If the dimensional tolerances and/or the mechanical properties obtained after the sintering (or heat treatment) of the MIM parts are unsatisfactory, a sizing or coining operation can be carried out.
The sizing operation, which is carried out at moderate compacting pressures, serves to improve the dimensional accuracy of the product. For a small batch of components, the primary pressing or compacting die can be used to carry out the sizing of the compact. Large batches of compacts are normally sized in a special die using an inexpensive sizing press.
The coining operation serves two purposes: improving the mechanical properties of the product and improving the dimensional tolerances. The mechanical properties can be improved only by increasing the density of the compact, which means high compacting pressures (higher than or equal to the primary compacting pressures). Thus, in general, coining requires a special die for the purpose, often of a higher quality than the primary die, because of the higher pressures and the adverse wear conditions.
When coining is involved, the sintering process carried out between the primary compacting and the coining operation is often incomplete and takes the form of pre-sintering for a short time and at a temperature considerably below the normal sintering temperature but sufficient to anneal the compact. After coining, the compact is fully sintered, producing a component with excellent mechanical properties and dimensional tolerances. If the requirements of the product are exceptionally high, a sizing operation may follow the coining operation.