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 shifting 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 and dense final product.
As the temperature climbs higher 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
There are two types of furnaces available for MIM production: continuous furnaces and batch furnaces. Continuous furnaces can debind and sinter in the same step. The temperatures reach near the melting temperature of the base metal, and these furnaces are only ideal 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 process gas is pumped through the furnace – these gases can be nitrogen, argon or hydrogen. Supporting equipment for this furnace includes a hydrogen generator, a nitrogen generator and a back-up emergency power generator for cooling the vessel during a power failure.
Issues during sintering
Problems can arise during sintering, which is why the design stage is critical to preventing defects in the final part. These problems include failing to account for gravity or friction. The resulting part can be warped. There are options that our engineers use to minimise this. Some of these options include spacers, adding a support rib to the part and coining. A part can also experience sagging issues. Using special setters that support the pieces most likely to sag in an upright position can resolve this issue. Parts can also be placed in a special ceramic tray, or they can include a runner.
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