Handbook of Metal Injection Molding (MIM) - common defects in metal injection molding

Prévia do material em texto

10
Common defects in metal injection molding
(MIM)
K.S . HWANG, National Taiwan University, Taiwan, R.O.C.
Abstract: Inconsistent product quality, including poor dimension control,
distortion, and internal and external defects, has tended to be
underestimated by MIM practitioners. These defects may originate in the
early processing steps, but they often do not manifest until after
debinding or sintering. Thus, the solutions are difficult to provide. This
chapter presents an overview of MIM defects. Explanations for these
defects are described and, where available, remedies and suggestions are
provided.
Key words: defects, powder/binder separation, dimensional control,
swelling, distortion.
10.1 Introduction
Powder injection molding comprises several processing steps, and defects
may occur in each step if care is not taken. The defects encountered could be
caused by mechanical factors, such as poor mold design and mold
manufacturing, or by processing related factors, such as incomplete
kneading, inadequate molding pressure, injection speed, holding pressure,
and non-optimized debinding and sintering parameters (Zhang et al., 1989;
Hwang, 1996). Some of these defects may originate in the early processing
steps but are difficult to identify because they do not manifest until after
debinding or sintering. In the following section, the defects that frequently
occur in each processing step are examined and their causes are explained.
Hopefully, with an understanding of the scientific background of the
defects, exhaustive trial and error experimentation can be avoided.
235
© Woodhead Publishing Limited, 2012
Josue
Destacar
10.2 Feedstock
10.2.1 Feedstock uniformity
The raw powders used for MIM are usually quite fine, mostly below 20 μm,
and thus agglomeration could be serious. When hard agglomerates, which
cannot be broken up during high-shear-rate kneading, are included in the
feedstock, the final sintered product could then contain inhomogeneous
microstructures. If the agglomerates are alloying element additions, highly
alloyed areas will develop and cause lean-alloyed regions, which will affect
the mechanical properties (Hwang et al., 2007). Moreover, the highly
alloyed region may also have low density after sintering, as shown in Fig.
10.1, since the agglomerated alloying elements, such as Mo, may not be
densified at the sintering temperature. Owing to unbalanced interdiffusion,
10.1 (a) Fracture surface showing Mo agglomerates; (b) large voids in
Mo-rich region.
Handbook of metal injection molding236
© Woodhead Publishing Limited, 2012
other alloying elements, such as nickel, when too large or agglomerated,
may also develop Kirkendall pores that are too large to be annihilated.
The uniformity of the feedstock constituents is always a concern in
kneading. If the kneading time and shear rate are insufficient, the metal
powders will not be uniformly distributed. Some pockets of organic binders
may also exist and cause blistering during the subsequent thermal debinding
process. In addition, with excessively long kneading time, decomposition or
evaporation of low-temperature binder components will occur. The
optimized kneading time can be determined by monitoring the power
input to the kneader. When the power decreases and becomes steady, the
feedstock is ready and should contain uniformly distributed powders and
binders. This uniform distribution and lot-to-lot consistency can be
confirmed using a pycnometer density meter or a capillary rheometer
(Kulkarni, 1997).
10.2.2 Recycled feedstock
To lower the manufacturing cost of MIM products, the gate, runner, sprue,
and green parts with defects are usually recycled. Two methods are usually
adopted by the MIM industries (Kulkarni and Kolts, 2002). The first
method is to add 30–50% recycled feedstocks to fresh materials, while the
other is to use 100% recycled materials. Unfortunately, these feedstocks
deteriorate as the number of recycling iterations increases. The main cause
of the deterioration of feedstock is oxidation of the binder components,
particularly in the case of paraffin wax, caused by the transformation of the
C–C chain to the C55O chain. The backbone binder, such as polyethylene,
also deteriorates during recycling (Cheng et al., 2009). These deteriorations
cause decrease of the intrinsic binder strength and weakening of the bonding
at the powder–binder interface and thus lower green strength. The loss of
the lubrication effect of the binder among powders also results in higher
viscosity. This means that the lot-to-lot dimensions and green densities will
be difficult to control (Kulkarni and Kolts, 2002). Consequently, the
molding parameters, including injection pressure and barrel temperature,
should be re-adjusted with recycled feedstocks. In addition, as a result of the
weaker bonding between powders and binders, more cracks and distortions
of production parts may result during solvent debinding with further
iterations of recycling, as shown in Fig. 10.2 for feedstocks recycled six and
eight times.
