1. Process characteristics of metal powder injection molding technology

Metal powder injection molding technology is the product of the integration and intersection of plastic molding technology, polymer chemistry, powder metallurgy technology and metal materials science. It uses molds to inject mold blanks and quickly manufacture high-density, high-precision products through sintering. , three-dimensional complex-shaped structural parts, which 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 process technology not only has the advantages of conventional powder metallurgy processes such as fewer steps, no or less cutting, and high economic benefits, but also overcomes the shortcomings of traditional powder metallurgy products such as uneven materials, low mechanical properties, and difficulty in forming thin walls and complex structures. It is especially suitable for mass production of small, complex and special metal parts.

2. Process flow of metal powder injection molding technology

Binder → mixing → injection molding → degreasing → sintering → post-processing.

1. Powder metal powder

The particle size of metal powder used in the MIM process is generally >0.5~20>μ>m>; theoretically, the finer the powder particles, the larger the specific surface area, making it easier to shape and sinter. The traditional powder metallurgy process uses coarser powders larger than >40>μ>m>. >

2. Organic adhesive

The function of the organic adhesive is to bond the metal powder particles so that the mixture has rheology and lubricity when heated in the barrel of the injection machine, that is to say, it is a carrier that drives the powder to flow. Therefore, the choice of binder is the carrier of the whole powder. Therefore, the sticky pull selection is the key to the whole powder injection molding. Requirements for organic adhesives:

1) The dosage is small, and the mixture can produce better rheology with less adhesive;

2) No reaction, no chemical reaction with metal powder during the process of removing the adhesive;

3) Easy to remove, no carbon remains in the product.

3. Mixing

Mix metal powder and organic binder evenly together to make various raw materials into injection molding mixture. 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 its equipment conditions are basically the same. During the injection molding process, the mixed material is heated in the barrel of the injection machine into a plastic material with rheological properties, and is injected into the mold under appropriate injection pressure to form a blank. The microcosm of the injection molded blank should be uniform, so that the product shrinks evenly during the sintering process.

4. Extraction

The organic binder contained in the blank must be removed before sintering. This process is called extraction. The extraction process must ensure that the adhesive is gradually discharged from different parts of the blank along the tiny channels between the particles without reducing the strength of the blank. The rate of binder removal generally follows the diffusion equation. Sintering Sintering can shrink and densify the porous degreasing blank into products with certain structure and properties. Although the performance of products is 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 parts with more precise size requirements, necessary post-processing is required. This process is the same as the heat treatment process of conventional metal products.

3. Characteristics of MIM process

Comparison between MIM technology and other processing technologies

The particle size of raw metal powder used in MIM is >2-15>μ>m>, while the particle size of raw metal powder used in traditional powder metallurgy is mostly >50-100>μ>m>. The finished product of the >MIM> process has a high density due to the use of fine powder. >MIM> process has the advantages of traditional powder metallurgy process, but the high degree of freedom in shape cannot be achieved by traditional powder metallurgy. Traditional powder metallurgy is limited to the strength and filling density of the mold, and the shape is mostly two-dimensional cylindrical.

The traditional precision casting dehydration process is a very effective technology for making products with complex shapes. In recent years, ceramic cores have been used to assist in the completion of finished products with slits and deep holes. However, due to the limitations of the strength of the ceramic cores and the fluidity of the casting liquid, , the process still has 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 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>g>10->hundreds of product shapes, three-dimensional complex shapes, two-dimensional simple shapes, mechanical properties.

Comparison between the MIM process and traditional powder metallurgy. The die-casting process is used in materials with low melting points and good fluidity of the casting solution, such as aluminum and zinc alloys. Due to material limitations, the products produced by this process have limited strength, wear resistance, and corrosion resistance. >MIM> technology can process more raw materials.

Although the precision and complexity of its products have improved in recent years, the precision casting process is still inferior to the dewaxing process and the MIM> process. Powder forging is an important development and has been suitable for mass production manufacturing of connecting rods. However, generally speaking, there are still problems with the cost of heat treatment and the life of the mold in forging projects, which still need to be further solved.

Traditional machining methods, recently relying on automation to improve their processing capabilities, have made great progress in effect and accuracy, but the basic procedures are still inseparable from 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 is far better than other processing methods, but because the effective utilization rate of materials is low and the completion of its shape is limited by equipment and tools, some parts cannot be completed by mechanical processing. On the contrary, MIM can effectively utilize materials without restrictions for the manufacturing of small, precision parts with difficult shapes. Compared with mechanical processing, the MIM process has lower cost and higher efficiency, making it highly competitive.

MIM technology does not compete with traditional processing methods, but makes up for the technical deficiencies or inability to produce defects of traditional processing methods. >MIM> technology can exert its expertise in the field of parts made by traditional processing methods. The technical advantages of MIM technology in parts manufacturing can form structural parts with highly complex structures.

Injection molding technology uses an injection machine to inject molded product blanks to ensure that the material fully fills the mold cavity, thus ensuring the realization of highly complex structures of parts. In the past, in traditional processing technology, individual components were first made 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 procedures. Compared with MIM and other metal processing methods, the product has high dimensional accuracy and does not require secondary processing or requires only 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. The dimensional tolerance of the parts is generally maintained at around ±0.1->±>0.3>. It is particularly important to reduce the processing cost of hard alloys that are difficult to machine and reduce the processing losses 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 uneven microstructure of the pressed blank, which will cause the pressed powder metallurgy parts to The shrinkage during the sintering process is uneven, so the sintering temperature has to be lowered to reduce this effect, resulting in large porosity, poor material compactness, and low density of the product, 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 distribution of the powder, thereby eliminating the uneven microstructure of the blank, so that the density of the sintered product can reach the theoretical density of the material. Under normal circumstances, the density of pressed products can only reach a maximum of 85% of the theoretical density. The high density of the product can increase the strength and toughness, improve the ductility, electrical and thermal conductivity, and improve the magnetic properties. High efficiency, easy to achieve mass production and large-scale production.

The metal mold used in MIM technology has a lifespan equivalent to that of engineering plastic injection molding molds. Due to the use of metal molds, it is suitable for mass production of parts. The use of injection machines to mold product blanks greatly improves production efficiency and reduces production costs. In addition, injection molded products have good consistency and repeatability, thus providing guarantee for large-volume and large-scale industrial production. It has a wide range of applicable materials and broad application fields (>iron-based, low alloy, high-speed steel, stainless steel, gram valve alloy, cemented carbide>).

There is a wide range of materials that can be used for injection molding. In principle, any powder material that can be poured at high temperatures can be made into parts by the MIM process, including materials that are difficult to process and materials with high melting points in traditional manufacturing processes. In addition, MIM can also conduct research on material formulas according to user requirements, manufacture any combination of alloy materials, and shape composite materials into parts. The application fields of injection molded products have spread across all fields of the national economy and have broad market prospects.