for additive manufacturing of metallic and ceramic
parts, the related categories will be: binder jetting (Exone), directed energy deposition
(Sciaky, Optomec, etc.), powder bed fusion (EOS, ARCAM, etc.), and sheet lamination
(Fabrisonic). These existing technologies have been applied in the industry for research
and production purposes such as for the manufacturing of end-use parts in Leap
engines in GE Aviation
1
.
The challenges of the aforementioned technologies in these categories are also
recognized during application. For example, the binder jetting approach is especially
susceptible to powder contamination. Directed energy deposition has lower resolution
due to the large melting pool, especially in cases of wire feeding. Powder bed fusion is
slow in speed. Sheet lamination (UAM) suffers lack of complexity and limited availability
of materials. Overall, a common challenge of these technologies is the expensive initial
investments, which block out small to medium businesses from accessing any
advanced manufacturing technologies in the field of metal and ceramics.
The proposed process of this research, Selective Separation Shaping (SSS), is meant
to overcome challenges faced by other AM technologies. SSS, a two-step
manufacturing process, is an additive manufacturing method for metals and ceramics
that does not fall into the aforementioned seven existing categories. The steps of this
process entail creating a custom tank of metal or ceramic powder that will then be
sintered to completion.
1.2. Introduction to Selective Separation Shaping (SSS)
SSS (Selective Separation Shaping) was invented and developed in University of
Southern California. Initially, SSS was developed for outer space constructions of in-situ
materials utilization (ISRU) [2,3], in a NIAC (NASA Institute of Advanced Concept)

1
http://www.ge.com/stories/advanced-manufacturing
3

project. The process has further proven its capabilities by manufacturing multiple
materials, including ceramics, metals, and polymers at an affordable cost with increased
speed compared with existing AM technologies in the market.
1.2.1. The SSS Process
SSS is a layer-based 3D printing approach that promises to deliver low cost 3D printing
of high quality parts for a variety of materials including ceramics, high melting point
metals, etc. The SSS process uses low cost furnace sintering and generates parts with
isotropic properties. The current challenges in making ceramic and metallic pieces at an
economic cost can be solved by this approach, and the expensive initial investments
associated with purchasing an industrial 3D printer can be significantly cut down.
Additionally, in contrast with other methods, the process may be used to manufacture
large scale parts without compromising the resolution.
1.2.2. How SSS works
SSS is a layer-based additive manufacturing technology associated with two powders
which require sintering after all the layers are printed. In the SSS process, a dry
separator powder (S-powder) is selectively deposited into a paved layer of the base
material (B-powder). The deposited S-powder, which has different joining conditions
compared with the B-powder, defines the boundaries of each cross section of the part.
In this research, the S-powder has a higher sintering point compared with the B-
powders. The S-powder boundaries form a de facto coating that surrounds the loose B-
powder representing the part after the printing is done. The green parts are then
removed from the build platform and placed in a furnace for sintering following proper
heating procedures. After being cooled down, the S-powder coating, which is not fused
during the sintering process, can be removed. After the part is separated, post
processing such as infiltration or heat treatment may be applied if necessary.
4

2. Fundamentals of Selective Separation Shaping
2.1. Introduction to Selective Separation Shaping
In the SSS process a separator powder (S-powder) is selectively deposited into the
base powder (B-powder) material. The S-powder forms a barrier that surrounds the
part’s cross section in each layer. After sintering, the S-powder remains not fused and is
easily removed to reveal the finished part. The process can be described by the
following steps shown in Figure 2-1:

Figure 2-1 Stages of the SSS part printing process
1. The platform is lowered one layer thickness down for the B-powder to be spread
over the previous layer. The S-powder deposition nozzle is then lowered into the
powder bed.
2. The nozzle is moved along the perimeter of the current cross section with a
selected offset and rotation so that the nozzle opening is always opposite the
direction of motion. Controlled vibration is applied to the nozzle for delivery of S-
powder at a desired rate.
3. After the layer is finished, the nozzle is raised above the platform with a desired
clearance.
5

4. To minimize powder waste, a binding liquid is printed outside the part cross
section to act as a temporary container for the loose powder during transport to
the furnace.
 Repeat steps 2-4 until all layers are processed.
5. Once the part has been completed, the green part will be transported to the
sintering furnace.
6. The sintered part is removed from the furnace and the S-powder is cleared away
to reveal the final part. The removal process can be done with ease because the
S-powder remains loose throughout the process.
The operating principle behind SSS is explained through the differences in sintering
temperature or chemical properties between the chosen separator and base material.
Most materials will be dependent on the difference of sintering temperature. This
difference is exploited in a sintering furnace where the base material sinters while the
separator material remains loose. A sintering profile correspondingly to the B-powder is
chosen that the B-powder is sintered to a planned extent, while the S-powder is not
fused. After this sintering stage is completed, the chamber is slowly cooled down to
room temperature. Figure 2-2 illustrates the strategy used in exploiting the temperature
difference between B-powders and S-powders. As long as the sintering profile is kept
between the sintering temperature of the B-powder and S-powder, part extraction can
be easily achieved.

Figure 2-2 Sintering process for SSS. The blue line represents the heating ramp; the red line represents sintering
temperature for the S-powder and the green line represents the sintering temperature for the B-powder.
6

Figure 2-3 illustrates a simplified model of the process where the black spheres
represent the B-powder and the white spheres represent the S-powder. After the B-
powder is spread, the whole top layer is filled by black spheres. The S-powder (white
spheres) is then selectively deposited into the B-powder layer only at the part boundary
as illustrated in Figure 2-3. During the sintering stage, the area occupied by black
spheres sinter and shrink to form the solid part while the white spheres remain their
original occupied volume. The curves represented by white spheres neither fuse to each
other nor to the neighboring black spheres. After sintering, if the sintered part is
completely surrounded by the S-powder, then it can be separated with ease from the
surrounding sintered sections through removal of the loose S-powder by compressed
air or by simply shaking.

Figure 2-3 Microscopic illustration of the SSS principle. (a) Simplified powder distribution before sintering; (b): B-
powder shrinks by sintering while S-powder remains untouched. (The dark spheres represent the B-powder and the
white spheres represent the S-powder)
The S-powder may be delivered inside the base powder by means of a thin conduit at
the end of nozzle made of a narrow hollow needle (such as a hypodermic needle shown
in Figure 2-4.a. Normally, granular materials form an arch in a tight conduit [4-6], as
shown in Figure 2-4.b. However, with the addition of vibration through a piezo electric
element, a controlled and continuous flow of powder can be achieved as illustrated in
Figure 2-4.c. Vibrating the conduit agitates the particles touching the inner wall of the
7

conduit and results in the breaking of the bridge and hence flows of the S-powders.
When vibration stops an arch pattern quickly returns and stops the flow.

Figure 2-4 The dry powder delivery system. (a): The inhibitor deposition system. (b) The arch pattern the stops
powder flow (c) The arch pattern broken by vibration
In order to deliver the S-powder along a desired path inside the base powder, the
nozzle must be inserted into the powder layer. A slot is made on the side of the nozzle
to allow S-powder deposition while the nozzle is moving forward. The nozzle slot is
always oriented opposite the direction of nozzle motion. Piezo-actuated vibration
enables the flow of S-powder. The deposited S-powder fill the momentary void space
created in the B-powder by the nozzle movement. In other words, the nozzle displaces
8

the base powder material as it moves through the powder and allows space for the
flowing S-powder to fill. The S-powder deposition width is approximately the inner
diameter of the nozzle. Therefore, a properly selected nozzle size determines the
resolution of the printing process, and hence the final part. Note that the depth of nozzle
insertion can be larger than the height (h) of the nozzle opening shown in Figure 2-5. In
other words, the nozzle opening is submerged into the B-powder. In this case, the
nozzle movement creates a tunnel inside the base powder, which collapses over the
deposited S-powder as the nozzle moves.

Figure 2-5 Nozzle opening
2.2. Preliminary results
The 3
rd
generation prototype SSS machine is shown in Figure 2-6. In addition to the AM
standard of the utilization of a powder tank, build tank, spreading mechanism, and three
axes of motion (X, Y, Z), the machine has a separate Z axis for the vertically positioning
and a R axis for orientation of the nozzle during deposition.
9

Figure 2-6 Prototype and scheme of SSS machine(3
rd
generation)
In preliminary experiments, 2.5D and 3D parts have been produced using both ceramics
and metals. Multiple samples have been demonstrated in Figure 2-7. A pattern of
10

interlocking ceramic tiles are shown in (a) and are made of Lunar regolith simulant
(JSC-1A)
2
with aluminum oxide (𝐴 𝑙 2
𝑂 3
) being the S-powder. Other samples are made of
metals: (b) a pair of pliers made of bronze; (c) a few samples made of 316L stainless
steel; (d) a pair of pliers made of H13 tool steel.

Figure 2-7 Samples printed using SSS (a) Interlocking tile patterns using lunar regolith simulant JSC-1A (b) A pair of
pliers using bronze (c) Multiples pieces printed with 316L stainless steel (d) A pair of pliers printed H13 steel

2
Purchased from www.orbitec.com, sifted to 200 um
11

2.3. Research focus
In this research, the focus is to build metallic and ceramic parts using SSS process.
Compared with commercialized AM technologies, the SSS approach of building ceramic
and metallic parts have the advantages of low equipment investment, high production
rate, working with multiple materials and capable of manufacture large scale parts. As
the SSS technology gets mature, more business can have access to the advanced
technology of AM at an affordable cost.
2.4. Research objectives
The objectives of this research are to prove the feasibility of SSS, to provide a
systematic method for selection of proper S-powder to pair with a given B-powder, to
provide an activation method for effective control of powder deposition on/off, and to
identify the proper parameters for control the S-powder deposition rate through a nozzle.
The feasibility of SSS will be demonstrated by the production of high quality ceramic
and metallic parts; a systematic method for selection of S-powder covering materials,
size and flowability will be detailed; the S-powder deposition rate controlled are to be
studied from the intrinsic properties of S-powders and the interaction between the S-
powder with the nozzles.
2.5. Research contributions
In the research, the author has successfully demonstrated the feasibility of the SSS
process by producing the samples in ceramics and metals, thus showing the great
potential of an alternative additive manufacturing process for metals and ceramics.
For scientific research, a detailed sifting process is provided to choose the candidate S-
powders. With this research, the candidates for S-powders have been narrowed down
to a few materials among thousands of powders available on the market.
The properties of the powders regarding flow rate have been studied to guide for the
selection of powders used as S-powders.
An effective deposition mechanism has been identified and realized in the experimental
setup. The factors of importance for controlling the flow rate have been identified and
12

used to improve the flow rate. The noises that disturb a stable flow are eliminated in the
machine design.
Overall, a systematic research study has been carried out for a new additive
manufacturing approach for ceramics, metals and multiple other materials and has been
proven feasible.

13

3. Background of AM of metallic and ceramic parts and the SSS
methodology
3.1. Review of existing AM technologies for metallic parts
There are four categories of technologies that have been commercialized for additive
manufacturing of metallic parts namely powder bed fusion, binder jetting, direct energy
deposition and sheet lamination. A fifth category, material jetting, is under development
in an Israeli company called Xjet.
3.1.1. Powder bed fusion (PBF)
Powder bed fusion, as the term suggests, involves selective fusion of powder lying on a
powder bed. Under this category, there are two main processes, Electron Beam Melting
(EBM) and Directed Metal Laser Sintering (DMLS).
In the process of powder bed fusion, a power beam of high density (an electron beam or
laser beam) is used to selectively melt metal powder representing the cross section of a
part layer. The process is depicted in Figure 3-1 and Figure 3-2. The selected area of
powder adsorbs the energy and forms the melting pool, which becomes solid as it cools
down [7,8]. The depth of the melting pool is larger than the current powder layer
thickness that neighboring layers have overlap and are firmly bonded together. Since
the power beam melts the powder completely, a printed part of near full density can be
obtained [9,10]. As the melting pool cools down rapidly, fine microstructures may be
achieved [11,12]. The parts have comparable strength to conventionally manufactured
parts [13,14]. The whole process is carried out either under high vacuum or under the
protection of inert gas [15,16]. When fusion of all layers is done, the part(s) are left to
cool down to a safe to handle temperature in a protective atmosphere such as in the
vacuum or in the inert gas.
14

Figure 3-1 Illustration of EBM process
3
Figure 3-2 Illustration of DMLS process of metallic parts
4

The processes under this category have been the mainstream approaches for additive
manufacturing of metals and have been used in the fields of aerospace & aeronautics,
automotive, energy, medical, tool and molding, etc.[17,18]. Some of the AM parts are
shown in Figure 3-3. In GE Aviation, the additive manufactured metallic parts have been
tested and installed in their Leap engines. An additive manufactured chromium-cobalt
part, which is stronger and 25% lighter, replaced the originally designed parts that was
welded together from more than 20 parts [19].
Despite the applications of parts in the industry, powder bed fusion technology has not
reached the stage of maturity for parts to be manufactured with confidence. Also, a lack
of understanding for the complex powder and beam interaction limits the material
available. Failures and defects still occur without any changes in the process
parameters [20]. Current research focuses on understanding the interaction of beams
with powders with the aim of quality control for AM parts [21,22].

3
www.arcam.com
4
www.eos.info
15

Figure 3-3 Acetabular cup (EBM) with SEM (Source: www.arcam.com)

Figure 3-4 Conventional design of the steel cast bracket (upper left) and design of the EOS titanium AM-made
bracket (lower right corner). Source: Airbus Group Innovations
16

Companies active in this category include Arcam AB, EOS, Concept Laser, SLM
Solutions, Renishaw and 3D Systems [1,23].
3.1.2. Binder jetting (BJ)
A binder jetting process selectively prints the binder onto the powder layers and post
processing of sintering is applied to strengthen the parts. In the printing stage, an
adhesive liquid is printed onto the selected area of the powder bed representing the
cross section of the part as shown in Figure 3-5. The printed part gains enough strength
to be moved into a furnace for sintering, where the binder is burned off and sintering is
realized. The sintered parts may be infiltrated by a lower melting temperature material
such as bronze which can infiltrate stainless steel parts [24,25]. This process is
commercialized by Exone [26] and Hoganas AB and was initially invented by MIT [27].
The printed binders are polymer based and may contaminate the materials in use,
especially if the powder in use is sensitive to contamination such as titanium based alloy.
Materials available for binder jetting include: bronze, stainless steel, cobalt alloy,
chromium alloy, and tungsten alloy.

Figure 3-5 Binder jetting process (Source: Exone)
17

BJ does not rely on support for overhang structures and can additive manufacture parts
with subtle structures. Compared to powder bed fusion, there is less thermal stress
during printing where the loose powder serve as a support. Some of the parts built are
shown in Figure 3-6.

Figure 3-6 Samples demonstrated by Exone (a) Impeller (b) Strainer plates (Source: Exone)

3.1.3. Directed energy deposition (DED)
DED involves the direction of energy into a focused region where the substrate and the
material deposited are melted while layers are added. There are two approaches for
DED: one is wire feeding based and the other is powder feeding based. Both are shown
in Figure 3-7 and Figure 3-8.
In DED, the high power electron beam or laser beam is focused on a narrow area where
the substrate and the deposited material forms a melting pool and solidifies as the beam
moves away. DED approaches do not require large amounts of powder in the powder
tank and are not limited to process on a flat surface, which enable in-situ repair of
broken parts [28]. In the case of powders, a laser beam is used to serve as the energy
source to melt the powders as used in practice by Optomec and BeAM. Because
multiple nozzles feeding different materials are available, functional gradient parts can
18

be made by changing the ratio of different powders during manufacturing. In the case of
wire feeding, an electron beam or a laser beam is used as the power supply as
practiced by Sciaky

[29,30]. Functional gradient parts are also available if two or more
wire feeders exist. The printing speed for wire feeding can be much faster and the cost
of wire is comparatively cheaper than powders of the same material ingredients
5
.

