how to deal with sink marks

Sink marks are caused by local contraction after gate sealing or injection.

It is an old problem in the process of injection molding that the surface of injection molded parts is concave or tiny. Dents are generally caused by the local increase of shrinkage rate caused by the increase of the wall thickness of plastic products. They may appear near the external sharp corners or abrupt changes in wall thickness, such as bulges, stiffeners or the back of supports, and sometimes in some unusual parts.
The root cause of dents is the thermal expansion and cold contraction of the material, because the coefficient of thermal expansion of thermoplastic is quite high. The degree of expansion and contraction depends on many factors, among which the properties of plastics, the maximum and minimum temperature range and the cavity packing pressure are the most important factors, as well as the size and shape of injection molded parts, cooling rate and uniformity. Depressions and shrinkage marks on plastic products are common molding defects.
Analysis and elimination of sink mark defects
1) Improper control of molding conditions
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 demoulding is too high, the temperature at the insert is too low or the feeding is insufficient, the surface of the plastic part will appear concave or orange peel like fine uneven. Therefore, it is necessary to increase the injection pressure and injection speed, increase the compression density of the melt, prolong the injection and holding time, compensate the melt shrinkage and increase the injection back impulse. But the holding pressure should not be too high, otherwise it will cause bumps.
If the depression and shrinkage mark occur near the gate, it can be solved by prolonging the holding time. The cooling time of the plastic part in the mold should be prolonged when the plastic part has a depression at the wall thickness.
If the local shrinkage of melt around the insert results in depression and shrinkage mark, which is mainly caused by the low temperature of the insert, it is necessary to increase the temperature of the insert.
If the nozzle hole of the injection molding machine is too small or the nozzle is locally blocked, the local loss of injection pressure will also cause depression and shrinkage marks. Therefore, the nozzle should be replaced or cleaned.
If the surface depression of plastic parts is caused by insufficient feeding, the feeding amount should be increased.
In addition, the cooling of the plastic parts in the mold must be sufficient. On the one hand, the melt temperature can be appropriately reduced by adjusting the barrel temperature; on the other hand, the setting of the mold cooling system can be changed to reduce the cooling water temperature, or on the premise of keeping the die surface and all parts of the die evenly cooled, the concave parts can be properly cooled.
Otherwise, when the plastic part is demoulded under the condition of insufficient cooling, it is not only easy to produce shrinkage depression, but also can cause local depression in the ejector pin due to hard demoulding.
2) Mold defects
If the cross-section of runner and gate is too small, the resistance of filling is too large, the setting of gate is asymmetric, the speed of filling is not balanced, the position of feed gate is not reasonable, and the bad exhaust of mold affects the feeding, feeding and cooling, or the wear of mold causes pressure relief, all of these will lead to the depression and shrinkage mark on the surface of plastic parts. Therefore, the cross-section of gate and runner should be appropriately expanded according to the specific situation The gate should be set at the symmetrical position as far as possible, and the feeding gate should be set at the thick wall of the plastic part.
If the depression and shrinkage mark occur far away from the gate, it is generally due to the poor flow of molten material in a certain part of the mold structure, which hinders the pressure transmission. For this reason, the structure size of the gating system should be enlarged appropriately, especially for the "bottleneck" which hinders the flow of molten metal, the runner section must be increased, and it is better to extend the runner to the concave part.
For thick wall plastic parts, wing gate should be preferred. In this way, for the plastic parts which are not suitable to set the gate directly on the plastic part and easy to produce residual deformation at the gate after molding, a wing shape can be attached to the plastic part, and then the gate can be set on the small wing. The gate on the small wing can be inverted gate and point gate, so as to transfer the concave defect of the plastic part to the wing, and then cut off the wing after the plastic part is formed.
In addition, we should often check whether there is wear relief or poor exhaust of the die, timely replace the consumable parts in the mold or improve the exhaust conditions of the mold.
3) The raw material does not meet the molding requirements
If the shrinkage rate of the molding material is too large or the flow performance is too poor, and the lubricant in the raw material is insufficient or the raw material is wet, the surface of the plastic part will produce depression and shrinkage marks. Therefore, for the plastic parts with high surface requirements, low shrinkage resin grade should be selected as far as possible.
If under injection depression is caused by poor flow of molten metal, proper amount of lubricant can be added to the raw material to improve the fluidity of molten material or increase the structural size of gating system.
If the surface of plastic parts is sunken due to the moisture of raw materials, the raw materials should be pre dried.
4) The structure design of plastic parts is unreasonable
If there is a big difference in the wall thickness of the plastic parts, it is easy to produce depressions and shrinkage marks in the thick wall parts due to the lack of pressure. Therefore, the wall thickness should be consistent as much as possible when designing the structure of plastic parts. For special cases, if the wall thickness difference of plastic parts is large, it can be solved by adjusting the structural parameters of gating system.