What are the main factors affecting mold shrinkage?

2021-03-23 12:55:23 admin

What are the main factors affecting mold shrinkage?






  Precision injection molding must be based on the premise that the mold can be made according to the required size. However, even if the mold size is constant, the actual size of the product varies due to the actual shrinkage. Therefore, in precision injection molding, the control of the shrinkage rate of plastic mold processing is very important. The appropriateness of the mold design governs the shrinkage rate, and it also varies with different resin batches. If the pigment is changed, the shrinkage rate will also vary. Depending on the molding machine, the setting of molding conditions, reproducibility, and the behavior of each molding cycle fluctuate, and the actual shrinkage rate fluctuates. Therefore, it is difficult to control the shrinkage.




   The main factors affecting the shrinkage rate:




  The size of the mold can be calculated by adding the size of the product to the shrinkage rate, so when designing the mold, the main factor of the shrinkage rate needs to be considered. These effects vary with changes in items such as resin and molding conditions.




   The main factors that affect the molding shrinkage rate are:




   1. Mold temperature




   Whether it is non-crystalline resin or crystalline resin, if the mold temperature is high, the shrinkage rate will increase. Precision molding must maintain the mold temperature at a specific temperature. When designing the mold, attention must be paid to the design of the cooling circuit.




  2, gate cross-sectional area




   Generally speaking, when the cross-sectional area of the gate is changed, the shrinkage rate also changes. The shrinkage rate becomes smaller as the gate size becomes larger, which is related to the fluidity of the resin.




  3, reinforcement material content




When glass fiber reinforced resin is used, the more glass fiber is added, the smaller the shrinkage rate of plastic mold processing. The shrinkage rate in the flow direction is smaller than the lateral shrinkage rate. According to the resin, the difference is larger. In order to prevent warping, it must Consider gate shape, gate location and gate number.




   4. Directionality




   Although there is a big difference in orientation, there is orientation for all resins. The orientation of crystalline resin is particularly large, and there are differences due to wall thickness and molding conditions.




  5, product wall thickness




  The thickness of the product wall also affects the yield. For non-crystalline resins, the shrinkage of the resin has a different tendency to affect the wall thickness. The larger the wall thickness, the higher the shrinkage, and vice versa, the shrinkage becomes smaller. For crystalline resins, extreme wall thickness changes must be avoided. In the case of a multi-cavity mold, if the cavity wall thickness is different, the shrinkage rate will also be different.




  6, resin pressure




   The resin pressure has a great influence on the shrinkage rate. If the resin pressure is large, the shrinkage rate becomes smaller and the product size becomes larger. Even in the same cavity, the resin pressure varies depending on the shape of the product, resulting in a difference in shrinkage. In the case of a multi-cavity mold, the resin pressure in each cavity is likely to be different, and as a result, the shrinkage rate of each cavity is also different.