Why MIM (metal injection molded) parts break.

 

About MIM

Metal injection molding MIM is a manufacturing process that is used to produce small integrated metal components. Common areas of use are Automotive, Medical and Firearms.
The process starts with a fine metal powder that is mixed with a plastic binder material creating a "metal slush" that is called feedstock.
The feedstock is injected into an injection mold. The molded part consists of the binder and the metal powder at this step.
Next the binder is removed in the debinding process which can be either a thermal or chemical process.
The now porous part is fragile and still about 15-20% larger than the finished part.
During the next step (sintering) the part is heated to create a metallurgical bond. At this point the part shrinks to its final size. The finished density of the material reaches 96-98%. The orientation of the part during this step is critical as it can affect the bonding.

Concerns

For designers and consumers alike the main concern is the density and consistency of the material throughout the part.
Knit lines are a huge concerns as this can become an area that contains voids or prevents the material from properly fusing together. The porous material structure can cause the part to not be as strong as standard material and even crack under loads. Knit lines are areas that are not filled properly when the feedstock is injected into the mold. Knit lines risk can be reduced by analyzing the mold flow. Designers should not use standard material design parameters for MIM parts as the reduced density also decreases the inherent strength of the part. MIM parts tend to be more brittle.
If a finite element analysis is conducted of a MIM component an adjusted (weaker) yield strength has to be used. MIM certainly has advantages such as creating complex shapes and combining two parts in one. Thus it certainly has its place in manufacturing today. But when strength counts proceed with caution.

Stamping Benefits over MIM

• Material density is inherently higher
• Material strength is not compromised
• Large experienced supplier base
• Stamping is easily scaled up for large production needs
• Equipment cost is significantly lower
• Stamped parts can be produced at a very low cost
• Material properties do not change so a ductile material does not become brittle
• Strength of material can be increased by coining, work hardening or heat treating.