Precise injection molding must be based on the premise that the mold can be made to the required size. However, even if the mold size is fixed, the actual size of the product will vary depending on the actual shrinkage. Therefore, in precision injection molding, the control of shrinkage is very important.
The appropriateness of the mold design controls the shrinkage rate, which 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 operation of each molding cycle fluctuate, and the actual shrinkage rate fluctuates. Therefore, it is difficult to control shrinkage.
1.Main factors affecting shrinkage
The size of the mold can be obtained by the product size plus the shrinkage rate, so the main factors of the shrinkage rate need to be considered in the mold design: resin pressure resin temperature mold temperature gate cross-sectional area injection time cooling time product wall thickness reinforcement material directional injection speed resin Pressure resin pressure has a great influence on the shrinkage rate. If the resin pressure is large, the shrinkage rate becomes small, and the product size is large. Even in the same cavity, the resin pressure varies depending on the shape of the product, so there is 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.
Mold temperature Whether it is non-crystalline resin or crystalline resin, if the mold temperature is high, the shrinkage rate will become larger. For precision molding, the mold temperature should be maintained at a specific temperature. When designing the mold, attention must be paid to the design of the cooling circuit. Gate cross-sectional area Generally speaking, when the gate cross-sectional area 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. . Product wall thickness Product wall thickness also affects the yield. For non-crystalline resins, the shrinkage rate of the resin has a different influence on the wall thickness. The larger the wall thickness, the larger the shrinkage rate. On the contrary, the shrinkage rate becomes smaller. For crystalline resins, it is necessary to avoid particularly large changes in wall thickness. In the case of multi-cavity molds, if the cavity wall thickness is different, the shrinkage rate will also be different. Reinforcing material content When Borui fiber reinforced resin is used, the more glass fiber is added, the smaller the shrinkage rate is, and the shrinkage in the flow direction The rate is smaller than the lateral shrinkage rate, and the difference is larger according to the resin. In order to prevent twisting and warping, the gate shape flying gate position and gate number must be considered. Orientation Although there is a large difference in orientation, there is orientation for all resins. The orientation of the crystalline resin is particularly large, and varies depending on the wall thickness and molding conditions. In addition, there is shrinkage after molding, the main factors are: internal stress relaxation and crystallization temperature and humidity
2.Possible measures
Runner, gate balance As mentioned above, the shrinkage rate varies with resin pressure. In the case of single-cavity molds with multiple gates and multi-cavity molds, to fill the mold in the same way, gate balance is necessary. Resin flow is related to the flow resistance in the runner, so it is best to balance the runner before taking the gate to balance. In order to make the setting of molding conditions easy, it is necessary to pay attention to the cavity arrangement. Since the molten resin introduces the heat into the mold, in the case of the general mold cavity arrangement, the mold temperature distribution is concentric with the gate as the center. Therefore, when selecting the cavity arrangement of the multi-cavity mold, it is necessary to take the balance of the runner and the concentric arrangement with the gate as the center.
3.Prevent molding deformation
The reason for molding deformation is that there is internal stress under uneven shrinkage, so it is necessary to prevent uneven shrinkage. In the case of a circular product with a hole in the center of the gear, a gate must be provided in the center. However, when there is a large difference between the shrinkage of the resin in the flow direction and the vertical direction, it has the disadvantage of ellipse. When a higher degree of roundness is required, a 3 or 6 gate is required. But need to pay full attention to the balance of each gate. When the side gate is used, the 3-point gate will increase the inner diameter of the cylindrical product. When the gate mark is not allowed on the outer surface and the end surface, the inner multi-point gate is rarely used, and good results can be obtained.