Common defects in metal injection molding 237
© Woodhead Publishing Limited, 2012
10.3 Molding
10.3.1 Flash
When high numbers of cavities are built in the mold, long runners are
required. This requires a high molding pressure to transport materials to
each cavity. When a high molding pressure is applied, flash usually occurs,
as shown in Fig. 10.3, unless the molds are perfectly clamped without any
clearance. Since the materials and shapes of the mold components are
different, different amounts of elastic deformation and, in some cases, even
plastic deformation, will occur and create clearance between tooling
components. As a result, materials will be forced into the clearances. This
problem becomes even more serious when large molds with large numbers of
cavities and undersized molding machines are used, because the stiffness of
the mold and the precision of the mold dimensions become more difficult to
control.
10.2 The surface condition of the specimen after solvent debinding in
508C heptane for 4 h. More defects can be found in the specimens
prepared with highly recycled feedstocks (6R and 8R).
10.3 Flashes occur when the feedstock is forced into the clearances
between mold components under high molding pressures.
Handbook of metal injection molding238
© Woodhead Publishing Limited, 2012
10.3.2 Residual stress
Another defect, which is caused during molding but does not appear until
the later processing steps of solvent debinding or thermal debinding, is
residual stress. This stress is frozen in owing to the rapid cooling by the
relatively cold mold surfaces. When this stress is relieved during heating in
the debinding stage, either in solvent debinding or in the early stage of
thermal debinding, distortion could occur. If a high injection pressure is
applied, distortion may even occur for parts sitting at room temperature for
a long time.
10.3.3 Binder/powder separation
Another common defect of cosmetic parts is a poor appearance around the
gate. As the feedstock passes through the narrow gate into the die cavity,
binder separation can occur due to powder migration from a high-shear-rate
region to a low-shear-rate region. This binder/powder separation becomes
more serious at the end of the filling stage and during the pressure holding
stage, wherein the feedstock flows slowly and the pressure builds up rapidly.
To fill in the clearance in the die cavity resulting from thermal shrinkage,
more feedstock is pushed into these openings by the holding pressure. Since
the movement of the metal powders in the feedstock is slow, due to the high
interparticle friction, the binder, which has a low viscosity, is forced to flow
through interparticle interstices. Thus, the binder content at the gate areas is
increased, causing molding defects, particularly at the gate, where the shear
rate is the highest. If the bonding betweenthe powder and the binder is
weak, separation will occur easily and form binder-rich areas at the surface
of the part near the gate, as shown in Fig. 10.4. This gate mark remains after
sintering, since the solid content in this region is low and thus the
appearance is different. Post-sintering surface treatment, such as sand
blasting or coating, is often applied to eliminate or alleviate this problem.
Development of new binders that can reduce the gate mark is still a focus of
research. Occasionally, the binder-rich area at the surface of the part may
delaminate from the bulk or form a hidden delaminated void near the
surface. When a post-sintering secondary operation, such as drilling, is
applied, these defects will then be noticed, as shown in Fig. 10.5. In some
cases, gate redesign helps to alleviate this powder/binder separation
problem. If the original gate has a narrow opening and is located at a
thin section, it can be enlarged and relocated to a thick section. As a result,
the level of binder/powder separation will be reduced and the flow marks
and weld lines will be minimized.
Common defects in metal injection molding 239
© Woodhead Publishing Limited, 2012
10.3.4 Other defects
Other defects in MIM parts are similar to those found in conventional
plastic injection molding due to improper molding parameters. When the
molding pressure or holding pressure is low, low green density areas and
incomplete filling can occur, as shown in Fig. 10.6. When combined with
poor venting design and early solidification at long runners or sprue,
internal voids may also develop in thick sections. After sintering, these
regions with low density or internal voids could form concave sink marks.
Weld lines and flow marks are caused by low feedstock temperatures, low
10.4 With a high shear rate at the gate, powder/binder separation
occurs and binder-rich gate marks form.
10.5 Powder/binder separation occurs and delamination is observed
after drilling.
