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Metal Powder Injection Molding (MIM) Explain

Powder injection molding technology is a new process developed from plastic injection molding technology and combined with powder metallurgy, which is divided into ceramic powder injection molding and metal powder injection molding (MIM). The most significant advantage of MIM technology is near net forming, which greatly reduces machining and material waste, and can solve the limitations and insufficient accuracy of traditional powder metallurgy processes in processing complex shaped parts. It is suitable for mass production of structurally complex metal components.

1. MIM process flow

Including: raw material mixing → granulation → injection molding → degreasing → drying → sintering → post-treatment. Mix metal and thermoplastic in a certain proportion, and add plasticizing additives such as paraffin. After heating the mixture, it is injected into the mold cavity through the injection molding machine feeding system while maintaining pressure to compensate for cooling shrinkage. After the component cools and solidifies to a certain strength, it is extruded to obtain a blank, which is then degreased and sintered to obtain the final product.

(1) Raw materials: various metal powders, such as iron powder, steel powder, aluminum powder, copper powder, nickel based alloy, tungsten alloy, hard alloy, titanium alloy powder, etc. Generally speaking, MIM powder is in the micrometer range (less than ten micrometers), with a spherical appearance and special requirements for powder characteristics (loose density, particle size distribution, compaction pressure, angle of repose).

(2) Mixing: Mix MIM powder evenly and apply adhesive on the surface to facilitate subsequent degreasing and sintering. Common mixing devices include internal mixers, dual planetary mixers, dual eccentric wheel mixers, single screw extruders, Z-type gear mixers, etc. Adhesives are usually made by mixing wax and plastic, with some surfactants added. They require a small contact angle with the powder, strong adhesion, no two-phase separation from the powder, a certain strength after cooling, and no cracking or bubbling of the green body during degreasing. 

(3) Injection molding: Injecting metal powder into green bodies of target shape and size, including dual loop injection, dual template injection, rodless injection, fully automatic injection, etc.

(4) Defatting: Defatting the green body, extracting and separating the binder, and then drying and evaporating the extractant. The methods include solvent degreasing, thermal degreasing, catalytic degreasing, etc. According to different degreasing methods, equipment is divided into solvent degreasing furnace, catalytic degreasing furnace, siphon degreasing furnace, etc. Among them, the catalytic degreasing process is relatively advanced.

(5) Sintering: Heating the defatted body at high temperature under vacuum or atmosphere to densify it. According to sintering pressure, it can be divided into atmospheric pressure sintering and pressure sintering; According to the sintering atmosphere, it can be divided into air sintering, hydrogen sintering, vacuum sintering, etc; According to the sintering temperature, it is divided into medium temperature sintering (<1000 ℃) and high temperature sintering (1100-1700 ℃).

2. Development of MIM Technology

Micro powder injection molding technology (micro MIM): Due to the application of ultra precision machining technology, small precision molds can be used to manufacture difficult materials and complex small products that cannot be processed after machining. The size of its products is mostly below 1mm, making it difficult to adjust the dimensional accuracy and surface smoothness of the products through post-processing techniques. The conventional MIM method uses powders with an average diameter of about 10 μ m, while micro MIM requires the use of powders with an average particle size of about 3 μ m to ensure that the obtained products have high surface smoothness.

Micro Metal Powder Injection Molding (μ - MIM): A technology that combines powder metallurgy and plastic injection molding by mixing metal fine powder with resin or paraffin, injection molding, degreasing, and sintering to manufacture metal products. Compared with powder metallurgy, μ - MIM utilizes finer metal powders to improve the density of sintered products, and can also manufacture small metal parts with complex shapes and higher precision.

Manufacturing metal porous materials: Metal porous materials have broad application prospects in filters, chemical reaction catalysts, heat exchangers, impact energy absorbers, collectors for lithium-ion batteries, electrodes for air purifiers and alkali ion water purifiers, and high brightness cathode tube electrodes for LCD monitors and reverse signal lights. However, traditional manufacturing techniques for metal porous materials are difficult to control in terms of porosity and pore size. During MIM molding, in addition to metal powder and binder, porous metal materials with controllable pore characteristics can be prepared by adding pore forming materials (such as resin powder) and controlling their particle size and addition amount.

Manufacturing non-ferrous metal products and antibacterial stainless steel: In addition to iron-based alloys such as stainless steel and carbon steel, MIM has been extended to non-ferrous metals such as pure titanium, titanium alloys, and tungsten alloys. Jiang Xiangcao and others studied the MIM process of ultrafine tungsten powder particles and used MIM to prepare tungsten components that can be used in fusion reaction devices. The average grain size is 10-15 μ m, and the main properties are basically the same as those of forged pure tungsten components. Chen Liangjian et al. used MIM technology to prepare nanoporous titanium, and coated gelatin and sustained-release nanospheres on the surface to manufacture nano medical implant fillers. Pang Wuji et al. studied the MIM manufacturing process for oral surgical tools, orthodontic brackets, hearing aid sound tubes, and knee cartilage implant parts. Indo MIM company in the United States uses MIM design to produce hearing aid sound tubes with a relative density of 7.65 g/cm3, a tensile strength of 480 MPa, a yield strength of 150 MPa, and a surface hardness of 100 HRB. The MIM method can be used to produce copper added stainless steel with excellent corrosion resistance and antibacterial properties. By adding excessive Cu elements and forming Cu precipitates, the stainless steel can maintain its antibacterial properties for a long time.

3. Application of MIM Technology

MIM plays an important role in the preparation of high-performance rare earth permanent magnet materials, rare earth hydrogen storage materials, rare earth luminescent materials, rare earth catalysts, high-temperature superconducting materials, and new metal materials such as Al Li alloys, heat-resistant Al alloys, superalloys, powder corrosion-resistant stainless steel, powder high-speed steel, etc. At present, MIM can manufacture automotive engine motor bearings, optical communication connectors, medical equipment parts, automotive industry consumables (ABS brakes, driving rods, fuel injectors, etc.), aviation accessories, daily consumer goods, military weapons, etc. In addition, MIM can produce multifunctional compositions with composite functions, such as magnetic and non-magnetic, magnetic response and corrosion resistance, constrained porosity and strong inertness, high thermal conductivity and low coefficient of thermal expansion, wear resistance and toughness, etc.