High-end molds must be based on the premise that molds can be manufactured according to the required size, but even if the size of the mold is constant, the actual size of the product will change due to actual shrinkage. Therefore, in precision injection molding, shrinkage control is very important.
Whether the high-end mold design is appropriate determines the shrinkage rate, and it also varies with resin batches. If you change the pigment, the shrinkage will also change. Depending on the molding machine, molding conditions, reproducibility, and behavior settings for each molding cycle, the actual shrinkage rate also changes, so it is difficult to control the shrinkage rate.
The size of the mold can be obtained by adding the product size to the shrinkage rate. Therefore, when designing high-end molds, the main factors affecting the shrinkage rate of the plastic mold need to be considered: resin pressure, resin temperature, mold temperature, gate cross-sectional area, injection time , Cooling time, product wall thickness, reinforcement material, orientation, injection speed, resin pressure.
1. Resin pressure: has a great influence on shrinkage. 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 will vary with the shape of the product, leading to differences in shrinkage. In the case of a multi-cavity mold, the resin pressure in each cavity may be different, and therefore, the shrinkage rate of each cavity is also different.
2. 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 keep the mold temperature at a specific temperature. When designing high-end molds, attention must be paid to the design of the cooling circuit.
3. Gate cross-sectional area: Generally speaking, when the cross-sectional area of ​​the gate changes, the shrinkage rate will also change. The larger the gate size, the smaller the shrinkage, which is related to the fluidity of the resin.
4. Wall thickness of high-end mold products: The thickness of the product wall will also affect the output. For amorphous resins, the shrinkage of the resin has a different tendency to the wall thickness. The greater the wall thickness, the greater the shrinkage rate, on the contrary, the shrinkage becomes smaller. For crystalline resins, it is necessary to avoid excessive wall thickness changes. In the case of a multi-cavity mold, if the cavity wall thickness is different, the shrinkage rate will also be different.
5. Reinforced material content: When using glass fiber reinforced resin, the more glass fiber added, the smaller the shrinkage rate. The shrinkage in the flow direction is smaller than the transverse shrinkage, and the difference is greater depending on the resin. In order to prevent twisting, flying and warping, the shape, location and number of gates must be considered.
6. Orientation: Although the orientation is very different, all resins have orientation. The orientation of crystalline resin is particularly large, and there are differences due to wall thickness and molding conditions.