Handbook of metal injection molding240
© Woodhead Publishing Limited, 2012
mold temperatures, or low barrel temperatures. When cold materials are
forced by high pressure to flow over mold surfaces, flow marks are left
behind, as shown in Fig. 10.7. When the flowing feedstock is separated into
two streams by a post and the two separated cold fronts meet again, weld
lines form, as shown in Fig. 10.8. These defects could also be related to
improper mold designs, such as long runners, improper gate locations,
improper venting ports, etc. The typical solution to these molding defects is
to prevent cooling of the feedstock before the end of molding by increasing
the barrel, nozzle, and mold temperatures and by redesigning the part to
avoid partitioning of the flowing feedstock stream in the die.
Most defects encountered during molding of MIM, including the ones
described above, are similar to those found in conventional plastic injection
molding and are listed in Table 10.1. Also shown are the causes and
remedies of these defects.
10.6 Incomplete filling occurs when the molding pressure is too low.
10.7 Flow marks occur when the feedstock temperature is too low.
Common defects in metal injection molding 241
© Woodhead Publishing Limited, 2012
10.8 Weld lines form when two cold fronts meet each other.
Table 10.1 Defects frequently found in molding
Defect type Possible causes Remedies
Flash Too high a pressure inside
the die, poor flatness of
mold surface along the
parting line, venting channel
too large
Use large tonnage machine,
proper tool making, use a
lower injection speed and
molding pressure, optimize
the switch point
Sticking in cavity Too high a molding
pressure, not enough
thermal shrinkage, early
ejection, improper mold
design or making
Use a lower injection speed,
molding/holding pressure,
and mold temperature,
increase cooling time,
eliminate undercut and
increase draft angle, adjust
ejection area and location,
redesign the binder
Sink mark Thermal shrinkage, low
density
Increase molding/holding
pressure and injection
speed, decrease mold
temperature, increase gate
area, add venting channels,
decrease speed when
passing thick sections
Voids Trapped gas, absorbed
moisture
Increase holding pressure,
decrease injection speed,
increase mold temperature,
increase gate area, move
gate to thick sections
Burn marks Overly heated binders Decrease injection speed
and feedstock temperature,
increase gate area, change
gate location
Handbook of metal injection molding242
© Woodhead Publishing Limited, 2012
10.4 Debinding
A successful debinding process usually consists of two or three stages,
during each of which one of the binder components is removed. This ensures
that the shape of the molded compact remains intact throughout the
debinding process. Minor binder components, such as plasticizers,
surfactants, coupling agents, and lubricants, are usually removed first.
Polymeric backbone binders, which account for 30–60% of the binder, are
the last type removed, and this late debinding stage is accompanied by light
sintering of particles.
Several debinding processes have been identified, including the two-stage
debinding process of solvent/water debinding followed by thermal debind-
ing, straight thermal debinding, and catalytic debinding. The 100% thermal
debinding is a slow process, since decomposed gas components that develop
in the core section cannot escape to the atmosphere effectively through any
pore channels. Defects are frequently encountered unless extremely slow
heating rates and long debinding times, such as several days, are applied.
Catalytic debinding is used for polyacetal-based materials, which also
contain about 10% polyolefin binder components such as polyethylene and
polypropylene. The polyacetal is thermally decomposed at around 1358C
into formaldehyde in a diluted nitric acid gas atmosphere. This decomposi-
tion is basically a direct solid–gas reaction. No liquid is involved and thus
the compact remains rigid throughout the debinding process. Moreover,
since the reaction occurs only at the binder–vapor interface, there is no
internal pressure build-up caused by decomposed gases. As a result,
distortion or slumping can be prevented and blistering, voids, and cracks
can thus be minimized. Although catalytic debinding can produce good
Table 10.1 (cont.)
Defect type Possible causes Remedies
Weld lines Cold feedstock in the die Increase injection speed,
mold temperature, and
feedstock temperature,
enlarge gate opening, add
venting channels or
overflow wells near weld
line locations, move gate
location, redesign parts to
avoid stream partition
Flow mark Cold feedstock in the die Increase injection speed,
mold temperature, and
feedstock temperature,
enlarge gate opening,
change gate location
Common defects in metal injection molding 243
© Woodhead Publishing Limited, 2012
quality debound parts in a very short debinding period, the more widely
used debinding process today is the two-stage debinding process of solvent
debinding followed by thermal debinding, for economic reasons. Thus, this
process will be used to illustrate the examples of debinding defects in more
detail.
