The document discusses metal injection moulding (MIM) or powder injection moulding (PIM), which involves mixing metal or alloy powders with a plastic binder and injecting the mixture into a mould to form a "green part". The binder is then removed through a debinding process to form a "brown part" which is sintered to densify it. Key steps include mixing/pelletizing powders and binder, injection moulding, debinding to remove the binder, and sintering to establish atomic bonding between particles and achieve final density. Debinding is often the most challenging step and can use thermal or solvent methods. The driving force for sintering is reducing surface

1. Powder Metallurgy
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Metal Injection Moulding (MIM) or
Powder Injection Moulding (PIM)
 MIM is a recent extension of conventional powder
metallurgy that offers a high capability of forming
complex and intricate parts using metal powders and still
takes the advantages of powder metallurgy.
 Non-metallic powders can also be used in the injection
moulding process. In this case, the process is called
powder injection moulding (PIM).

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 Mix fine a metal or alloy powder (or powders) (particle size:
usually 2-20 mm) with a plastic binder which is normally a
mixture of low molecular weight thermoplastics and wax.
 The volume fraction of powder (Powder Loading) is up to 50%
 Pelletize the powder/binder mixture
 Injection mould the powder/binder mixture into a part with
desired shape at temperatures similar to the required
temperatures for plastic injection moulding (150-300oC). The
part at this point is called “green part”.
 Remove the binder (debinding) from the green part. The part at
this point is called “brown part”
 Sinter the brown part which is a powder compact.
MIM process:

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Schematic flowchart of MIM process

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MIM Process Flow
Mixing
Moulding
Debinding
Sintering

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Debinding
 Debinding is the most time consuming, and often
most challenging step of the MIM process.
 Debinding Methods:
 Thermal method: heating the injection moulded parts to
temperatures above the volatilization temperature of
the binder
 Solvent extraction method: use a suitable solvent to
dissolve the binder

8. Functions of Sintering
 To turn a powder compact into a true single-piece solid
material with desired strength by establishing atomic
bonding between particles.
 To reduce the fraction of cavity in the powder compact to
a desired level.
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Sintering Temperature
 The temperature at which the powder compact is
sintered is sintering temperature.
 The sintering temperature normally needs to be
>0.8Tm when pressure is not used to assist the
sintering (pressureless sintering)
 Tm is the melting point of the powder in Kelvin scale.
 The sintering temperature can be lowered
substantially by applying pressure during sintering
(pressure assisted sintering).

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 Sometime, if a powder mixture is sintered, the
sintering temperature may be slightly above the
melting point of one powder in the powder
mixture.
 This is to obtain some liquid phase to accelerate
the sintering process.
 Such process is called liquid phase sintering.

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What is the driving force for the
sintering process?
 The driving force for the sintering process is
the reduction of surface energy of the powder
compact by reducing the surface area of the
powder particles.

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What occur during sintering?
 Lubricant and binders burn off during heating.
 Atoms diffuse between neighbouring particles
through contacts, and in this way atomic bonds
between powder particles are established.
 The gaps between particles change from continuous
pores to isolated pores and then the pores shrink or
disappear. -->The density increases.
 Chemical reactions may also occur during sintering.

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1400oC
1550oC
1650oC
Ti(Al,O)/Al2O3 Composite
Dark phase: Al2O3
White phase: Ti(Al,O)

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Porous Materials
(i) Morphology: type of porosity needed (open versus closed), amount of
porosity needed size scale of porosity desired, total internal surface
area of material required;
(ii) Metallurgy: metal or alloy or microstructural state required;
(iii) Processing: possibilities for shaping the foam or cellular solid or for
manufacturing composites between the foam and conventional sheets or
profiles;
(iv) Economy: cost issues, suitability for large volume production.

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Bram et.al ,Advanced Engineering Materials
2000;2:196.