Material selection of metal injection molding mold
Writer： Admin Source： MDM Date：2022-06-10
Injection mold and plastic injection mold mold is exactly the same, mold material can refer to plastic injection mold mold to choose: general, dynamic template, fixed mold? The push plate can be made of 45 steel with a hardness of 180~250HBS. Other sheets can be made of 45 steel or Q235 steel. For the mold frame with high precision requirements, all sheet materials can be treated with 45 steel quenching and tempering, or even with Cr12 type of micro deformation die steel, quenching treatment. Guide columns and guide sleeves can be hardened to 50~55HRC by T8A or hardened to 56~60HRC by 20 steel carburized 0.5~0.8mm thick.
When the injection mold works, it generally bears the alternating load of 20~50MPa, accompanied by the alternating temperature of hot and cold. In ultra-precision injection molding, the forming pressure used can even exceed the normal use of several times. The service life of injection mold is usually tens of thousands or even hundreds of thousands of times, so the mold should have enough strength and stiffness.
Generally, the mold cavity of metal injection mold is made of hardened mold with hard filling of 58~62HRC, so it is necessary to pay attention to the stress concentration caused by sharp corners, grooves, incisions and machining defects. These defects can greatly reduce the fatigue strength of the die.
For the cavity part, the material selection mainly considers wear resistance, quenching shape stability, corrosion resistance and machining performance. Because the erosion and wear of metal injection material on the mold is much more serious than that of ordinary plastics, wear resistance is the most basic requirement of the cavity of metal injection mold used for mass production, and the hardness of the cavity is generally required to be 58~62HRC. Tool steel is the preferred material for cavity due to its comprehensive strength, hardness, toughness, hardenability, corrosion resistance and machining performance. Commonly used materials are alloy die steel such as Cr12, Cr12MoV, Cr12MoV, Cr2Mn2SiWMoV, Cr6WV and high-speed steel W18CrV, W6Mo5Cr4V2, etc.. Injection moulds are generally complex in shape, usually using ultra-precision electric spark, wire cutting and other electrical machining methods, which require uniform material structure, high hardenability and quenching shape stability. Metal injection materials often produce corrosive gases at injection
temperature, and corrosion resistance is also a top requirement for mold materials.
In special cases, such as sample and small batch parts of fast belt production, in order to simplify the process, Q235 steel, 45 steel. Aluminum alloy, zinc-nickel alloy, etc., are also commonly used to make some cavities. This will greatly shorten the sample development cycle. But such molds generally can not be used for mass production.
In short, the choice of mold must be based on the batch of products, the shape of parts, precision, the processing means and technology, with the degree of difficulty of heat treatment, and the convenience of material sources will be considered comprehensively.
Metal injection mold is generally used for the production of small parts, the mold size is very small. In normal use, as long as the use area of the template does not exceed 60% of the length and width, and the depth compensation exceeds 10%, strength calculation can not be carried out. In addition, the mold shape is more complex, the actual force in the cavity is also very complex, even if a variety of assumptions, it is difficult to use elastic mechanics and finite element method to get the results. Therefore, the tolerance method of magnifying
safety factor is often adopted in die design.
However, for larger products or ultra-high pressure precision forming process, if possible, or should be calculated cavity strength. Prevent overflow or even damage due to insufficient strength and excessive deformation. The strength calculation of simple shape cavity can refer to the design manual of injection mold for plastic mold design. Complex cavities can be analyzed and calculated using finite element or professional
software.
In order to ensure the accuracy of products, the following points should be paid attention to in mold design:
1) Reasonably design the magnification factor of the mold. On the premise of ensuring smooth forming, the amount of binder should be reduced as far as possible.
2) Reasonable mold structure. Precision size parts should have a certain hardness, and easy to replace; Thetolerance adopted is reasonable, the upper limit of tolerance should be adopted for forming blank hole as far as possible, and the lower limit of tolerance should be adopted for forming blank shaft as far as possible. The process structure of the parts is reasonable to ensure the machining accuracy.
3) Reasonable selection of the form and location of the gate.
4) Reasonable selection of ejection way, as far as possible to make the ejection uniform and smooth, reducedeformation.
5) The mold shall be equipped with a temperature regulation system to ensure uniform filling and cooling as faras possible to reduce density heterogeneity.
6) Reasonable selection of cavity number and layout, try to use uniform symmetrical layout, to ensure balancedfilling. Special precision products, should minimize the number of cavities.
7) Reasonable clearance to avoid flaking and burr.
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