Figure 3-7 Wire based EBAM process illustration (a) Schematic (b) Manufacturing in process (Source: Sciaky.com)

Figure 3-8 Powder based LENS process illustration (a) Schematic (b) Manufacturing in process (Source:
Optomec.com)

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19

The challenges of the DEM include low surface smoothness and difficulty in the
manufacturing of parts with overhang structures. A typical accuracy of 0.25 mm and a
surface smoothness of 25 𝜇𝑚 are very difficult to achieve, especially in the case of wire
feeding system. Therefore, a DED system is commonly assisted with a CNC machine
for post processing [31].
3.1.4. Sheet lamination
Sheet lamination in metal additive manufacturing takes advantage of ultrasonic welding
where the thin foils are welded and CNC milled to achieve desired shapes [32]. Sheet
lamination in metallic additive manufacturing has been commercialized by Fabrisonic
TM

and is termed as UAM (Ultrasonic Additive Manufacturing), specifically. In UAM process,
a piece of thin metallic foil is welded onto previous layers by ultrasonic welding and then
CNC milling is used to take off the extra area as illustrated in Figure 3-9.

Figure 3-9 Process of UAM (Source: Fabriconic)
UAM process has the flexibility of fabricating laminate or embedded parts. The metallic
foils are added one by one, and each foil does not depend on the choice of the other
that the laminate of different materials can be achieved as illustrated in Figure 3-10
(a)[33]. Ultrasonic welding also makes embedding of another structure possible such as
in the example of embedded SiC fibers in metal laminate as illustrated in Figure 3-10 (b).
Sensors can also be embedded while the process is going by being placed into the
previously designed cavity.

20

Figure 3-10 Samples printed (a) Aluminum-titanium laminate (b) SiC fibers embedded in metal laminate (Source:
Fabrisonic)
The UAM process can handle basically any material that is ultrasonically welded. The
drawback of this technology includes its inability to manufacture parts with overhang
structures; an elongated manufacturing time with parts of complex shapes as CNC
milling is used to take off extra materials.
3.1.5. Material jetting
An Israeli company called Xjet is using material jetting technology to deposit
nanoparticles and sinter at low temperature as illustrated in Figure 3-11. Such a
technology is in the progress of being commercialized. The description from XJet is as
follows:
XJet’s system print heads deposit an ultra-fine layer of liquid droplets which contain
stochastic metal-nanoparticles onto the system build-tray. The required support is
generated automatically by the system. Inside the system’s build envelope extremely
high temperatures cause the liquid ‘jacket’ around the metal nanoparticles to evaporate.
This results in strong binding of the metal with virtually the same metallurgy as
traditionally-made metal parts.
21

Figure 3-11 XJet illustration(a) and matrial properties(b) (Source: Xjet)
After the deposition is complete, a relatively low temperature heating source such as at
300 °C can be applied to vaporize the solvent and sinter the nanoparticles [34]. The
obtained part will have very high resolution and has a minimum requirement for manual
intervention.
The challenges facing this approach may include the high cost of raw materials and
limited amount of material availability. The nano-particles and the technology to reserve
the nanoparticles are not cheap and will add significant cost to the manufacturing
process. Additionally, sensitive metal may have a high chance of being contaminated by
the solvent.
3.1.6. Selective Inhibition Sintering (SIS)
All the aforementioned approaches for making metallic parts have high up-front
equipment cost and/or high operation cost, and a limited printing size. Under such a
background, a low cost approach called Selective Inhibition Sintering (SIS) has been
developed by Dr. Khoshnevis to address the limitations of 3D manufacturing of metal
parts [35-38]. The SIS technology does not belong to any of the existing 7 categories.
The SIS process is fast and guarantees uniform part quality. As the base powder layer
is paved, the inhibitor solution is deposited along the part layer profile, often along a 2D
border line that shortens the printing time as shown in Figure 3-12. The inhibitor solution
is deposited layer by layer. As the printing process is completed, the green part is
moved into a sintering furnace, where the inhibitor coating serves as a temporary
22

sacrificial mold, separating the parts from the redundant powder. As the sintering
process is completed, the part is taken out from the furnace and the redundant
materials are removed by means of sand blasting. The fabrication resolution depends
upon the precision of deposition of inhibitor and base powder particle size and does not
degrade as part size increases.

Figure 3-12 SIS process 1. Powder pavement; 2. Inhibitor deposition; 3. Sintering in the furnace; 4. Part separation
SIS has been proven to produce high quality parts using low melting temperature alloys.
In the experiments, low melting temperature material such as bronze coupled with
sucrose as the inhibitor have been tested successfully to produce parts of complexity as
shown in Figure 3-13. During the sintering process, sucrose decomposes into carbon
which covers the bronze particles and prevents sintering in the printed area. The
effective element from sucrose during sintering inhibition is carbon. Therefore materials
that reacts with carbon cannot be inhibited, such as in sintering of stainless steel.
23

Also for alloys with high melting temperature, the inhibitor dissolved in the solution may
not be sufficient for sintering inhibition.

Figure 3-13 Bronze samples produced (a) Bracelet (b) Wrench
3.2. Review of existing AM technologies for ceramics
Compared with the additive manufacturing of metallic parts, the ceramic technologies
are relatively new and have few commercialized machines available in the market. Due
to the fact that ceramics are insulators and poor in heat conduction, many approaches
successful in metal prove to be difficult in ceramic printing. EBM cannot be used for
ceramics, which are mostly electrically non-conductive. The process of laser melting
proves to be very difficult and is still under investigation as cracks easily form due to
high stress caused by low thermal conductivity. The commercially available methods for
ceramic printing and those under research are listed below.
3.2.1. Powder bed fusion
3.2.1.1. Selective laser sintering
Selective sintering of ceramics utilizes a laser beam to melt the binder part of binder-
coated structural ceramic powder in the printing process [39-41]. The green part is then
sintered in the furnace where the binder is burned off and the ceramic part is obtained
[42]. Because the binder occupies a fair ratio of the volume in the printing, the sintered
parts suffer from substantial shrinkage, affecting printing precision, and may crack in the
sintering process. The binder can be premixed with the ceramic powder or printed on
the ceramic powder. For certain approaches, the binder is mixed with the powder to
24

form a printable material [43]. For others, the liquid binder is printed onto the thin layer
of ceramic powder and bonds the powder into the shape of the part [44]. Post-
processing might include the burning off of the binder based upon requirements. In
some applications of bone scaffolds, a biocompatible binder is used [45].
The parts made from this process are mainly used for molding or aesthetic purposes
since the parts are not strong enough [46]. The shrinkage rate in this way depends upon
the volume of binder used [47].
3.2.1.2. Direct Laser Melting
Direct laser melting of ceramic powder is still at research stage, thus significant success
have not been demonstrated. Research work includes the study of the transient
interaction and temperature gradient during the process, the study of the ceramic
candidates that are suitable for this process [48,49], and the study of preheating the
powder to reduce the thermal stress [50].
Prior research has analyzed the thermal conduction and the strength of the material and
stress during the process. The analysis has been concentrated primarily on silicon
dioxide. The research of silicon dioxide direct melting is carried out without significantly
preheating the powder bed [51].
The other approach is to preheat the powder bed, which is the case in the process of
selective laser melting of metal. In this research, the powder bed is preheated up to
1600 ° C throughout the printing process and the stored powder is preheated before
paving as illustrated in Figure 3-14. Since the preheating temperature gets close to the
melting temperature of the ceramic, the balling phenomenon (the melted powder
forming into a series of spheres due to surface tension) is very severe and the obtained
parts do not result in good quality [50]. While preheating did eliminate the thermal stress
that no obvious cracks was observed.
25

Figure 3-14 Selective laser melting of ceramics with preheating scheme (Source: Fraunhofer, ILT)
3.2.2. Binder jetting
The binder jetting approach for ceramics is similar to that of metals. The binder is
printed to bond the structural ceramic powder together, which will then be processed
further in the sintering furnace[52]. Unlike metals, some of the ceramics may undergo
stable sintering in the atmosphere. Currently Exone is offering the binder jetting process
of ceramic materials with the available materials of: silica, zicon, etc.
One challenge facing the binder jetting approach is the possibility of contaminating the
powder with the binders. In the pyrolysis process, the decomposed binder may react
26

with the raw materials and form stable materials, which are considered to be
contaminants [53].
3.2.3. VAT photopolymerisation
The vat photopolymerisation based method takes advantage of the existing setup of
DLP (digital light projection). In DLP, photosensitive resin is initially a liquid of
monomers, which becomes solid polymers once exposed to light with photon energy
above a certain threshold [54]. The ceramic powder is mixed with the resin and the
printing process is almost the same process as that in DLP. The obtained 3D printed
green parts have the ceramic powder frozen within [55,56]. The post-processing burns
off the resin and sinters the ceramic powder following a proper sintering profile. Lithoz
and Admatec have commercialized this process to make ceramic parts of high
resolution [57]. Again, since the resin takes around 25% of the weight, shrinkage is
common in the post-processing of sintering. The projection based technology poses a
restriction in the size for the parts.
3.2.4. Selective inhibition sintering
SIS has been tested for the inhibition of ceramics but the results are not satisfying. First,
the solution based inhibitor does not generate enough ceramic particles to weaken the
printed area and cannot serve as an inhibitor for separation. Second, the printed area
has too much liquid and needs to be sintered by slowly raising the temperature,
otherwise, the water vaporization makes the part very porous, weakening the overall
structure as seen in Figure 3-15.
27

Figure 3-15 Inhibition test on ceramics (a) Failed inhibition (b)Microscope of the printed area
3.3. Critique of the existing approaches
The aforementioned approaches for additive manufacturing of metals have either been
commercialized or are near commercialization. Few commercialized machines can be
found in the market due to the technology still being immature and in the exploratory
phase. The commercialized technologies have been utilized in the medical, industry and
research fields where large corporations such as GE and NASA can afford the high
initial investment. While for small to medium sized companies, the high initial cost (~$1
million) of a 3D printer as well as the limited availability of materials pose a high risk.
According to a Stratasys survey from 700 serious, professional users, the factors that
prevent them from using additive manufacturing (not limited to metal or ceramics) are
ranked from high to low as: equipment costs, limited materials, manufacturing costs and
post-processing requirements
6
as illustrated in Figure 3-16.
The questions asked were:
Which of the following challenges do you see your company facing for using additive
manufacturing today?
Which of the following challenges do you see your company facing for using additive
manufacturing in the next three years?

6
StratasysDirect.com
28

Figure 3-16 Factors that affect wide adoption of additive manufacturing (Stratasys)
3.3.1. High Cost
Due to expensive up-front investment and high operating and maintenance cost
presently, metal 3D printing equipment are mainly used by large corporations or
institutes like NASA, GE, medical facilities and universities [58,59].
For powder bed fusion, the two representative companies are Arcam AB (founded in
1997) and EOS (founded in 1989), which use EBM and DMLS respectively. The price of
the machine from Arcam AB is about $1 million plus strict environmental requirements
and extra accessories to the equipment. The price for the EOS machine is
approximately the same level but operating it is higher as the inner gas is constantly
consumed.
29

For DED approaches with an integrated CNC machine forming a hybrid system, the
price is more than $1 million, plus the cutting tools wearing off in the CNC machine. The
binder jetting machine from Exone has a large price range from $200K to $1 million.
The Lithoz system for ceramics costs more than $300K and the micrometer scale
ceramic powders are very costly.
3.3.2. Limited available materials
For additive manufacturing of metals and ceramics, the available materials are limited
due to the complex physics behind the processes.
In the category of powder bed fusion, the interaction of beams with powder is so
complex that extensive experiments must be carried out to determine the proper
parameters for each new powder[60]. For a powder type that is engineered and proven
to work for a specific machine, consistency has to be closely maintained for chemical
composition, powder size and powder distribution. Such strict requirements actually
restrict a lot of material selection outside the candidates and contribute to the high cost
of 3D printable metals and ceramics as discussed before. The approved metallic
materials for EOS include 13 kinds of powders
7
.
Binder jetting in the process used by Exone may contaminate the final products in the
cases of both metals and ceramics, therefore materials that may react with the binder
cannot be available for processing.
The UAM process has the advantages of welding almost any material that can be
welded, while the supply of metallic foils are limited. The drawback beyond materials
availability also includes that the UAM process cannot manufacture parts with overhang
structures, which are imperative for many parts.
3.3.3. High manufacturing cost
High manufacturing cost can be attributed to different factors in different processes but
are mainly attributed to low production rate, high cost of materials as well as
consumption of inert gas or degradation of tools. Low production rate can be the result
of slow scanning speeds in the case of EBM and DMLS and long cooling times in the

7
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30

printing chamber. In the process of UAM, the welding process is fast but is hindered by
the slow milling process, especially with parts of complex structures. In DED where the
precision is low, a CNC milling is necessary which increases the time for the overall
process. The high selectivity in the interaction of beams and powder deposited also
requires expensive high quality powder. In the case of DED, where CNC milling is
applied to improve the part accuracy, the tools may degrade faster in the case of
objects printed in titanium and other hard materials.
3.3.4. Limited Build Volume
The current approach with the high power beam suffers from both the limited build
volume due to the divergence of the power beam at large deflection angles and the
requirement for maintaining uniform temperature.
By the laws of physics, the laser beam and electron beam have almost spherical focus
planes therefore defocus will pose as a problem with the flat building platform. As a
large part manufacturing is needed, the area further away from the center requires a
large reflection angle. The increased deflection angle results in serious defocus of the
beam [61]. The defocused beam will present two main problems: 1) Reduced resolution
and accuracy resulting from the enlarged beam spot size, which cannot achieve the
high resolution where the narrow beam can; 2) Reduced power density which may not
sinter or melt the powder to the designed level and hence cause defects. The size
problem can be solved by installations of multiple beams at the expense of high
equipment cost, such as in the case of SLM solutions.
Binder jetting and material jetting have the potential for making very large parts, but
binder jetting incurs a high risk for part contamination and material jetting has high raw
material costs. The cost of building a part at the scale of meters with nanoparticles such
as in Xjet technology is unaffordable in the industry.
3.3.5. Overall Comparison
As a summary of the above sections, the pros and cons of the mentioned approaches
are listed in Table 3-1 and Table 3-2 for metals and ceramics, respectively.
31

In Table 3-1, the features of metal additive manufacturing are compared. As the table
shows, for selective melting and selective sintering using an electron beam or laser
beam, machine price, operating and maintenance fee are all high. The binder based
approach can cut the cost of laser or electron beams, but the obtained parts can be
contaminated with the remnants of the binder. Admittedly, the existing commercialized
machines have their merits and are providing service to a section of the market while
the market is not fully satisfied.
Table 3-1 Comparison of different approaches for 3D Printing of Metals
Features Powder bed fusion Binder
Jetting
Sheet
Lamination
Material
Jetting
New approaches
EBM DMLS BJ UAM Xjet SIS SSS
Resolution Medium High Medium High High Medium Medium*
Build Speed Low Low High Medium-High Medium HIGH HIGH
Affordability Low Low Low Low Low High High
Available
materials
LOW LOW Medium High Low Low High
Free of
contamination
High High Low High Low High High
Overhang
capability
High High High Low Not sure High High
Build Volume Low Low High High Low High High
Isotropy Low Low High High Not sure High High
Note: in all the features High is preferred by the customers
*The resolution of SSS will be improved as the technology matures
Only the commercially available ceramic additive manufacturing technologies are
compared in Table 3-2.The binder based approaches suffer from the effects of the
32

binder because they occupy a certain volume during the process of printing. In the latter
state of binder pyrolysis, the occupied volume will be released in the form of vacancy
and shrinkage will occur during this stage. The obtained parts also may suffer from
contamination of the binder. The SSS approach is a promising technology that can build
high quality ceramic parts at an affordable the market cost.
Table 3-2 Comparison of different approaches for 3D Printing of Ceramics
Features Powder bed
fusion
Binder
Jetting
VAT
Photopolymerization
New
Approaches
SLS BJ VP SSS*
Resolution Medium Medium High Medium
Build Speed Low High Low High
Affordability Low Low Low High
Available materails High Medium Medium High
Free of
contamination
Low Low Low High
Overhang capability High High Medium High
Build Volume Medium High Low High
Isotropy Medium High High High
Note: * The resolution of SSS will improve as the technology matures
The advantages of SSS are reflected in the innovative approaches adopted in SSS. By
using separation sintering, a furnace can be used to replace the costly electron beam or
laser beam, which can significantly cut down the initial investment for equipment. By
using an S-powder with a higher melting temperature (therefore high sintering
temperature), all the other materials which have a lower sintering temperature may be
used as B-powders. The nonselective separation principle of SSS makes more material
available for manufacturing. The existing sintering profile in use from other fields, such
33

as powder metallurgy, can be used directly in the sintering step of SSS. Using dry
powder deposition where a mechanical system is employed, the resolution remains the
same for a large part manufacturing as in the case of a small part. Depositing only along
the perimeter of the part, the layer printing time is only linearly proportional to the size of
the part.
Overall, the SSS approach provides one platform that promises to be capable of printing
multiple materials including ceramics and metals. As the principle explains, the printing
equipment does not have to be changed when shifting from metals to ceramics, which
eases the adoption of new separation powders and base powders. The sintering
process involved is the same to those that have been extensively studied in the past,
making the post-processing step of SSS straight-forward.

