Abstract: Metal powder injection molding technology is a new type of near-net forming technology in the field of powder metallurgy, which introduces modern plastic injection molding technology into powder metallurgy. The metal powder process flow is binder mixing → injection molding → debinding → sintering → post-processing.
Metal Powder Injection Molding Technology Process Features
The metal powder injection molding technology is a product of multiple disciplines such as plastic forming technology, polymer chemistry, powder metallurgy technology, and metal material science. It uses molds to inject molded blanks and quickly manufacture high-density, high-precision, three-dimensional complex-shaped structural parts through sintering. It can quickly and accurately materialize design ideas into products with certain structural and functional characteristics, and can directly mass-produce parts, which is a new change in the manufacturing technology industry. This technology not only has fewer conventional powder metallurgy process steps, no or less cutting, and high economic benefits, but also overcomes the disadvantages of traditional powder metallurgy technology, such as uneven material, low mechanical properties, difficult to form thin walls, and complex structures. It is particularly suitable for mass production of small, complex, and special metal parts.
Metal Powder Injection Molding Technology Process:
Binder → Mixing → Injection Molding → Debinding → Sintering → Post-Processing.
1. Metal Powder: Generally, the particle size of metal powders used in Metal Injection Molding (MIM) technology is greater than 0.5-20μm. According to theory, the finer the particles, the larger the specific surface area, which makes them more suitable for molding and sintering. However, the specialized powder metallurgy process adopts coarser powders greater than 40μm.
2. Organic Binder: The organic binder adheres to metal powders to make the mixture have rheological and lubricating properties when heated and flowing in the injection molding machine cylinder. In other words, the binder acts as the carrier of the entire powder mixture. Therefore, selecting the right binder is crucial for the injection molding process. The requirements for organic binders are:
1) Small amount: a small amount of binder can give the mixture good rheological properties.
2) Non-reactive: the binder should not react chemically with metal powders in the process of removing the binder.
3) Easy to remove: there should be nothing left of the binder in the final product.
3. Mixing: The metal powder and organic binder are evenly mixed to form a mixed material for injection molding. The uniformity of the mixed material directly affects its flowability and injection molding process parameters, as well as the density and other properties of the final material. The injection molding process is similar to plastic injection molding in principle, and the equipment conditions are basically the same. During the injection molding process, the mixed material is heated in the injection machine barrel to become a rheological plastic material, and is injected into the mold under proper injection pressure to form a green compact. The microstructure of the green compact produced by injection molding should be uniformly consistent, so that the product can uniformly shrink during sintering.
4. Extraction: Before sintering, the green compact must remove the organic binder contained in it through a process called extraction.
The extraction process must ensure that the binder is gradually discharged from different parts of the green compact along the small channels between the particles without reducing the strength of the green compact. The rate of binder removal generally follows the diffusion equation. Sintering can prevent the porous green compact from shrinking to densify into a product with certain organization and properties. Although the properties of the final product are related to many process factors before sintering, in many cases, the sintering process has a great, or even decisive, impact on the metallographic structure and properties of the final product.
5. Post-processing: For precision parts with strict size requirements, necessary post-processing is required. This process is the same as
the heat treatment process for conventional metal products.
Characteristics of MIM Process Compared to Other Processing Technologies
The particle size of the raw materials used in MIM process is between 2-15 μm, while the particle size of the raw powder used in traditional powder metallurgy is mostly between 50-100 μm. MIM products have high density due to the use of fine powders. MIM process has the advantages of traditional powder metallurgy, while having a higher degree of shape freedom. Traditional powder metallurgy is limited by the strength and filling density of the molds, and most shapes are two-dimensional cylindrical types.
Traditional precision casting and debinding process is an effective technology for producing complex shapes, and in recent years, the use of ceramic cores can produce finished products with narrow slits and deep holes. However, due to the strength of the ceramic cores and the flowability of the casting liquid, this process still faces some technical difficulties. In general, this process is more suitable for manufacturing large and medium-sized parts, while MIM process is more suitable for small and complex-shaped parts.
Comparison of Manufacturing Processes
Comparison Item | MIM Process | Traditional Powder Metallurgy
-----------------------|---------------|-------------------------------------
Particle Size | 2-15 μm | 50-100 μm
Relative Density | 95-98% | 80-85%
Product Weight | ≤ 400 g | 10s - hundreds
Product Shape | Three-dimensional complex shape | Two-dimensional simple shape
Mechanical Properties | Varies depending on materials | Limited by materials in batch process
Comparison of MIM Process and Traditional Powder Metallurgy Process
The die casting process is used for materials such as aluminum and zinc alloys with low melting points and good casting fluidity,
while the strength, wear resistance and corrosion resistance of batch process products are limited by the materials used. MIM process
can process more types of raw materials.
