Powder metallurgy uses sintering process for making various parts out of metal powder. The metal powder is compacted by placing in a closed metal cavity (the die) under pressure. This compacted material is placed in an oven and sintered in a controlled atmosphere at high temperatures and the metal powders coalesce and form a solid. A second pressing operation, repressing, can be done prior to sintering to improve the compaction and the material properties.
The properties of this solid are similar to cast or wrought materials of similar composition. Porosity can be adjusted by the amount of compaction. Usually single pressed products have high tensile strength but low elongation. These properties can be improved by repressing as in the following table.
Powder metallurgy is useful in making parts that have irregular curves, or recesses that are hard to machine. It is suitable for high volume production with very little wastage of material. Secondary machining is virtually eliminated.
Typical parts that can be made with this process include cams, ratchets, sprockets, pawls, sintered bronze and iron bearings (impregnated with oil) and carbide tool tips.
Design Considerations
•Part must be so designed to allow for easy ejection from the die. Sidewalls should be perpendicular; hole axes should be parallel to the direction of opening and closing of the die.
•Holes, even complicated profiles, are permissible in the direction of compressing. The minimum hole diameter is 1.5 mm (0.060 in).
•The wall thickness should be compatible with the process typically 1.5 mm (0.060 in) minimum. Length to thickness ratio can be upto 18 maximum-this is to ensure that tooling is robust. However, wall thicknesses do not have to be uniform, unlike other processes, which offers the designer a great amount of flexibility in designing the parts.
•Undercuts are not acceptable, so designs have to be modified to work around this limitation. Threads for screws cannot be made and have to be machined later.
•Drafts are usually not desirable except for recesses formed by a punch making a blind hole. In such a case a 2-degree draft is recommended. Note that the requirement of no draft is more relaxed compared to other forming processes such as casting, molding etc.
•Tolerances are 0.3 % on dimensions. If repressing is done, the tolerances can be as good as 0.1 %. Repressing, however, increases the cost of the product.