POWDER INJECTION MOLDING
POWDER INJECTION MOLDING
• The Powder Injection Molding (PIM) process is said to be a
combination of conventional powder metallurgy and plastic
injection molding technology because it brings together the
diversity of materials of conventional powder metallurgy
and the geometric freedom of part design associated with
thermoplastic injection molding.
• The combination of these technologies allows for complex
shapes to be produced to near full-density for high
performance applications.
• Some of the advantages of PIM over conventional powder
metallurgy are that it allows more design flexibility, closer
tolerances and gives more uniform shrinkage during
sintering.
• Historically, the concept of injection molding
originated from the metal die casting industry
wherein the technology of injecting molten metal
into closed dies to obtain desired shapes evolved.
• Later, this molding concept was successfully
exploited in the plastics industry.
• Powder injection molding is an extension of the
plastic injection molding process.
PROCESS OUTLINE:
• The PIM process begins by mixing selected
powders with a suitable combination of binders,
lubricants and plasticizers. The mixture, usually
termed the ‘Feedstock’, may be either granulated
or may be used in the form of a thick slurry for
injection molding into the desired shape,
employing conventional plastic injection molding
practices.
• The shaped parts removed from the die must have
an adequate green strength and stiffness to be
handled. Subsequently, the binder and other
unwanted additives are removed from the compact
by a process known as ‘debinding’.
• Then, the parts are sintered to densify the resulting
porous compacts to yield near-net shape and high
performance components. The parts shrink
considerably during sintering and the final part is a
reduced version of the as-molded green shape.
• The product can be used without further processing
or it may be further densified, heat-treated,
machined or surface treated.
 As noted in the figure, the basic stages involved in forming
a component by PIM include the following.
1) selecting and tailoring a powder for the process;
2) mixing the powder with a suitable binder for producing
either a homogeneous slurry or granular pellets of mixed
powder and binder (i.e. feedstock);
3) injection molding of the feedstock to obtain the required
shape;
4) removing the binder from the molded parts (debinding);
5) densifying the compact by high-temperature sintering;
6) post-sintering processing, as appropriate, including
machining, heat treatment, surface finishing or further
densification.
Schematic Diagram of MIM
MATERIALS FOR PIM
The feedstock for powder injection molding consists
of two components:
a) the particulate materials and
b) the binder and other additives.
PARTICULATE MATERIALS:
• metal, alloy, ceramic or cermets powders.
• There are some apparent conflicts in the powder
characteristics needed.
• For example, an irregularly shaped or ligmental
powder will raise the viscosity of feedstock mixture
and yield a lower packing density during injection
molding. This requires more binder (i.e. low powder
loading) and results in more sintering shrinkage;
however, due to particle interlocking, the shapes
produced will have a high green strength, increased
compact strength after binder removal and better
shape retention during sintering.
• On the contrary, a spherical powder results in high
packing density and offers comparatively less
resistance to flow during injection molding, thereby
minimizing the amount of binder that must be added
as well as reducing the sintering shrinkage.
However, because there is no mechanical
interlocking between spherical particles, the green
part will have a comparatively lower strength,
especially after removal of the binder.
• Fine powder
• Fine powders have the advantage of better sinterability and
ease of molding and they expose the injection molding
machine screw to less risk of damage.
• In addition to faster sintering kinetics, fine powders also
result in more homogeneous microstructures since diffusion
lengths and times for migration of alloying constituents will
decrease with reduced particle size.
• Finer powders also allow for more intricate geometries,
thinner walls, sharper edges and better surface finish in
parts.
• Additionally, the smaller particles, because of comparatively
higher inter-particle friction, exhibit a desirable increase in
the compact strength during debinding that reduces the
possibility of distortion or slumping in processing.
• However, very fine particles cause difficulty in
attaining a high packing density because of
agglomeration.
• On the other hand, coarser powders generally give
lower sintered densities and larger residual pores
and also pose difficulties in molding.
• To obtain a high sintered density in the final
product, a high packing density is required in the
green compacts.
• In general, the lower the initial packing density, the
greater the sintering shrinkage needed to attain a
high final density.
BINDERS AND OTHER ADDITIVES:
• The binder is not only a necessary aid for promoting
viscous flow in the feedstock during injection
molding, but it should also maintain the shape of the
green part after removal from the mould.
• After molding, the binder has to be removed
carefully without impairing the integrity of the
molded part. Even though the binder is just an
intermediate processing aid it has considerable
influence on the success of the PIM process.
• A binder system that may produce excellent flow
characteristics during molding, but presents
difficulties during its subsequent removal, is not
suitable for PIM purposes.
• Generally, a binder system consists of the following
components:
1) Major binder component:
• This is responsible for holding the compact in shape during
debinding until the commencement of sintering and
determines to a large extent the final binder properties. It is
normally a high molecular weight thermo-plastic; such as,
polystyrene, poly- propylene and paraffin wax.
