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What's Metal injection molding (MIM)?

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2018-08-17 16:20:00
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Metal Injection Molding (MIM) is a new manufacturing process technology that combines powder metallurgy with plastic injection processes. To mix the metal powder with the polymer binder, this is called feedstock. 

the polymer imparts its viscous flow characteristics to the mix, aiding in the uniformity of forming, cavity filling and powder loading. After the injection, it’s Debinding, to remove the polymer 

binder from the metal. 

Then go into sintering process, the purpose is to reduce the empty space to approximately 1%-5%, heat treatment or machining. The sintered product not only has the same complex shape and high precision as the product obtained by plastic injection molding, but also has physical, chemical and mechanical properties close to the forging or even CNC parts. In traditional machining technology, for complex parts, it is usually to disassemble and make a single part and then assemble it. After using metal injection molding technology, it can be considered to integrate it into a complete single part, which greatly reduces production process. The steps simplify the machining process, save costs and increase efficiency. This technical feature makes this process particularly suitable for the mass production of small, precise, three-dimensional shapes and metal parts with special performance requirements.

Metal injection molding technology not only has the advantages of less conventional powder metallurgy process, no cutting or less cutting, high economic efficiency, but also overcomes the traditional powder metallurgy process products with low density, uneven material, low mechanical properties, and is not easy to form thin wall and complicated structure. Disadvantages, especially suitable for mass production of small, complex and special metal parts.

Polymer BinderPolymer Binder is the core of metal injection molding (MIM) technology. In metal injection molding, the polymer binder has the two basic functions of enhancing fluidity to be suitable for injection and maintaining the shape of the compact. In addition, it should be easy to remove, non-polluting and non-contaminating. Due to its toxicity and reasonable cost, various polymer binder have emerged. In recent years, it has gradually designed the bonding from the experience to the degreasing method and the polymer binder function..

The polymer binder is generally composed of a low molecular component and a polymer component plus some necessary additives. The low molecular component has low viscosity, good fluidity and easy to remove; the polymer component has high viscosity and high strength, and maintains the strength of blank. The two are properly proportioned to achieve high powder loading, resulting in high precision and high uniformity. Commonly used polymer binders are: thermoplastic systems (paraffin based, oil based and thermoplastic polymer based) and gel systems.

Introduce the process of metal injection molding

Mixing in metal injection molding

Mixing is the process of mixing metal powder with a binder to obtain a uniform feedstock. Since the nature of the feedstock determines the properties of final injection molded product, the process of mixing is very important. This involves various factors such as the manner and sequence of binder and powder addition, the mixing temperature, and the characteristics of the mixing device. This process has been stuck at the level of experience. An important indicator for evaluating the quality of the mixing process is the uniformity and consistency of obtained feed.

The mixing of MIM feedstock is done under the combined effect of thermal and shear forces. The mixing temperature should not be too high, otherwise the binder may decompose or the viscosity of binder may be separated due to too low viscosity. The shear force varies depending on the mixing method. Z-shaped impeller mixer, double planetary mixer, double cam mixer, all  are suitable for preparing a mixture with viscosity range of 1-1000 Pa?s.

Mixing method generally involves adding a high melting point component to melt firstly, then cooling, adding a low melting point component, and then adding the metal powder in portions. This can prevent decomposition of the low melting point component, and the addition of metal powder in batches can prevent torque increasing due to rapid cooling and reduce the equipment loss.

For feeding method of different particle size powders, the Japanese patent introduces: firstly add 15-40um water atomized powder into the binder, then add 5-15um powder, and finally add the powder degree ≤5um powder, thus obtained low shrinkage of final product. In order to apply a layer of binder around the powder uniformly, the metal powder may be directly added to the high melting point component, the low melting component may be added too, and finally remove the air. 

Keywords in the article: Metal Injection Molding MIM
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