Analysis and elimination methods for injection molding dents and shrinkage defects

Dents and shrinkage marks are caused by local internal contraction caused by gate sealing or injection of missing materials.
The occurrence of dents or micro dents on the surface of injection molded products is an old problem during the injection molding process. Dents are generally caused by a local increase in the shrinkage rate of plastic products caused by an increase in wall thickness. They may appear near external sharp corners or sudden changes in wall thickness, such as protrusions, reinforcing ribs, or behind supports, and sometimes also in some uncommon areas. The root cause of dents is the thermal expansion of materials, because the coefficient of thermal expansion of thermoplastics is quite high. The degree of expansion and contraction depends on many factors, among which the performance of plastic, the maximum and minimum temperature range, and the cavity pressure are the most important factors, as well as the size and shape of injection molded parts, cooling rate, and uniformity. Dents and shrinkage marks on plastic products are common molding defects. As shown in the figure:
Analysis and elimination methods for dents and shrinkage defects
1、 Improper control of forming conditions
1. If the injection pressure is too low, the injection and holding time is too short, the injection rate is too slow, the material temperature and mold temperature are too high, the cooling of the plastic part is insufficient, the temperature during demolding is too high, the temperature at the insert is too low, or the supply is insufficient, all of these can cause the surface of the plastic part to have small concave or orange peel like unevenness. In this regard, it is necessary to appropriately increase the injection pressure and injection speed, increase the compression density of the melt, extend the injection and pressure holding time, compensate for melt shrinkage, and increase the injection backlash. But the pressure should not be too high, otherwise it will cause protrusions.
2. If dents and shrinkage marks occur near the gate, they can be solved by extending the holding time. When the plastic part produces a depression at the wall thickness, the cooling time of the plastic part in the mold should be appropriately extended.
3. If dents and shrinkage marks are caused by local shrinkage of the melt around the insert, which is mainly due to the low temperature of the insert, efforts should be made to increase the temperature of the insert.
4. If the nozzle hole of the injection molding machine is too small or the nozzle is locally blocked, it can also cause dents and shrinkage marks due to excessive local loss of injection pressure. For this, the nozzle should be replaced or cleaned.
5. If the surface of the plastic part is dented due to insufficient supply, the supply amount should be increased.
In addition, the cooling of the plastic parts inside the mold must be sufficient. On the one hand, the temperature of the material barrel can be adjusted to appropriately reduce the melting temperature; On the other hand, it is possible to change the setting of the mold cooling system, reduce the temperature of the cooling water, or strengthen the cooling of the depressed areas while maintaining uniform cooling of the mold surface and various parts as much as possible. Otherwise, if the plastic part is demolded under insufficient cooling conditions, it is not only easy to produce shrinkage and depression, but also can cause local depression on the top rod of the plastic part due to hard demolding.
2、 Mold defect
1. If the flow channel and gate section of the mold are too small, the mold filling resistance is too large, the gate setting is asymmetric, the mold filling speed is uneven, the feeding port position is not set properly, and the poor exhaust of the mold affects the feeding, feeding and cooling, or the mold wear causes pressure relief, all of which can cause dents and shrinkage marks on the surface of the plastic part. Therefore, based on the specific situation, the gate and gate section should be appropriately expanded, and the gate position should be set at the symmetrical position as much as possible, The feeding port should be set at the thick wall of the plastic part.
2. If dents and shrinkage marks occur far from the gate, it is generally due to poor flow of molten material in a certain part of the mold structure, which hinders pressure transmission. In this regard, the structural size of the mold pouring system should be appropriately expanded, especially for the "bottleneck" that hinders the flow of molten material. It is necessary to increase the cross-section of the runner, and it is best to extend the runner to the area where the depression occurs.
For thick walled plastic parts, wing gate should be preferred. In this way, for plastic parts that are not suitable for directly setting the gate on the plastic part and are prone to residual deformation at the gate after forming, a wing shape can be attached to the plastic part, and the gate can be set on the small wing. The gates set on the small wing can use inverted gates and point gates, thereby transferring the concave defects of the plastic part to the small wing. After the plastic part is formed, the small wing can be cut off.
In addition, the mold should be regularly checked for wear, pressure relief, or poor exhaust, and easily worn parts in the mold should be replaced in a timely manner or the exhaust conditions of the mold should be improved.
3、 The raw materials do not meet the molding requirements
1. If the shrinkage rate of the forming raw material is too high or the flow performance is too poor, as well as the lubricant inside the raw material is insufficient or the raw material is damp, it will cause dents and shrinkage marks on the surface of the plastic part. Therefore, for plastic parts with high surface requirements, resin grades with low shrinkage should be selected as much as possible.
2. If insufficient injection depression is caused by poor flow of the molten material, an appropriate amount of lubricant can be added to the raw material to improve the fluidity of the molten material, or the structural size of the pouring system can be increased.
3. If the surface of the plastic part is dented due to the moisture of the raw material, the raw material should be pre dried.
4、 Unreasonable structural design of plastic parts
If the wall thickness of the plastic part varies greatly from place to place, the thick wall area is prone to dents and shrinkage marks during molding due to insufficient pressure. Therefore, when designing the shape and structure of plastic parts, the wall thickness should be as consistent as possible. For special cases, if there is a significant difference in the wall thickness of the plastic part, it can be solved by adjusting the structural parameters of the pouring system.