Although the precision casting process has improved its accuracy and complexity in recent years, it still lags behind the wax removal
process and MIM> process because of material limitations in terms of strength, wear resistance, and corrosion resistance.
Powder forging is an important development and has been used for the mass production of connecting rods. However, in general, the cost of heat treatment and the life of the mold in forging still remain problematic and need further solutions.
Traditional mechanical processing methods have greatly improved their processing capacity through automation in terms of results and accuracy, but the basic process still revolves around step-by-step processing (>turning, planing, milling, grinding, drilling, polishing, etc.) to complete the shape of the part. The processing accuracy of mechanical processing methods far exceeds other methods, but some parts cannot be machined due to low utilization of material and limitations on equipment and tools. In contrast, MIM can effectively utilize materials, is not limited, and is suitable for the manufacturing of small, highly complex parts. Compared to mechanical processing, MIM has lower costs and higher efficiency, and is highly competitive.
MIM technology is not in competition with traditional processing methods, but rather compensates for their shortcomings or defects in manufacturing. MIM technology can play to its strengths in the field of parts manufacturing where traditional processing methods are used. MIM technology has technical advantages in the production of highly complex structural components. Injection molding technology ensures that the material fully fills the mold cavity, which guarantees the realization of high-complexity structures. Previously, in traditional processing technology, individual elements were made first and then assembled into components, but with MIM technology, they can be
integrated into complete single parts, greatly reducing the process and simplifying the processing program.
Compared with other metal processing methods, MIM products have high dimensional accuracy, and require no secondary processing or only a small amount of precision processing. Injection shaping technology can make complex structural parts without the need for further processing, which is particularly important for reducing the processing cost of hard alloys that are difficult to machine and reducing the losses associated with processing precious metals. The microstructure of the product is uniform, the density is high, and the performance is good.
During the pressing process of powdered metal products, the uneven distribution of pressure caused by the friction between the die wall, the powder and the powder will lead to the uneven microstructure of the pressed blank. This results in uneven shrinkage during the sintering process and therefore it is necessary to reduce the sintering temperature to minimize this effect, which results in high porosity, poor material density and low density, seriously affecting the mechanical performance of the product. In contrast, the injection molding process is a fluid molding process. The presence of a binder ensures the uniform arrangement of the powder, which eliminates the unevenness of the blank's microstructure, so that the sintered product can achieve its theoretical density. Generally, the highest density that can be achieved by pressing products is only 85% of the theoretical density. Higher density products can increase strength, improve toughness, improve ductility, conductivity, and thermal conductivity, enhance magnetic properties, and are efficient and easy to achieve large-scale production.
The metal molds used in MIM technology have a service life comparable to that of engineering plastic injection mold. Due to the use of metal molds, it is suitable for mass production of parts. By using an injection machine to form the product blank, the production efficiency is greatly improved and the production cost is reduced. The consistency and repeatability of injection molded products provide a guarantee for mass production and scale production. The range of applicable materials is wide, and the application fields are extensive (> iron-based, low-alloy, high-speed steel, stainless steel, Kovar alloy, hard alloy>).
Almost any powder material that can be high-temperature sintered can be made into parts by the MIM process, including difficult-to-machine materials and high-melting-point materials in traditional manufacturing processes. In addition, MIM can also perform material formula research according to user requirements, produce any combination of alloy materials, and mold composites into parts. The application fields of injection molded products have spread across all sectors of the national economy and have broad market prospects.
Applications of metal powder injection molding technology:
1. Computers and related devices: Parts such as printer components, magnetic cores, pivot pins, and drive components.
2. Tools: Parts such as drill bits, blades, nozzles, gun drills, spiral milling cutters, punching heads, sockets, wrenches, electrical tools, hand tools, etc.
3. Household appliances: Parts such as watch cases, watch chains, electric toothbrushes, scissors, fans, golf club heads, jewelry chains, ballpoint pen clips, blades, and knife heads.
4. Medical equipment parts: Parts such as dental braces, scissors, forceps.
5. Military parts: Parts such as missile fins, gun parts, warheads, explosive devices, and detonators.
6. Electrical parts: Parts such as electronic packaging, micro motors, electronic components, and sensor components.
7. Machinery parts: Parts such as knitting machines, textile machines, edge trimming machines, and office machinery.
8. Automobile and ship parts: Parts such as clutch inner rings, fork sleeves, distributor sleeves, valve guide pipes, synchronizer hubs, and safety airbag components.
