In my last blog post, we focused on the feedstock that is required to perform metal injection molding (MIM). Today we 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. We then take this blended mix and process it through a twin screw extruder. The paraffin binder is known as the primary binder and the plastics are the secondary binder. The mixer, as I 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.
The metal powder is mixed with the 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 green state) to a finished sintering condition by 16-21%. We call this ratio our powder loading. There are two different ways that MIM materials are typically mixed, 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. It is mixed until the metal powder is uniformly coated with the binders. This mass is then cooled and pelletized. Both these mixing processes are batch and mixed the metal powder is uniformly coated with the binders. This pelletized mix known as feedstock is now ready for the molding machine.
The planetary mixer uses a batch process and is slower. It produces inconsistent blends and more variables then other mixers that are available. The tubular mixer, which is also a batch process, is the mixer that OptiMIM uses. It has a faster throughput and creates consistent blends with fewer variables.
OptiMIM does all of the compounding in-house. There are many advantages to compounding in our own facilities. We are able to create custom blends for customers who have very specific requirements or want a particular metal for their part. It is also lower cost – since we keep the materials that we need in-house, there is no need for a third party to mix metals and compound the material. We are also able to not only match shrinkage of different materials but also of existing tooling. This provides an overall better and more consistent part for the customer.
Part Three of this series will focus solely on the molding piece of the MIM process. What questions do you have for me about compounding? Contact our team for more information, or sign up to stay informed!
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