The shrinkage problems of modified ABS, modified PP, and modified PC plastic parts (surface shrinkage and internal shrinkage) are all defects caused by insufficient melt replenishment when the thicker parts are cooled. We often encounter situations where the shrinkage problem cannot be solved by increasing the pressure, increasing the water inlet, and prolonging the injection time.

Among the commonly used raw materials, due to the fast cooling rate, the shrinkage problem of PC materials is the most difficult to solve, and the shrinkage and shrinkage problems of PP materials are also more difficult to deal with.

Therefore, when encountering serious shrinkage problems of thick and large parts, some unconventional injection molding techniques need to be adopted, otherwise it will be difficult to solve the problem. In practical production, we have explored a set of more effective techniques to deal with this difficult injection molding problem.

First of all, under the premise of ensuring that the injection molded parts are not deformed, the method of shortening the cooling time as much as possible is adopted to make the injection molded parts come out of the mold early at high temperatures. At this time, the temperature of the outer layer of the injection molded part is still very high, and the skin is not too hardened, so the temperature difference between the inside and the outside is relatively small, which is conducive to the overall shrinkage, thereby reducing the concentrated 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, and the degree of internal shrinkage and surface shrinkage can therefore be reduced.

Next, if it is to solve the shrinkage problem, it will be discussed below. As for the improvement of the surface shrinkage problem, the method of solving the shrinkage cavity after the injection molded part is out of the mold at high temperature is different.

The shrinkage problem occurs because the mold surface heats up and the cooling capacity decreases. The surface of the injection molded part that has just been solidified is still soft (unlike the PC part after the mold is harder, it is easy to produce shrinkage holes), and the interior that has not been completely eliminated The shrinkage cavity forms a vacuum, which causes the surface of the injection molded part to compress inward under the pressure of atmospheric pressure. At the same time, the shrinkage force is added to cause the shrinkage problem. And the slower the surface hardening speed, the easier it is to produce shrinkage, such as PP material, and vice versa.

Therefore, after the injection molded part is ejected from the mold early, it must be properly cooled to maintain a certain hardness on the surface of the injection molded part, so that it is not prone to shrinkage. However, if the shrinkage problem is more serious and moderate cooling cannot be eliminated, it is necessary to take the method of freezing water chilling to quickly harden the surface of the injection molded part to prevent shrinkage, but internal shrinkage holes will still exist. For materials with a softer surface like PP, due to the effect of vacuum and shrinking force, injection molded parts may still have shrinkage, but the degree of shrinkage has been greatly reduced.

While taking the above measures, if the method of extending the injection time instead of the cooling time is used, the improvement of surface shrinkage and even internal shrinkage will be better.

When solving the shrinkage problem, because the mold temperature is too low, it will increase the degree of shrinkage. Therefore, it is best to cool the mold with machine water. Do not use frozen water. If necessary, the mold temperature should be increased. When the temperature rises to 100 degrees, the improvement effect of shrinkage will be better. But if it is to solve the shrinkage problem, the mold temperature cannot be raised, but needs to be lowered.

Finally, sometimes the above methods may not completely solve the problem, but it has been greatly improved. If the problem of surface shrinkage must be completely solved, adding an appropriate amount of anti-shrinking agent is also a last resort. Of course, transparent parts cannot do this.

If there are still sink marks on the surface of thick-walled parts, or plastic parts such as offset walls are encountered, the introduction of gas-assisted injection molding will be solved.

Gas-assisted injection molding is a novel plastic molding technology that introduces high-pressure gas into the thick-walled part of the part to produce a hollow section inside the injection part, complete the filling process, realize gas pressure retention, and eliminate product sink marks.

The traditional injection molding process cannot combine thick and thin walls to form, and the parts have high residual stress, are easy to warp and deform, and sometimes have sink marks on the surface. The newly developed gas-assisted technology successfully produces thick-walled and partial-walled products by hollowing out the thick-walled interior, with excellent appearance and surface properties and low internal stress. Lightweight and high strength.

The structure and mold design of gas-assisted products have been successfully developed, including gating system, air intake mode 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 products Defect diagnosis and elimination technology, gas-assisted process special material technology.

Televisions, home appliances, automobiles, furniture, daily necessities, office supplies, toys, etc. have opened up new application areas for plastic molding. Gas-assisted injection molding technology is particularly suitable for pipe-shaped products, thick walls, and partial walls (made of different thickness sections). Parts) and large flat structural parts.

Gas auxiliary equipment: including nitrogen generation and booster system, pressure control unit and air intake components. The gas-assisted process can be fully integrated with the traditional injection process (injection molding machine).

Reduce the weight of modified ABS, modified PP, modified PC products (saving materials) can be increased by 40%, shorten the molding cycle (save time up to 30%, eliminate sink marks, improve yield; reduce injection pressure up to 60%, the use of small Tonnage injection molding machine produces large parts, reducing operating costs; mold life is extended, manufacturing costs are reduced, and more stable structures such as thick roots, thick ribs, and connecting plates can be used, which increases the freedom of mold design.

Wuxi Jiahong Plastics Technology Co., Ltd. has nearly 30 years of experience in R&D and pelletizing production of PVC granules, and experience in R&D and production of modified engineering plastics; professional technical service team can provide customers with one-stop modified plastic system solutions. If you want to know more about the product, please log in to our official website: www. js-plastics. com, consult online customer service or call the hotline. Landline: 0510-68755207 Mobile: 15190220696, we will serve you wholeheartedly.

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