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Metal Injection Molding Metal injection molding, also called "MIM," is a low cost, high volume manufacturing process that produces custom metal parts near to net shape. It combines the way injection
molded plastics are formed with powdered
metal sintering to create metal or ceramic parts which are stronger, denser and more capable of complex geometric shapes than most forged or die
cast metal parts. A relatively new invention in manufacturing, metal injection molding is a fast-growing manufacturing process which combines the flexibility of the injection molding process (which until very recently was used only with rubbers and plastics) with the strong structural integrity of sintered powdered metal. The strength, economy and capacity for complex shapes demonstrated in injection molded metal and ceramic parts are sought-after attributes becoming more widely utilized by telecommunications, automotive, medical and dental instrumentation, industrial parts, orthodontics, firearms, hardware and lock components, computer and electrical application industries, among many others. Injection molded metal parts are formed from sintered powdered metal, just as are regular powdered metal parts. The powdered metal or ceramic powder used in the injection molding process must, however, become "plasticized" in order to flow through the injection molding machine, which is the same machine used for plastic
injection molding. Polymers called “binders” and sometimes wax are added to the powdered metal base, creating what manufacturers call "feedstock." Sometimes feedstock is formed into pellets for easy feed; the pellets are fed through a hopper into a screw conveyor where they are sheared and friction heated until the material becomes malleable, or "plasticized," then it is extruded into a closed die at the screw conveyor's opposite end. The tools used here range from simple designs to multi-cavity molds. The molded part is allowed to cool until it has solidified, then it is ejected and the mold is filled again. These molded but not yet sintered parts are called "green parts." Green parts' polymers must be extracted through a process called "debinding." Debinding can be done chemically (as through a solvent bath) or thermally, and sometimes the sintering process is even sufficient to break down green parts' polymers, dissipating them during the sintering process. Green parts are sintered between 2200 and 2550 degrees Fahrenheit and shrink 15-20% in the process as the pores where polymers were close, creating densities between 95 and 98%, almost as high as that of wrought metals. This is done in a high temperature sintering oven. Despite the shrinking, closer and more accurate tolerances can be achieved with injection molded metals than with die-casting or forging. This is because the plasticity of the feedstock and the nature of the closed injection mold allow greater design freedom than more restricted methods. Parts which must be cast or forged in multiple pieces can often be molded and sintered as a single part, cutting down on expensive secondary assembly processes. After this process, the parts are always examined for quality. Other post-processing operations like machining, heat treating or plating take place at this time to enhance properties and reach higher tolerances. Ferrous-based alloys are most commonly used in metal injection molding, as well as low
alloys, stainless
steels, copper, chrome, nickel
alloys, semimetals, intermetallic compounds and magnetic
ceramics. Injection molding is easier and faster than the process used to form green sand castings, and the sintered parts require very little or no machining to reach their finished state, greatly cutting down on secondary finishing costs such as drilling, etching or assembling. The densities injection molded metals achieve not only give parts much higher strength, but also higher corrosion resistance and, for magnetic
compounds, improved magnetic qualities. Injection molded metal parts are usually limited to 100 grams due to feedstock costs and the difficulty of maintaining the structural integrity of the part during the sintering process. Although some short runs may still be more economical than die-casting or forging, injection molded metals are usually manufactured in long runs to balance high overhead mold tooling costs.
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