34

4. Research Methodology
The most crucial step in the SSS process is the separation of the part from the
redundant materials defined by the S-powder separation walls. A primary objective of
SSS is the ability to work with any powder that can be furnace sintered. The research
focuses on the ease of separation by the S-powder deposition. The B-powder is
characterized by its chemical composition, sintering temperature, and powder size. The
S-powder is characterized by its chemical composition, melting temperature, shape,
size, density and repose angle as a reflection of flowability.
After targeting the optimal pair of S-powder and B-powder, the research moves on to its
next stage, which involves finding the factors that affect the deposition rate of the S-
powder from the nozzle and controlling the deposition rate as the process requires.
The factors that affect S-powder deposition rate include the properties of S-powder, the
methods for enabling and disabling powder flow, and the parameters used in the
actuator to control powder flow rate.
In this research, the following activities are carried out to develop the SSS process from
a concept to a technology that is proven feasible by produced samples.
1. Theoretical analysis supplemented by complete instruction for the selection of S-
powder paired with commonly used B-powders. (A detailed explanation is given
in the following chapter.)
2. Fabrication of the SSS machine to check the feasibility of the SSS concept and
production of samples with selected S-powders.
3. Design of software for control of the SSS process including the motion system
and the powder deposition system (as illustrated in the scheme of Figure 4-1).
4. Design of experiments to figure out the proper parameters for stable S-powder
deposition.
35

Figure 4-1 Scheme of SSS Printing Process on Top Level
4.1. Research plan
The research has been carried out following the listed research plan:
 Analytical analysis and experimental runs to select the proper S-powders;
 Identification of the effective vibration method compatible with SSS process;
 Investigation of the factors affecting the S-powder deposition rate through the
nozzle;
 Testing of the SSS process by fabricating samples with comprehensive
consideration of printing speed, deposition rate and S-powder selected.
36

4.2. Major stages of SSS research
 Successful testing of selected S-powder in separation B-powder candidates. In
this stage, the selected S-powder is used to separate the B-powder in a
controlled experiment.
 Identification of the compatible powder control actuator with the SSS process.
The actuator that controls the S-powder deposition will satisfy the requirement of
minimizing the effects on positional accuracy and with a stable powder flow rate
control.
 The standalone S-powder deposition test proves to control factors that enable a
stable, long lasting S-powder deposition. This stage shows that the deposition
flow rate can be stable and also that the separation line in the printing will have a
uniform width, thus enabling a smooth surfacing of the parts.
 Fabricating samples that have satisfying surface quality. This is the final testing
stage of the SSS process in which the samples can be separated easily while
having a smooth surface quality.
4.3. Research methodology
4.3.1. Analysis
Analysis is carried out to preclude all experiments to quickly figure out a narrowed
direction for experiments. The search for S-powders and vibration methods are
accelerated by working with the laws of physics and chemistry.
As a consideration for the S-powder candidates, the required properties are chemical
inertness, stability in air, high melting and sintering temperatures, and non-reactivity
with the base materials throughout the sintering cycle. Analysis narrows the searching
into a more specific field and quickly points to the proper candidates currently in use.
There are known methods for activation of powder flow. Through an understanding of
the requirements of SSS process, a stable and non-interruptive activation approach with
piezo enable vibration has been chosen. The following experiments also prove the
effectiveness of this choice.
37

4.3.2. Experimental verification
Experiments verify the prediction provided by the theoretical analysis and provides
feedback for further theoretical analysis. The theoretical analysis provides targeted
candidates that are affordable for experimental verification. For example, the theoretical
analysis narrows the searching of S-powder materials with a high melting temperature
into the metal and oxide families where around 10 candidates are to be tested.
Experiments nevertheless reflect all the factors in the real practices and may reveal
some factors which are not considered or not foreseen during analytical stages.
In the experiments of S-powder selection, the experiments expose the compaction in
the S-powder as an important factor which is not recognized in the first place. Further
experiments are improved to reduce the effect of compaction. Experiments also provide
new insight and better understanding of the laws behind. Such as in the powder
deposition process, the arch pattern that holds the powder from free flow is found to
have discrete deposition properties. Such findings may serve in other field with high
accurate powder feeding.
4.3.3. Future development
Based on the analysis and experimental research, crucial factors will be seen and laws
behind the operating principles may be revealed. The factors that deserve further study
for maturation of the SSS process in quality control will be suggested in the end of this
research.

38

5. Selection of S-powders materials
After sintering, the part is separated from the surrounding material along the separation
walls. Easy separation is a pre-requisite for smooth parts surface without the risk of
fracture. A difficult separation commonly means the part is connected with the
surrounding materials which may cause roughness or fracture in the case of ceramic
additive manufacturing. As mentioned above, separation depends on the S-powders to
remain loose after sintering. For most of the cases, the separation is due to higher
sintering temperatures of S-powders compared to that of B-powders. Other standards
for S-powder selection may include: low cost, easy accessibility, non-toxicity
(preferably), and also no chemical reaction with the B-powders. The sintering profile is
accordingly set to ensure a desired sintering degree of B-powder while leaving S-
powder untouched. If the difference between the sintering temperature of S-powder and
B-powder is large, then there will be more space in choosing the sintering profile.
5.1. General analysis of S-powder candidates
The criterion of high melting temperature (correspondingly high sintering temperature
[62]) can be used as the first criterion used to filter out the improper candidates of S-
powders. In the industrial market, there are numerous powders available, ranging from
daily used flour powder to specially used gold powders.
From a systematic view, all powders are solid and there are four types of crystalline
solids [63]:
 Ionic solids: made up of positive and negative ions and held together by
electrostatic attractions (e.g. MgO).
 Molecular solids: made up of atoms or molecules held together by Van der Waals
force, or hydrogen bonds (e.g. Ice).
 Atomic solids: made up of atoms held together by covalent bonds (e.g. SiC);
 Metallic solids: made up of metal atoms held together by metallic bonds (e.g. Fe).
For the nature of bonding force, the molecular solids has the lowest strength and lowest
melting and decomposing temperature. All organic candidates, composed of
carbohydrates fall in this category are not considered for S-powders. The atomic solids
39

contain largely carbides, nitrides and are mostly used for cutting and surface
smoothening. These materials are not stable under high temperature and there are less
spherical powders available in the market.
Oxides from the ionic category have high melting temperature members such as
magnesium oxide, aluminum oxide, zirconium oxide and these are stable in the ambient
environment. Their spherical powders are also easily accessible in the market.
Some metals have a very high melting temperature and can work with other metals in
the protected environment. For example, tungsten powder has the highest melting
temperature compared with other metals and its spherical powder is easily accessible in
the market.
Based on the above analysis, candidate materials are restricted to metals and oxides.

Figure 5-1 Melting temperature of metals and oxides. (a) Metal by element (b) Ceramic Oxides

5.1.1. S-powder candidates-metal
The most used metallic materials in the industry include aluminum, copper, iron,
titanium and their respective alloys. All these materials have melting temperatures
below 2,000 ° C as illustrated in Figure 5-1.a. In the figure, the elements having
exceptional high melting temperatures are molybdenum, tungsten and uranium.
40

Uranium is radioactive and thus not within the consideration. Molybdenum and tungsten
have a high melting temperature and may be good candidates of S-powders.
Among the remaining metallic materials, chromium has the highest melting point at
1,907 ° C. The others have lower melting temperature at approximately 1,500 ° C. While
implied by the operating principle of SSS, an S-powder only needs to have a higher
sintering temperature compared with its paired B-powder, therefore, stainless steel
powder may serve as S-powder for bronze as B-powder.
Tungsten is a heavy metal with an extremely high melting temperature. The bulk
material density is 19.25𝑔 /𝑐 𝑚 3
and the melting temperature is 3422 °C at one
atmospheric pressure. Tungsten has a melting temperature that exceeds the melting
temperature of all other metals by a large margin. The MP of tungsten is about 800°C
higher than that of molybdenum at 2623°C as shown in in Figure 5-1.
Besides tungsten, other materials can also be used as the S-powder provided that it has
a higher sintering temperature that is comparable to that of the B-powder in use. For
example, stainless steel powder of H13 (Carpenter Powder Products8) with 𝐷 50
=
45 𝜇𝑚 as illustrated in Figure 5-3, may be used as S-powder to pair with a bronze
powder as the B-powder. The criteria of choosing the S-powder is checked as followed.
Firstly, H13 powder and bronze do not react to form new alloys. Secondly, the sintering
experiment was carried out at 800 °C and the sintering temperature of H13 started at
about 1100 °C. Experiments showed that stainless steel powder succeeds to separate
the part with ease Figure 5-4.

8
Carpenter Powder. Lot 015-186, Alloy: H13. Size: -45u/15u
41

Figure 5-2 Printed pattern of different S-powder (a) W-90 (b) H13

Figure 5-3 SEM image of H13 powder Figure 5-4 A part built with S-powders half of H13, half of
W90
5.1.2. S-powder candidates-ceramics
The melting temperature for ceramic oxides are plotted in Figure 5-1.b, the ceramic
oxides that have melting temperature above 2,000 ° C include aluminum oxide (Al2O3),
beryllium oxide (BeO), calcium oxide (CaO), zirconium oxide (ZrO2), magnesium oxide
(MgO), thorium dioxide (ThO2). They are ordered in increasing melting temperatures.
Among them, beryllium oxide is toxic and of high cost, calcium oxide reacts with carbon
dioxide in the air and thorium dioxide is rare not to mention it is radioactive. Therefore,
the applicable candidates are alumina, zirconia and magnesia, all of which are of low
cost, of different sizes and shapes, and can be mass produced industrially. Therefore,
the appropriate candidates are narrowed to alumina, zirconia and magnesia.
42

Because there are no chemical reactions, the only relevant factor is the sintering
temperature. Alumina, zirconia and magnesia, which have a high sintering temperature
are guaranteed to work as S-powders for a lot ceramics and most metals. Experiments
carried out have confirmed the effectiveness of them as S powders.
Despite the fact that ceramic powders function well as S-powder in separation, low
density and large cohesive forces have prevented them from being deposited fast under
current experimental setup. Study on the flow rate of powder is covered in the followed
chapters. The other high density and strong flow rate candidates are metallic powders
molybdenum and tungsten. Molybdenum is less commonly used and therefore tungsten
is the optimal choice.
5.2. Candidates of S-powder materials with B-powders being metal
High temperature ceramics and metals may be used as S-powders for available metallic
powders as B-powder.
Experiments have been carried out to check the capability of proposed S-powers on
metallic B-powders. Bronze powder (AcuPowder 5890) is tested first for its low sintering
temperature with the paired S-powder being alumina. The sintering profile of Bronze is
as illustrated in Figure 5-5.

Figure 5-5 Sintering profile of bronze powder (AcuPowder 5890)
43

Experiments show that alumina is efficient in separating sintered bronze part as
illustrated in Figure 5-6. To figure out what types of B-powders may work with alumina
powder as S-powder, it is necessary that the sintering temperature of alumina is found.
In SSS, the deposited S-powder is not compacted, so the same type of loose alumina
powders are placed into the furnace for sintering.

Figure 5-6 Bronze sintering with alumina as S-powder (a) Printing pattern (b) Separated sample
Alumina powder stays loose after being heated to a temperature as high as that of the
melting temperature of stainless steel. Samples of alumina powders are held at a
temperature of 1,500 ° C for four hour in the ambient environment. The sample has a
mean size of 56.2𝜇𝑚 . After the sintering process, no sign of sintering for the alumina
powder is shown as illustrated in Figure 5-7. SEM image in Figure 5-7 does not show
any sign of sintering either.
44

Figure 5-7 Sintering test of alumina powder (a) Loose powder pile (b) SEM image
Tungsten as an S-powder is tested with 316L stainless steel (45𝜇𝑚 ) as B-powder. A
spherical powder is used for deposition and the separation wall thickness is about
500 𝜇𝑚 . The green part is sintered to a temperature of 1200°C and kept for 1 hour. The
cooled down part can be separated easily and the obtained part has smooth surface as
illustrated in Figure 5-8.

Figure 5-8 316L steel sintering with tungsten as S-powder (a) One layer printing (b) A separated part
45

Because the sintering process of metal is required in a vacuum condition, the
untouched S-power may be reused if filtering of the tungsten powder can remove any
possible contaminations. If necessary, cheaper materials like steel powders can be
used to act as the S-powder for multiple other metallic powders.
In this experiment, tungsten is chosen as the S-powder. With its melting temperature
being the highest of all metals, tungsten can actually be used as the S-powder for all
other metals.
5.3. Candidates of S-powder materials with B-powders being ceramics
Ceramics used as B-powders can be separated by either S-powders of tungsten or
higher sintering temperature oxides.
In the cases where tungsten is used, the sintering process must be carried out in a
vacuum furnace. Tungsten has a high MP of 3422 °C, but it reacts with oxygen at high
temperatures. Its oxide has a MP of merely 1473 °C, which is lower than commonly
used ceramic B-powders such as alumina, silica. A requirement of the vacuum may
inevitably increase the cost of the process, but will work well for ceramics where
vacuum is preferred.
The alternative choice is to use stable ceramic powder as the S-powder. Alumina has a
melting temperature of 2072 °C and may serve as the S-powder for lower MP ceramics.
For example, in the project of lunar ISRU (In-situ resource utilization) construction,
alumina is used as the S-powder for separation of lunar regolith simulant (JSC-1A
9
) as
illustrated in Figure 5-9. Magnesium oxide has a melting temperature of 2856 °C and
zirconium oxide has a MP of 2715 °C. Therefore, the two of them can also be used as
S-powders in separating other low MP ceramics.

