The previous article focused on the feedstock required for metal injection moulding (MIM). This article discusses the first step of the MIM process – compounding. In short, compounding involves mixing metal powder, plastic binders and paraffin binders in a mixer. 1 This blend is then processed through a twin-screw extruder. The paraffin binder is known as the primary binder, while the plastics are the secondary binder. As 1 noted in the feedstock article, the mixer blends these ingredients so that the material has uniform density throughout the batch, which is the first key step in process control.
Metal injection moulding 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 powder size and the required tooling shrinkage. MIM parts can shrink from their original moulded condition (known as the green state) to the finished sintered 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 at room temperature or under heat. When heated, the material is mixed at 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 pelletised. Both mixing processes are batch processes, ensuring that the metal powder is uniformly coated with the binders. The resulting pelletised mix, known as feedstock, is then ready for the moulding 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 required materials 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 focuses on the moulding aspect of the MIM process. For more information or enquiries about compounding, contact the team.
Other articles in the series:
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