10.4.1 Solvent debinding
In the solvent debinding process, cracking and slumping are frequently
observed if the process is not well executed. Since the solvent is usually
heated in order to increase the debinding rate, the minor components, such
as paraffin wax and lubricant, become soft or even melt. As a result,
warpage and slumping frequently occur, particularly when the part has a
complicated shape with overhanging sections. To alleviate this problem,
using a stronger backbone binder and a lower debinding temperature is
usually helpful. A modification of the part shape to give better support for
thin or extended sections is also frequently employed.
When parts are subject to solvent debinding, cracking may also occur,
even though the part is soft. This problem is mainlycaused by the swelling
of polymers when solvents penetrate into the binder. The amount of
expansion depends on the temperature, and the types of binder component
and solvent used. Figure. 10.9 shows the in situ length change of a 100mm
long plate immersed in heptane at 40, 50, and 608C using a laser dilatometer
(Lin and Hwang, 1998). The part expands and the amount increases as the
10.9 Amount of swelling of molded specimens increases with
increasing temperature.
Handbook of metal injection molding244
© Woodhead Publishing Limited, 2012
temperature increases, in particular when the temperature exceeds the
melting point of a binder component. The main cause of the expansion is the
reaction between the solvent and the backbone binder. This has been
confirmed by using compacts with and without soluble binder components,
which show similar amounts of swelling for both specimens (Hu and
Hwang, 2000). The addition of isopropyl alcohol reduces the amount of
expansion by retarding the penetration of the heptanes (Fan et al., 2008).
The amount of expansion also increases as the total binder content
increases. These observations suggest that the solvent debinding tempera-
ture and the amount of binder should be reduced if distortion occurs.
Parts with different cross-section thicknesses also behave differently (Hu
and Hwang, 2000). Thin sections expand quickly in the early stage, while
thick sections expand slowly, as shown in Fig. 10.10, owing to the
constraints from the inner section of the part. As a result, distortion may
occur early in the debinding stage because of the different amounts of
expansion in different sections of the part.
10.4.2 Thermal debinding
Debinding defects are often observed after thermal debinding. However, the
causes of these defects are not necessarily related to the thermal debinding
10.10 Effect of thickness on the swelling during solvent debinding.
Common defects in metal injection molding 245
© Woodhead Publishing Limited, 2012
process per se. They could originate from problems in the kneading,
molding, or solvent debinding processes, which are amplified and
manifested during thermal debinding.
Thermal debinding defects are frequently seen when the applied heating
rate is too fast. In most cases, the defects are caused by the fast
decomposition of the binder components. Adsorbed water has also been
shown to produce defects when it is converted to steam. When the
decomposed gas molecules cannot escape fast enough to the ambient
through interconnected pore channels, they cause blisters or even bloating
holes if the green body is plastic. When the green body is more rigid or the
interparticle friction is low, cracking will occur.
It is widely recognized that the heating rate should be slow to prevent the
formation of defects. This heating rate can be determined using thermo-
gravimetric analysis (TGA), which shows the critical temperature ranges
where binder components decompose. The heating rate through these
ranges should be slow or have a holding period to prevent drastic binder
burn-off. However, it should be noted that the debinding behavior of parts
in a production furnace with a heavy loading is quite different from that
used in TGA. The temperature range where slow heating or holding is
required is usually higher than that measured in TGA. It is also
recommended that the gas flow rate be increased and a short flow path be
used to help carry away the decomposed gases. When a vacuum furnace is
used, this means that a high partial pressure should be applied during
thermal debinding. The entrance and exit of the gas should be designed in
such a way that the flow path over parts is short in order to maintain a
laminar flow, which removes decomposed gas more effectively than does a
turbulent flow.
As thermal debinding proceeds to the end, only a small amount of
backbone binder is left. This remaining backbone binder is mostly located at
interparticle necks. The capillary force induced tends to rearrange particles
and thereby to induce internal stress. A feedstock using high tap density
powder and high solids loading could help to alleviate these stresses. If the
content of the backbone binder is high or the percentage of the soluble
binder removed is low, distortion could occur because too much liquid
binder is present during thermal debinding.