2) Minor binder component:
• This is generally added to alter the viscosity of the binder
system and is removed early in the heat treatment
(debinding). If this component, which is usually a thermoplastic or an oil, forms a continuous, separate phase, then its
removal would create pore channels throughout the molded
product to facilitate the removal of the evolving gas(es).
Examples are paraffin wax and beeswax.
3) Plasticisers:
• These minor additives are included to enhance the
moldability of the feedstock mixture. Examples are
carnauba wax and glycerin.
4) Processing aids:
• Some minor additives (surfactants) are included to
improve the wetting between the binder and powder.
Other minor components may be used to facilitate
release of the product from the mould.
• German has classified binders into five different
categories, as given below:
1) thermoplastic compounds,
2) thermosetting compounds,
3) water-based systems,
4) gellation systems, and
5) inorganics.
PREPARATION OF FEEDSTOCK:
• The first processing step in the manufacture of a PIM part
is to produce an appropriate feedstock.
• The selected powders are mixed in precise proportions
with suitable thermo-plastic binders and other additives.
• These polymeric additions often comprise as much as 3040 volume percent of the feedstock.
• To obtain high density green compacts and reproducible
results, it is essential to optimize the composition of the
powder-binder mixture.
• In general, it is considered best to use the minimum
amount of binder that gives good flow behavior since the
binder must eventually be removed from the powder.
However, too little binder results in a highly viscous
mixture causing mould filling difficulties and voids
formation. During debinding, these voids can cause
cracking due to internal vapour build-up, as a result of
degradation of polymers.
• On the other hand, an excess of binder is wasteful
and requires comparatively longer debinding times
and also results in greater dimensional shrinkage
during sintering. During molding, excess binder can
separate from the powder, leading to
inhomogeneities in the molded compact and
possible dimensional control problems.
• Moreover, excessive binder loading levels usually
cause deformation, sagging and blistering during
subsequent binder removal stage.
• The ideal corresponds to the case where particles are
coated with a uniform and very thin layer of binder
and with no voids in the binder.
• Thus, it is necessary that the binder fills all of the
void space between the particles while maintaining a
reasonably low viscosity.
MIXING
• Once a powder and binder have been selected, the next
concern is to mix these ingredients to prepare a
homogeneous feedstock on both large and small scales of
size.
• The homogeneity of the feedstock composition is crucial,
since inhomogenieties cannot be removed by subsequent
processing.
• Compositional variation on a large scale of size, i.e. on a
scale of size that is a moderate fraction of the molding, will
lead to non-uniform shrinkage on a similar scale of size and
the distortion of the molding during sintering.
• A very homogeneous mixture on a small scale of size, e.g.
100 particle scale, is important as it determines the
homogeneity of porosity on the same scale of size after the
binder is removed and this determines how the material
shrinks during sintering on this small scale.
• Variations in porosity from place to place on a small scale
lead to the formation of enlarged pores.
• An ideal mixing operation should result in a uniform
distribution of powder in a matrix of binder on all
appropriate scales of size with the binder filling the holes
between the particles and coating each powder particle.
This will ensure a thin liquid film between the particles at
the injection molding temperature, thus promoting good
rheological properties, and will result in maximum packing
of powder in the molding cavity.
• There is a possibility of segregation when the mixture has
particles of different sizes, shapes, or densities. As the
particle size decreases there is greater inter-particle
adhesion and friction, making the problem of
agglomeration more acute but reducing segregation.
• An important requirement in mixing is to break up
agglomerates to attain uniform particle packing and
porosity on a small scale of size. Failure to do this
can affect the final microstructure causing both
residual porosity and non-uniform grain size.
• Agglomerates can also decrease the packing
efficiency, which may increase the viscosity of the
feedstock mixture.
• The agglomeration of the powder can also be
reduced by the addition of appropriate dispersing
and coupling agents within the binder system.
• The mixing variables that affect the homogeneity of
a feedstock can be broadly classified into four
categories, namely those associated with the powder,
binder, the mixer and the way of ingredients are
added to the mixer.
TYPES OF MIXERS
• Different types of mixers
• These include mixers incorporating plug extrusion,
double planetary, twin-screw extrusion, sigma-blade,
Z-blade, milling, and impeller concepts.
• Consequently, there can be a wide variation in the
quality of the mixtures. Several problems in the
molding, debinding and sintering stages can be
traced to improper mixing procedures.
MOLDING:
• In its simplest form, molding consists of heating the
feedstock pellets/granules to a sufficiently high
temperature such that the binder is melted and
making the mixture fluid, then forcing this mixture
into a cavity where it cools and forms the compact
shape.
• The objective is to attain the desired shape free of
voids or other defects and with a homogeneous
distribution of powder.
• The feedstock must therefore have sufficiently low
viscosity at the molding temperature to flow freely
into the mould and subsequently to leave minimal
residual stresses.
Feedstock pellets and worms for molding
• PIM processors have adopted the principles and
equipment used in plastics injection molding.
