MIM Process Series Part 6: Sintering

Sintering is the crucial final step in the Metal Injection Molding (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 like machining or surface coating to meet specific application requirements.

Types of Furnaces

There are two kinds of furnaces that are available for MIM production: continuous furnaces and batch furnaces. Continuous furnaces can debind and sinter in the same step. The temperatures reach near melting temperature of the base metal, and these furnaces are only ideal for high volume manufacturing. The batch furnaces also reach temperatures near the melting temperature of the base metal but have a much shorter process time then continuous furnaces. These furnaces run under vacuum and during the process a flow gas is pumped through the furnace – these gases can be nitrogen, argon or hydrogen. There is also supporting equipment for this furnace, including a hydrogen generator, a nitrogen generator, and a back up emergency power generator for cooling the vessel in times of a power failure.

Issues During Sintering

There are issues that can occur during sintering, which is why the design step of our business is problems with the final part. These problems include not taking into consideration 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. There can also be sag issues with a part. Using special setters that support the pieces most likely to sag upright can solve this issue. Parts can also be set in a special ceramic tray, or they can include a runner.

After exploring the MIM process through this blog series, are there any remaining questions? Perhaps there's an aspect that wasn't fully covered or a specific topic of interest. If so,

contact the OptiMIM engineering team.

You may also be interested in:

MIM Series Part 1

MIM Series Part 2 – Feedstock

MIM Series Part 3 – Compounding

MIM Series Part 4 – Molding

MIM Series Part 5 - Debinding