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Metal powder injection - forming technology in the 21st century


Metal Powder Injection Molding (MIM) is a new type of powder metallurgy near net forming technology that introduces modern plastic injection molding technology into the field of powder metallurgy. The basic process is as follows: first, the solid powder is uniformly mixed with the organic binder, granulated, and then injected into the mold cavity for solidification under heating and plasticization (~150 ℃) using an injection molding machine. Then, the binder in the formed billet is removed by chemical or thermal decomposition methods, and finally, the final product is obtained by sintering and densification. Compared with traditional processes, it has the characteristics of high precision, uniform organization, excellent performance, and low production cost. Its products are widely used in industrial fields such as electronic information engineering, biomedical equipment, office equipment, automobiles, machinery, hardware, sports equipment, watch industry, weapons, and aerospace. Therefore, it is widely believed internationally that the development of this technology will lead to a revolution in component forming and processing technology, known as the "most popular component forming technology today" and the "forming technology of the 21st century".
Parmatech, a California based company, invented this technology in 1973. In the early 1980s, many European countries and Japan also invested great effort in researching this technology, which was rapidly promoted. Especially in the mid-1980s, this technology has achieved rapid development since its industrialization, increasing at an astonishing rate every year. So far, more than 100 companies in more than ten countries and regions such as the United States, Western Europe, and Japan are engaged in product development, research and development, and sales of this technology. Japan is very competitive and has shown outstanding performance, with many large companies participating in the promotion of MIM industry, including Pacific Metal, Mitsubishi Steel, Kawasaki Steel, Kobe Steel, Sumitomo Mining, Seiko Epson, Datong Special Steel, and others. At present, there are more than 40 companies in Japan specializing in the MIM industry, and their total sales value of MIM industrial products has already surpassed Europe and directly caught up with the United States. So far, more than a hundred companies worldwide have been engaged in the product development, research and sales of this technology, making MIM technology the most active cutting-edge technology field in the new manufacturing industry. As a pioneering technology in the world's metallurgical industry, MIM technology represents the main direction of powder metallurgy technology development.
Metal powder injection molding technology is a product that integrates multiple disciplines such as plastic molding technology, polymer chemistry, powder metallurgy technology, and metal materials science. By using molds to inject molded blanks and rapidly manufacture high-density, high-precision, and three-dimensional complex shaped structural parts through sintering, design ideas can be quickly and accurately materialized into products with certain structural and functional characteristics, and parts can be directly mass-produced, It is a new transformation in the manufacturing technology industry. This process technology not only has the advantages of fewer conventional powder metallurgy processes, no or less cutting, and high economic benefits, but also overcomes the shortcomings of traditional powder metallurgy products, uneven materials, low mechanical properties, difficulty in forming thin-walled and complex structures. It is particularly suitable for large-scale production of small, complex, and special requirements of metal parts. Process flow: binder → mixing → injection molding → degreasing → sintering → post-treatment.
Powder metal powder
The particle size of metal powder used in MIM process is generally between 0.5 and 20 μ M; In theory, the finer the particle, the larger the specific surface area, making it easier to shape and sinter. However, traditional powder metallurgy processes use materials larger than 40 μ Coarse powder of m.
organic adhesive 
The function of organic adhesive is to bond metal powder particles, making the mixture rheological and lubricating when heated in the injection machine barrel, which is the carrier that drives the flow of powder. Therefore, the choice of adhesive is the carrier of the entire powder. Therefore, the selection of adhesive pull is the key to the entire powder injection molding process. Requirements for organic adhesives:
1. The use of less adhesive can improve the rheological properties of the mixture;
2. No reaction, no chemical reaction with metal powder during the removal of adhesive;
3. Easy to remove, without residual carbon in the product.
Mixing materials
Mix metal powder and organic adhesive evenly to make various raw materials into injection molding mixtures. The uniformity of the mixture directly affects its fluidity, thus affecting the injection molding process parameters, as well as the density and other properties of the final material. The process of injection molding in this step is consistent with the process of plastic injection molding in principle, and its equipment conditions are also basically the same. During the injection molding process, the mixture is heated into a plastic material with rheological properties inside the injection machine barrel, and injected into the mold under appropriate injection pressure to form a blank. The microstructure of the injection molded blank should be uniform and consistent, so that the product shrinks uniformly during the sintering process.
The organic binder contained in the formed blank must be removed before sintering, and this process is called extraction. The extraction process must ensure that the adhesive is gradually discharged from different parts of the blank along the small channels between the particles, without reducing the strength of the blank. The removal rate of binders generally follows the diffusion equation. Sintering and sintering can cause porous degreased blanks to shrink and densify into products with certain structure and properties. Although the performance of the product is related to many process factors before sintering, in many cases, the sintering process has a significant and even decisive impact on the microstructure and properties of the final product. For parts with precise size requirements, necessary post-processing is required. This process is the same as the heat treatment process for conventional metal products.
The characteristics of MIM technology and its comparison with other processing technologies
The raw material powder used by MIM has a particle size of 2-15 μ m. However, the particle size of traditional powder metallurgy raw powder is mostly between 50-100 μ M. The finished product density of MIM process is high due to the use of fine powder. The MIM process has the advantages of traditional powder metallurgy, but high degrees of freedom in shape cannot be achieved by traditional powder metallurgy. Traditional powder metallurgy is limited by the strength and filling density of molds, and the shape is mostly two-dimensional cylindrical.
The traditional precision casting drying process is an extremely effective technique for producing complex shaped products. In recent years, ceramic core assistance has been used to complete products with narrow and deep holes. However, due to the strength of the ceramic core and the limitations of the fluidity of the casting liquid, this process still faces certain technical difficulties. Generally speaking, this process is more suitable for manufacturing large and medium-sized parts, while the MIM process is more suitable for small and complex shaped parts. Comparison of Project Manufacturing Process MIM Process Traditional Powder Metallurgy Process Powder Particle Size( μ m) 2-1550-100 relative density (%) 95-9880-85 product weight (g) less than or equal to 400 grams 10-hundreds product shape three-dimensional complex shape two-dimensional simple shape mechanical performance advantages and disadvantages, comparison between MIM process and traditional powder metallurgy method Die casting process is used in materials with low melting point and good casting fluid fluidity, such as aluminum and zinc alloys. The products of this process have limitations in strength, wear resistance, and corrosion resistance due to material limitations.