Due to molecular stretching and shearing, in the molding process, residual stress will be left in the product, the residual stress will warp the product, and even crack the part. Mold steel in the processing will also have residual stress; this residual stress will make injection mold deformed or cracking.
The requirement of steel mold is very high.The right choice of steel would decide the die life, machining, precision great. The requirements of the mold steel depend on the injection mold structure, mold life, molding plastic, performance, appearance, dimensional and accuracy.
The general rules are as follows:
- easy to buy;
- excellent mechanical processing;
- easy polishing, beautiful surface;
- good strength, toughness and wear resistance;
- no internal defects such as holes;
- easy heat treatment, small deformation;
- excellent heat resistance, low thermal expansion coefficient;
- good welding performance;
- excellent corrosion resistance for heat-sensitive plastic
When the dimensional accuracy requirements are high, injection mold maker should use the steel after heat treatment or with small stress deformation. In recent years, due to significant mass injection molding production, plastic mold steel also requires quenching.
The following describes several major properties of steel:
- Wear resistance is the most important requirement of the mold steel, for the all injection mold life, precision, material removal (such as flash) after heavy load.Wear resistance varies from the number of alloying elements, internal stress. Wear resistance is generally proportional to the hardness. Therefore, increasing mold cavity surface hardness can solve the wear problem.Often the glass fiber is added to increase plastic material strength and wear resistance. Also, conductive additives can be added to the plastic as a conductive one, for the protection of radio interference, but also it could combine with a ferrous salt magnet to make magnets plastic. These plastic usages are expected to expand in the future. A variety of plastic additives adding is bound to increase. The biggest problem for These additives added is the increased fiber friction in the cavity, it resulting in mold cavity wear and tear and affecting the dimensional accuracy of products. Therefore, in order to expand the life of the injection mold, it is necessary to use a steel having excellent wear resistance.
- In the injection molding, the mold cavity surface roughness directly affects the plastic part surface roughness.For transparent products, cavity, core surface mirror polishing is a need; mold steel polishing has become a major problem. For the polishing performance, mold steel must have:
- no sand holes (PIN HOLE);
- no abrasions;
- very few non-metallic objects;
- uniform organization.
- fine grain size (no coarse grain);
- appropriate hardness;
- appropriate mechanical properties.
Machining of steel
- Injection mold building processing can be divided into machining (such lathe, general milling, CNC milling, grinding, pliers, carving, polishing), electrical processing (such as electrode processing, wire cutting processing).The choice of mold steel must take into account the life of the mold and mold processing. From the view of the machinability of the mold steel, it is preferable to use a low hardness steel which can be machined conventionally, but it is hard to ensure the life of the mold and the accuracy of the cavity. Therefore, in practical work, it is often using good pre-hardened (quenching and tempering) mold steel as the mold inserts. For The precision injection molds and long-life mold building, pre-hardened steel can be used for the core, cavity and steel nitriding could improve the core, cavity wear resistance.
- When halogen-containing compounds such as polyvinyl chloride (PVC), polyacetal (aqueous formaldehyde solution), low foaming resin (gas, carbon monoxide) and other resins are used, it often releases corrosive gas to damage Mold.
- For mold corrosion resistance, the best way is to use corrosion-resistant steel, such as S136H, NAK90 and so on. Beryllium copper is also a good corrosion resistance materials.
- In order to prevent mold erosion, sometimes chrome plating is used in the cavity.