MIM Series Part 3: Compounding

The previous blog post focused on the feedstock required to perform metal injection molding (MIM). This post will discuss the first step of the MIM process – compounding. In short, compounding is the process of taking the metal powder, plastic and paraffin binders and mixing these ingredients in a mixer. 1 This blended mix is then processed through a twin screw extruder. The paraffin binder is known as the primary binder and the plastics are the secondary binder. The mixer, as 1 alluded to in the feedstock post, blends these ingredients so the material has a uniformed density throughout the batch which is the first key step in process control.

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

MIM Feedstock

The metal powder is mixed with plastic and paraffin binders at a ratio of approximately 40% binder and 60% metal. This percentage can vary based on the powder size and desired tool shrinkage. MIM parts can shrink from their original molded condition (known as the green state) to a finished sintering condition by 16-21%. This ratio is known as powder loading. There are two common methods for mixing MIM materials: using a planetary mixer or a tubular mixer. These mixers blend the material, which can be done at room temperature or heated. When heated, the material is mixed to a temperature that causes the binders to melt. Mixing continues until the metal powder is uniformly coated with the binders. The mass is then cooled and pelletized. Both mixing processes are batch processes, ensuring the metal powder is uniformly coated with the binders. The resulting pelletized mix, known as feedstock, is then ready for the molding machine.

The planetary mixer uses a batch process and is slower. It produces inconsistent blends and introduces more variables than other available mixers. The tubular mixer, which is also a batch process, is the preferred method used by OptiMIM. It offers faster throughput and creates consistent blends with fewer variables.

Compounding

Compounding is handled in-house at OptiMIM. This approach offers several advantages. Custom blends can be created for customers with specific requirements or who need a particular metal for their part. Costs are also lower, as the materials needed are kept in-house, eliminating the need for a third party to mix metals and compound the material. Additionally, it is possible to match the shrinkage of different materials as well as existing tooling, resulting in a better and more consistent part for the customer.

Part Three of this series will focus on the molding aspect of the MIM process. For more information or inquiries about compounding, contact the team or sign up for updates.