Select powder types according to product technical requirements and service conditions, and then decide powder particle size. The powder particle size of metal powder injection molding is generally 0.5-20μm; theoretically, the smaller the powder particle, the bigger specific surface area is, the bigger the cohesion between particles is, and easy for molding and sintering. Traditional powder metallurgy technology uses thick powder bigger than 40μm. The powder selection shall be beneficial for mulling, injection molding, degreasing and sintering. But it is often contradictory. MIM material powder such as tungsten mim shall be very thin, the price of MM raw material powder is usually high and some even up to 10 times of traditional PM powder price. This is a key factor of limiting widely use of MIM technology. At present, method for producing MIM raw material powder is mainly ultra-high pressure water atomization method and high-pressure gas atomization.
Adhesive is core of MIM technology. Adhesive increases liquidity for fitting two basic functions, namely injection lim molding and position compact shape. Besides, it has features such as easy to remove, pollution-free, non-toxic, reasonable cost, etc. So, all kinds of adhesive appear. In recent years, the development direction is from selection only by experience to designing adhesive system specifically according to degreasing method and function request. Adhesive is usually composed by low molecular component, high molecular component and some necessary addition agents. Low molecular component has low viscosity, good liquidity and easy to remove; high molecular component has high viscosity, high strength and keeping molding blank intension. Match the two components at proper proportion to get high powder loading capacity, and finally gain products of high precision and high uniformity. General adhesive groups are: thermoplastic system (paraffinic base, oil base and thermoplastic polymer base), gel system, thermosetting system and water-soluble system.
Gate shall be at higher place in space to reduce melt rubbing material feeding in molds, reduce injection pressure and reduce possibility of product deformation. Besides, design of shining point is very important too. Shining point is used for loading during cold charge injection process, in order to keep product consistency.
Metal powder injection molding technology of metal injection molding machine manufacturer shrinks greatly during sintering because large amount of adhesive is used. The linear shrinkage can reach 13% to 25%. Thus, there is a problem of deformation control and size accuracy control. Especially because most MIM products are special-shaped parts of complicated shapes, the problem is more prominent. Even charging is a key factor for size progress and deformation control of the final sintering products. High powder tap density can decrease sintering shrinkage, and is beneficial to proceed sintering process and control size accuracy.
EN
