Sintering is the crucial final step in the metal injection moulding (MIM) process, where "brown parts", which have already undergone debinding, are transformed into solid, finished components. These parts are carefully placed on specialised ceramic trays designed to minimise movement during the intense heat of the sintering process. The trays are then loaded into a high-temperature furnace, where the parts are gradually heated. This controlled heating allows any remaining binder material to evaporate, ensuring a pure, dense final product.
As the temperature rises within the furnace, it reaches a point where the individual metal particles within the part begin to fuse together. This fusion process, driven by atomic diffusion, solidifies the component and gives it its final strength and form. During sintering, the part typically shrinks by about 20%, a critical factor that engineers must consider during the initial design phase. Once the sintering process is complete, the furnace cools and the parts are carefully removed. At this stage, they are considered finished, although some parts may undergo additional processing, such as machining or surface coating, to meet specific application requirements.
Types of furnaces
Two types of furnace are available for MIM production: continuous furnaces and batch furnaces. Continuous furnaces can perform debinding and sintering in the same step. Temperatures approach the melting temperature of the base metal, and these furnaces are only suitable for high-volume manufacturing. Batch furnaces also reach temperatures near the melting temperature of the base metal but have a much shorter process time than continuous furnaces. These furnaces operate under vacuum and, during the process, a gas flows through the furnace – these gases can be nitrogen, argon or hydrogen. Supporting equipment for these furnaces includes a hydrogen generator, a nitrogen generator and a backup emergency power generator for cooling the vessel during a power failure.
Issues during sintering
Issues can occur during sintering, which is why the design stage is crucial in preventing problems with the final part. These problems include failing to consider gravity or friction. The resulting part may be warped. Our engineers use options to minimise this. These options include spacers, adding a support rib to the part and coining. Parts can also experience sagging. Using special setters to support the areas most likely to sag in an upright position can resolve this issue. Parts can also be placed in a special ceramic tray or include a runner.
Other articles in the series:
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