9
JSC-1A is purchased from www.orbitec.com
46

Figure 5-9 SSS printed interlocking tile units made of JSC-1A (a) Printed pattern on base powder (b) Sintered part
before separation (c) Interlocking pattern formed by separated sintered parts
If zirconia or magnesia is used as the B-powder, tungsten powder may be used as the
S-powder for easy separation. Using ceramic powders as S-powder can in fact have
real world application. In places where only ceramic is accessible, such as in the case
of space colonization, magnesia is abundant while tungsten is less common [2].
5.4. S-powder selection for B-powders of super high melting temperature
It is difficult to find a proper S-powder to pair with B-powder materials of high melting
temperature, such as in the case where tungsten is to be sintered. If the melting
temperature of S-powder and B-powder is close, the difference may not be enough to
separate the sintered part as both powder may be sintered at the same time. The
melting temperature of magnesia is about 140 °C higher than zirconia and it will be risky
to use magnesia as S-powder for zirconia as B-powder. For tungsten powder, it is vastly
more difficult to find a material which can be made into a powder and deposited
successfully while having higher melting point compared with tungsten.
A chemical method may be used to assist the separation in the case of tungsten as B-
powder. Tungsten, not only has a high melting temperature, it is also inert to some acid,
such as hydrochloride acid. In the experiments, 10.00 grams of magnesium oxide and
10.00 grams of tungsten powders are put into a test vile which has abundant
hydrochloride acid. The material is left untouched in a test bin for 12 hours. In about 10
minutes, magnesium oxide was completely dissolved in the acid solution while tungsten
powder did not shown any change in the end of the experiment. The powder was left in
its container with the presence of strong ventilation for 2 days for fully vaporization.
47

Afterwards, the measured tungsten powder was seen to still be 10.00 grams. The
experiment proved that magnesium oxide can be used as the S-powder for the sintering
of tungsten powder and also that the separation can be assisted with hydrochloride acid
after the sintering is complete.

Figure 5-10 Hydrochloric acid with tungsten powder and magnesium oxide (L) MgO (R)W90
The same idea can be applied with magnesia and zirconia. Alumina has a comparable
lower melting temperature but alumina can dissolve in a strong alkaline solution while
the other two do not. Therefore, after the part is printed with aluminum oxide and
sintered in the furnace, the part will first be submerged in an alkaline bath for a period of
time to remove the fused alumina powder as the S-powder.
5.5. Summary
The selection of S-powders is based on the principles of easy separation. A powder of
higher sintering temperature may be the S-powder of other materials with lower
sintering temperatures that are used as B-powders. Separation with this methodology
benefits from no post processing. For powders where proper S-powders cannot be
48

found, a chemical method may be used to solve the problem, as previously explained in
the case of tungsten. The magnesium oxide used as an S-powder can be later removed
from the sintered part by acid.
Overall, for ceramics, if an ambient environment or low cost production is concerned,
aluminum oxide, zirconium oxide and magnesium oxide will be used based on their
accessibility and sintering temperature of the B-powder. For experiments where a
vacuum condition is preferred, ceramic powder can still be used; although tungsten may
also be applicable in the vacuum.
For metals, the powders have to be processed in a vacuum condition. Therefore
tungsten can be safely used as the S-powder without being oxidized. For large batch of
industrial applications, the used S-powder can be recycled with ease.
In later chapters, the flowability and stability of the S-powders will be analyzed to
achieve the desired printing speed.

49

6. Consideration of S-powder size and separation line width for
effective separation
Like the macroscopic inhibition effect in SIS, in SSS the parts are separated by the
existence of “separation walls” (multiple layers of separation line). The separation walls
are formed by the S-powder deposited during printing process and remain loose after
the green parts are sintered in the furnace. To effectively separate the parts but not to
pollute the parts by penetration, or coarsening the parts, the S-powder size and
separation line width shall be in the right range, neither too large nor too small.
6.1. Analysis on S-powder size selection
When powders of different sizes are put together as neighbors, the small powders tend
to fill the vacancy formed in the large powder structures, and they mix. This is first
principle that governs the size selection of S-powders or B-powders. A typical non-
compacted powder has a porosity of 30%~40% or a bulk density of 60~70%. To simplify
the problem the porosity can be considered to be the vacancy in the packing structure
formed by the powders. Without loss of generality, the largest powders will be
considered only as the structural powder. Since if the other powder can penetrate into
the vacancies of largest powders, the material is polluted. Therefore, a mono sized
model can be used. The packing structures are analyzed to figure out the allowed S-
powder size range. The packing efficiency of different crystal structures are: face
centered cubic about 74%, body centered cubic about 68% and simple cubic about 52%.
The bulk density of powder is close to that of primitive cubic and body-centered cubic.
6.1.1. Minimum required size of S-powders in consideration of slipping into B-powders
regions using primitive cubic model
The bulk density ratio of the loose powder is between the packing efficiency of SCC and
BCC structures. The respective packing efficiency and the largest powder that can slip
into the voids of the packing structures are listed in Table 6-1.

50

Table 6-1 Lattice parameters of SCC and BCC models
Lattice Structure Packing efficiency Largest allowed insertion size in D
SCC 52% 0.414D
BCC 68% 0.155D

Figure 6-1 Illustration of simple cubic model (rod sphere model & sphere model)
As illustrated by the simple cubic model in Figure 6-1, a smaller powder may slip into
the void of the powder lattice. Denote the edge of the cube to be D, which is the
diameter of the sphere. The largest sphere that can slip into the voids has a diameter of:
Equation 6-1 𝒅 = √𝟐 𝑫 − 𝑫 ≈ 𝟎 . 𝟒𝟏𝟒𝑫
Similarly, for body centered cubic model, the edge is denoted by 2/√3D, and that
largest sphere has a diameter of:
Equation 6-2 𝒅 = 𝒎𝒊𝒏 {
𝟐𝑫
√𝟑 − 𝑫 ,
𝟐 √𝟐 𝑫 √𝟑 − 𝑫 } ≈ 𝟎 . 𝟏𝟓𝟒𝑫
In fact, the loose powder pile will not follow any crystal structures, the simple cubic
structure is a close structure compared in consideration of porosity. The largest
powders that may slip into the B-powder region has a diameter 41% that of the largest
B-powders. As most powders of 325 meshes (<45 𝜇𝑚 ) are used in 3D printing field, the
powders that can slip into B-powder is 18.63 um. Without consideration of the actual
51

powder flowability, S-powders with a size below 18.63 um should not be used for B-
powders of 45 𝜇𝑚 .
Experiments show that small S-powders do flow into the voids of large B-powders. A
glass powder with a 500 𝜇𝑚 diameter is used and the tungsten powder of 45 µ m (W45)
is chosen as seen Figure 6-2.a. Tungsten powders flow into the voids formed by glass
powders and adapt the shape locally. While the same tungsten powders deposited into
ceramic powder of 45 𝜇𝑚 stay with sharp edges and keep a near rectangle shape. If the
B-powder has a larger cohesive force, such as in the example of Figure 6-2.b, the
powder do not collapse after the nozzle passes by. The complete gap will be filled by
the B-powder.

Figure 6-2 Cross section of tungsten powder deposited into powders (a) Glass powder of about 500 𝝁𝒎 (b) Ceramic
powder of 45 𝝁𝒎
6.1.2. Maximum size required of S-powders in consideration of B-powders
interconnection
The above section talked about the case that too small S-powder may slip into B-
powder voids and stay as contamination for the final parts. Therefore, S-powders should
be larger than a minimum value. While too larger S-powder may have their voids
occupied by B-powder. If the B-powder diameter is smaller than 41% of the S-powders
as calculated in previous section, B-powder may slip into the voids of the S-powders
and connect the part section across separation walls. For the most used B-powders with
a size of 45 𝜇𝑚 , to contain B-powders inside, the S-powders should have a size above
52

110 𝜇𝑚 . The 110 𝜇𝑚 S-powder is rarely used as the separation line width will be too
large.
B-powders may slip into the S-powders voids and bond with materials on both sides of
S-powders which may cause failure in separation. As shown in Figure 6-3 (a), the B-
powders sitting in the voids of S-powders may serve as a bridge connecting B-powders
on both sides of S-powder separation wall, causing a failure in the final part separation.
While if the S-powder wall is comparatively thick as in Figure 6-3 (b), the B-powders will
not be able to connect with each other.

Figure 6-3 B-powders in S-powders vacancies (a) Materials connected on both sides for thin S-powder wall (b)
Materials separated by thick S-powder wall
Even if the separation walls are tunneled by B-powders, the final part separation may
not necessary fail due to the weak bonding of B-powders through separation walls. In
the sintering process, the S-powders will stay in their position and the tunnel has to go
through the holes in B-powders. The neck formed in this case will be hindered by the
existence of S-powders and cannot grow up. Thus the formed tunnel may not be strong
enough to fail the separation. While B-powder filling up S-powder voids will reduce the
surface smoothness.
6.1.3. S-powder selection in consideration of parts surface quality
As the S-powders are deposited into the B-powders layers, the spatial distribution of S-
powder neighboring the B-powder will be reflected in surface smoothness after
53

separation. An indent of hemisphere remains after the sphere is removed, like pushing
a sphere into a piece of plasticine. If post processing with machining is required, the
parts can just be designed with enough margins to be machined off. Otherwise, a
surface quality concern will be passed on to the S-powder selection. Considering the
scenario that all the S-powders line up and have a roughness as in Figure 6-4. The best
surface smoothness is the value of the radius and the general cases will be worse than
that by a factor of 2.

Figure 6-4 Surface roughness caused by S-powder patterns
In combination of the above analysis, S-powders selection should avoid B-powder
contamination, interconnection and meet surface quality. For the most used 45 𝜇𝑚
sized B-powders, the S-powder should be between 18.6 um and 109 um and meet the
specific surface quality.
6.1.4. Exceptional dense B-powder by multi-modal powder mixtures
The premise of the above analysis is the large porosity ratio that may allow S-powder to
flow into the holes. For the powder mixed from different sized powders with a specific
ratio, the powder porosity may be reduced significantly. In a study on laser selective
sintering, the density of thin layers increases from 53% to 63% when adding 30% of fine
powder to the coarse powder, with a coarse-to-fine ratio of 1:10 [64]. A ternary powder
54

mixture can reach a loose density of 94%, which does not leave too much space for
small S-powder to fill. In such cases, the voids of B-powders are already occupied that
S-powder is not going to cause any contamination. In this case, the only concern is to
prevent B-powder from penetrating into S-powders and forming inner connection tunnel.
The other exception is that the powder in use are very cohesive, it can be the B-powder,
or S-powder, or both. The above analysis is based on the assumption that the powder
can flow with ease that small powders will tend to flow into the voids formed in large
powders. In reality, many powders are so cohesive that penetration into other powder
voids will be impossible. For very cohesive powder, a vertical wall may even stand
without support, as shown in Figure 6-5. As the nozzle passes by, the gap made by the
nozzle stays in the steel powder of A11 (D50 about 20𝜇𝑚 ) which has high cohesion.
Therefore, if the B-powder in use is cohesive that flowability is very poor, then there is
no limit for the upper size of the S-powders in consideration of B-powder connected
though the separation walls. While small S-powders may still flow into the vacancies of
B-powders. If the S-powder in use is cohesive, then there is no lower limit for the size of
S-powder but small B-powder will still flow and fill the vacancies formed in S-powders.

Figure 6-5 Powder of different cohesion from large to small (a)A11 (b)316L (c) Bronze
55

6.2. Thinnest separation wall thickness for effective part separation
The effective separation wall thickness also considers the possibility of powder
penetration, but is determined by the flowability of S-powder in the chosen conduit,
which is covered later. If a thin layer of film at micron meters scale, is used to divide B-
powder into two sections, then the final part is guaranteed to be easily separable.
The thickness of the powder cannot be smaller than the diameter of the largest powders.
Previous analysis indicates that for B-powders of 325 meshes (45 um), the S-powder
size choice should be larger than 18.6 um and the theoretical thinnest wall is 18.6um. A
singer layer powder deposition is not easily achievable from thin conduit deposition, and
thicker separation wall is more reliable for separation. An experiment is carried out to
test the analysis. A lunar regolith simulant JSC-1A sifted to below 250 𝜇𝑚 (purchased
from orbitec.inc) and an alumina powder sifted to 74 um are used as the B-powder and
S-powder respectively. Depositing for a large area for verification is difficult and less
accurate, a powder spreading method is used.
 A 5mm thick layer of B-powders is paved onto the platform;
 A 100 um thick layer of S-powder (alumina) is paved above;
 Another layer of 5 mm thick layer of B-powders is paved above the S-powder;
 Sinter the parts in the furnace;
 Check the ease of separation.
The experiment shows that the separation is very easy. Actually, due to thermal stress,
the B-powder sections separated by the paved S-powder film curled up and led to
automatic separation in Figure 6-6. Despite the fact that during S-powder spreading,
some coarse particle of B-powder protrudes through the S-powder layer, the part is still
easy to separate. Theoretically, as long as the S-powder forms a layer that separates
the B-powders, the part is separable.
56

Figure 6-6 Effective separation thickness test (a) S-powder spread between two B-powder layers (b) curled top B-
powder layer
6.3. Summary
In summary, the selection of S-powder size shall be based on the principle of no
contamination and easy separation. The guideline for the S-powder size is to be about
the same size as that of the B-powders. The thinnest separable thickness will be larger
than the size of the S-powder diameter but proven effective with a value of 100𝜇𝑚 .

57

7. Properties of S-powder in a nozzle
In the SSS setup, the S-powder is deposited through a nozzle into the previously paved
B-powder layers. Several factors affect the dry powder deposition of the S-powder,
namely the choice of the S-powder (material, shape, density, size, moisture, etc.), the
type of nozzle (size, length, etc.), the methods of powder flow on/off control, and the
parameters related to that powder control method.
In this chapter, the properties of S-powder deposition through a nozzle are studied. The
covered factors include the intrinsic flowability of the powders as reflected by repose
angle; the position where an arch pattern forms in a nozzle and how fast the arch
pattern rebuilds after an agitation is applied; the proper selection of nozzle sizes to pair
with the S-powder for stable while fast deposition.
7.1. Intrinsic flowability of powders
The flow rate of any given powder is dependent on its material properties, which include:
material species, shape, size, bulk density, humidity and porosity (related with the
compaction rate). Specifically, material species, shape, size, and bulk density are static
properties. Humidity and porosity, on the other hand, are dependent on the
environment and compaction rate respectively.
The flow rate of the powder is a direct reflection of the forces within the powder. The
powder as a cluster of particles do not have surface tension. Therefore, if the
interparticle forces diminish suddenly, gravity will drive the powder to spread to a thin
layer thickness, like a liquid wets on a flat surface. With the interparticle forces, the
powder may maintain different shapes. The interparticle forces affecting the flow of
powders include mechanical interlocking, capillary force, static force, and Van der
Waals force.
7.1.1. Mechanical interlocking
The powder composing of many particles are mostly not spherical, some of them are
quite irregular. Particles of irregular shapes can include structures similar to hooks or
loops. Such irregular particles may be interlocked and increase the friction where there
is a trend of powder flow. As seen in Figure 7-1, particles with concave and convex
58

shapes may fit with each other, thereby stopping powder flow when forces are applied
onto the pile of powder. Tungsten powders C110
10
show such instances of irregular
shapes and hence poor flowability is measured.
While conducting powder deposition experiments, the results show that powder C110 is
incapable of flowing through a nozzle of inner diameter 500𝜇𝑚 . In contrast, while
conducting experiments for tungsten powder W45
11
of spherical shapes, the powder
flow smoothly through the same 500𝜇𝑚 nozzle. The properties of these two powders
are listed in Table 7-1.

Figure 7-1 Mechanical locking of powders (a) Scheme of interlocking (Source: Freeman Technology) (b) SEM
microscopic images of C110 Tungsten powder
Table 7-1 Comparison of C110 and W45

Tap
density
D10/𝜇𝑚 D50/𝜇𝑚 D90/𝜇𝑚 Flow under gravity in a 500 𝜇𝑚 nozzle for 60 s
C110 9.09 ~34.2 ~46 ~70 ~0.1g
W45 10.8 24.6 33.8 49.1 ~15g

10 C110 tungsten powder is purchased from Buffalo Technology
11
W45tungsten powder is purchased from Tekna.inc
59

The size and the density do not show significant differences for the two powders, thus
the clogging of powder flow for C110 can be attributed to its irregular shapes. The SEM
images of the spherical tungsten powder W-45 is shown in Figure 7-2. The flow rate is
not affected despite the small concentration of impurities in the powder.

