Factors Influencing Precision Injection Mold Precision

1. Selection of mold materials

To make precision injection molding molds, mold steel with small deformation (micro-deformed steel) is the best choice. Through the analysis of the severely deformed mold, it was found that the steel used to make 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 the matrix), it is necessary to choose mold steels with less carbide segregation when manufacturing sophisticated molds.

2. Mold structure and size design

Uneven or unreasonable structure thickness will not only easily leads to uneven force of the injection mold, but also easily causes different thermal and organizational stresses between various parts of the mold due to heating, then the volume expansion of each part is different. Therefore, when designing an injection mold, the thickness gap between the molds should be minimized. For example, using symmetrical structure, designing smooth transition structure at the junction of thickness and thinness part of the mold.

In addition, the elastic deformation of the mold itself and the size of the cavity will affect the accuracy of the mold. In traditional injection mold design, the cavity size design mainly considers the molding shrinkage rate of the product, and generally does not take the elastic deformation of the mold itself into consideration. Although increasing the wall thickness of the mold can reduce the molds' own elastic deformation caused by force and heat. But when you determine the size of the mold you also need to consider the installation, processing difficulties and the cost (including processing costs and material costs) of the mold.

3. Mold manufacturing process

(1) Heat treatment heating process. When the plastic injection mold and molding is heated, even in the same mold, the temperature of each part will not be uniform. When the precision 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. The larger grains can increase the hardenability of the steel, leading to greater internal stress during quenching and cooling. Therefore, after the rough machining of the mold and before the semi-finish machining, 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 technology. Grinding is a key process of precision injection molding. In order to prevent the occurrence of grinding deformation and grinding cracks (even micro-cracks), the choice of grinding wheel and feed rate should be considered when formulating precision grinding processing technology.