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MIM metal injection molding sintering process

1 Basic concepts of MIM

MIM, metal injection molding technology, has become a rapidly developing and most promising new forming technology in the field of powder metallurgy, and is known as one of the “most popular metal parts forming technologies in the world”.

Metal injection molding, referred to as MIM (metal injection molding), is a method of mixing metal powder and binder for injection molding. The selected powder is first mixed with a binder, and then the mixture is granulated and injection molded into the desired shape. The binder is removed by degreasing and sintering to obtain the desired metal product, or after subsequent molding, Surface treatment, vacuum heat treatment, mechanical processing and other methods make the product more perfect.

2 MIM process

2.1 Granulation

Granulated fine metal powder is mixed with paraffin binders and thermoplastics in precise proportions. The mixing process takes place in a special mixing device that is heated to a certain temperature to melt the adhesive. In most cases, mechanical mixing is used until the metal powder particles are evenly coated with the binder and cooled to form particles (called feedstock) that can be injected into the mold cavity.

2.2 Injection

The granular raw materials are fed into the machine for heating, and injected into the mold cavity under high pressure to obtain a green body through injection molding. The process is very similar to plastic injection molding. The mold can be designed with multiple cavities to improve productivity. The shrinkage of metal parts during sintering should be considered when designing the mold cavity size.

2.3 Degreasing

Degreasing is the process of removing the binder from the green embryo. After degreasing, the brown body is obtained. This process is usually completed in several steps and is sintered. After most of the binder is removed, the remaining part can support the part into the MIM vacuum degreasing sintering furnace. Degreasing can be accomplished by many methods, the most common method being solvent extraction. The degreased parts are semi-permeable, and the residual binder during sintering can easily evaporate.

2.4 Sintering

The degreased embryo is placed in a high-temperature, high-pressure controllable furnace. Slowly heat the brown body under gas protection to remove residual adhesive. After the binder is completely removed, the brown body is heated to a very high temperature and the gaps between the particles disappear due to the fusion of the particles. The brown blank shrinks directionally to its designed dimensions and transforms into a dense solid, resulting in the final product.

3 Advantages of MIM sintering

MIM combines the advantages of powder metallurgy and plastic injection molding, breaks through the limitations of traditional metal powder molding processes in product shape, and uses plastic injection molding technology for mass production and efficient forming of complex-shaped parts, becoming a high-quality and precise modern manufacturing The near-net shape technology of parts has unparalleled advantages over conventional powder metallurgy, machining and precision casting.

3.1 Highly complex parts can be formed

Compared with other metal forming processes such as sheet metal stamping, MIM such as powder forming, forging and machining can form parts with highly complex geometries.

Generally speaking, MIM can also complete complex part structures that plastic injection molding can achieve.

Taking advantage of this feature, MIM has the opportunity to combine multiple parts that were originally formed by other metals into one part, simplifying product design, reducing the number of parts, and thus reducing the assembly cost of the product.

3.2 High material utilization rate

MIM molding is a near-net shape process. The shape of the parts is close to the final product shape and the material utilization rate is high. This is particularly important for the processing loss of precious metals.

3.3 The parts have uniform microstructure, high density and good performance

MIM is a fluid molding process. The presence of the binder ensures the uniform distribution of the powder, thereby eliminating uneven microstructure of the blank, so that the density of the sintered product can reach the theoretical density of its material.

Generally speaking, MIM can reach 95% to 99% of the theoretical density. High density can increase the strength, toughness, ductility, electrical and thermal conductivity of MIM parts, and improve magnetic properties.

The density of parts pressed by traditional powder molding can only reach up to 85% of the theoretical density. This is mainly due to the friction between the mold wall and the powder and between the powder and the powder, which causes uneven distribution of the pressing pressure, which leads to The microstructure of the blank is uneven, which will cause the pressed powder metallurgy parts to shrink unevenly during the sintering process. Therefore, the sintering temperature has to be lowered to reduce this effect, resulting in large porosity, poor compactness, and low density of the product, which is serious. Affects the mechanical properties of parts.

3.4 High efficiency, easy to achieve mass and large-scale production

MIM uses injection machines to mold green products, which greatly improves production efficiency and is suitable for mass production. At the same time, injection molded products have good consistency and repeatability, thus providing guarantee for mass and large-scale industrial production.

3.5 Wide range of applicable materials and broad application areas

Metal materials suitable for MIM are very wide. Any powder that can be sintered at high temperature can be made into parts by the MIM process, including materials that are difficult to process and materials with high melting points in traditional manufacturing processes. Metal materials that MIM can process include low alloy steel, stainless steel, tool steel, nickel-based alloy, tungsten alloy, cemented carbide, titanium alloy, precision ceramics, etc.