Figure 7-2 SEM image of W45
In SSS, the fast deposition rate corresponds to fast printing speed and powder with fast
deposition rate is preferred. Therefore, irregular powders that may form interlocking will
not be considered.
7.1.2. Cohesive force
Mechanical interlocking exists only in the powders of irregular shapes. In contrast,
cohesive forces including capillary force, static force and Van der Waals force exist
between all powders.
60

Capillary force is due to the surface tension of the liquid bridge between particles. For
powders of high moisture levels, the condensed vapor will form liquid bridges among
powders as seen in Figure 7-3; such capillary forces can be hundreds of times stronger
than the gravitational force [65,66]. While in SSS, the S-powder is kept in a controlled
dry container. In general, the experimental environment is kept at low moisture levels in
order to circumvent and ignore any capillary force.

Figure 7-3 Scheme of capillary force (Source: Freeman Technology)
Static force is the result of particles charged with static electricity. In the SSS
experimental setup, powder is initially static in the powder reservoir. When vibration is
applied on the nozzle, the powder in the nozzle moves down together and there is
limited chance for generating static charges. Static force should have an
inconsequential role in SSS. Experiments have been carried out to test if static force
affects the powder deposition. A neutrally charged rubber rod was used to check if any
deposited powder was adsorbed onto it. The results of the experiment confirmed that
the powders did not adsorb onto the rubber band as a proof that the powder is not
charged.
Such an experiment confirmed that the static force will not be of any significant effect to
the powder flow. Analysis from other researchers also point out that static force is
insignificant compared with Van der Waals force [67].
61

Thus, the force of interest in the research is Van der Waals force. Van der Waals force
exists between powders and increases with reduced distances.
According to Hamaker’s calculation in 1937 [68], the Van der Waals' interaction energy
between spherical bodies of radii R1 and R2 and with smooth surfaces was
approximated (using London's famous 1937 equation for the dispersion interaction
energy between atoms/molecules as the starting point) by:
Equation 7-1 𝑼 (𝒛 ; 𝑹 𝟏 , 𝑹 𝟐 ) = −
𝑨 𝟔 (
𝟐 𝑹 𝟏 𝑹 𝟐 𝒛 𝟐 −(𝑹 𝟏 +𝑹 𝟐 )
𝟐 +
𝟐 𝑹 𝟏 𝑹 𝟐 𝒛 𝟐 −(𝑹 𝟏 −𝑹 𝟐 )
𝟐 + 𝐥𝐧 [
𝒛 𝟐 −(𝑹 𝟏 +𝑹 𝟐 )
𝟐 𝒛 𝟐 −(𝑹 𝟏 −𝑹 𝟐 )
𝟐 ])
A: Hamaker coefficient, a constant (~10
−19
− 10
−20
J) that depends on the material
properties (it can be positive or negative in sign depending on the intervening medium)
z: The center-to-center distance; i.e., the sum of R1, R2, and r (the distance between the
surfaces): .
In the limit of close-approach, the spheres are sufficiently large compared to the
distance between them; i.e., , thus equation (1) for the potential energy
function simplifies to:
Equation 7-2 𝑼 (𝒓 ; 𝑹 𝟏 , 𝑹 𝟐 ) = −
𝑨 𝑹 𝟏 𝑹 𝟐 (𝑹 𝟏 +𝑹 𝟐 )𝟔𝒓

The Van der Waals force between two spheres of constant radii (R1 and R2 are treated
as parameters) is then a function of separation since the force on an object is the
negative of the derivative of the potential energy function:
Equation 7-3 𝑭 𝒗𝒘
(𝒓 ) = −
𝑨 𝑹 𝟏 𝑹 𝟐 (𝑹 𝟏 +𝑹 𝟐 )𝟔 𝒓 𝟐
The force is linear to the size of the powders and inversely linear to the square of the
distance between the powders. As the powders get smoother and smaller, the force
increases rapidly. Calculations show that Van der Waals force is more than 1000 times
the gravitational force for glass beads with a density of 2.5 𝑔 /𝑐 𝑚 3
and a diameter of 50
𝜇𝑚 [67].
In Chapter 5, a few ceramic and metallic powders have been identified as the
candidates of S-powders. The SEM images of the powders tested are displayed in
Figure 7-4.
62

Figure 7-4 SEM image of ceramic powders (a) sa 0180 (b) sa0109 (c) alumina (d)zirconia
As observed from the SEM images, the tiny ceramic particles tend to adhere to each
other and the surface of the larger particles, which causes them to cluster together. This
is a reflection of the large ratio of cohesive force to gravitational force. The microscopic
images of the tungsten powders are shown in Figure 7-5. As the density increases, in
powders of the same particle size, the gravitational force plays a more significant role.
For example, the spherical tungsten particles, which are the same size of the ceramic
powder, are generally observed to be looser and not clustered.
63

Figure 7-5 SEM microscopic images of tungsten powder. From left to right: W25, W45, W90
To have an explicit understanding of the flow properties of the powders, repose angles
were measured. A standard ASTM Designation: C 1444 requires the use of a greater
quantity powder. As measured powder may be contaminated and cannot be reused, it
will be a large waste of powder. It is also important to note that the measured repose
angle flowing through a funnel does not entirely reflect the practical situation for SSS in
which powder flows through a thin conduit under vibration. A lab built facility was
subsequently used to replace the standard facility for measurement of all powder
repose angles.
A syringe nozzle with an inner diameter of 10 mm and a syringe needle of gauge 18
with an ID of 0.9 mm were used. The tip of the needle is 1 cm away from the plate
where powder is deposited. For each test, about 5 cc of powder is fed into the syringe.
The vibration frequency of 4 kHz is applied to assist the flow. The properties of the
powders tested are illustrated in Table 7-2.

64

Table 7-2 Material properties of powders
Powder types Materials Density/
g/cc
Upper Sieve
Size/um
D50/um Repose angle
SA0180
12
Alumina 1.90 75 30 64.7
SA0109
13
Alumina 1.92 15 2.5 75.3
Alumina
14
Alumina 1.82 90 56.2 55.0
Zirconia
15
Zirconia 2.84 100 33.8 60.1
W-25
16
Tungsten 10.4 30 19.1 44.8
W-45 Tungsten 10.8 50 33.8 37.5
W-90 Tungsten 10.7 100 62.3 20.7
C110 Tungsten 7.5 100 46 48.5
The measured repose angles are plotted according to the powder size.

Figure 7-6 Static repose angle v.s. Powder size and type

12 Purchased from Industrial Powder
13 Purchased from Industrial Powder
14 JT Baker® 0537-01 | 500g
15 Provided by Saint Go-bain
16 Purchased from Teka.inc
65

Figure 7-7 Lab built repose angle testing facility for fine powders

As observed in Figure 7-6, repose angles generally increase with decreased powder
sizes. For spherical tungsten powders, as D-50 decreases from 62.3 𝜇𝑚 to 19.1𝜇𝑚 , the
repose angle increases from 20.7° to 44.8°, the repose angle is roughly linear with the
powder size. For spherical ceramic powders, the same trend is observed where the
increased powder size yields reduced repose angles. For irregular alumina powder
which does not have sharp edges, it also roughly lies on the line for ceramic powders.
Powders of larger densities tend to have smaller repose angles, which can be explained
by the fact that the repose angle is the balance between the cohesive force and
gravitational force. The relationship between the repose angles with the powder sizes
can be attributed to Van der Waals force.
66

7.2. Arch formation and breaking
Powder in a nozzle may form an arch pattern to stop powder flow, which may be
disturbed under agitation to allow powder flow. In this experiment, a regular hypodermic
needle with a blunt end is used as the nozzle for experiments. On the lower end of the
nozzle, there is an opening as illustrated in Figure 7-8.
7.2.1. Identification of arch formed position
The formation of the powder arch may be in the lower thin conduit part of the nozzle or
may form just at the connection where the plastic cup narrows into the needle.

Figure 7-8 Deposition nozzle in SSS (a) Scheme (b) Nozzle with powder deposition on

67

If the arch forms in the thin conduit (the thin pipe section), the length of the conduit will
determine the strength required to break the arch. As the conduit is uniform, the arch
shall form everywhere inside the conduit. Therefore, the longer the nozzle, the more
difficult it is to break all the arch to allow powder flow. Two experiments have been
conducted to verify whether this assumption is correct.
 The powder deposition rate of two nozzles of the same size but different lengths
are to be compared. A resonant frequency of 4.786 kHz with the piezo disc
activated vibration is used for a deposition period of 30 seconds. Both nozzles
have an inner diameter of 240 𝜇𝑚 with lengths 0.5 inches and 0.25 inches,
respectively. The powder used for the experiment is W-25. Results show that the
length of the nozzles is insignificant in the powder deposition rate.

Figure 7-9 Flow rate with respect to the nozzle length
 An alternative approach to verify the aforementioned assumption is by direct
observation of the powder flow. A transparent glass conduit made from a
capillary tube is used to replace the original steel conduit. The new nozzle, which
has a transparent conduit, is used for observation of the arch formation. The
inner diameter of the glass conduit is 310𝜇𝑚 . A similar experiment setup is used
68

where a piezo disc is used to vibrate the nozzle to enable powder flow. In the
experiments, it is observed that when the vibration stops, the excess powder in
the conduit continues to flow and is discharged until there is no powder in the
conduit.

Figure 7-10 Nozzle with a transparent glass conduit
From these two experiments, it can be concluded with confidence that the arch is not
built inside the thin conduit at the end of the nozzle. One possible explanation is that the
inner wall of the nozzle is very smooth, thus the friction is not strong enough to bear the
weight of the powder. Another explanation is that the powder flown into the conduit is
already loosely packed, thereby making the formation of an arch less likely.
Therefore, any arch formed in powder deposition in SSS is at the upper level where the
nozzle narrows to the thin conduit. Effective activation to break the arch should be close
to this position.
7.2.2. Arch pattern building up time
From the previous analysis, the position where the arch formed was identified. Another
question that remains to be answered is how fast the arch can rebuild once it is broken
by vibration. If the rebuilding period takes a very long time, the powder will continue to
flow after the agitation is disabled and such flow may be unstable. If the arch rebuilding
takes a very short time, a high frequency of vibration will be needed to achieve a high
69

flow rate. In the experiment, it was observed that after the vibration stopped, some
powders such as W90 in the nozzle of 240 𝜇𝑚 inner diameters flow for a short time to
stop while other powders such as W25 in the same nozzle stops immediately.
Experiments have been carried out to study the time it takes for the arch to build up. A
longitudinal vibration mode using the piezo disc is set up. The frequencies tested and
the deposited weight is shown in Table 7-3. For runs 1 Hz to 20 Hz, the period is 300
seconds. For the rest of the frequencies, the running time is 30 seconds. In the table, all
the weight is regulated to the value of 30 seconds.
Table 7-3 Deposited W90 weight vs. frequency in 30 seconds
Frequency/Hz 1 5 10 20 100 200 400 800 1600 3200 6400 12800 25600
Weight/mg 8.4 39 75.6 152.2 159 168 181 170 172 177 172 175 173

70

Figure 7-11 Deposition weight of W90 vs. Frequency (a) Frequency from 1 to 20Hz (b) Frequency from 1Hz to 200Hz
(c) Frequency from 1 Hz to 25.6kHz

Experiments show that at low frequencies the powder flows like discrete droplets. From
Figure 7-11 (a), it can be seen that the deposited powder weight is linear to the
frequency. The linear relationship reveals that for each vibration the arch pattern is
broken and a fixed amount of powder is deposited until the arch pattern rebuilds. By
linearly fitting the plots, a linear trend becomes very clear and each tap (on/off of one
71

vibration) generates a flow of 0.25𝑚𝑔 . The linear relationship is apparent until the
frequency reaches 20 Hz.
At the top and bottom of the square wave, the voltage is held constant and the
displacement of piezo disc is zero. It is in the slope area that the vibration causes a
displacement and enables the powder to flow as illustrated in Figure 7-12. From the
oscilloscopic image of the square wave, it can be seen that the descending slope is as
short as 23.4𝜇𝑠 . When the slope ends, the piezo holds its position until another slope is
produced.

Figure 7-12 Oscilloscopic image of the square wave (a) waveshape (b) descending slopes
72

The deposition experiments show that the deposited weight is linear until it reaches 20
Hz. In each cycle, there are two slopes. Therefore, the arch rebuilding time should equal
to about 25𝑚𝑠 . From the beginning where a slope breaks the arch pattern to enable the
powder flow until the end where the arch pattern is rebuilt, the full cycle is found to be
25𝑚𝑠 .
Here, the frequency at which a complete set of powder arches can just be rebuilt
without interference is defined as the full arch frequency.
If the frequency is less than 20 Hz, each cycle can be completed and the deposition
weight should be linearly proportional to the frequency. For frequencies higher than 20
Hz, the vibration is turned on again before the arch is rebuilt. As the arch rebuilding
process is nearly complete, the flow rate slows down and the reapplication of the
agitation increases the flow rate again, thereby causing the deposition rate to increase a
bit more with the flow rate. As the frequency becomes much higher, such as in the case
of 25.6 kHz, the arch rebuilding cannot start, which causes the powder to flow at its
maximum allowed flow rate.
In the case of the W90 powder flowing in the nozzle of ID 240𝜇𝑚 , the arch rebuilding
takes 25𝑚𝑠 . Because the frequency is far below the resonant frequency of 4.8 kHz, the
vibration is not strong, which reveals that such an arch is also weak. Comparative weak
vibration can start the powder flow through the conduit.
In the experiments of W25, the same experimental setup is used. The nozzle has an ID
of 240𝜇𝑚 with a length of 0.5 inches. The same experimental setup is used. Unlike what
was observed with the W90 powder, the W25 powder flow was almost zero until the
frequency reached about 800 Hz where the powder flow increased quickly to its near
maximum level. As the vibration frequency increased, the powder flow rate quickly
reduced to nearly zero at 12.8 kHz as seen in Figure 7-13.
Table 7-4 Deposited W25 weight vs. frequency in 30 seconds
Frequency/Hz 100 200 400 800 1600 3200 4786 6400 12800 25600
Weight/mg 2 3 21 620 720 730 900 720 10 1
73

Figure 7-13 Deposition weight of W25 vs. Frequency from 1Hz to 25.6kHzHz
The sharp increases in the flow rate with the frequency reflects the breaking of the arch
pattern, which has a threshold. As the displacement of the piezo disc peaked at its
resonant frequency, the displacement reduced away from the resonant frequency. The
descend time for the square wave is seen to be almost the same, from 23.4𝜇𝑠 at 100Hz
to 20.0𝜇𝑠 at 1.6 kHz and to 12.4𝜇𝑠 at 12.8 kHz as illustrated in Figure 7-14. The
acceleration is nearly linear to the displacement of the piezo disc, which is further
determined by the frequency.
74

Figure 7-14 The descend time for different frequencies (a) 100 Hz (b) 1.6kHz (c) 12.8kHz
75

