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MIM Technology to make tiny parts

The latest smart phones, small high-performance UAV engines, micro robot gearboxes or other mechanical structures all demonstrate the demand for system miniaturization in the world today. Traditional processing technology takes a long time, has large defects and low yield. However, if the MIM process is adopted, the precision and high-strength parts can be formed at one time, thus effectively reducing the assembly cost. MIM process can complete the forming of workpiece at one time by degreasing and sintering in vacuum furnace after metal granulation, thus greatly reducing the manufacturing cost.

Take a micro gear as an example. This kind of gear can be well used in the transmission structure of unmanned aerial vehicles and small robots. Using MIM to manufacture such gears is the most cost-effective price method at present. The process flow is as follows: powder and binder – mixing – granulation – injection molding – debonding agent – sintering – subsequent treatment – product. Here are the procedures:

Powder and binder:

The particle size of metal powder used in MIM process is generally 0.5-20 (Pm). The finer the particles are, the larger the specific surface area is, which is easy to form and sinter. At present, the main methods of producing powder for MIM include water atomization, gas atomization and carbonyl method. Each method has its advantages and disadvantages. Powder selection should be comprehensively considered from MM technology, product shape, performance, price and other aspects. Currently, water atomized powder and aerosol powder are usually used together. The former improves compaction density and the latter maintains shape retention. The selection of binder directly affects the mixing, injection molding, degreasing and other processes, and has a great impact on the quality, degreasing, dimensional accuracy, alloy composition, etc. of injection molded blanks. Generally, it is selected according to the process, such as thermoplastic binder, which is composed of paraffin high density polyethylene.

Mixing, pelleting and injection molding

After the powder and binder are determined, they should be mixed. Mixing can improve the fluidity of the powder and complete the dispersion. Commonly used mixing devices include twin-screw extruder, Z-shaped impeller mixer, double planet mixer, etc. The feeding rate, mixing temperature and rotating speed during mixing will affect the mixing effect. Then use the injection machine for injection molding. The key problems of injection molding are product design and mold design. The aim is to obtain the required shape of defect free forming blank, and the injection defects should be strictly controlled. The internal defects of injection molded blanks can be detected by ultrasonic testing technology.

Degreasing and pre sintering

The degreasing process includes pre degreasing and thermal degreasing. The main equipment is the vacuum degreasing furnace produced by SIMUWU. Pre degreasing can quickly remove part of the binder components with low melting point, and produce a large number of connecting holes in the blank, thus speeding up the degreasing speed of thermal degreasing. The thermal degreasing process shall be reasonably determined according to the thermal decomposition characteristics of the binder components to prevent defects such as blistering and cracking of the degreased billet due to excessive degreasing speed. Stainless steel powder is very sensitive to carbon content, so reducing atmosphere should be selected to prevent carbon residue due to decomposition of binder. The heating speed is generally lower than 1 ℃/min, and the atmospheric vacuum degreasing furnace is used to remove the polymer component high-density polyethylene of the binder and form a connecting hole. The polymer components in the binder can be completely decomposed after rapid heating and heat preservation, and the degreasing and pre sintering of the blank can be completed.

Vacuum sintering

Sintering is the last process in MIM process, which plays a role in making products dense and uniform in chemical properties, and improving their mechanical and physical properties. Although the sintering method and principle of MIM are the same as those of traditional powder metallurgy, due to the use of a large number of binders in metal powder injection molding, the shrinkage during sintering is very large, and the linear shrinkage generally reaches 12% – 18%. There is a problem of deformation control and size control. The stable shrinkage can be obtained by several sintering experiments on simple moulds.

Sintering is carried out in RVS vacuum sintering furnace. The sintering temperature should be as stable as possible. The sintering temperature fluctuates by tens of degrees Celsius, which can cause the sintering density to fluctuate by 10% and the shrinkage to change by 3%. Therefore, high-quality vacuum sintering furnace must be used.

The RVS vacuum sintering furnace and RVS-D vacuum degreasing sintering furnace provided by SIMUWU are high-quality products for processing MIM process. It has the characteristics of good temperature uniformity, high temperature control accuracy, high efficiency of glue removal system, etc. The combination of two week furnace type can efficiently carry out MIM process. SIMUWU focuses on the manufacturing of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to providing customers with the most convenient and fast experience.