For solvent debound parts, the heating rate can be quite fast, owing to the
presence of interconnected open pores. With more and larger interconnected
pore channels, the gases generated during the thermal debinding can escape
to the ambient without causing blistering or cracking. Fan et al. (2009) used
a model to predict the minimum amount of binder removal required during
solvent debinding for producing interconnected pores in the middle section
of products with different thicknesses. For example, to create open pores in
the middle section of a part with 4.2 wt% soluble binder, the minimum
Handbook of metal injection molding246
© Woodhead Publishing Limited, 2012
debinding fraction needed is about 59% of the total amount of soluble
binder, despite the thickness of the part, yielding a local debinding fraction
of 37% and a porosity of 8.5% at the center. However, with too much
residual binder and too few open pore channels, the large amounts of gases
generated during thermal debinding may not be able to escape to the
surface. Moreover, a subsequent study by the same group found that the
remaining binders in the solvent debound parts redistribute within the
compact during thermal debinding (Fan and Hwang, 2010). As the solvent
debound parts are heated above the melting points of the binder
components, capillarity drives binders into fine pore channels and
interparticle contact areas in order to reduce the total surface energy. As
a result, the open pores in the middle section are blocked again, as shown in
Fig. 10.11. With too much binder and few interconnected pore channels,
these regions behave similarly to those occurring when as-molded parts are
subjected to thermal debinding directly, without solvent debinding. Thus, to
improve the efficiency of thermal debinding, the amount of binder removal
during solvent debinding must be increased so that the amount of the
remaining binder is too little to flow into and fill in the pores in the middle
section.
Experimental results have shown that this minimum amount of binder
removal for the prevention of binder redistribution increases as the
thickness of the part increases. For parts with 4.20 wt% soluble binder
and a thickness of 6mm, the suggested minimum amount of binder removal
for solvent debinding is 79%, or 3.32 wt% of the total weight of the part,
when a heating rate of 58C/min is used (Fan and Hwang, 2010). This value is
higher than the 59% required for creating open pores in the center of the
part during solvent debinding. Since the minimum amount of the binder
that needs to be removed depends on the thickness of the part and the
10.11 Fluorescence dye penetration test of 6mm thick specimens with
68% of the soluble binder removed, showing that the binder
redistributes and fills open pore channels at the center of the part after
being heated to 1208C.
Common defects in metal injection molding 247
© Woodhead Publishing Limited, 2012
heating rate of the thermal debinding, a general number of 90% is
recommended for a safe start.
The exfoliation phenomenon, which is caused by the delamination of a
skin layer from the main body, has also been reported (Woodthorpe et al.,
1989; Zhang et al., 1989). This has been attributed to the binder-rich surface
caused by incorrect molding procedures in combination with a fast heating
rate during thermal debinding. One postulation is that, as the feedstock
cools in the die cavity, the material shrinks owing to the volume shrinkage of
the metal powder and binder components, in particular the paraffin wax.
This shrinkage leaves a smallclearance between the part and the die wall,
which allows further penetration of the feedstock. Since the clearance is
small, the easiest material to fill this gap in the pressure holding stage of the
molding process is the low melting material, such as paraffin wax. The other
possibility causing the binder-rich surface is the increased binder/powder
ratio at the surface due to the emerging flow of the binder from the interior
during thermal debinding. With a high binder content and less interparticle
friction, a layer, or skin, could detach from the main body. With a fast
heating rate, this phenomenon may worsen if the outward flow of the binder
is accelerated by the high pressure of the decomposed gas in the core of the
part.
To facilitate explanation, Table 10.2 presents a summary of the various
types of defects found after solvent and thermal debinding, the possible
sources of the problems, and the recommendations for their removal
(Hwang, 1996).
Several techniques have been developed to alleviate the above defects of
cracking, blistering, and distortion that occur during thermal debinding.
They include the use of slow heating rates in the temperature range where
binder components decompose or evaporate, high gas flow rates, higher
binder removal percentages during solvent debinding, and supports or
fixtures for overhanging and intricate sections. The use of more irregular
powders also helps to increase the green strength and allows the use of a
faster debinding rate. However, these powders will have an adverse effect on
feedstock flowability and sinterability.
