Metal Injection Molding vs. Powder Metallurgy
As a commercial process, metal injection molding (MIM) is fairly new. It offers economic and technical advantages not available using other procedures.
The basic metal injection molding procedure involves blending a polymer with an extremely fine (10-20 micrometer) metal powder. The blended material is injection molded using the same type of equipment and tools employed by the plastics industry. The molded (or green) parts are placed in a low temperature oven for the initial stages of decomposition of the organic material. They are then transferred to a high temperature furnace for final polymer removal and complete sintering, using controlled temperatures and protective atmospheres.
If desired, carbon can be added at this point by methane additions to the atmosphere while the parts are still porous. Components shrink about 15 percent during the sintering operation. Parts are normally ready for use immediately after sintering, although secondary operations such as tumbling or tapping may be added.
MIM offers two major advantages over conventional powder metallurgy processing. The most obvious is three-dimensional design flexibility. In theory, any shape which is injection molded in plastic can also be done in metal. The myriads of injection molded plastic toys give an idea of design freedom.
The second big advantage is high density. Conventional structural powder metal parts are produced at densities of 75 percent to 90 percent. MIM products are typically 93 percent to 97 percent dense, which means higher strength, better corrosion resistance, and the elimination of interconnected porosity. For those who need even higher density, SSI's "fully" dense process produces parts at over 99 percent density, subject to the same geometrical limitations as conventional P/M.)
Because of the high density of MIM components, properties approach those of wrought material. This means they can compete with other metal forming operations, even for demanding applications. Quantities, exact geometry and physical requirements will finally determine whether MIM is the most attractive alternative.
The fully dense process for P/M high speed tool steel and stainless steel from SSI Technologies involves powder compaction in metal dies and vacuum sintering to near full density. The tool steel and stainless steel powders used are specially prepared to closely control the chemical composition, particle size, particle distribution and powder cleanliness.
Using the powder metal process, compacts of near-net shape are pressed, using very high pressures in precision made dies. he parts are then vacuum sintered to the fully dense state by liquid phase sintering. The liquid phase means that, at a controlled temperature, a small amount of liquid is formed. This liquid provides the path for the solid phase, or the undissolved atoms, to rearrange and achieve near full densification. The correct temperature and time at temperature are critical parameters which must be maintained in order to realize product densities of 98 percent or better of theoretical density.
In the high speed tool steels, the presence of complex carbides in the hardened and tempered matrix gives the material qualities of wear and abrasion resistance that make it suitable for wear resistant uses, as well as for cutting tool applications. The characteristics needed in a wear resistant high-speed steel are fine, uniformly dispersed alloy carbides in a fine-grained, tempered martensitic matrix.
In the annealed condition, the material possesses the ability to be readily machined or ground to final dimensions without distortion (in these operations or in subsequent heat treating operations). The material is amenable to conventional heat treating and surface treatments.
The fully dense P/M stainless steels combine excellent corrosion resistance and mechanical properties, similar to wrought stainless steels, with the material savings and reduced machining costs of near-finished-shape part production. The stainless steels can be supplied in the "as sintered" condition or "solution heat treated and quenched" condition. The latter heat treatment step considerably improves the corrosion resistance and ductility of the material at a small cost differential.
Fully dense stainless steels should be considered where corrosion by chemicals, weather or water is a significant concern, where high temperature oxidation is a factor, or where machining costs and scrap losses are excessive by present production techniques.
- There are few geometrical limitations, although certain types of complexity increase both tooling cost and unit price. As in plastic injection molding, it helps to have uniform wall thicknesses.
- Tolerances are in the 0.3 percent to 0.5 percent range.
- Total size should not be much larger than a golf ball, although it may be possible to stretch that a bit.
- Economically, a competitive edge usually goes to high volume parts that are small and have relatively complex geometry.
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