What is sink mark?
Dent and sink mark are caused by local shrinkage caused by gate sealing or lack of material injection.
The depression or micro depression on the surface of injection molded products is an old problem in the process of injection molding. Dents are generally caused by the increase of the shrinkage rate of plastic products caused by the increase of the wall thickness. They may appear near the external sharp corner or the sudden change of the wall thickness, such as the bulge, the back of the stiffener or the support, and sometimes in some uncommon parts. The root cause of the dent is the thermal expansion and cold shrinkage of the material, because the coefficient of thermal expansion of thermoplastic is quite high. The degree of expansion and shrinkage depends on many factors, among which the plastic properties, the maximum and minimum temperature range and the cavity packing pressure are the most important factors, and the size and shape of the injection parts, the cooling rate and uniformity are also the influencing factors. Dents and shrinkage marks on plastic products are common molding defects. As shown in the figure:
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 material supply is insufficient, the surface of the plastic part will appear concave or orange peel like micro unevenness. Therefore, it is necessary to increase the injection pressure and injection speed, increase the compression density of the melt, extend the injection and holding time, compensate the melt shrinkage and increase the injection recoil. But the packing pressure should not be too high, otherwise it will cause convex mark.
1. If the depression and shrinkage mark occur near the gate, it can be solved by extending the holding time. The cooling time of the plastic part in the mold should be extended appropriately when the plastic part is concave at the wall thickness.
2. If the local shrinkage of the melt around the insert causes depression and shrinkage mark, which is mainly caused by the low temperature of the insert, we should try to increase the temperature of the insert.
3. If the nozzle hole of the injection molding machine is too small or the nozzle is partially blocked, the local loss of injection pressure will also cause depression and shrinkage mark. In this regard, the nozzle should be replaced or cleaned.
4. If the surface of plastic parts is depressed due to insufficient feeding, the feeding amount should be increased.
In addition, the cooling of 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 temperature of the cooling water, or the cooling of the concave parts can be properly strengthened on the premise of maintaining the uniform cooling of the mold surface and all parts as far as possible. Otherwise, when the plastic part is demoulded under the condition of insufficient cooling, it is not only easy to produce shrinkage depression, but also local depression of the plastic part in the ejector due to hard demoulding.
2) Mold defects
If the cross-section of runner and gate is too small, the mold filling resistance is too large, the gate setting is asymmetric, the mold filling speed is not balanced, the position of feed gate is not reasonable, and the mold exhaust affects the feeding, feeding and cooling, or the mold wear causes the pressure relief, it will lead to the surface depression and shrinkage mark of plastic parts. Therefore, the gate and gate cross-section should be appropriately expanded according to the specific situation The gate position 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. Therefore, the structure size of the gating system should be appropriately expanded, especially for the “bottleneck” which hinders the melt flow, 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 parts and are easy to produce residual deformation at the gate after molding, a wing body can be attached to the plastic parts, and then the gate can be set on the winglet. The gate on the winglet can use the reverse gate and the point gate, so as to transfer the concave defects of the plastic parts to the winglet, and then cut off the winglet after molding.
In addition, it is necessary to frequently check whether there is wear relief or poor exhaust of the mold, and timely replace the consumable parts in the mold or improve the exhaust condition of the mold.
3) The raw material does not meet the molding requirements
1. If the shrinkage rate of molding raw materials is too high or the flow performance is too poor, and the lubricant in the raw materials is insufficient or the raw materials are wet, the surface of the plastic parts will produce depressions and shrinkage marks. Therefore, for plastic parts with high surface requirements, low shrinkage resin grade should be selected as far as possible.
2. If the underfill depression is caused by the 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 increase the structural size of the gating system.
3. If the surface of plastic parts is depressed due to the moisture of raw materials, the raw materials should be pre dried.
4) Unreasonable shape and structure design of plastic parts
If the wall thickness of plastic parts varies greatly, the thick wall parts are prone to depression and shrinkage marks 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 the wall thickness of plastic parts is different, it can be solved by adjusting the structural parameters of the gating system.
About JasonMould Industrial Company Limited
Jasonmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.
Person: James Yuan
Company: JasonMould Industrial Company Limited
Add: LongGang Village,LongXi Town,BoLuo County,HuiZhou City,GuangDong Province, China
Email: [email protected]