According to the principle of material solidification, the reason for the formation of internal shrinkage holes and surface shrinkage of the injection molded part is that the melt needs to continuously shrink during the cooling process. When the shrinkage is concentrated to the position where the injection molded part is finally solidified, and the melt cannot be supplemented, It will cause the problem of internal concentrated shrinkage holes or surface shrinkage.
There are two forms of cooling shrinkage for injection molded parts: internal concentrated shrinkage and external overall shrinkage (shrinkage from the periphery of the injection molded part to the inside), and internal concentrated shrinkage is the motive force for forming shrinkage holes. Since the sum of the two shrinkages is equal to the total shrinkage of the injection molded part, when the overall external shrinkage increases, the amount of internal shrinkage will decrease, then the power to form the internal shrinkage cavity and the resulting diameter will decrease; When the amount of external overall shrinkage decreases, the amount of internal concentrated shrinkage will increase, and the greater the power and diameter to form internal shrinkage holes.
Therefore, to solve the problem of internal shrinkage of transparent plastic parts, we must try to increase the amount of external overall shrinkage, try to reduce the internal shrinkage of injection molded parts, and at the same time supplement the shrinkage as much as possible to reduce the shrinkage of injection molded parts. Under the space, the purpose of reducing shrinkage holes is achieved.
To achieve these requirements, conventional injection molding processes such as increasing injection pressure and increasing injection time cannot achieve this effect. Generally, it can only solve the problem of shrinkage of transparent plastic parts to a lesser degree. For serious shrinkage problems There is nothing you can do, especially for the shrinkage of PC parts.
The reason why the shrinkage problem of PC parts is difficult to solve is mainly due to the solidification characteristics of PC materials-the speed of cooling and hardening is fast.
First of all, because the PC material needs a large amount of melt to supplement during the solidification process, the gate is easily blocked due to the fast cooling speed, so that the injection molded parts cannot be compensated by the external melt, thus leaving a large It can become the shrinkage space of shrinkage hole.
Importantly, because the outer shell hardens quickly, the appearance has become a hard shell, and the inside is still not solidified completely. Due to the supporting effect of the rapid hardening of the shell (the hardening degree of ABS and other transparent materials at high temperature is much worse than that), it hinders the overall shrinkage of the injection molded parts, so that the shrinkage of the centralized shrinkage is much greater than the overall shrinkage. Moreover, according to the formation mechanism of shrinkage cavities and depressions, the slower the surface hardening, the easier it is to cause depressions. On the contrary, the faster the surface hardening, the easier it is to cause shrinkage cavities.
Combining the two factors, the formation of internal concentrated shrinkage holes has become an inevitable trend for PC parts, and it is full of power, which is incomparable to other transparent materials. Therefore, the shrinkage of PC injection molded parts has become a problem in injection molding.
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