Designing for MIM

Metal injection molding (MIM) is growing as the manufacturing method of choice for certain types of steel parts from medical to automotive. BASF's Mark Torgerson delves into MIM as a process.

What is MIM?

MIM is a metal-forming process that uses standard plastics injection molding equipment to form a part. Very complex steel and stainless-steel parts can be formed in a single injection molding shot.

What other steps are required?

Let's start with the granules that are injection molded. The MIM raw material, or feedstock, is a blend of ultra-fine metal powders and a thermoplastic binder. The feedstock can be processed in standard injection molding equipment to form a molded part. Next, a majority of the binder is removed by using a catalytic process. The part is then heated (sintered) at a high temperature to fuse the metal particles together. As the particles fuse, the part shrinks by a very predictable percentage to a near full-density metallic part.

Why use MIM as a manufacturing method?

Thousands of MIM parts are in production for use in devices including cell phone and laptop hinges, buttons and clips, medical instruments and automotive parts. The producers of these parts likely view MIM as the process that provides the best overall value for these products. Three factors drive most decisions concerning MIM. First, MIM requires a mold and, therefore, a sufficient production quantity must justify this investment. Second, MIM raw materials are more expensive than wrought metals used for machining or casting processes. Finally, the MIM process allows very complex parts with excellent surface finish to be produced easily with injection molding.

Do the rules for plastics injection molding apply to MIM?

Most do. However, some limits exist due to the higher viscosity of the highly loaded thermoplastic. Also, shear must be minimized so the metal powder distributes evenly in the part. Otherwise, cracks may develop during sintering.

What role does BASF play in the MIM industry?

BASF is a supplier of standardized MIM feedstocks sold under the Catamold® brand. Catamold is based on a very strong thermoplastic polymer binder ideally suited for high-volume automated production. Standardized feedstock results in consistent batch-to-batch quality, but also means projects can be moved from one MIM part producer to another without investing in a new mold, a common problem with proprietary feedstock formulations.