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AN INFLUENCE OF SS316L’s PARTICLE SHAPE TO THE INJECTION
MOLDING OF COMPOSITE BINDER FEEDSTOCK
K. R. Jamaludin1, N. Muhamad2, M. N. Ab. Rahman2, S. Y. M. Amin2, S. Ahmad2, M. H. I. Ibrahim2, I.
Murtadhahadi2, N. H. M. Nor2
1
Dept of Mechanical Eng., College of Science & Technology, University Technology Malaysia, Intl
Campus, Kuala Lumpur, Malaysia
2
Precision Process Research Group, Dept. of Mechanical and Materials Engineering, Faculty of
Engineering, National University of Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia
*khairur@citycampus.utm.my
ABSTRACT: Metal injection molding (MIM) has emerged as a viable method of producing complex shaped parts
at a competitive cost. The MIM process uses a combination of powder metallurgy and injection molding
technologies to produce net-shape parts and comprising of five main sub processes: raw materials selection
(powder/binder), feedstock preparation, injection molding, debinding, and sintering. One of the advantages of MIM
is its ability to produce parts with complex geometry without machining. However, most of the MIM parts are
produced with fine and spherical shaped metal powder (gas atomized), which is expensive in cost. Thus, this paper
presents an investigation to study the particle shape’s influence to the injection molding performance as well as the
green body characteristics. The investigation is vital because the author attempts to seek a possibility of minimizing
the production cost by substituting such existing metal powders with nearly spherical but still irregular shaped
metal powder. The investigation begins with a rheological properties of the feedstock followed with the green body
characteristics, and finally ends with analysis of variance of the processing parameters that influences the green
strength and green density. The investigation discovered that the green characteristics of the green body produced
with water atomized feedstock are satisfactory as its green characteristics are the equivalent as the one produced
with gas atomized feedstock.
KEYWORDS: Metal injection molding, Particle shape, Gas atomized powder, Water atomized powder
1. Introduction
Metal injection molding (MIM) is a near-net shaping
technology that combines the advantages of plastic
injection molding and conventional powder metallurgy. It
is a cost effective technology for fabrication of small
intricate and precise parts in large quantities [1-3]. Four
typical steps for the MIM process are mixing, injection
molding, debinding and sintering. Initially, a binder with
suitable formulation is mixed with the metal powder to
form a feedstock. During molding, the feedstock is
injection molded to produce green parts with required
shapes. The molded parts then undergo a debinding step
where the binder is removed. After debinding, the parts
are subjected to a sintering step and final products with
required properties are obtained [4-5]. In recent years,
MIM has gained extensive popularity from the material
science and industrial field due to its preponderances in
fabrication area.
Nowadays, a broad range of stainless steels are
available as MIM powders. Most often they are produced
by gas atomization or by water atomization. Figure 1 (a)
shows the morphology of the spherical gas atomized
powder while Figure 1 (b) 2 presents the irregular shaped
water atomized powder. Table 1 gives the corresponding
particle size distributions. Both kinds of powders have
their advantages and disadvantages.
Gas atomized powder has a higher packing density
and thus needs less binder for injection molding leading
to low shrinkage and distortion during sintering. The
spherical morphology also guarantees a more isotropic
shrinkage as the particles can not take a preferred
direction during injection molding. The disadvantages of
this kind of powder are its higher price and the lower
strength of the molded part during debinding. After the
stabilizing binder has been removed the spherical
particles easily flow if sintering has not begun yet. Any
handling or vibration can destroy the brown part in this
state. The advantage of the water atomized powder lies in
its lower price. Also, the brown strength of parts
produced with this kind of powder is fairly high as the
irregular particles cannot flow past each other.
Disadvantages are the low tap density resulting in high
sintering shrinkage as well as the tendency of the
irregular particles to slightly align during injection
molding and thus causing anisotropic shrinkage. This is
why some companies ‘activate’ the water atomized
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powder by milling which produces a less irregular,
slightly rounded powder [6].
The objective of this paper is to present the influence
of SS316L’s powder particle shape to the green compact
characteristics. The injection temperature and injection
pressure has been indentified as factors that influences
the green characteristics [7, 8]. An analysis of variance is
used to investigate the significant and contributions of
each factors to the green compact characteristics. Beside
that, the rheological behavior of the feedstock is also
presented. The rheological property of the feedstock is
important because it will influence the green part
characteristics as well as a molding performance.
