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What Are The Main Factors That Affect The Die Shrinkage Rate During Precision Injection Molding?
May 10, 2018

What are the main factors that affect the die shrinkage rate during precision injection molding?

Precision injection molding must be based on the fact that the mold can be made according to the required size. However, even if the die size is fixed, the actual size of the product varies depending on the actual shrinkage.

Therefore, in the precision injection molding, the control of the shrinkage rate is very important. The appropriate or not the shrinkage rate of the mold design varies with the different batch of resin. If the pigment is changed, the shrinkage rate is also different. Because the molding machine is different, the setting of forming conditions, the reproducibility and the movements of each molding cycle fluctuate, and the actual shrinkage rate fluctuates, so the control of shrinkage is difficult.

The main factors affecting the shrinkage rate

The die size can be calculated by the product size plus shrinkage rate, so the main factors of shrinkage should be taken into account when designing the die. These effects vary depending on the resin and molding conditions.

The main factors affecting the shrinkage rate are as follows:

1. Mould temperature

Whether the non crystalline resin or crystalline resin, the mold temperature is high, the shrinkage rate is bigger. Precision molding should mold temperature to maintain at a specific temperature. Attention must be paid to the design of cooling circuit in mould design.

2, gate area

In general, shrinkage is also changed when the gate area is changed. Shrinkage rate decreases with increasing gate size, which is related to the fluidity of resin.

3, enhance the content of material

When the glass fiber reinforced resin is used, the more glass fiber is added, the smaller the shrinkage rate is, the shrinkage rate of the flow direction is smaller than the transverse shrinkage, and the difference of the resin is larger. In order to prevent the distortion of the warping, the position of the gate and the number of gate must be taken into consideration.

4, directionality

Although there are great differences in orientation, there is directivity in all resins. The directivity of crystalline resin is very large, which is different due to wall thickness and molding conditions.

5. Wall thickness of products

The thickness of the product wall also affects the yield. For the non crystalline resin, the influence of the resin on the shrinkage of the wall thickness is different, the wall thickness is large, the shrinkage rate is large, and contractile rate becomes smaller. For crystalline resins, wall thickness must be avoided. In the case of multi cavity modes, if the wall thickness of the die cavity is different, the shrinkage rate will also be different.

6, resin pressure

Resin pressure has a great influence on shrinkage rate. If resin pressure is large, shrinkage rate becomes smaller and product size is larger. Even with the first mock exam cavity, resin pressure because of the product shape varies, resulting in differences of shrinkage. In the case of multi cavity mold, the resin pressure in each mold cavity is easily different, and the shrinkage rate of each cavity is different.

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