How to choose the right mold steel ?
In the choice of plastic mold steel, according to the following conditions and gradually considered, and finally make a decision.
(1) the production of plastic parts: mold is a highly efficient production tools. Each paid mold life, directly related to the cost of parts. When designing a mold, in addition to calculating the number of cavities per mold, it is necessary to consider its service life. Batch size is small, the requirements of the steel can be lower; and large quantities, you must choose high-quality steel, to ensure the service life and try to avoid duplication of production mold.
(2) dimensional accuracy of plastic parts: plastic parts of the dimensional accuracy, largely depends on the mold manufacturing accuracy, it also determines the qualified rate of parts. For high precision (SJ / T10628-95 3,4 grade accuracy) and ultra-high precision (SJ / T50628-95 1,2 level accuracy) of the plastic parts, should use high-quality die steel.
(3) the complexity of parts: the more complex parts, cavity processing more difficult, and therefore must use good cutting performance steel. Parts of high complexity, the size of large, more processing parts. So the stress deformation of the processing must be considered.
(4) the size of parts: the greater the workpiece, the greater the amount of cutting the cavity. With cutting the amount of knife cutting, the cutting stress is also large. Therefore, for the largest mold of the best choice of cutting steel. Parts of the hour, small cavity size, the use of the tool (mainly milling cutter) low strength, the amount of cutting is very small. Selection of steel should be selected uniform texture, alloy carbide distribution of fine and symmetrical steel. Small molds for pre-hardening and then processing, to consider the possibility of processing.
(5) the appearance of parts requirements: plastic parts such as the appearance of decorative pieces, the surface quality of good or bad can greatly affect the product sales. Where the plastic parts of the appearance of high demand plastic parts, the best choice of vacuum melting or electroslag smelting steel, in order to achieve the best cavity polishing effect.
◆ Polish polishing: the production of transparent plastic products, especially optical instruments, for the molding mold polishing (ie, mirror processing) requirements are high. Factors that affect the performance of the mirror are:
(1) steel metallurgical quality: not easy to use steel. VAD, VAR, ESR and other processes, can improve the purity of steel, thereby enhancing the polishing performance.
(2) Tissue nonuniformity: the effect of microscopic segregation. The measure is to master the tapping technology and the ingot after the casting quality; full forging to ensure the density; are heat treatment or ESR.
(3) Hardness: high hardness is conducive to mirror polishing. Heat-treated surface carburizing or decarburization results in deterioration of surface quality, affecting polishing performance.
(4) polishing technology: to avoid the process of polishing the surface contact stress is too high. Otherwise the softer base will be eroded, leaving the carbide free, and then leaving a defect on the surface of the steel.
6, the hardness of steel
(HB), Rockwell (HRC), Vickers (HV), and Richard (HL) hardness, among which HB and HRC are more commonly used.
● HB wide range of applications, supply status commonly used, Cu, Al is also available. HRC is suitable for characterizing high hardness materials such as heat treatment hardness. The difference between the two is different from the hardness of the probe, Brinell hardness of the probe for the ball, and Rockwell hardness of the probe for the diamond.
● HB and HRC are interchangeable under certain conditions. The conversion formula can be roughly recorded as: 1HRC ≈ 1 / 10HB.
● HV - for microanalysis, Cu, Al are also available.
● HL-portable hardness tester, easy to measure, but the thickness of the sample requirements.
8, hardenability and hardenability hardenability depends mainly on the chemical composition of steel and quenching before the original state of the organization; hardening depends mainly on the carbon content of steel. For most of the cold work die steel, hardenability is often one of the main considerations. For hot work die steel and plastic mold steel, the general mold size, especially the manufacture of large mold, the hardenability is more important. In order to reduce the quenching deformation, it is often possible to use a quenching medium having a low cooling capacity, such as air cooling, oil cooling or salt bath cooling, in order to obtain a desired hardness and hardened layer for various molds which are complicated in shape and are liable to generate heat treatment deformation. Depth, you need to use a better hardenability of the mold steel.