2. Methodology
2.1 Materials
Metal powder used in this study is SS316L gas atomized
powder and water atomized powder which supplied by
ANVAL Sweden and Epson Atmix Corporation, Japan
respectively. The SEM image and particle size analysis of
the powder is as shown in Figure 1 and Table 1
correspondingly; the D50 indicates the median particle
size of the powder. Note that the gas atomized powder is
more spherical than water atomized powder.
a) Gas atomized powder
b) Water atomized
powder
Figure 1 SEM image of the SS316L powder (2000 ×)
Table 1 Particle size (µm)
D10
Gas atomized
powder
5.780
Water
atomized
powder
4.985
D50
D90
11.225
19.840
15.052
34.747
In order to facilitate the SS316L powder to flow easily
into the mold cavity, polymer based binder is used to
bind the powder particles together. Hence, the binder will
act as a temporary vehicle for homogenously packing a
powder into a desired shape and therefore hold the
particles in that shape from the beginning of sintering.
Although the binder should not dictate the final
composition of molded material, it has a major influence
on the success of MIM processing [9]. The major goal of
the binder system is to provide the necessary flowability
and to enable filling of a cavity during the injection
molding process. The binder is subsequently removed
from the mixture during debinding and the first stage of
the sintering process [10]. The volumetric percentage
(v/o) of powder in the mixture is termed as “powder
loading” of the feedstock. This value has a large effect on
virtually all properties of the feedstock.
Consequently, a binder system based on polyethylene
glycol (PEG) is used in the investigation. The minor
component is polymethyl methacrylate (PMMA) and,
stearic acid (SA) is added as the surface-active agent. The
binder composition consists of 73 % PEG + 25 % PMMA
+ 2 % SA based on the weight fraction.
2.2 Experiment Procedure
In the investigation, SS316L powders were mixed
with binders in a sigma blade mixer for 190 minutes at 70
o
C. After mixing, the paste was removed from the mixer
and was fed into a strong crusher for granulation. The
rheological characteristic of the feedstock was
investigated using Shimadzu 500-D capillary rheometer
with a die of L/D = 10, while the injection molding is
performed with Battenfeld BA 250/50 CDC injection
molding machine.
3. Results & discussion
3.1 Rheological behavior of the feedstock
Viscosity is the most important parameter that judges the
rheological behavior of the MIM feedstock. For example,
high viscosity will make it hard to form molded
components while low viscosity will make the binder
separate from the powder. In this study, feedstock with a
powder loading of 63 v/o is used. After granulating,
viscosity of the feedstock was tested at the temperature of
130 °C and 140 °C and the results is as shown in Figure
2.
It is acknowledged that viscosity is the internal
friction force of running liquid and polymer melt is often
considered as pseudo-plastic fluid. At definite
temperature, viscosity decreases with the increase of the
shear rate that can be expressed as follows:
(1)
where ߟ is the apparent viscosity, is the shear rate, K is
a coefficient and n is a flow behavior index (< 1). Figure
2 demonstrates the relationship between shear rate and
the apparent viscosity. The apparent viscosity is
decreasing while the shear rate is increasing and this
indicates the pseudoplastic behavior of the feedstock. The
melt viscosity is also decreasing when the injection
temperature is increasing due to more binders are able to
wet the powder particle and thus minimizing the interparticle friction between powders. At the mean time,
Figure 2 also shows that the water atomized powder is
more viscous than of the gas atomized powder. This
happens due to higher inter-particle friction generated by
the non spherical and ligamental shaped of water
atomized powder particles.
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This is due to the powder-binder separation occurred in
the water atomized powder feedstock at high
temperature. The reason is supported by the rheological
result shown in Figure 2 that, there is a little increase of
the water atomized melt viscosity at 140 °C especially at
high shear rate. The increase of viscosity at high shear
rate indicates the occurrence of powder-binder
separation.
Figure 2 Apparent viscosities versus shear rate of the
SS316L feedstock at powder loading of 63 v/o.
The pseudoplastic behavior of the melt is shown by
the flow behavior index in Table 2. The pseudoplastic
behavior index is associated with feedstock sensitivity to
the shear stress. The small flow behavior index
demonstrates higher melt sensitivity to the shear stress.
Table 2 shows that the flow behavior index for both
powders are increasing when the injection temperature is
increases to 140 °C. This indicates that the shear
sensitivity decreases when the injection temperature is
increased. Moreover, the water atomized powder
feedstock also demonstrates higher flow behavior index
than of the gas atomized powder feedstock and this
indicates water atomized powder feedstock is less
sensitive compared to the gas atomized powder
feedstock.
