Mold selection
To make precision injection molds, mold steel with small deformation (micro-deformed steel) must be used as much as possible. Through metallographic analysis of the severely deformed mold, it was found that the steel used for manufacturing the mold contained a large amount of eutectic carbides distributed in blocks and bands. Since the expansion coefficient and elastic modulus of the carbides in this steel are significantly smaller than the matrix (where the expansion coefficient is about 30% lower than that of the matrix), it is necessary to choose mold steels with less carbide segregation when manufacturing sophisticated molds.


Mold structure and size design
Uneven or unreasonable structure thickness not only easily leads to uneven force and strain of the mold, but also easily causes different thermal and organizational stresses between various parts of the mold due to heating, which makes the volume expansion of each part different. Therefore, when designing an injection mold, the gap between the thickness of the mold should be minimized, a symmetrical structure should be adopted, and a smooth transition structure design should be adopted at the junction of thickness and thinness. In addition, the elastic deformation of the mold itself and the size of the cavity will affect the accuracy of the mold. In the traditional compression mold design, the cavity size design only mainly considers the molding shrinkage rate of the product, and generally does not take the mold itself. The elastic deformation is taken into account. Although simply increasing the mold wall thickness can reduce the mold's own elastic deformation caused by force and heat, the mold size design also needs to consider the mold installation, ease of processing and cost (including processing costs and material costs).


Mold manufacturing process
(1) Heat treatment heating process When the mold is heated, the temperature of each part will not be uniform even in the same mold. When the complex mold is heated, it should be heated slowly below the phase transition point. Generally, the deformation caused by vacuum heat treatment is much smaller than that of salt bath furnace heating and quenching. The quenching heating temperature is directly proportional to the grain size of the steel. Larger grains can increase the hardenability of the steel, leading to greater internal stress during quenching and cooling. In addition, because complex molds are generally made of medium and high alloy steel, too high temperature during quenching will increase the amount of retained austenite in the structure due to the low Ms point (martensite transformation starting temperature), which will increase the deformation of the mold after heat treatment. . The residual stress generated during machining of the mold and the stress generated by quenching will be superimposed together, thereby increasing the deformation of the mold after heat treatment. Therefore, after the rough machining of the mold and before the semi-finish machining, an annealing treatment should be carried out to remove the stress; at the same time, the quenching temperature should be reduced, and staged quenching should be used to reduce the residual stress after quenching.
(2) Machining process Grinding is a key process of injection mold processing. In order to prevent the occurrence of grinding deformation and grinding cracks (even micro-cracks), the choice of grinding wheel and feed rate, as well as the clamping of the workpiece, should be considered when formulating the precision grinding process.