From 800 Hz, it can be seen that a linear relationship does not exist between the
deposited weight and the frequency, which means that the interruption of the new arch
breaking starts before the arch has time to complete its cycle. Therefore, the arch
rebuilding time should be greater than 625𝜇𝑠 .
For the deposition of W25, the arch pattern has a strong structure which can only be
broken by a comparatively large acceleration. The rebuilding time of the arch pattern is
buried by the effect of the acceleration. The rebuilding time may be found by increasing
the applied voltage, which in turn increases the overall acceleration.
From the results and comparison of the W90 and W25 deposition tests under the same
parameters, it can be concluded that a stronger arch is more selective on frequencies. If
the frequency is low enough to allow the arch pattern breaking and rebuilding process to
be completed, then the deposited powder will be discrete. Additionally, the deposited
weight will vary linearly to the frequency, provided that the acceleration is large enough
to break the arch.
7.3. Nozzle range for effective powder flow switch on/off
A powder flow can be switched on/off by vibration if the right pairing of powder and
nozzle is chosen. As shown in section 7.2.1, the arch build up on the opening of the
conduit can spontaneously stop the powder flow. Vibrations satisfying certain criteria
have the ability to break the arch pattern and thus allow the powder to flow freely under
gravity.
For a given powder and nozzle, there exists a maximum inner diameter (ID) that an arch
can form to stop the powder flow and a minimum inner diameter that the powder can
pass through as illustrated in Figure 7-15.
76

Figure 7-15 Maximum of the arch spread
The maximum ID can be calculated according to the cohesive force as in
Equation 7-4 𝑾 =
𝒇 𝒄 𝑯 (𝜽 )
𝝆𝒈

𝐻 (𝜃 ): A constant dependent on the hopper opening’s geometry.
𝜌 : The density of the powder.
𝑔 : The gravity constant.
𝑓 𝑐 : Reduction of yield stress.
Although the calculated value gives a suggested quantity, there are practical limitations
due to assumptions and imperfections in the nozzle. Experiments are subsequently
carried out to determine the true values. In the analysis, the candidate powders have
been filtered to a few materials including alumina, zirconia and tungsten. The minimum
inner diameter (ID) of the nozzle can be assumed to be the size of the largest powder in
the powder distribution.
Equation 7-5 𝑾 𝒎𝒊𝒏 < 𝑾 < 𝑾 𝒎𝒂𝒙
Given the fixed approach for vibration application, as described previously, there is a
minimum tube size in which the vibration may break the arch and allow for the free flow
77

of powder. In this experiment, the vibration is vertically applied onto the lower bottom of
the nozzle.
Under a given vibration, the theoretical calculation of the minimum conduit inner
diameter that allows the powder to flow through can be found. The acceleration from the
vibration can then be measured. The achieved minimum value will be a function of
acceleration, nozzle material, and powder properties.
Equation 7-6 𝑾 𝒎𝒊𝒏𝒇 = 𝒇 (𝑷 , 𝒂 , 𝑵 )
𝑃 : powder, which contains the powder shape, distribution, compact ratio, density and
other necessary parameters.
𝑎 : is acceleration applied by vibration. The acceleration is further determined by the
wave shape, amplitude, frequency and etc.
N: is the nozzle in use, which further contains the material of the nozzle, the
smoothness of the material and the opening angle of the nozzle on the upper level.
In the research, further experiments are carried out to empirically find the minimum ID of
the nozzle as illustrated in Table 7-5.
Table 7-5 Flow Rate of Powder vs Nozzle Gauge Based on 2 Minutes Test
ID/µ m

Powders
Nozzle Gauge vs Flow Rate
410 381 340 260 240 210
SA0109 0 0 0 0 0 0
SA0180 25 14 9 10 1 0
ZrO2 275 56 34 5 0 0
Al2O3 700 228 150 43 20 8
W90 leaking leaking leaking 1400 600 54
W45 leaking leaking leaking leaking leaking 800
W25 16350 8864 8652 4732 3648 1204
78

On the other hand, the arch should be strong enough to survive the inevitable and
unavoidable vibration from the translation and rotation motion of the stage. An arch
formed that can barely hold the weight of the powder will break easily and leak with a
small amount of vibration, such as acceleration in rotation or acceleration in the motion.
Such a leakage may cause contamination if it falls in the part section.
Ideally, a well-designed rotational and translational system will not generate vibration
during motion. Unfortunately, acceleration constantly exists in rotational motion as well
as in the beginning and ending of a linear motion. Experiments have observed that
rotational force may lead to powder leakage in the absence of active vibration from the
piezo disc. An illustration of such effect is shown in Figure 7-16 where the centrifugal
force causes a shear force on the powder, which attempts to shift inside and may cause
the powder on top of the arch to break. The start and stop of a translational motion is
also associated with an acceleration. Such an acceleration may break the arch pattern if
the arch is not strong enough.

79

(b) (c)
Figure 7-16 Bridging illustration and forces on the powders (a) Rotational force (b) Vertical motion (c) Horizontal
motion
It has been observed that horizontal acceleration has a smaller tendency to cause
leakage compared with a rotational acceleration or vertical acceleration. As expected, a
vertical acceleration going up equals an increased force on the arch. Passing the limit
will cause the arch to collapse and thus, the powder will flow down.
Therefore, it takes the combination of a shaking effect during motion and acceleration of
the motion to find out the maximum nozzle size for a nozzle that will not leak in the
working condition.
Equation 7-7 𝑾 𝒎𝒂𝒙𝒗 = 𝑭 (𝑷 , 𝒂 , 𝑵 , 𝒔 )
P: the properties of powder in use;
a: Acceleration experienced in the printing process;
N: Nozzle properties including the material, excluding the size;
s: Shaking effects in the motion, which is to be minimized.
Therefore, the proper nozzle range for use is:
Equation 7-8 𝑾 𝒎𝒊𝒏𝒇 <𝑾 < 𝑾 𝒎𝒂𝒙𝒗
80

From experiments shown in Table 7-5, W25 powders may work with nozzle sizes
between 210 𝜇𝑚 and 380 𝜇𝑚 with enough stability. While for W45, the only nozzle in
which it does not leak easily is with 210 𝜇𝑚 , which limits its selections.
The first standard in choosing a nozzle sizes for a certain powder is the successful
formation of an arch pattern. The preferred powder flow rate is found to satisfy the
minimum requirement for the printing speed. In Figure 7-17, the flow rate of powders
with respect to the nozzle sizes are plotted.

Figure 7-17 The flow rate of powders vs the inner diameter of nozzles. (Only the read curve uses the right axis on the
right)

7.4. Potential applications
As this research points out for powder deposition can be enabled with the proper
acceleration. If the frequency of the control signal is lower than full arch frequency, the
deposited powder will be proportional to both the deposition time and the deposition
81

frequency. As the experiments show, an accurate deposition of 250 𝜇𝑔 can be achieved.
Tungsten powder has a very large density therefore powders of smaller densities may
achieve a deposition accuracy of approximately 20 𝜇𝑔 .
For a given powder, there will have certain nozzle sizes that the powder will not flow
without vibration but responds selectively to the applied frequencies.
Such a finding can be very useful in other fields where an accurate deposition of powder
is required such as deposition of high potent drugs in pharmaceutics.

82

8. Methodology of Vibration
For the SSS process, a stable deposition of S-powder into the B-powder layers is the
prerequisite of quality control. Powder confined in a large nozzle will flow freely under
the effects of gravity and can be stopped by a mechanical switch when needed;
contrastingly, the powder in a thin nozzle will be arrested from free flow due to a
spontaneous arch pattern built up. Active vibration has to be applied on the nozzle
when deposition is needed.
The most optimal methods for powder deposition should generate stable powder flow
without disturbing the paved powder layers. There are several approaches for effective
control of powder flow. A gas assisted powder flow has been used extensively in LENS
[69](laser engineering net shaping) systems. With the flow of gas, dry powder is focused
into the melting pool. Gas is used both for powder flow and powder focus. While such
an approach cannot be used in SSS since pressured gas will disturbe the paved powder.
In SSS, the nozzle is submerged under the powder layers, if a compressed air system is
used to assist powder flow, the compressed air will rush out from the tip of the nozzle
and blow away surrounding powders. Other potential deposition methods used in SSS
include a solenoid activated plunger in a larger nozzle of free flow, and a magnetic
sphere driven in a medium nozzle of easy flow. Ultimately, the most effective powder
deposition method seen thus far, is driven by vibrations through a piezoelectric actuator.
For both the plunger activated and magnetic sphere activated powder flow control, it is
required that the gravity is the main driving force for the powder to flow down. The
applied interruption is mainly responsible for stop the powder from flowing. The applied
force can assist in switching the powder flow on and/or off. From previous chapters, it
was demonstrated that powders generally tend to flow easily under the effects of gravity
in a nozzle with larger ratios of nozzle inner diameter (ID) to powder diameter.
8.1. Plunger activated flow control
For a nozzle with a sufficiently large nozzle ID, the powder flows down freely under
gravity and a plunger is used to stop the powder flow. A thin nozzle may either get
clogged due to powder contamination or due to compaction that accumulates during
83

deposition; a larger nozzle can circumvent such issues. Free flow can be guaranteed
with assisted modes of vibration; for example, tapping on the nozzle. To stop the
gravitational induced flow, a plunger with a hemispherical tip is inserted to block the
opening in the bottom of the nozzle, as seen in Figure 8-1.

Figure 8-1 Plunder down and up under the magnetic force for powder switch of/on (a) Off (b) On
In the experiment, W-90 powder with a mean size of 62.3 μm and a maximum powder
size of 100 μm is placed into a nozzle of 500 μm inner diameter. The powder flow can
be continuous and reliable with the assisted tapping system. The experimental results,
illustrated in Figure 8-2, show that a flow rate of 50.8 𝑚𝑔 /𝑠 is achieved and that the flow
rate is relatively stable with an overall fluctuation of 9.3% for a 6 hour testing period.

84

Figure 8-2 Flow Rate of W90 in 500um inner diameter nozzle

To stop the powder flow, the solenoid is activated that the plunger shifts downwards and
blocks the powder flow. As the response is expected to be immediate, the force applied
on the plunger is larger. To revive the powder flow, the solenoid is deactivated, thus
allowing the spring to pull up the plunger. The aforementioned approach worked a
majority of the time, but there were observed instances where the flow rate significantly
slowed down or the nozzle may even be clogged. An investigation showed that as the
plunger shifts down fast and compressed the powder, a cake of the powder formed
(Figure 8-3). After the force is released, the powder cake remained and even the
assisted tapping was not sufficient to break the powders.
85

Figure 8-3 Caked powder by plunder compaction
Experiments yielded that ceramic powders used such as alumina and zirconia were
more susceptible to becoming clogged. For plunger controlled free flow, the nozzle
diameter is required to be larger, and the powder flow is almost completed driven by
gravity. An alternative method to control powder flow is through a magnetic sphere that
rotates inside the nozzle, which can stop the powder flow without applying too much
compaction force. On the other hand, the rotation of the sphere inside the nozzle assists
in breaking apart clogs in the powder, thus easing the powder flow.
8.2. Magnetic sphere activated powder flow control
In this experimental design, a spherical magnet is placed into a nozzle, which is in the
center of a pair of coils taken off of a stepper motor. The coils are positioned in such a
way that when a constant current is applied through them, the magnetic sphere is pulled
down to cover the bottom of the nozzle, which in turn, stops the flow rate as illustrated in
Figure 8-4. If an alternating current runs through the coils, such as in the pattern that
drives stepper motors, the magnetic sphere will rotate and levitate up to open the
powder flow. While the sphere rotates inside the nozzle, the sphere actually agitates the
powder to enable easier flow. The repeating on/off switch appears stable and such a
mechanism was used in the experiments for bronze powders.
.
86

Figure 8-4 Powder switch on/off by rotation and dragging down of a magnetic sphere
Generally speaking, experiments have run smoothly except in the instances when the
nozzle was magnetized. A magnetized nozzle works well with non-ferrous powder;
however, when ferrous elements are concerned, the powder is observed to become
adsorbed to the tip of the nozzle, which leads to magnetized powder clusters clogging
the powder flow as illustrated in Figure 8-5. Also, the coils surrounding the nozzle
generated strong magnetic field that disrupted the paved B-powder

Figure 8-5 Magnetized nozzle tip clogged by steel powder

87

The deposition system is targeted to work with almost all materials. Ferrous materials,
in particular, are a large proportion of the materials consumed in the industry. Therefore,
the magnetic sphere driven method was discontinued.
Deposition with large nozzles have several defects in the SSS process as described
below:
 A fluctuation in a larger nozzle will cause a larger defect. For example, as shown
in this experiment, a 10% change in flow rate for a nozzle of 500 𝜇𝑚 will lead to a
change of 50 𝜇𝑚 in the separation line width, which will be further reflected in the
surface smoothness of the final parts.
 As the nozzle makes space for the S-powder, it disrupts the B-powders as well
as the deposited S-powders surrounding. If the nozzle is too large, the B-powder
around it will also be disrupted. Experiments show that the minimum features in
SSS is roughly twice the outer diameter of the nozzle.
 A large nozzle will cause unnecessarily large amounts of S-powder usage.
Excessive powder consumption requires larger storage in the S-powder reservoir
and more frequent changes in the S-powder reservoir, which is not preferred.
For future large scale part fabrications (like for construction), a larger nozzle will be
necessary and a plunger may be effective to control the powder flow rate. In this case,
the force applied from the solenoid may be studied to simply stop the powder flow
without applying any extra force of compaction. Some mechanisms such as a small
blade may assist in deconstruction of the powder cake.
For this research specifically, high resolution manufacturing is preferred, thus a smaller
nozzle will be used.
8.3. Piezo disc driven
For powders in a smaller nozzle, the arch pattern is so strong that neither gravity nor
weak vibration can break that arch pattern to enable powder flow. Because the powder
is at the level of tens of microns, the application of micrometer level may be proficient
enough to break down the arches. A piezo actuator may serve to provide a controlled
88

vibration that functions with the spontaneous formation of the arch to control the powder
flow.
An electric unit made from piezoelectric material expands or shrinks when a voltage is
applied on its poles. By applying a changing voltage, the unit will vibrate corresponding
to its signal. In the research, various types of wave shapes and directions of vibrations
were tested.
A thin piezo disc which vibrates along its axis is chosen as the actuator. The piezo disc
used in this experiment has the parameters, which is shown in Table 8-1:
Table 8-1 Parameters of the piezo disc

ID OD Thickness Weight
Resonant
Frequency
Copper based 20mm 27mm 0.5mm 2.1g 4.786kHz

The resonant frequency is measured with an Agilent equipment Agilent Precision
Impedance Analyzer 4294A as illustrated in Figure 8-6.