Table 2 Flow Behavior index
Powder type
Temperature
Gas atomized
Water atomized
130 °C
140 °C
130 °C
140 °C
Flow behavior
index, n
0.57
0.69
0.68
0.84
3.2 Green body characteristics
The green part characteristic presented in this paper is the
green strength and green density. The inter-particle
friction between metal powders and binders that hold the
powder particles in its matrices is a major factor that
influences the green strength. In addition, the irregular
and ligamental shaped water atomized powder as shown
in Figure 3 are stronger than of the gas atomized
powders. This occurs due to the particle shape of the
water atomized powder is providing more friction to the
powder particle and thus improves the green strength.
However, the water atomized compact become weaker
when the injection temperature is increases and a vice
versa happens to the gas atomized compact as it become
stronger when the injection temperature was increased.
Figure 3 Green strength versus injection temperature
Furthermore, Figure 4 shows the injection temperature
influence to the compact apparent density. The figure
shows that the water atomized compact has higher green
density than of the gas atomized compact. Mean while,
water atomized compact demonstrates significant
reduction of the apparent density due to powder-binder
separation. The gas atomized compact exhibits an
increase of the apparent density when the injection
temperature is increases.
At the mean time, Figure 3 and Figure 4 also show
that the injection pressure has its influence to the green
strength. Both figures demonstrate that the green strength
and green density are proportional to the injection
pressure.
Figure 4 Green densities versus injection temperature
3.2 Analysis of variance for the green part
characteristics
An important technique for analyzing the effect of
categorical factors on a response is to perform an analysis
of variance. The analysis decomposes the variability in
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the response variable amongst the different factors.
Depending upon the type of analysis, it may important to
determine: (a) which factors have a significant effect on
the response, and/or (b) how much of the variability in
the response variable is attributable to each factor.
Moreover, study of the analysis of variance table
helps to determine which factors need control and which
do not. The interactions between factors in the
experiment can quantitatively determined by this analysis
[11, 12]. As shown in Table 3, the interaction of injection
pressure and injection temperature (A×B) to the gas
atomized powder green strength is 23.88 %. This
indicates that any changes to both factors will influence
the gas atomized powder green strength. In addition, by
comparing the variance ratio, Fn shown in Table 3 with
the F statistics, it is indicates that only A×B is significant
at moderate significant level of 5 %. However, the main
factors, A and B do not show any significant to the green
strength. Note that factors with a variance ratio, Fn less
than the F statistic at 10 % significant level is considered
not significant.
Table 3 Analysis of variance for the injection
temperature and injection pressure influence to
the green strength of gas atomized powder
Factor
Sum of
squares,
Sn
Variance,
vn
Variance
ratio, Fn
A (Inj.
Pressure)
B (Inj.
Temp.)
0.323
0.323
0.282
7.020
2.340
2.038
AxB
12.596
4.199
3.656
E
18.375
1.148
T
38.314
F
Contribution
statistic percentage, Pn
F(0.1)=
3.05
F(0.1)=
2.46
F(0.05)=
3.24
Table 4
Analysis of variance for the injection
temperature and injection pressure influence to
the green strength of water atomized powder
Factor
Sum of
squares,
Sn
Variance,
vn
Variance
ratio, Fn
F statistic
Contribution
percentage,
Pn
A (Inj.
Pressure)
B (Inj.
Temp.)
AxB
0.133
0.133
0.147
F(0.1)=3.18
-5.34
0.601
2.857
0.301
1.428
0.332
1.578
F(0.1)=2.81
F(0.1)=2.81
-8.37
7.24
e
10.859
0.905
T
14.450
100
The analysis of variance for the green density is
shown in Table 5 and Table 6. Table 5 shows that only
main factor (injection temperature, B) is significant to the
green density of the gas atomized powder compact while,
in Table 6 the interaction of A × B is significant to the
green density of the water atomized powder compact.
Though, both factors are significant at 10 %, which is the
lowest significant level. Additionally, Table 6 indicates
that the interaction of A × B is 20.86 % compared to that
shown in Table 5 for the main factor B is only 18.52 %.
Table 5 Analysis of variance for the injection
temperature and injection pressure influence to
the green density of gas atomized powder
Sum of
squares,
Sn
Variance,
vn
Variance
ratio, Fn
F statistic
Contribution
percentage,
Pn
0.013081
0.013081
0.978799
F(0.1)=3.05
-0.08348
2.570365
F(0.1)=2.46
18.52132
0.248365
F(0.1)=2.46
-2.15
Factor
9.33
A (Inj.
Pressure)
B (Inj.
Temp.)
0.103051
0.03435
AxB
0.009957
0.003319
e
0.213831
0.013364
T
0.33992
23.88
68.94
100
Furthermore, Table 4 shows the analysis of variance
for the green strength of water atomized powder compact.
