Metal powder injection molding technology
Metal Powder Injection Molding Technology (MIM) is a new powder metallurgy near net formation technology that is introduced into the field of powder metallurgy.
The basic process is as follows: firstly, the solid powder and the organic binder are uniformly kneaded, and after granulation, the film is solidified by injection molding in a plasticizing state (~150 ° C), and then chemically or thermally. The decomposition method removes the binder in the shaped body and finally densifies by sintering to obtain the final product. 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 electronic information engineering, biomedical devices, office equipment, automobiles, machinery, hardware, sports equipment, watch industry, Weapons and aerospace and other industrial sectors. Therefore, it is widely believed that the development of this technology will lead to a revolution in the forming and processing technology of parts, and is known as “the hottest part forming technology of today” and “forming technology of the 21st century”.
History and current situation
Parmatech, California, was invented in 1973. In the early 1980s, many countries in Europe and Japan also invested a lot of energy to start researching the technology and quickly promote it. Especially in the mid-1980s, this technology has achieved rapid development since it was industrialized, and it is increasing at an alarming 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, development, and sales of the process technology. Japan is very active in competition and has outstanding performance. Many large companies are involved in the promotion of MIM industry, including Pacific Metal, Mitsubishi Steel, Kawasaki Steel, Kobe Steel, Sumitomo Mine, Seiko-Epson, Datong Special steel, etc. At present, there are more than forty companies specializing in the MIM industry in Japan, and the total sales value of MIM industrial products has already surpassed that of Europe and directly pursued the United States. So far, more than 100 companies around the world have been engaged in the product development, research and sales of this technology. MIM technology has thus become the most active frontier technology field in the new manufacturing industry, and is represented by the pioneering technology of the world metallurgical industry. The main direction of powder metallurgy technology development MIM technology.
Metal powder injection molding technology is a multi-disciplinary and cross-product of plastic molding technology, polymer chemistry, powder metallurgy technology and metal materials science. It can mold injection blanks and quickly produce high density and high precision through sintering. The three-dimensional complex shape structural parts can quickly and accurately transform the design ideas into products with certain structure and functional characteristics, and can directly produce parts in batches, which is a new revolution in the manufacturing technology industry. The process technology not only has the advantages of less conventional powder metallurgy process, no cutting or less cutting, high economic efficiency, but also overcomes the defects of traditional powder metallurgy process products, uneven material, low mechanical properties, and difficulty in forming thin walls and complicated structures. It is especially suitable for mass production of small, complex and special metal parts. Process binder → mixing → injection molding → degreasing → sintering → post-treatment. 
Powder metal powder
The metal powder used in the MIM process generally has a particle size of 0.5 to 20 μm; in theory, the finer the particles, the larger the specific surface area, and the ease of molding and sintering. Conventional powder metallurgy processes use coarser powders larger than 40 μm.
The function of the organic adhesive is to bond the metal powder particles, so that the mixture is heated in the barrel of the injection machine to have rheology and lubricity, that is, a carrier that drives the flow of the powder. Therefore, the choice of binder is the carrier of the entire powder. Therefore, the stick selection is the key to the entire powder injection molding. Requirements for organic binders:
1. The dosage is small, and the binder can produce better rheology with less adhesive;
2. No reaction, no chemical reaction with metal powder during the process of removing the binder;
3. Easy to remove, no carbon remains in the product.
The metal powder and the organic binder are uniformly blended together to make various raw materials into a mixture for injection molding. 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. Injection molding This step process is consistent with the plastic injection molding process in principle, and the equipment conditions are basically the same. During the injection molding process, the mixture is heated into a plastic material having rheology in the barrel of the injection machine and injected into the mold under appropriate injection pressure to form a blank. The injection molded blank should be uniform in microscopicity so that the article shrinks uniformly during the sintering process.
The shaped blank must be removed from the organic binder contained in the blank prior to sintering, a process known as extraction. The extraction process must ensure that the binder is gradually discharged from the different portions of the blank along the microchannels between the particles without reducing the strength of the blank. The rate of removal of the binder generally follows the diffusion equation. Sintering allows the porous defatted blank to shrink to densify into a product having a certain texture and properties. Although the properties of the article are related to many process factors prior to sintering, in many cases the sintering process has a large, even decisive, effect on the metallurgical structure and properties of the final article.