Figure 8-6 Agilent Precision Analyzer 4294A for resonant frequency
89

The peripheral of the disc is fixed by a holder. The maximum vibration is in the center of
the disc. A piezo disc can generate vibration with an amplitude of micrometers scale
and at a large range of frequencies.
In powder flow through a thin nozzle, a direct vibration transferred to the nozzle is seen
to be the most effective form of deposition. When applying vibration onto a nozzle, there
are three main directions that should be considered: lateral vibration, longitudinal
vibration and diagonal vibration, (which is a combination of the previous two).
 Lateral vibration
In this experimental setup, a piezo disc is placed in such a way that the direction of
vibration is perpendicular to the length of the nozzle.
(a) (b) (c)
Figure 8-7 Piezo vibration applied perpendicular to the nozzle (a) Front view (b) Perspective view (c) Powder view
 Longitudinal vibration
In this experimental setup, a piezo disc is placed in such a way that the direction of
vibration is along the length of the tube. A central hole is made on the disc which the
nozzle passes through. The disc is placed flush against the nozzle with a certain
pressure to allow the vibration to be steadily transferred to the powder.
90

(a) (b) (c)
Figure 8-8 Piezo vibration applied longitudinally along the nozzle (a) Front view (b) Perspective View (c) Powder view
 Diagonal vibration
A diagonal vibration is a combination of the lateral vibration and longitudinal vibration.
The experimental setup has a piezo disc placed with 45 ° angle relative to the length of
the nozzle. The vibration is transferred to the nozzle through a metallic bar. In the center
of the bar, a hole is made in which the nozzle passes through.
(a) (b)
Figure 8-9 Piezo vibration applied diagonally along the nozzle (a) Front view (b) Powder view

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In the experiments, vibration applied from all three directions are tested and the results
found bear similarities to previously held studies [70-72]. Experimental results show that
longitudinal vibration yielded the largest flow rate, followed by the diagonal vibration.
The lateral vibration yielded the smallest flow rate.
For this observations, there are a few possible explanations.
 The literature points out that the lateral displacement actually has a low flow rate
as a result of the powder bouncing onto the wall of the tube [73]. As the powder
touches the wall of the nozzle, the friction slows down the powder movement. As
illustrated in Table 8-2, under lateral vibration, the flow rate of SA0180 is almost
zero and the flow rate for zirconia is less than 5% of that for longitudinal vibration.
Such a large difference cannot be justified by friction in the case of SSS setup.
 The proposed explanation is that the longitudinal vibration effectively breaks the
arch. As proven in the previous chapter, the arch pattern builds up at the upper
level of the conduit connected to the nozzle. The arch pattern is surrounded with
powder in all directions except in the free space below the arch. Therefore,
longitudinal vibration that moves vertically can enable powder to flow down until
the arch rebuilds.
Table 8-2 Flow rate of powder vs. Vibration direction
Weight/mg Mass of powder flow over 2 minutes period
Direction SA0180 SA0109 Zirconia Alumina
Lateral 2 0 10 50
Longitudinal 30 0 275 650
Diagonal 5 0 45 200

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8.4. Advantages of piezo disc controlled powder flow
Compared with all the other approaches for powder deposition, the piezo disc driven
powder flow has the most advantages and is selected as the ultimate deposition method
for SSS. The advantages include:
 High resolution deposition. The vibration generated by the piezo disc can break
strong arch patterns so that the S-powders can be stored and deposited through
nozzle of small ID and thus a small OD. The smaller OD will have less disruption
to the neighboring material and enables printing of fine features.
 Fast response speed. The vibration of the piezo disc is controlled by electric
signals which can be run at high frequencies such as 100 kHz. The on/off switch
for the vibration can be accurately controlled without any delay. The mode of the
vibration can also be changed at real time which may be useful when a changing
deposition rate is necessary.
 No contact with the powder inside the syringe. The powder flow is sensitive to
contamination, which may form the nucleus of clogging. For the plunger based
powder flow control, the plunger will move repeatedly inside the powder and
contaminants such as scape may form and clog the powder. Even when no
contamination is introduced, this is still troublesome when powder refilling is
necessary. For the piezo disc method, the powder tank can be changed
independent of any interaction with other parts of the system.
 The piezo disc powder control is compact and occupies little space. The piezo
disc in use is small, thin, and can be compacted even further in industrial
application. Such a compact design will help make the system compact and
provide more space should multiple deposition nozzles be added and
simultaneously flow.

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9. Powder deposition rate control
Through identification of proper powder species and vibration methods, the parameters
of their interactions are to be determined to achieve the required deposition rate. The
purpose of this study is to reach the maximum stable deposition rate of S-powder so
that printing speed can be maximized. In this experiment, design of experiments are first
carried out to find the factors responsible for flow rate and stability, including nozzle size,
waveform shapes and frequency.
9.1. DOE for determining the significant factors that affect volumetric flux
in a confined conduit
A powder confined in a thin conduit can be activated for free flow under vibration. The
flow rate has been observed to respond to conduit size, frequency, etc. To understand
the factor of importance for control the flow rate, deposition experiments are carried out.
9.1.1. Initial experiments to identify the factors of importance
The factors under study include: inner diameter of the conduit, wave shape and
frequency. The powder for testing is Bronze (grade: 5807C). A full factorial design at 3
levels is applied to this set of experiment. The details of the factors and levels are
represented in the table below:
Table 9-1 Levels of the Full Factorial Design

Level
Factors 0 1 2
A: Needle size 260 240 210
B: Shape Square PS NS
C: Frequency 250 1000 4000

The order and level are listed as shown in Table 9-2:
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Table 9-2 Flow rate experiments and order of each run
Factors Response: Flow rate ( insert unit) Response order of replicates
Run A B C R1 R2 R3 R1 R2 R3
1 0 0 0 0.004 0.017 0.004 5 2 5
2 0 0 1 0.165 0.205 0.632 9 4 1
3 0 0 2 0.875 0.49 0.363 1 7 9
4 0 1 0 0.009 0.005 0.003 8 9 2
5 0 1 1 0.313 0.042 0.027 2 5 7
6 0 1 2 0.102 0.428 0.119 6 8 3
7 0 2 0 0.003 0.002 0.002 3 6 4
8 0 2 1 0.01 0.036 0.032 7 3 8
9 0 2 2 0.221 0.861 0.229 4 1 6
10 1 0 0 0.009 0.007 1.296 9 5 1
11 1 0 1 0.285 0.267 1.199 5 2 4
12 1 0 2 0.346 0.311 1.289 1 6 7
13 1 1 0 0.005 0.0007 1.344 7 3 2
14 1 1 1 0.065 0.043 0.613 3 4 5
15 1 1 2 0.143 0.117 1.211 6 8 8
16 1 2 0 0.002 0.778 1.402 2 1 3
17 1 2 1 0.007 0.006 1.445 8 7 6
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18 1 2 2 0.209 0.229 0.487 4 9 9
19 2 0 0 1.044 g 0.002 0.059 1 4 8
20 2 0 1 0.886 0.186 0.057 5 1 5
21 2 0 2 0.932 0.18 0.425 2 6 1
22 2 1 0 .005 g 0.002 0.002 3 8 2
23 2 1 1 0.086 0.009 0.015 7 9 4
24 2 1 2 0.963 0.116 0.068 4 7 9
25 2 2 0 0.003 0.002 0.002 8 5 3
26 2 2 1 0.12 0.003 0.004 6 3 7
27 2 2 2 0.755 0.281 0.087 9 2 6

Using R to analyze the data and an ANOVA table is generated as shown:

Figure 9-1 ANOVA analysis for factors of importance
96

From the ANOVA table Figure 9-1, it can be seen that at a confidence level of 95%, the
factor of the conduit size is the only factor of importance matters. Such a conclusion
does not agree with our observations that the flow rate changes with the frequency. The
suspected cause is some large noise actually buries the information.
To further study these phenomena and reduce the noise among different runs, we have
randomized the replicate runs in the experiments. By plotting the deposition rate again
the running orders, it can be seen that the deposition rate decreases with the increased
running orders as illustrated in Figure 9-2. Generally, it can be seen that flow rates
quickly descend to a low flow rate from an initially large value. The red plot in the 3
rd

replicate have all the powder refilled after the previous run and it yields a much larger
value. Considering that powder compacts from vibration each time as the experiments
continue, it is hypothesized that the damping effect in the flow rate is due to compaction.

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Figure 9-2 Plot of deposition rate against running order for all replicates

9.2. Study on the role of compaction
According to the hypothesis, the powder flow may be affected by the compaction of
powder. Experiments are carried out to study the compaction effect on the powder. The
experimental setup is as shown in Figure 9-3. A glass syringe is used to store the
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powder. The piezo disc is put under the powder as in the case of the SSS machine
setup. The opening of the conduit is clogged with glue. Vibration is turned on to
compact the powder and the height of the powder is measured. A paired experiment
where the same amount of powder filled in the syringe is allowed to deposit the powder
and the deposited powder weight is measured.

(a)

(b)

(c)
Figure 9-3 Scheme of experimental setup for compaction study (a) Experiment setup (b) Measured height of S-
powder over time (c) the correlated S-powder flow rate. The red line represents the stabilized flow rate after 45
minutes of vibrational compaction
These experiments show that the powder flow rate decreases with the increased
compaction rate. As shown in Figure 9-3 (b), the height of the powder inside the syringe
reduces rapidly in the beginning and stabilizes at about 88% of its original height. The
quick compaction rate in the beginning is also reflected in the powder deposition rate as
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in Figure 9-3 (c), which has almost the same shape to that of the compaction curve. A
stable flow rate is eventually achieved as a stabilized compaction rate is reached.

More experiments are carried out to understand the compaction rate with respect to
powder species, frequencies. Experiments show that the mean compaction rate for
bronze powder (below 45 micron meters) is 5% while that value for W-45 (tungsten
below 45 micron meters) is 2.5% for 60 seconds compaction as in Figure 9-4. For an
extra 60 seconds vibration following the initial 60 seconds vibration, the increased
compaction rate for bronze is 11% for bronze but 20% for tungsten powder. As for
different frequencies, bronze powders reach maximum compaction around 2 kHz while
tungsten reaches its maximum around 1.5 kHz.

Figure 9-4 Compaction rate of (a) bronze and (b) W-45
Experiments also show that for the already compacted powder, a shift to another
vibration waveform may change the compaction rate. Such finding means that a
compacted powder may be loosened again by vibration [74] to increase the powder flow
rate. While in SSS, it is desired to have a constant compaction rate for the powder
deposition.
9.2.1. A buffer zone for stable compaction rate
The powder stored in the syringe gets compacted as a result of the vibration and
reduces the flow rate. A specially designed divider is then used to provide a buffer zone
for powder flow may enable fast and stable powder low.
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In the experimental setup of SSS, the powder volume stored in the tube is divided into
two sections using the inserted special divider shown in Figure 9-5. When the S-powder
fills the tube, the divider bears the pressure of the powder above its upper surface, while
the remaining powder falls through the gap between the outer diameter of the divider
and the inner wall of the tube. The powder in the lower level is therefore not subjected
to the pressure of the powder height in the upper level since the loose powder will be
discharged before compaction occurs. The lower cavity created by the divider enables
the powder to consistently flow regardless of the total powder volume in the tube.

Figure 9-5 Scheme of secondary stage serving as a buffer zone for powder deposition
Experiments show that the flow rate is much more stable with installation of the
secondary stage. The powder in use is W25, and the nozzle has an ID of 240 𝜇𝑚 . There
is still fluctuation in flow rate, but the deviation from the average is about ± 3.5% as seen
in Figure 9-6. To have a more stable flow rate, the design of the secondary stage needs
to be optimized.

101

Figure 9-6 Deposited powder weight for every 30 seconds with continuous deposition
9.3. Powder contamination
With the secondary stage, the powder deposition rate is more stable but nozzle clogging sharply
reduces the deposition rate. Observation under the microscope shows that such sharp reduction
in the flow rate is due to powder contaminations. These contaminations tend to be from powder
adsorption of fibers and other impurities.
9.3.1. Powders clustered around contaminants
The powder volume used in the experiment may introduce in contaminants mixed
during powder production, delivery and preparation for applications. The contaminants
take different forms, such as dust or fibers. In this research, a fiber contaminant is
studied which may block the thin tube delivery process.
Experiments that have powder clogged the nozzle show how the contaminants may
quickly clog the powder deposition. As seen in Figure 9-7, the microscopic observation
show that during deposition, a fiber may move down with the powder flow and stay
entrenched the opening of the nozzle. The fiber absorbs fine powders and partially
102

covers the opening area. As more powders adsorb to the fibers, the opening shrinks to
an extent that almost no powder can flow through where deposition fails

Figure 9-7 Development of clogging in the nozzle from contaminants (a) Clean nozzle (b) Fiber moved to the top of
the nozzle (c) Half of the nozzle clogged (d) Complete clogging (these pictures are taken from different runs)

Interaction of powders with fibers is studied to help understand how clogging occurs in
thin conduit deposition. In the experiment, all the powder samples are sifted with a 90
𝜇𝑚 sieve before use, and the contaminants with larger sizes will be removed. However,
fibers with a diameter smaller than the sieve openings but larger length may pass
through the sieve, causing future contamination issues. The force responsible for
clogging observed is Van der Waals force as previously explained.

103

Figure 9-8 Microscopic pictures 25 um sized powder absorbed to fiber surface. Left: a cluster of powder formed on
fibers. Right: powders absorbed on a single fiber. From top to bottom (25 um, 45 um, 90 um)
104

Compared with the length of the fiber to the size of the powder, the length of the fiber is
considered to be infinite in the model. Experiments show smaller powders easily cluster
around the fibers due to the Van der Waals forces. The powder used in the experiments
includes W-25, W-45 and W90. In the experiment, fibers are mixed with powders with a
weight ratio of 1:1000. The microscopic pictures are illustrated below in Figure 9-8. For
W25, the tungsten powders adhered to the fiber packed the fiber very closely; for W45,
the powders are observed to cover the fiber sparsely; for W90, there is almost no
powder that is adhered to the fiber.
The clogging problem occurs frequently with the W25 and W45 but is rarely observed
for W90. As the microscopic image shows, the W90 powders are not adsorbed to the
fibers. The powder is also heavy compared to the fiber and the powder flow tends to
bring down the fiber sitting at the nozzle opening. In the experiments, it is observed that
a slowdown in the powder flow of W-90 returns to a fast flow after a short while without
any interference.
In sum, a fiber or other nucleates can be the seed of clogging, which may lead to a
failure of the delivery process. For better quality control, the proper screening will be
given to remove the contaminants from the powder source and a sealed cartridge will
be used for powder storage.
9.4. Identification of factors of importance
By eliminating the noise of compaction and contaminants, the factors of importance can
be identified. In the experiments, the two controlled factors are wave shape and
frequency. The wave shapes may affect the powder flow by applying different
acceleration on the nozzle while the frequency may change the vibration amplitude.
9.4.1. Effects of frequency on powder deposition
The vibration amplitude is related with the frequency applied on the piezo disc. As the
frequency gets closer to the resonant frequency, the vibration becomes stronger. In the
experiment, W25 tungsten powder is put in the syringe with a nozzle of 240 𝜇𝑚 inner
diameters. At each frequency, the vibration sustained for 30 seconds. The deposition
105

rate shows a strong dependence on the frequency. The deposition rate is small at low
frequency and peaks at the resonant frequency.

Figure 9-9 Deposition of W25 with respect to the frequency
9.4.2. Effects of wave shape on powder deposition
An arbitrary waveform generator is used to generate different wave shapes and the
response of the powder deposition is studied. The wave shape affects the acceleration
of the vibration, such as in the case of a sine wave; the acceleration is expected to be
smaller than in the case of a square wave, where the change is much sharper. The
tested wave shapes are sine wave, square wave, triangle wave, saw wave, negative
saw wave and Gaussian wave as illustrated in Figure 9-10.

106

Figure 9-10 Wave shapes for driving the piezo discs
The powder used is W25 and the nozzle in use has an inner diameter of 240 𝜇𝑚 . The vibration
period is set to 30 seconds. For each wave shape, three repetitions are carried out and the
order is randomized.
107

Figure 9-11 W25 deposition rate as a response of wave shape
The results indicate that deposition rate does not show any dependence on wave
shapes except for the triangle wave. The wave shapes after amplification are measured
with an oscilloscope for analysis. The observed wave shapes do not keep their original
shape and are rectified by the piezo disc in use as Figure 9-12. The ascending time for
the wave shape is about the same value, from 34 𝜇𝑠 to 54 𝜇𝑠 , except for the triangle
wave has a long ascending time of about 80 𝜇𝑠 . The ascending time is a reflection of
the acceleration and longer acceleration time corresponds to a smaller acceleration.