The table shows that none of the factors are significant
but the interaction of A × B shows very little contribution
(7.24 %) to the green strength of the water atomized
powder compact. Although these factors are not
contributed to the green strength of the water atomized
powder, however it may influenced by some other factors
such as powder loadings, mold temperature, holding
pressure and etc.
106.463
-8.86558
90.42774
100
Table 6 Analysis of variance for the injection
temperature and injection pressure influence to
the green density of water atomized powder
Factor
Sum of
squares,
Sn
Variance,
vn
Variance
ratio, Fn
F statistic
Contribution
percentage,
Pn
0.0003813
0.0003813
0.2330953
F(0.1)=3.18
-3.7962595
-1.2191629
A (Inj.
Pressure)
B (Inj.
Temp.)
0.0028691
0.0014345
0.8768518
F(0.1)=2.81
AxB
0.0101673
0.0050837
3.1073686
F(0.1)=2.81
e
0.0196319
0.001636
T
0.0330496
20.86353
84.151892
100
4. Conclusions
The influence of the SS316L’s particle shape influence to
the injection molding of MIM’s green compact is
discussed in this paper. The authors have found that there
are influences of the powder particle shape to the
injection molding performance.
CAFEO 26
The rheological investigation found that the water
atomized powder feedstock exhibits higher viscosity than
of the gas atomized powder feedstock. In addition, the
flow behavior index of the feedstock shows water
atomized powder feedstock demonstrates lower shear
sensitivity.
Moreover, water atomized powder compact was found
to be stronger and denser than the gas atomized powder
feedstock. This is for the reason that the irregular shapes
of the water atomized powder provides more interparticle friction to the powder matrix and thus makes it
stronger than the gas atomized powder compact.
The analysis of variance also shows that the
interaction of A×B is significant to the green strength of
the gas atomized powder compact and green density of
the water atomized powder compact respectively. Besides
the main factor B is only significant to the green density
of the gas atomized powder compact.
Acknowledgements
My deepest appreciation and gratitude to National
University of Malaysia for the research grant, UKM-KK02-FRGS0013-2006 and University Technology of
Malaysia and Ministry of Higher Education, Malaysia for
the PhD scholarship.
References
[1] German, R. M, Hens, K. F. & Lin, S. T. 1991. Key
issues in powder injection molding. Bulletin of
American Ceramic Society. 70(1): 294-302.
[2] Mutsuddy, B. C. & Ford, R. G. 1995. Ceramic
Injection Molding. London: Chapman & Hall: 1-22.
[3] German, R M. 1993. Technological barriers and
opportunities in powder injection molding. Powder
Metallurgy. 25(4): 165-169.
[4] German, R.M. 1990. Powder Injection Molding.
Metal Powder Industries Federation, Princeton, NJ.
[5] Loh, N. H., Tor, S. B. & Khor, K. A. 2001.
Production of metal matrix composite part by
powder injection molding. Journal of Materials
Processing Technology. 108(3): 398-407.
[6] Hartwig, T., Veltl, G., Petzoldt, F., Kunze, H.,
Scholl, R. & Kieback, B. 1998. Powders for Metal
Injection Molding. Journal of the European Ceramic
Society 18: 121 l-1216.
[7] Jamaludin, K. R., Muhamad, N., Rahman, M. N.
Ab., Amin, S. Y. M. & Murtadhahadi . 2008.
Analysis of Variance on the Metal Injection Molding
parameters using a bimodal particle size distribution
feedstock. Proceeding of the International
Conference in Mechanical and Manufacturing
Engineering 2008. Johor Bahru Malaysia.
[8] Jamaludin, K. R., Muhamad, N., Rahman, M. N.
Ab., Amin, S. Y. M., Murtadhahadi, Ismail M. H.
2008. Injection molding parameter optimization
using the Taguchi method for highest green strength
for bimodal powder mixture with SS316L in PEG
and PMMA. Proceeding of the World Powder
26-29 November 2008, Bangkok Thailand
Metallurgy and Particulate Materials 2008,
Washington DC.
[9] German, R.M., and Bose, A., 1997. Injection
Molding of Metal and Ceramic, Metal Powder
Industries Federation , Princeton, N.J.
[10] L. Kowalski and J. Duszczyk, Specific heat of metal
powder-polymer feedstock for powder injection
molding. Journal of Materials Science Letters, Vol.
18 (1999) 1417-1420.
[11] Ranjit, R. A primer on the Taguchi method, 1990,
Society of Manufacturing Engineers, Michigan.
[12] Park, S.H, Robust Design and Analysis for Quality
Engineering, Chapman & Hall, London 1996.