Most MIM parts are nearly finished after sintering but some require sizing or coining operations to improve dimensional or structural results. Sizing and Coining operations are used to reduce dimensional variation and/or improve part properties. Although Steels and Stainless Steels don’t usually change as much, sizing can decrease dimensional variation and coining can increase density and thereby strength.
Sizing is a postsintering operation used to correct size, warping, distortion and other dimensional defects during sintering. Sizing can also be used to improve the surface finish and wear properties of the surface. Normally, sizing is done by forcing the sintered components into a die of slighdy smaller dimensions, which plastically deforms and smoothens the faces in contact with the die. Sizing of parts such as piston, bearings and cams are done to reduce tolerances to within 0.001 inch per inch horizontally and 0.002 to 0.003 inch/inch vertically or even better. The pan tolerance after sizing is dependent on pan shape, size as well as dimensions. The forces involved in sizing are small and, therefore, the improvement in density is minimal.
There are two methods of sizing, namely, positive sizing and negative sizing. Positive sizing occurs when an oversize component with a hole smaller than the core rod is pressed into the die. Negative sizing occurs when an undersize component is put into the sizing die and its hole is larger than the core rod. The punches are used to apply pressure on the part to reduce the length of the part considerably and consequently pressing the part to the die walls and core rods. A thin-walled bushing which is oversize on both outside and inside diameter after sintering can be subjected to positive sizing on the outside and negative sizing on the inside.
In sizing, the amount of deformation is small and hence the force required for deformation is less in comparison to the compaction force. The subsequent improvements in density as well as mechanical properties are also limited.