10.4.3 Binder residues
The binder residues left from debinding have been paid little attention in the
literature (Zhang et al., 1989). These residues could be critical for some
structural or magnetic parts. Oxide residues from high-density polyethylene
and metal stearates have been reported in several studies. These oxides may
come from catalysts, which are used as Ziegler–Natta initiators for
polymerization of polyethylene and isostatic polypropylene, while some
other oxides may come from metal atoms in stearates. The metal atoms can
Handbook of metal injection molding248
© Woodhead Publishing Limited, 2012
usually be dissolved in the matrix during sintering. However, reactive metals
such as titanium and aluminum will react with oxygen or water vapor in the
metal powder or sintering atmosphere, forming stable metal oxides. These
oxides or dissolved elements could influence the mechanical properties of
structural parts. These residues may not always be detrimental to MIM
parts. It has been demonstrated that residual Ti from high-density
polypropylene helps densification and increases the strength (Lu et al.,
1996). Magnesium from magnesium stearate has also been shown to
Table 10.2 Defects frequently found in debinding
Defect type Possible causes Remedies
Cracks (solvent
debinding)
Swelling of binder
components, poor bonding
between binder and
powder, low strength of the
backbone binder, too high a
molding pressure, large
differences in section
thicknesses
Change the type and
composition of solvent or
binder, use a lower injection
speed and molding
pressure, redesign parts
with smaller differences in
section thicknesses, use
lower debinding
temperatures
Bending/
distortion
(solvent
debinding)
Residual stress from
molding, lack of support for
overhanging sections,
entrapped air
Bake between 50 and 908C,
use fixtures, adjust molding
parameters
Corrosion/stain
(solvent
debinding)
High acidity of solvent,
humid environment
Replenish solvents or use
new ones, leave parts in a
dry atmosphere
Cracks/blistering
(thermal
debinding)
Overly fast heating,
absorbed water in
feedstock, insufficient
binder removal for solvent
debinding, poor binder
distribution, low solid
content
Use slow heating rates,
extend solvent debinding
time, use longer kneading
time and adjust binder
components, keep feedstock
dry, use higher gas
sweeping rate and shorter
flow path
Bending/
distortion
(thermal
debinding)
Overly fast heating,
insufficient binder removal
for solvent debinding, lack
of support for overhanging
sections, insufficient
interparticle friction, too
much binder
Use slow heating rates,
extend solvent debinding
time, use fixtures or sands
for the support, use higher
gas sweeping rate, use
more irregular powders,
increase solids loading
Exfoliation
(thermal
debinding)
Wax separation to the
surface, overly high heating
rate
Extend solvent debinding
time, use slower heating
rate, bake below 1008C
Source: (Hwang, 1996).
Common defects in metal injection molding 249
© Woodhead Publishing Limited, 2012
improve the sintered density and bending strength of injection molded
alumina by forming fine and uniformly distributed magnesia in the alumina
matrix (Hwang and Hsieh, 2005).
Another residue is carbon soot left from thermal debinding. With an
increase in the carbon content, the corrosion resistance of stainless steels
such as 316L will deteriorate due to the formation of chromium carbide and
Cr-lean areas. If an excessive amount of carbon is present, the melting point
decreases and may cause local distortion or melting of the part. For Kovar
(Fe–29Ni–17Co), the coefficient of thermal expansion will also increase and
cause adverse effects on glass–metal sealing (Tokui et al., 1994). To ensure
complete debinding without leaving carbon residues, a high gas sweeping
rate during thermal debinding is recommended. With the use of continuous
furnaces, such as pusher or walking beam furnaces, a high dew point in the
debinding zone is also recommended to facilitate the removal of the carbon
residues. It should be noted that carbon has a high diffusion rate into iron
and that such diffusion starts at about 8758C, depending on the carbon
content and the phase transformation temperature. Thus, thermal debinding
and carbon removal should be completed before parts reach this
temperature.