For parts with more precise dimensions, the necessary post-processing is required. This process is the same as the heat treatment process of a conventional metal product.
MIM process characteristics
Comparison of MIM process with other processing techniques
The raw material powder used by MIM has a particle size of 2-15 μm, while the conventional powder metallurgy raw powder powder has a particle size of 50-100 μm. The finished product density of the MIM process is high due to the use of fine powder. The MIM process has the advantages of the traditional powder metallurgy process, and the high degree of freedom in shape is not achieved by conventional powder metallurgy. Traditional powder metallurgy is limited to the strength and packing density of the mold, and the shape is mostly a two-dimensional cylindrical type.
The traditional precision casting de-drying process is an extremely effective technique for making complex shape products. In recent years, the finished products of slits and deep holes can be completed by using ceramic core assist, but the strength of the ceramic core and the fluidity of the casting liquid are limited. The process still has some technical difficulties. In general, this process is suitable for making large and medium-sized parts, and small and complex shapes are more suitable for MIM. Comparison Project Manufacturing Process MIM Process Traditional Powder Metallurgy Process Powder Particle Size (μm) 2-1550-100 Relative Density (%) 95-9880-85 Product Weight (g) Less than or equal to 400g 10 - Hundreds of Product Shapes 3D Complex Shapes Two-dimensional simple shape mechanical performance.
The MIM process and the conventional powder metallurgy method are used in materials such as aluminum and zinc alloys, which have low melting points and good fluidity of the casting fluid. The products of this process are limited in terms of strength, wear resistance and corrosion resistance due to material limitations. MIM processes can process more raw materials.
The precision casting process, although the accuracy and complexity of its products have increased in recent years, but it is still not comparable to the dewaxing process and the MIM process. Powder forging is an important development and has been applied to the mass production of connecting rods. However, in general, the cost of heat treatment and the life of the mold in the forging process are still problematic and still need to be further solved.
Traditional machining methods, and recently increased their processing capabilities by automation, have made great progress in terms of effect and precision, but the basic procedures still cannot be separated from machining (turning, planing, milling, grinding, drilling, polishing, etc.) ) to complete the shape of the part. The machining accuracy of machining methods is much better than other machining methods, but because the effective utilization of materials is low, the completion of the shape is limited by equipment and tools, and some parts cannot be machined. On the contrary, MIM can effectively utilize materials without restriction. For the manufacture of small and difficult-shaped precision parts, the MIM process is relatively cost-effective and highly competitive compared to machining.
MIM technology does not compete with traditional processing methods, but rather compensates for the technical inadequacies or inability of traditional processing methods. MIM technology can be used in the field of parts made by traditional machining methods. The technical advantages of the MIM process in the manufacture of components enable the formation of structural components of highly complex structures.
The injection molding process technology uses the injection machine to inject the product blank to ensure that the material fully fills the mold cavity, thus ensuring the realization of the high complex structure of the part. In the past, the traditional processing technology was first made into individual components and then assembled into components. When using MIM technology, it can be considered to be integrated into a complete single part, which greatly reduces the steps and simplifies the processing procedure. Compared with other metal processing methods, MIM has high dimensional accuracy and does not require secondary processing or a small amount of finishing.
The injection molding process can directly form thin-walled and complex structural parts. The shape of the product is close to the final product requirements, and the dimensional tolerance of the parts is generally maintained at ±0.1-±0.3. In particular, it is particularly important to reduce the processing cost of hard alloys that are difficult to machine and to reduce the processing loss of precious metals. The product has uniform microstructure, high density and good performance.
