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Vacuum Sintering Manufacturing of MIM Stainless Steel

Metal injection molding is gaining more and more attention because it can produce high-performance metal parts with complex shapes at a relatively low cost. MIM technology has unique advantages in preparing near-net-shape parts with complex geometry, uniform structure and excellent performance.

Injection-molded stainless steel has a sintered density of about 98%, and can achieve a mirror-bright finish after polishing. Sintering properties are comparable to forging, so it can be used for decorative and functional parts.

In the medical device industry, MIM has successfully developed hundreds of stainless steel orthodontic brackets, and the annual output of tens of millions of brackets in the United States is produced by MIM. The cost can be reduced by 60% compared with traditional processing methods, and it has become a replacement technology for traditional processing methods. In addition, it can also be used as medical equipment such as surgical scissors. In order to prevent allergic reactions caused by nickel, the injection molding technology of nickel-free stainless steel has also been developed.

Another application of injection-molded stainless steel is the production of watch parts. Injection-molded stainless steel cases and watch chains have been developed with a dimensional accuracy of ±0.02mm. Comparing watch parts produced by MIM method with other methods (such as powder pressing/sintering, stamping, precision casting, etc.), the parts using MIM process are the best in life, precision and strength.

In the field of electronic products, MIM technology has been vigorously developed. MIM stainless steel has been used in structural parts in computer hard drives, spacers for computers. With the popularity of smart phones, a large number of MIM precision parts have been used in mobile phones. Including structural supports, various interfaces, and camera stabilizers, etc.

In addition, daily electric toothbrush gears, combustion chambers of automobiles, and rotors in military missile warhead safety devices are all made of MIM stainless steel.

Stainless steel MIM process:

1 raw powder

MIM has requirements on the particle size, sphericity, bulk density and tap density of the raw material powder. The preparation methods of stainless steel powder for injection molding are mainly gas atomization method and water atomization method. The powder obtained by the former method is thicker, with good sphericity, low carbon and oxygen content, and higher powder loading can be obtained, but the cost is relatively high. The price of water atomized stainless steel powder is lower, the powder is finer, and the output rate of fine powder is high. But the sphericity is poor. The carbon and oxygen content are high, and the powder loading is low.

2 Binder and degreasing

The design and selection of the binder is always combined with a specific degreasing method. There is no essential difference between MIM stainless steel binders and other alloy powder systems, such as paraffin-based binders, oil-based binders, and polymer-based binders can all be used for injection molding of stainless steel. If you want to increase the powder loading, you can improve the powder shape, particle size and particle size distribution by ball milling to increase the maximum powder loading.

Stainless steel can be degreased by thermal degreasing, solvent degreasing, siphon degreasing, catalytic degreasing and other methods, thermal degreasing is the most commonly used method. Since the stainless steel powder contains alloy elements such as chromium and copper that are easily oxidized and carbonized, it is necessary to choose a suitable heating system and atmosphere. In order to prevent defects such as bubbling and cracking, or to produce oxides, carbides and residual carbon. Generally, the temperature is raised slowly at a rate lower than 1°C/min before 200°C to remove some low-molecular components such as paraffin wax. Rapidly raise the temperature to 400-500°C at a min rate of 400-500°C to decompose the polymer components in the binder and complete the degreasing process.

If the polymer components and the atmosphere are not selected properly, residual carbon will easily be generated and the performance of the sintered body will be affected. Vacuum thermal degreasing is widely used because there are few impurities in the atmosphere, and the degreasing products are easy to remove, thereby reducing oxidation or carbonization. The vacuum degreasing rate is several times that of other thermal degreasing rates, which is conducive to the rapid removal of binders. Matching with the vacuum debinding method, using SIMUWU’s RVS vacuum debinding sintering furnace can greatly reduce the production cycle.3 Reduction and vacuum sintering of stainless steel

3 Reduction and vacuum sintering of stainless steel

Stainless steel has a reduction process before sintering, and chromium in stainless steel is easily oxidized, which affects the sintering densification process. It is generally reduced in H2 or in vacuum. The sintering temperature of 316L is generally 1350-1390°C, and the sintering heating rate is generally about 5-10°C/min. If the temperature rises too fast, it is easy to cause shrinkage cavity due to too fast shrinkage, resulting in low sintered density. If the sintering temperature is too high, it is easy to produce too much liquid phase, resulting in defects such as collapse. The sintering temperature of 17-4PH is 1300-1350°C. In addition to preventing the volatilization and oxidation of chromium, it is also necessary to prevent the volatilization and oxidation of copper.

Selection of MIM sintering equipment: RVS vacuum degreasing sintering furnace provided by SIMUWU is a high-quality product for processing MIM process. It has the characteristics of good temperature uniformity, high temperature control precision, and efficient degumming system. SIMUWU company focuses on the manufacture 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 air 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 is committed to giving customers the most convenient and efficient experience.