10.5 Sintering
10.5.1 Appearance and discoloration
One of the advantages of the MIM process is its capability to produce parts
with shiny surface finishes that have small surface roughness. To achieve this
goal, high density must be obtained, and the metal surface must be free from
reaction with sintering atmospheres to avoid the formation of oxides,
nitrides, or other reaction products. Consequently, the dew point or oxygen
content in the atmosphere must be sufficiently low, and the amount of
hydrogen or the degree of vacuum must be sufficiently high, to reduce the
metal oxides on metal powder surfaces. Otherwise, fine oxide particles, such
as silicon dioxides, can easily form. When nitrogen is contained in the
atmosphere, such as dissociated ammonia, it will react with the chromium in
stainless steels, forming chromium nitrides, which deteriorate the corrosion
resistance of stainless steel. When a high vacuum is used at high
temperatures, chromium will evaporate, with a resultant reduction in
corrosion resistance. Thus, a partial atmosphere of argon or pure argon is
often used for high-temperature sintering. These sintering concerns and
solutions are similar to those for press-and-sinter parts.
Handbook of metal injection molding250
© Woodhead Publishing Limited, 2012
10.5.2 Dimensional control and distortion
High density, above 95% of the theoretical density, is usually required for
MIM parts. To achieve this density, fine powder and high-temperature
sintering are used most of the time. In some cases, liquid phase sintering,
including supersolidus liquid phase sintering, is required. Unfortunately,
with the typically low solids loading in MIM parts, less than 70 vol%,
preservation of the geometry is a challenge. This problem is an even greater
challenge in prealloyed powders for which the solidus and liquidus lines are
flat and the temperature difference between these two lines is small. These
characteristics make the amount of liquid very sensitive to the temperature
variationin the sintering furnace, as indicated by the phase diagram and the
lever rule. A good example is the SKD11 or D2 tool steels. The temperature
must be controlled to within ±58C. If the temperature is too high, too much
liquid is formed; then gravitational forces cause distortion or even slumping,
while too small an amount of the liquid will not densify the compact
(German, 1997).
Several solutions have been reported. A recent patent discloses prealloyed
tool steel powders doped with 2–5% niobium (Soda and Aihara, 2007). The
niobium forms NbC and inhibits grain growth during liquid phase sintering.
With fine grains, the thickness of the liquid layer at the grain boundaries
decreases, which makes particle rearrangement more difficult (Liu et al.,
1999). The addition of carbides has also proven effective to open up the
sintering window with the same underlying mechanism (Chuang and
Hwang, 2011).
Similar to the distortion problems found in solvent debinding, over-
hanging sections, steps, and narrow openings, as shown in the schematic
diagram of Fig. 10.12, may also distort during sintering due to gravitational
force or uneven amounts of shrinkages in different regions. This problem
becomes even more serious when liquid phase sintering or heavy metals such
as tungsten alloys are used. The use of high solids loading and more
irregular powders helps to a certain degree. But sintering fixtures are often
required to prevent warping, and dummy bridges, which are removed by
mechanical methods after sintering, are often designed into the part to
prevent opening or narrowing of the long slots during sintering.
10.6 Conclusion
Although considerable time and effort has been spent to resolve the
problems encountered during the fabrication of MIM parts, and the
technology itself has advanced significantly since the birth of the MIM
process, defects still frequently occur during the daily practices of kneading,
injection molding, debinding, and sintering. Some examples are gate marks,
Common defects in metal injection molding 251
© Woodhead Publishing Limited, 2012
sink marks, distortions, cracking, blistering, and poor dimensional control.
Some of these problems have been resolved, and the underlying basics are
understood, as briefly described above. However, many challenging
problems remain, and in-depth understanding of the root causes and the
fundamentals of the processes has yet to be attained. There is still much to
learn about the residual stress in molded articles, powder/binder separation,
the flow behaviors of binders during molding, binder redistribution behavior
during thermal debinding, and shape retention during supersolidus liquid-
phase sintering.
10.7 References
Cheng, L.H., Hwang, K.S., and Fan, Y.L. (2009), ‘Molding properties and causes of
deterioration of recycled powder injection molding feedstock’,Metallurgical and
Materials Transactions A, 40A, 3210–3216.
