Metal injection molding technology is developed from the powder injection molding technology of ceramic parts. It is a new type of powder metallurgy near-net molding technology. The main production steps of metal injection molding technology are as follows: metal powder and binder mixing-granulation-injection molding-debinding-sintering-subsequent processing-final product. This technology is suitable for mass production, high performance, Small-sized powder metallurgy parts with complex shapes, such as those used in the Swiss watch industry to produce watch parts.
In recent decades, MIM technology has developed rapidly, and the applicable material systems include: Fe-Ni alloy, stainless steel, tool steel, high specific gravity alloy, cemented carbide, titanium alloy, nickel-based superalloy, intermetallic compound, alumina , Zirconia, etc. The metal injection molding technology requires the powder particle size to be below the micron level and the shape is nearly spherical. In addition, there are also certain requirements for the powder’s loose density, tap density, powder aspect ratio, natural slope angle, and particle size distribution. At present, the main methods of producing powder for metal injection molding technology are: water atomization method, gas atomization method, and carbonyl method. Commonly used powder grades for stainless steel metal injection are: 304L, 316L, 317L, 410L, 430L, 434L, 440A, 440C, 17-4PH, etc.
The production process of the water atomization method is:
Selection of stainless steel raw materials-melting in an intermediate frequency induction furnace-composition adjustment-deoxidation and slag removal-atomized powder making-quality inspection-screening-packaging and storage. The main equipment used is: intermediate frequency induction melting furnace, high pressure Water pumps, fully enclosed powder mills, circulating water pools, screening and packaging equipment, testing instruments, etc.
The production process of the gas atomization method is:
Selection of stainless steel raw materials-melting in an intermediate frequency induction furnace-composition adjustment-deoxidation and slag removal-atomized powder making-quality inspection-screening-packaging and storage. The main equipment used is: intermediate frequency induction melting furnace, nitrogen Source and atomization device, circulating water pool, screening and packaging equipment, testing equipment, etc.
Each method has its own advantages and disadvantages: the water atomization method is the main powdering process, which is efficient and economical in large-scale production. It can make the powder finer, but the shape is irregular, which is conducive to shape retention, but the stickiness used There are more binding agents, which affect the accuracy. In addition, the oxide film formed by the high-temperature reaction of water and metal hinders sintering. The gas atomization method is the main method for producing powders for metal injection molding technology. The powders it produces are spherical, with low oxidation degree, less binder required, and good formability, but the yield of extremely fine powder is low, the price is high, and the The shape is poor, and the C, N, H, O in the binder affect the sintered body.
The powder produced by the carbonyl method has high purity, stable beginning, and extremely fine particle size. It is most suitable for MIM, but it is limited to powders such as Fe and Ni, which cannot meet the requirements of varieties. In order to meet the requirements for powders used in metal injection molding technology, many milling companies have improved the above methods, and have also developed micro-atomization, laminar atomization and other powder-making methods. Nowadays, water atomized powder and gas atomized powder are usually mixed. The former increases the tap density and the latter maintains shape retention. At present, water atomized powder can also be used to produce sintered bodies with a relative density greater than 99%. Therefore, larger parts only use water atomized powder, and smaller parts use gas atomized powder.