108

109

Figure 9-12 Amplified wave shapes as measured from the piezo disc. From top to down the wave shapes are: sine,
square, triangle, saw, negative saw and Gaussian. Left: two complete cycles of waveforms. Right: amplified
ascending slopes. The ascending times are: 54.4𝝁 𝒔 , 42.2 𝝁𝒔 , 79.6 𝝁𝒔 , 42.2 𝝁𝒔 , 34 𝝁𝒔 ,36.6 𝝁𝒔

9.5. Flow rate calculated as a function of printing speed, layer thickness
and separation wall thickness
9.5.1. The maximum flow rate of powder under gravity
In this experiment, vibration breaks the arch pattern to enable the powder flow, but it is
gravity that drives the flow directly.
As the powder is static on top of the nozzle opening therefore the flow starts with zero
speed and accelerates in the nozzle.
Under ideal assumption that there is no friction between the powder and the wall of the
nozzle, the time takes to reach the bottom of the nozzle is:
Equation 9-1 𝒕 = √
𝟐 𝒉 𝒈
The average speed is:
Equation 9-2 𝒗 =
𝒉 𝒕
Given a certain period of 𝑇 a powder length of L transverses through the nozzle:
Equation 9-3 𝑳 = 𝒗𝑻 = √
𝒈𝒉
𝟐 𝑻
The volume of the powder flowing through the nozzle therefore is:
110

Equation 9-4 𝑽 =
𝝅 𝑫 𝟐 𝟒 𝑳 = 𝝅 𝑫 𝟐 √
𝒈𝒉
𝟑𝟐
𝑻
Knowing the density of the powder, the weight can also be calculated:
Equation 9-5 𝑾 = 𝝆𝑽
The ideal volumetric flow rate is 1.14 × 10
−2
𝑐 𝑚 3
/𝑠 . Using W25 powder with an
apparent density of 10.4 g/𝑐 𝑚 3
, a syringe nozzle with a diameter of 240 𝜇𝑚 and a
length of 0.5 inches, the highest possible deposited powder over 30 seconds should
weigh 3.4 grams, which is about 3 times the value of the maximum measurement.
The real deposition rate is about 1/3 of the calculated value can be explained by the
friction between the powder and the wall of the nozzle, which slows down the powder
outflow. In addition, the rate of powder feeding into the upper opening may be
insufficient.
9.5.2. Maximum printing speed as a calculated result of flow rate
For a given printing speed, layer thickness and separation wall thickness, the ideal
powder flow rate is fixed. A nozzle fully buried by the B-powder will yield a separation
wall with a thickness of the opening height. The printing speed is the speed that the
nozzle moves; ideally, a faster printing speed is a desired for accelerated part
completion. The upper limit of speed is determined by the point at which high print
speeds disturb the neighboring powders and disrupt nearby printing patterns. The
separation wall width is close to the ID of the nozzle in use. By adjustment of the nozzle
insertion depth into the B-powders layers, the separation wall width can also be
changed.
Equation 9-6 𝑽 = 𝒘 ∙ 𝒕 ∙ 𝒗

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Figure 9-13 Parameters of printing process including deposition depth, separation width and printing speed
From the ideal volumetric flow rate of the powder, the maximum printing speed can be
obtained. In the case of W25, the maximum volumetric flow rate is 1.14 × 10
−2
𝑐 𝑚 3
/𝑠
and the calculated maximum printing speed is: 18.7 𝑖𝑛 /𝑠 . With the measured deposition
rate being 1/3 of ideal deposition rate, the current maximum printing speed is about 6
𝑖𝑛 /𝑠 . If the friction between the powder and the inner surface of the nozzle can be
eliminated, the fast printing speed can reach 18 𝑖𝑛 /𝑠 .
9.6. S-powder separation wall width
In the SSS printing process, the deposited powder mechanically interacts with the base
powder within which it is deposited. Flow stoppage occurs if the movement of the nozzle
is not consistent with the deposition rate. In the experiments, it has been observed that
blocking occurs more easily when the nozzle is inserted into the layer than when the
nozzle is clearly above the powder layer. Experiments are carried out to understand the
relationship between the separation line width with nozzle insertion depth and printing
speed. An analytical model is proposed to predict the separation line width with a
reasonable degree of accuracy given a specific flow rate.
As the nozzle moves forward in the B-powder and delivers the S-powder, the separator
and the base powders compete to fill in the gap that the moving nozzle leaves behind.
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The opening in the nozzle is in the center of the gap and the two sidewalls of the gap
slide down (collapse under the force of gravity) into the gap. We assume that the flow
rate of the S-powder is fast enough that if there is no B-powder collapse the gap would
be filled completely with the S-powder, as shown in Figure 9-14.a. While the S-powder
fills the gap, the slope for the collapsing B-powder is assumed to maintain a constant
angular value of the repose angle. This angle depends on the nature of the B-powder
and the height of the opening slot. The filling process starts from the bottom until it
reaches the top level of the nozzle opening.

Figure 9-14 Dynamic filling of the gap (a) Without gap wall collapsing (b) With gap wall collapsing
Note that in Figure 9-14.a the black dotted area is the B-powder and the yellow square
is the back view of the nozzle (or the initial shape of the gap made immediately behind
the nozzle). The black triangles within the yellow rectangle in Figure 12.b are the filled
areas made by the B-powder in of the gap by the base powder. The blue dotted area is
the S-powder. The white rectangular area is the opening of the nozzle.
In the real situation, the gap collapses and is simultaneously filled with the separator
powder and the base powder as illustrated in Figure 9-14.b. The separator rushes out of
the nozzle, spreading out from the center to both sides, and the walls of the gap
collapse from the side to take up the space in the center. In other words, both B-powder
and S-powder compete to take the volume of the gap behind the moving nozzle.
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Let us assume that flow rate of the S-powder from the nozzle is 𝑽 𝒔 . Let us further
assume that the cross section of S-powder volume flowing out of the nozzle is a
trapezoid and the slope is of the repose angle 𝜷 𝑠 β
p
and the volume is 𝑽 𝒔 𝒕 and d is half
the value of the frontier boundary of the trapezoid minus ID of the opening at time t. The
volume equation is given as:
Equation 9-7 𝑽 𝒔 𝒕 = (𝑰𝑫 . 𝒅 . 𝒕𝒂𝒏 𝜷 𝒔 + 𝒅 𝟐 𝒕𝒂𝒏 𝜷 𝒔 )𝜹𝑳
In the above equation ID is the top edge of the trapezoid formed by the flow of separator
powder, and ID+2d is the bottom edge. The length of the gap travelled is 𝛿𝐿 .
While the S-powder flows out of the nozzle, at the same time the B-powder continues to
collapse to fill the bottom of the gap at the filling rate of Vb. At time t, the collapsed base
material would move toward the center of the gap by the distance Db, therefore the
volume of the gap taken by the B-powder is:
Equation 9-8 𝑽 𝒃 𝒕 =
𝟏 𝟐 𝑫 𝒃 𝟐 𝒕𝒂𝒏 𝜷 𝒃 𝜹𝑳
The slope angle of collapsed base powder is assumed to be 𝛽 𝑏 .
Assuming that both opposite walls collapse and spread at the same speed and
magnitude, then at time Tm the collapsing of the walls meets the incoming S-powder.
Equation 9-9 𝟐 𝑫 𝒃 + 𝟐 𝒅 + 𝑰𝑫 = 𝑶𝑫
OD is the outer diameter of the nozzle.
Equation 9-10
𝑽 𝒔 𝑽 𝒃 =
(𝑰𝑫 .𝒅 .𝐭𝐚𝐧 𝜷 𝒔 +𝒅 𝟐 𝒕𝒂𝒏 𝜷 𝒔 )
𝟏 𝟐 𝑫 𝒃 𝟐 𝐭𝐚𝐧 𝜷 𝒃 V
p
V
b
=
(ID.r.tan β
p
+r
2
tanβ
p
)
1
2
D
b
2
tan β
c

The separation line with d may be calculated by substituting the values of Vs, Vb and
the angles of 𝛽 𝑏 and 𝛽 𝑠 , and solving Equation 9-8 and Equation 9-9.
Consider two extreme situations, one is the B-powder is so cohesive that B-powder
does not collapse therefore 𝑉 𝑏 = 0, then from Equation 9-9 yields the width 𝑤 = 𝐼𝐷 +
2𝑑 = 𝑂𝐷 . The width of the separation wall width is the OD of the nozzle. Such a value is
reasonable as the whole gap is filled with S-powders.
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The other situation is that the B-powder flow so fast, that compared with S-powder, the
flow rate of 𝑉 𝑏 = +∞. From Equation 9-9 and Equation 9-10 yields the separation line
width 𝑤 = 𝐼𝐷 . Such a value is also reasonable, as the S-powder flow is protected by the
wall of the nozzle that it can occupy the area defined by the inner diameter but there is
no time for it to spread outside the nozzle.
The width of the separation line width therefore will be between the ID and the OD of
the nozzle.
The spreading rate of the base material is influenced by the nozzle insertion depth and
the nozzle movement speed. The above equations represent a static prediction model
for the separation line width. In real practice, however, the collapsing pattern may be
more complex than described by the static model. Experiments are carried out to
observe the influence of printing speed and insertion depth. For the purpose of
observation, the prototype nozzle opening height is 800 𝜇𝑚 much larger than a typical
layer thickness of 100 𝜇𝑚 . The nozzle in use has an ID of 260 𝜇𝑚 and OD of 510 𝜇𝑚 .
The insertion depth is defined as the distance from the upper opening of the nozzle to
the B-powder surface. B-powder is a bronze powder of 45 microns and S-powder is
W45.
Table 9-3 Separation wall width as a response of insertion depth and printing speed
Depth/inch 0.05 0.05 0.1 0.1 0.15 0.15 0.2 0.2
Speed/units 500 100 500 100 500 100 500 100
Average Width/𝝁𝒎 338 452 237 354 248 252 214 247

The separation wall width shows that the separation wall width reduces with increased
insertion depth and increased printing speed as shown in Table 9-3. This can be
explained by the facts that the higher the insertion depth, the taller the wall will be on
both sides of the gap made by the nozzle. The increased height will increase the flow
rate on the B-powder. As the printing speed increases, the deposited S-powder may not
be sufficient to fill the gap and the remained area will be filled by the B-powder. In an
extreme case, if the printing nozzle is moving at an infinite speed, the powder deposited
115

will be almost zero everywhere behind its trace. In some cases, the average separation
wall width is smaller than the ID of the nozzle. Such observations can be explained by
the opening height. The height is about 800 𝜇𝑚 , as the S-powder flows down to the
bottom of the nozzle, the B-powder which has very good flowability actually spreads and
occupies the space of the S-powder. As the opening height is reduced to the height of
one layer thickness 100 𝜇𝑚 , this observation will disappear.
The next step is to have computational simulation using discrete element method
(DEM)[75,76] to model the real time, dynamic deposition process. Compared with the
experiments, the desired separation wall width can be achieved.
9.7. Experimental verification
With the experimental and analytical analysis from the above sections, experiments are
carried out to build parts. The B-powder in use is bronze powder 5807C and the S-
powders used are W-25 and W-90.The wave shape in use is square, the frequency is 3
kHz and the nozzle has an ID of 240 μm. The secondary stage is used where a stable
flow rate is achieved. The printing speed is 1inch/s.
The part with the best surface smoothness has been made with W25 with a layer
thickness of 50μm. The surface smoothness is comparable to that of the machine part
as seen in Figure 9-15. As W25 tends to clog due to contamination, small and simple
parts are tested.

Figure 9-15 Bronze samples printed with W25 at a layer thickness of 50 𝝁𝒎
116

Samples printed with a stable W90 powder deposition can produce parts of good quality
as seen below. The printed parts show smooth surface and sharp edges Figure 9-16.

Figure 9-16 One layer of deposited S-powder pattern and final product for pliers (layer thickness 100 𝝁𝒎 , W90)
Steel parts of different shapes are also printed using W90 as the S-powder. The B-powder size
is 45 𝜇𝑚 and is purchased from Carpenter Powder.

Figure 9-17 Steel samples printed with W90
117

A close look at the samples show the surface quality of the parts made with W90 with a
layer thickness of 100 𝝁𝒎 .

(a)

(b)
Figure 9-18 Steel samples printed with W90 of high surface quality (a) Bronze sample (b) Steel sample

118

10. Future Study
In this research, a promising additive manufacturing approach for ceramics and metals
is demonstrated. A SSS prototype is built and has proven the concept of SSS by
additive manufacturing of ceramic and metallic parts. The demonstrated samples have
shown comparable surface quality to that of commercialized machines in the market. To
have this technology develop to its full capability, the suggested future work may focus
on the following directions.
10.1. Customization of deposition nozzles
The design, shape and material of the design in use affect the strength of the arch and
the deposition rate of S-powder. By customization of the nozzle, the strength of the arch
pattern for a specific S-powder may be controlled in an expected range such that the
powder deposition can be controlled in a narrow range and mode vibration. The
vibration from translational and rotational motion of the system will not interfere with the
deposition.
As the powder deposition can be controlled with higher accuracy, the manufactured part
surface will be improved. Currently, the deviation of the powder deposition may vary by
about 10% as experiments demonstrated. If the deviation can be controlled within 1%,
the surface smoothness can be at the level of powder scale. For a powder size of 25𝜇𝑚 ,
the surface smoothness can be better than 12.5𝜇𝑚 .
10.2. Application of active S-powder delivery
In the current experimental setup, SSS relies on gravity to bring down the powder. As
the industry of powder delivery has demonstrated, the powder can be delivered by a
proper vibration mode. The active mode of S-powder delivery will speed up the printing
process and further reducing the printing time. Active powder delivery will also serve for
the purpose of printing in the space with zero gravity.
10.3. Deposition of fine S-powders
As the analysis points out, finer S-powders will be the prerequisite of the printed parts.
In the current experimental setup, fine S-powder deposition tends to clog the nozzle, as
119

fine powders are prone to cluster around any nucleus. Using sealed clean tank for
powder storage, the contaminants can be avoided. Also, the installation of active S-
powder delivery will assist in deposition of fine S-powders, which are expected to have
comparative large cohesion force.
10.4. Deposition of S-powder over large areas
For deposition of S-powder over a large area or 3D printing of a large part, a single
nozzle may not suffice. A multi-nozzle S-powder print-head is currently under
development for rapid blanketing of horizontal separation surfaces. Multiples deposition
heads may also work together to speed up the printing process.

120

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Abstract (if available)

Abstract SSS is a layer-based 3D printing approach that promises to deliver low cost 3D printing of high quality parts for a variety of materials including ceramics, metals, etc. In the SSS process a separator powder (S-powder) is selectively deposited into the base powder (B-powder) material, which is the composition of the final part. The S-powder forms a barrier that surrounds the part’s cross section in each layer. The green part is moved into a furnace for sintering. After sintering, the S-powder is not fused to the B-powder and is easily removed to reveal the finished part. ❧ In the application fields, the challenges in making ceramic and metallic pieces at an economic cost can be solved by this approach, and the high initial investments associated with purchasing industrial additive manufacturing equipment can be significantly cut down. The distinct separation based shaping process provides a larger availability of materials. Additionally, in contrast with other methods, the process may be used to manufacture large scale parts without compromising the resolution. ❧ In the SSS process, the selection and deposition of the S-powder are the key factors that determine the quality of SSS produced parts. The thesis presents a guideline to the selection of the S-powder materials and powder sizes corresponding to the chosen B-powders. Different methods for S-powder delivery are discussed and the most effective method is identified for the experimental setup. The powder deposition process is analyzed and experimental findings generate a stable deposition rate with the chosen parameters. The produced samples demonstrated easy separation and high quality of surface smoothness.

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Asset Metadata

Creator Zhang, Jing (author)

Core Title Selective separation shaping: an additive manufacturing method for metals and ceramics

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Industrial and Systems Engineering

Publication Date 12/21/2017

Defense Date 05/03/2016

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag 3D printing,additive manufacturing,dry powder deposition,OAI-PMH Harvest,selective separation shaping,selective separation sintering

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Khoshnevis, Berok (committee chair), Chen, Yong (committee member), Wang, Qiming (committee member)

Creator Email jzhang.thu.usc@gmail.com,zhan947@usc.edu

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