Process tips for shrinkage problems of thick-walled injection molded parts

The shrinkage problem of plastic parts (surface shrinkage and internal shrinkage) is caused by the defect of insufficient melt replenishment when the thicker parts are cooled. We often encounter the situation that no matter how to increase the pressure, increase the water inlet, and extend the injection time, the shrinkage problem just can't be solved.

Among the commonly used raw materials, due to the fast cooling speed, the shrinkage problem of PC material can be the most difficult to solve, and the shrinkage concave and shrinkage problem of PP material is also more difficult to deal with.

Therefore, when it comes to thick and large pieces of more serious shrinkage problems, it is necessary to take some unconventional injection techniques, otherwise, it is difficult to solve the problem. In practical production, we have found a set of effective techniques to cope with this difficult problem of injection molding.



First of all, under the premise of ensuring that the molded parts are not deformed, we adopt the method of shortening the cooling time as much as possible, so that the molded parts can be released early under high temperatures. At this time, the temperature of the outer layer of the injection molded part is still very high, the skin is not too hardened, so the temperature difference between inside and outside is not very large, which is conducive to the overall shrinkage, thus reducing the concentration of shrinkage inside the injection molded part. Since the overall shrinkage of the injection molded part is constant, the more the overall shrinkage, the smaller the concentrated shrinkage, the internal shrinkage, and surface shrinkage can be reduced.

Next, if we want to solve the problem of shrinkage, we will discuss it in the following section. As for the improvement of the surface shrinkage, after the injection molded part is released from the mold at high temperature, it is different from the solution of shrinkage.

The shrinkage problem arises because the mold surface is warmed up, the cooling capacity decreases, the surface of the injection molded parts just solidified is still soft (unlike PC parts out of the mold surface hard, very easy to produce shrinkage), the internal shrinkage is not completely eliminated due to the formation of a vacuum, resulting in the surface of the injection molded parts compressed inward under the pressure of atmospheric pressure, coupled with the role of shrinkage force, the shrinkage problem is thus created. The slower the surface hardening speed, the easier it is to produce shrinkage, such as PP material, and vice versa, the easier it is to produce shrinkage.

Therefore, after the injection molded part is released early, it should be properly cooled so that the surface of the injection molded part can maintain a certain degree of hardness so that it is not easy to produce shrinkage. However, if the shrinkage problem is more serious, moderate cooling will not be able to eliminate it, we must take the method of freezing water cooling so that the surface of the injection molded parts can be quickly hardened to prevent shrinkage, but the internal shrinkage will still exist. For soft materials such as PP, due to the vacuum and shrinkage force, the injection molded parts may have shrinkage, but the degree of shrinkage has been greatly reduced.

In addition to the above measures, if we adopt the method of extending the injection time instead of cooling time, the improvement of surface shrinkage and even internal shrinkage will be better.

In solving the problem of shrinkage, because the mold temperature is too low will aggravate the degree of shrinkage, so the mold is best to use machine water cooling, do not use frozen water, if necessary, but also to increase the mold temperature, such as injection molding PC material mold temperature to 100 degrees, the improvement effect of shrinkage will be better. But if it is to solve the problem of shrinkage, the mold temperature should not be raised, but need to be lowered.

Finally, sometimes the above methods may not be able to completely solve the problem, but there has been a big improvement, if you must completely solve the problem of surface shrinkage, the appropriate amount of shrinkage prevention agent is also the last resort to have a better solution. Of course, transparent parts can not do so.

If there are still shrinkage marks on the surface of thick-walled parts, or if you encounter off-wall plastic parts, then the introduction of gas-assisted injection molding will be the solution.

Gas-assisted injection molding is a novel plastic molding technology that introduces high-pressure gas into the thick-walled part of the part to create a hollow section inside the injection molded part, completely filling the process, achieving gas pressure retention, and eliminating the shrinkage marks of the product.

The traditional injection molding process cannot combine the thick and thin walls together, and the parts have high residual stress, are easy to warp and deform, and sometimes have shrinkage marks on the surface. The newly developed gas-assisted technology can successfully produce thick-walled and off-walled products by hollowing out the inside of thick walls, and the products have excellent surface properties in appearance and low internal stress. Lightweight and high strength.

Now we have developed successful gas-assisted product structure and mold design including pouring system, air inlet method and air channel distribution design technology, gas-assisted injection molding process design technology, gas-assisted injection molding process design technology, gas-assisted injection molding process computer simulation technology, gas-assisted injection molding product defect diagnosis and elimination technology, and gas-assisted process special material technology.

Gas-assisted injection molding technology is especially suitable for pipe-like products, thick-walled, partial-walled (parts with different thickness sections), and large flat structure parts.

Gas-assisted systems include nitrogen generation and booster systems, pressure control units, and air intake components. The gas-assisted process can be fully integrated with the conventional injection molding process (injection molding machine).

The weight reduction of products (material saving) can be as high as 40%, shorten the molding cycle (save time up to 30%, eliminate shrinkage, improve the yield; reduce the injection pressure up to 60%, can be used to produce large parts with small tonnage injection molding machine, reduce operating costs; mold life extension, lower manufacturing costs, also can be used such as thick root, thick rib, connecting plate and other more stable structure, increased freedom of mold design.