MIM Series Part 2: Feedstock

A previous blog post – MIM 101 – provided a high-level overview of the MIM process. As promised, this post in the series focuses on feedstock. The MIM process involves four steps: compounding, moulding, debinding, and sintering. Feedstock, the end result of compounding, is essential to the entire MIM process. Selecting the right powder mixture ensures the best possible part is manufactured.

To learn more about the MIM process, keep reading or contact our team of engineers.

What Is Feedstock?

Metal Injection Molding utilises a blend of metal powders combined with a plastic and wax binder. This mixture, known as feedstock, forms the foundation of the final part. OptiMIM takes pride in mixing its own feedstock, allowing for a diverse selection of metals for customer use.

Customers can choose from a variety of metal mixtures. Some common feedstocks include NiFe, 316SS, 420SS, 17-4SS, 4140, titanium, and copper. OptiMIM also offers custom powder blends to achieve specific mechanical properties, high-temperature resistance, and weight requirements.

Manufacturing the MIM powder

Metal powder is typically manufactured in two ways: water atomisation and gas atomisation. Water atomisation involves pouring molten metal through a nozzle and spraying it with water jets to create metal droplets. These droplets are then quenched with water and collected at the bottom of a tank. The rapid cooling results in rough and irregularly shaped particles with better "brown part" strength and consistency during sintering, but also leads to higher oxidisation and oxygen levels.

Gas atomisation is similar, but uses an inert gas instead of water to atomise the molten metal into fine droplets. These droplets cool as they fall in an atomising tower, resulting in spherical particles with a high level of cleanliness, better powder distribution, and superior oxygen and carbon control. However, this process can lead to poor "brown part" strength and sintering issues like sag and drag.

After atomisation, the particles are separated and classified by size, typically ranging from 4 to 25 microns for MIM applications. Screening and air classification are two common methods. Screening utilises various sized screens to separate particles, while air separators use a rising column of air to sort heavier, denser particles from finer ones. Smaller particles generally cost more, though some manufacturers have refined their processes to produce them at higher yields. To ensure accurate particle size distribution, a particle size analyser is used for quality checks.

Mix sheets guide the combination of metal powders, wax, and plastic binders in specific ratios to achieve proper shrinkage. These ingredients are mixed, blended, and processed through a twin-screw extruder to create feedstock pellets.

Part Three of this series will focus on compounding. Any questions regarding feedstock? Anything that needs further explanation? Contact a team of engineers for more information.