Chuang, K.H. and Hwang, K.S. (2011), ‘Preservation of geometrical integrity of
supersolidus-liquid-phase-sintered SKD11 tool steels prepared with powder
injection molding’, Metallurgical and Materials Transactions A, 42A, in press,
doi: 10.1007/s11661-010-0593-8.
Fan, Y.L., Hwang, K.S., and Su, S.C. (2008), ‘Improvement of the dimensional
stability of powder injection molded compacts by adding swelling inhibitor into
the debinding solvent’, Metallurgical and Materials Transactions A, 39A, 395–
401.
Fan, Y.L., Hwang, K.S., Wu, S.H., and Liau, Y.C. (2009), ‘Minimum amount of
10.12 Parts with long overhangs, steps, and slots require sintering
fixtures and dummy bridges to prevent sagging and opening/closure of
the slots.
Handbook of metal injection molding252
© Woodhead Publishing Limited, 2012
binder removal required during solvent debinding of powder injection molded
compacts’, Metallurgical and Materials Transactions A, 40A, 768–779.
Fan, Y.L. and Hwang, K.S. (2010), ‘Defect formation and its relevance to binder
content and binder redistribution during thermal debinding of PIM compacts’,
Proceedings of the 2010 PM World Congress, EPMA, Shrewsbury, UK, vol. 4,
pp. 595–601.
German, R.M. (1997), ‘Supersolidus liquid-phase sintering of prealloyed powders’,
Metallurgical and Materials Transactions A, 28, 1553–1567.
Hu, S.C. and Hwang, K.S. (2000), ‘Length changes and deformation of PIM
compacts during solvent debinding’, Metallurgical and Materials Transactions
A, 31A, 1473–1478.
Hwang,, K.S. (1996), ‘Fundamentals of debinding processes in powder injection
molding’, Reviews in Particulate Materials, 4, 71–103.
Hwang, K.S. and Hsieh, C.C. (2005), ‘Injection-molded alumina prepared with Mg-
containing binders’, Journal of the American Ceramics Society, 88, 2349–2353.
Hwang, K.S., Wu, M.W., Yen, F.C., and Sun, C.C. (2007), ‘Improvement in
microstructure homogeneity of sintered compacts through powder treatment
and alloy designs’, Materials Science Forum, 534–536, 537–540.
Kulkarni, K.M. (1997), ‘Dimensional precision of MIM parts under production
conditions’, International Journal of Powder Metallurgy, 33(4), 29–41.
Kulkarni, K.M. and Kolts, J.M. (2002), ‘Recyclability of a commerical MIM
feedstock’, International Journal of Powder Metallurgy, 38, 43–48.
Lin, H.K. and Hwang, K.S/. (1998), ‘In-situ dimensional changes of powder
injection molded compacts during solvent debinding’, Acta Materialia, 46,
4303–4309.
Liu, J., Lal, A., and German, R.M. (1999), ‘Densification and shape retention in
supersolidus liquid phase sintering’, Acta Materialia, 47, 4615–4626.
Lu, Y.C., Chen, H.C., and Hwang, K.S. (1996), ‘Enhanced sintering of iron
compacts by the addition of TiO2’, in Sintering Technology, eds R.M. German,
G. L. Messing, and R.G. Cornwall, Marcel Dekker, New York, USA, pp. 407–
414.
Soda, Y. and Aihara, M. (Mitsubishi Steel Mfg Co. Ltd) (2007), ‘Alloyed steel
powder with improved degree of sintering for MIM and sintered body’,
US Patent 7,211,125.
Tokui, K., Sakuragi, S., Sasaki, T., Yamada, Y., Ishihara, M., Nakayama, M., Kuji,
I., and Nomura, M. (1994), ‘Properties of sintered Kovar using metal injection
molding’, Journal of Japan Society of Powder and Powder Metallurgy, 41, 671–
675.
Woodthorpe, J., Evans, J.R.G., and Edirisinghe, M. J. (1989), ‘Properties of ceramic
injection moulding formulations, part 3, polymer removal’, Journal of Materials
Science, 24, 1038–1048.
Zhang J.G., Edirisinghe, M.J., and Evans, J.R.G. (1989), ‘A catalogue of ceramic
injection moulding defects and their causes’, Industrial Ceramics, 9, 72–82.
Common defects in metal injection molding 253
© Woodhead Publishing Limited, 2012