Metal injection molding is about a 25-year-old process. Essentially, what it does is combine the technology involved in the plastic injection molding process with a metal powder component at the micron level. Please note that this is also sometimes called “sintered metal,” which also promises a high level of dimensional accuracy, part density, freedom from air pockets and requires little or no additional processing, such as finish machining.
The process itself involves metal powders, which can be carbonized iron or inert gas atomized powder mixed with thermoplastic binders, creating a mixture that is approximately 60% of the total mixture volume. The parts are part of an expensive mold with “spruing,” which would be familiar to anyone who has ever built a plastic model kit, and which permits the flow of material into the mold, filling the voids and crevices of the mold with extreme accuracy.
This mixture is then fed into automated molding furnace machines at between 100 to 250 deg. C. Typically, there can be a shrinkage rate of 20-30 percent on all dimensions of the part, which strikes the layperson as excessive, yet the finished articles are quite dimensionally consistent as they are removed robotically from the cooled mold. The plasticizer used to fill the mold evenly is removed by high temperatures or chemically. After this, the parts are hardened and finished for the application.
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