During the pressing process, due to the friction between the mold wall and the powder and the powder and the powder, the pressing pressure distribution is very uneven, which leads to the unevenness of the pressed blank in the microstructure, which will result in the pressing of the powder metallurgy. The shrinkage during the sintering process is uneven, so the sintering temperature has to be lowered to reduce this effect, so that the product has large porosity, poor material density, and low density, which seriously affects the mechanical properties of the product. On the contrary, the injection molding process is a fluid molding process. The presence of the binder ensures the uniform arrangement of the powder, thereby eliminating the unevenness of the microstructure of the blank, and thus the density of the sintered product can reach the theoretical density of the material. In general, the density of the pressed product can only reach 85% of the theoretical density. The high compactness of the product can increase the strength, strengthen the toughness, improve the ductility, the electrical and thermal conductivity, and improve the magnetic properties. High efficiency and easy to achieve high volume and large scale production.
The metal mold used in MIM technology has the same life as the engineering plastic injection molding tool. Due to the use of metal molds, MIM is suitable for mass production of parts. Since the product blank is formed by using an injection machine, the production efficiency is greatly improved, the production cost is lowered, and the consistency and repeatability of the injection molded product are good, thereby providing a guarantee for mass production and large-scale industrial production. Wide range of applicable materials, wide application areas (iron-based, low-alloy, high-speed steel, stainless steel, valve alloy, carbide).
The materials that can be used for injection molding are very wide. In principle, any high-temperature-castable powder material can be made into parts by MIM process, including difficult-to-machine materials and high-melting materials in traditional manufacturing processes. In addition, MIM can also conduct material formulation research according to user requirements, manufacture alloy materials of any combination, and shape composite materials into parts. The application fields of injection molded products have spread all over the national economy and have broad market prospects.
Performance and cost
The MIM process uses micron-sized fine powder, which not only accelerates the sintering shrinkage, but also helps to improve the mechanical properties of the material, prolong the fatigue life of the material, and improve the resistance, stress corrosion resistance and magnetic properties. The basic properties of some MIM materials are listed in Table 1. Material density g/cm3 hardness tensile strength MPa bending strength MPa elongation % coercivity (A/cm) iron-based alloy 98Fe2Ni7.4187HRB552----5.5----92Fe8Ni7.5088HRB560----8--- -95.5Fe2NiCu0.5Mo7.4099HRB682----3.3----Stainless steel 3047.4242HRB520----20----3167.6042HRB520----20----Carbide YG614.60----- ---1460----173YG814.50--------1680----124YT1510.45--------1140----117 tungsten alloy 90% W17.90320HV30920-- --6----93%W18.30310HV30900----10----97%W18.50350HV30880----6----.
Note: * This data is a relative density MIM process cost analysis for ultra-hard, too brittle difficult to cut materials or geometric shapes, segregation or contamination of the material during casting, the use of MIM process can significantly save costs. For example, in the case of a typewriter printing component guide rod, it is usually required to have 14 passes to the upper process; and the MIM process requires only 6 processes, which can save about half of the cost. As the ratio of material cost/manufacturing cost increases, the potential cost is more likely to decrease. Therefore, the smaller and more complex the parts, the better the economic benefits will be. From the above analysis, it can be seen that the potential of MIM molding is very large.
Technical application field
1. Computer and its auxiliary facilities: such as printer parts, magnetic core, striker pin, drive parts;
2. Tools: such as drill bits, cutter heads, nozzles, gun drills, spiral milling cutters, punches, sleeves, wrenches, electric tools, hand tools, etc.;
3. Household appliances: such as watch cases, bracelets, electric toothbrushes, scissors, fans, golf clubs, jewelry chain rings, ballpoint pen clamps, cutting tool bits and other components;
4. Parts for medical machinery: such as orthopedic frames, scissors, tweezers;
5. Military parts: missile tails, gun parts, warheads, hoods, and credit parts;
6. Electrical parts: electronic packaging, micro motors, electronic parts, sensor parts;
7. Mechanical parts: such as loose cotton machine, textile machine, crimping machine, office machinery, etc.;
8. Automotive marine parts: such as clutch inner ring, fork sleeve, distributor sleeve, valve guide, synchronization hub, airbag parts, etc.
The future development of powder injection molding is mainly in the material and design efforts, using the advantages of this process to help customers improve product design and reduce costs, thereby expanding the application of powder injection molding.