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MIM tools can be simply divided into mold base and chamber.


Metal injection mold bases are the same as plastic injection ones. The material of movable platen and thrust plate is usually #45 steel, thermal refining hardness is 189--250HBS. Other plate material can be #45 steel or Q235 steel. For those high requirements mold cases, all plate material can use #45 steel, or even the micro-deformation tooling steel like Cr12 to anneal. The guide pins and guide bushings can use T8A annealing to 50--55HRC or #20 steel carburizing thickness to 0.5--0.8mm and annealing to 56--60HRC.


When making MIM tools, the general alternating is 20--50 Mpa with cool and hot change. When making super precision MIM tools, the molding press is several times than normal ones. MIM tools file can be hundreds of thousands times, so it must be tough and hard enough.


The hardness of MIM tools mold chamber is usually 58--62HRC, so need to pay much attention on the sharp edges, grooves, cut and the stress concentration caused by the defects during machining. These defects will largely reduce the fatique strength.

So as to the mold chamber, when choosing the material, we mainly need to consider the abrasive resistance, annealing shape stability, corrosion resistance and machining performance. Due to the metal erosive wearing against MIM molds is rather worse than to plastic molds, abrasive resistance to MIM molds is the basic and essential requirement. Normally, the HRC is 58--62. Tooling steel is the main choice because of its comprehensive intensity, hardness, toughness, hardenability, corrosion resistance and machining performance. The common tooling steel includes Cr12, Cr12Mov, Cr2Mn2SsiWMoV, Cr6WV and high speed steel W18CrV, W6Mo5Cr4V2 etc..

 

MIM tools always have complex shape. They are usually made by high precision electrospark, wire-electrode cutting etc.. It demands that material should have good microstructure uniform, hardenability characteristic and high annealing shape stability. Because when injecting materials into the mold, there will be corrosion gas, it also demands good corrosion resistance.

 

In particular cases such as samples testing and small batch production, in order to simplify the process, we can also use Q235, #45 steel, aluminum alloy and nickel alloy to make the chamber. That will largely shorten the development time, but these molds cannot be used for large batch production.

In brief, to design and choose MIM tools need to according to the parts quantity production, parts shapes, precision and machining technologies, the difficulty of heat treatment and material etc..

 

MIM parts are usually small so the molds size is small as well. Generally, you do not need to calculate the intensity only if the using mold plate percentage is not longer, wider than 60% nor deeper than 10% from the whole mold plate. What is more, the molding shape is complex, so as to the chamber stress ability. Even if having several assumes, it is also hard to get the result by elastomechanics, and finite element. Thus, molds usually be designed by amplifing safety factor method.

 

But for big products or parts which need high precision press, it is necessary to do intensity calculation, in order to prevent flash or damage due to less strength or large deformation. To calculate simple shape mold chambers, we can refer to plastic molds design. For complex chambers, we can use finite element or professional soft to analyze. In order to ensure the precision of parts, mold design need to pay attention to the following facts: 

1, Rational design the mold amplification coefficient. In the premise of successful forming, minimize the volume of binder. 
2, Rational mold structure. Precision parts need a certain intensity, easy replacement and rational tolerance. Forming blank holes need positive tolerance while blank axis need negative tolerance. Parts technology structure must be proper to ensure machining precision. 
3, Rational sprue form and position.
4, Rational push-out. Make the thimble uniform and smooth and deduct the deformation.
5, Molds need with temperature adjustment system. Ensure uniform filling and cold-down. Deduct the density heterogeneity. 
6, Rationally choose chamber quantity and lay out. Use uniform symmetry lay out to ensure balance filling. For high precision parts, we had better deduct the chamber quantity. 
7, Rational gap. Avoid trimming and spurs. 

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