• Since the material properties and molding
requirements of PIM parts are substantially different
from those of plastics, the injection molding
machines and techniques for PIM parts require
modification.
• Avoid premature freezing.
• Some differences in mould design, and especially in
the gating system, are needed to accommodate the
highly viscous PIM feedstock.
• To obtain reproducible results, molding is often carried out
under strictly controlled conditions of temperature of
feedstock and of pressure and flow rate which are influenced
by the configuration of the barrel, runners, gates and moulds.
• Control of these parameters within the required levels can
now be achieved with the advent of closed-loop computerbased, control systems applied to an injection molding
machine.
• In general, the minimum barrel temperature should be above
the melting point of the highest melting component of the
binder.
• The maximum mould temperature should be below the
lowest recrystallisation temperature among the binder
components.
• A high temperature difference between barrel and
mould results in a large thermal shrinkage of molded
parts thus requiring a higher packing pressure or
longer packing time to offset the thermal shrinkage.
However, the powder mixture quickly freezes in the
mold, rendering a longer packing time ineffective.
• Defects occurring during the molding stage include
voids, short shots, jetting, poor ejection, parting line
flash, surface blistering, cracking, formation of weld
lines and sink marks, cold flow patterns, warpage,
non uniform densities and poor dimensional control.
Some of the defects, such as sink marks, exterior
cracks and voids are apparent by visual observation.
• Unfortunately, most of the defects only become apparent
after debinding or sintering. These defects may be
eliminated through adjustments in the time, temperature,
and pressure parameters of the molding cycle.
• However, it requires extensive trial-and-error experiments
to determine the proper molding conditions.
• An injection pressure which is too low leads to weld lines
and cold flow patterns on the surface, whereas too high a
pressure can result in residual stresses or even cracking after
mold release.
• Moreover, a pressure release directly after mould filling
would cause a back flow from the molding into the
plastification unit and should be avoided.
Overview of a horizontal injection molding
machine and key components
DEBINDING:
• The binders and additives used in the molding steps
are removed by a processing which has come to be
known as “debinding”.
• Thus, a major difference between polymer injection
molding and powder injection molding occurs after
molding, when the binder is removed from the
compact prior to sintering.
• German has identified six debinding techniques
which may be used in PIM, broadly categorized as
either solvent or thermal processing, as shown in
Figure.
• The debinding process may be a combination of
more than one of these processes.
• Commercially, total thermal debinding and solvent
extraction followed by thermal debinding are the
two popular debinding methods.
• Recently, another technique known as ‘catalytic
debinding’ has been reported whereby acidic vapors
are used to remove the binder from green compacts
produced from BASF feedstocks.
• Solvent debinding generally offers better shape
retention as well as a shorter cycle time compared to
thermal debinding.
• The binders used in the common solvent debinding
approach have an inherent disadvantage in
moldability, making it difficult to mould highly
complex components.
• In addition, there is an environmental pollution
problem with the commonly used debinding
solvents.
• However, the recent trend is to use water-soluble
binders, thus water leaching is employed to remove
a major proportion of the binder, which is quite a
safe practice.
• Thermal debinding uses very low heating rates and
thus requires long times.
• High heating rates can result in distortion, slumping
or even failure of the component.
• For thermal debinding, concurrent use of wicking
enhances the shape retention. However, the removal
of wicking powder after debinding is a timeconsuming practice; therefore, the use of wicking
may not be practicable in some commercial
production.
• Several methods have been developed in order to
overcome the problems associated with classical
thermal debinding.
• They are mainly based on the principle that a major
fraction of the binder is removed chemically
followed by a short thermal debinding treatment.
• With the solvent extraction method, the green
component is immersed in a suitable organic fluid,
which dissolves the binder partially.
• This results in the formation of open pore channels.
Thus, the remaining binder fraction can be removed
relatively easily and in a shorter time through the
open pore structure by thermal means.
• In a recent development a highly concentrated acid
is added to the gas atmosphere in order to remove a
binder based on polyoxymethylene by catalytic
phase erosion, known as catalytic debinding.
• During debinding this binder develops
formaldehyde.
• One problem with these methods is that they have to
use special devices and/or chemical substances,
which are particularly hazardous for health and the
environment.
• The debinding rate depends on several factors
including compact size, packing density, particle
size, porosity, binder chemistry, debinding
mechanism, heating rate, solvent or atmospheric
composition and flow rate and placement in the
debinding apparatus.
SINTERING
• The ISO definition of the term ‘sintering’ reads:‘The thermal treatment of a powder or compact at a
temperature below the melting point of the main
constituent, for the purpose of increasing its strength
by bonding together of particles’.
• The definition by Thummler from the point of view
of physical chemistry is:
‘Sintering is a thermally activated mass transport
process which leads to strengthening of particle
contacts and/or a change in porosity and pore
geometry accompanied by a reduction of the free
energy. A liquid phase can take part in the process.’