INTRODUCTION TO METAL INJECTION MOULDING (MIM)
Metal injection moulding (MIM) is a manufacturing technique in which a metal powder-based feedstock is injected into moulds to produce complex moulded parts with high reproducibility. The feedstock is a mixture of powdered metal and a polymeric binder that holds the material together.
Before the components can be used, the binder must be removed and the component solidified; this is achieved by subjecting the components to heat treatment through debinding and sintering in a furnace.
1. Produce feedstock
The production of feedstocks for metal injection moulding (MIM) involves mixing fine metal powders with a binder system to create a mouldable mass. First, metal powders with a specific particle size and morphology are selected to achieve the desired mechanical properties and high sintering density. These powders are then mixed with a thermoplastic, polymer-based binder, which serves as the backbone for shaping and holds the metal powder together. The mixing process must be carefully controlled to ensure even distribution of the binder and to achieve the desired viscosity of the feedstock for the injection moulding process.
2. Injection moulding components
The injection moulding process begins by heating the feedstock in the injection unit of an injection moulding machine to bring it to a flowable state. This molten mass is then injected under high pressure into a precisely manufactured mould (tool or injection mould). The mould corresponds to the negative contour of the part to be manufactured. After injection, the mass cools down inside the mould and solidifies, with the part taking on the exact shape of the mould. After cooling, the solid part is ejected from the mould. The part formed in this way, known as a ‘green part’, still contains the binder and must be debonded and sintered in subsequent process steps to obtain the final metal component.
3. Debinding and sintering
Debinding is the process in which part of the binder is first removed from the injection-moulded green part. This can be done catalytically with a mixture of catalysts, solvents and water, or thermally by heat treatment in a furnace under a modified atmosphere. The remaining binder is then thermally decomposed at elevated temperatures. After debinding, the part is referred to as a ‘brown part’. Most common binder systems use a combination of chemical and thermal debinding to remove both the majority and the remaining backbone binder.
During sintering, the next step after debinding, MIM components are heat treated to compact their structure and increase their strength. This is done by heating the parts in a furnace or under a suitable atmosphere to temperatures just below the melting point, which causes the atoms to diffuse and the material to compact. The parts shrink by about 15-22% during this process. Precise temperature control and an inert or reducing atmosphere are crucial to prevent deformation and oxidation.
4. Further processing
After the sintering process, MIM parts may undergo further post-processing to meet the final specifications. This includes polishing to reduce surface roughness and achieve the desired surface quality. Quality checks, such as dimensional checks, hardness tests and microscopic analyses, are carried out to ensure that the parts meet the design requirements. If necessary, heat treatments can also be carried out to improve mechanical properties, surface coatings to increase corrosion resistance or the application of decorative layers. Assembly processes or joining with other components can also be part of the further treatment to complete the final product.