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Advantages and process flow of MIM metal powder injection molding


MIM is the abbreviation of Metal Injection Molded. The skills of metal powder injection molding are products of multi-disciplinary penetration and intersecting, such as plastic molding technology, polymer chemistry, powder metallurgy technology, and metal data science. There is a very practical explanation about MIM: design the metal parts like plastic injection parts; use the advantages of plastic injection to quickly replicate, the metal parts are injection molded, and then they are made into metal solid parts through a thermal process.
   Although MIM is formed by metal powder, it cannot be separated from plastic. Injection molding pellets are composed of metal powder and plastic, and the plastic is mainly used for bonding and smoothing. Through the plastic coating effect, the wear of the metal powder on the screw is greatly reduced, and the fluidity of the injection melt is increased, which then makes the metal injection possible.

1. Six advantages make MIM highly valued:
(1) Structural parts with highly messy structure can be formed
Traditional metal processing generally involves turning, milling, planing, grinding, drilling, boring and other processing of metal plates into products; it is difficult to achieve a messy structure for this type of product due to technical and time costs. MIM uses an injection machine to inject the molded product blanks to ensure that the material fills the mold cavity, which also ensures the realization of the high messy structure of the parts.

(2) The product has uniform micro-arrangement, high density and good function
Under normal circumstances, the density of the restricted product can only reach 85% of the theoretical density; the product density obtained by MIM technology can reach more than 96%.

(3) High efficiency, easy to realize mass production and large-scale production
The metal mold used by MIM technology has a life span that is appropriate for the mold of engineering plastic injection molding tool. Because of the use of metal molds, MIM is suitable for mass production of parts.

(4) Wide range of applicable materials and broad application fields
Iron-based, low-alloy, high-speed steel, stainless steel, gram valve alloy, cemented carbide, etc. are all suitable for MIM forming.

(5) Significant saving of raw materials
Generally, the utilization rate of metal processing and forming metal is relatively low. For example, the utilization rate of the metal shell of LeTV MAX mobile phone is less than 10%, and most of the aluminum alloy becomes debris. MIM can greatly improve the utilization rate of original data, theoretically 100% use.

(6) MIM process uses micron-level fine powder
Not only can accelerate the sintering shrinkage, help improve the mechanical function of the material, extend the fatigue life of the material, but also improve the resistance, stress corrosion resistance and magnetic function.

2. MIM process:
1. Mixing and granulation
    The mixing is to mix the metal powder and the organic binder uniformly, so that all kinds of raw materials become the mixture for injection molding. Granulation is to extrude the mixture into granules. The particle size of the metal powder used in the MIM process is generally 0.5-20¦Ìm. In theory, the finer the particles, the larger the specific surface area, and the easier it is to form and sinter. The effect of the organic binder is to bond the metal powder particles, so that the mixture has rheology and smoothness after heating in the barrel of the injection machine, that is, the binder is the carrier that drives the powder to flow. Therefore, the choice of binder is the key to the entire powder injection molding. Common binders are: PP, PE, EVA, PEG and POM.

2. Injection molding
   This type of injection molding is not much different from ordinary metal injection molding, but the screw is required to be more wear-resistant.

3. Degreasing (also called 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 binder 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 the binder generally follows the dispersion equation.

4. Sintering
    Sintering can shrink and densify the porous degreased blank into a product with a certain arrangement and function. Although the function of the product is related to many process factors before sintering, in many cases, the sintering process has a great and even decisive influence on the metallographic arrangement and function of the final product.

5. Secondary treatment
    For parts with finer dimensions, necessary post-processing is required. This process is the same as the heat treatment process of conventional metal products. The more processes used for secondary processing are fine CNC.