The shrinkage problem of plastic parts (surface shrinkage and internal shrinkage) is due to the defects caused by insufficient melt supplement when the thicker part is cooled. We often encounter that no matter how to increase the pressure, increase the water inlet, extend the injection time, the shrinkage problem can not be solved.

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

Therefore, when the thick large parts are more serious shrinkage problems, it is necessary to take some unconventional injection molding skills, otherwise it is difficult to solve the problem. In the practice of production, we have explored a set of more effective skills to deal with the difficult problem of injection molding.

First of all, under the premise of ensuring that the injection molding parts do not deform, the method of shortening the cooling time as far as possible is adopted to make the injection molding parts out of the mold early at high temperature. At this time, the temperature of the outer layer of the injection parts is still very high, the skin is not too hardened, so the temperature difference between the inside and outside is relatively not very large, which is conducive to the overall contraction, thereby reducing the concentrated contraction inside the injection parts. Because the overall shrinkage of the injection parts is unchanged, the more the overall shrinkage, the smaller the concentration of shrinkage, and the degree of internal shrinkage and surface shrinkage can be reduced.

Then, if in order to solve the shrinkage problem, will be discussed in the following, as for improving the problem of surface shrinkage, injection parts after high temperature mold with the solution of shrinkage is different.

Concave problem, is due to the mold surface temperature rise, cooling capacity decline, just solidified injection molding surface is still soft (unlike PC parts after the mold surface is hard, easy to produce shrinkage holes), not completely eliminated due to the formation of a vacuum, resulting in the injection molding surface under the pressure of atmospheric pressure inward compression, at the same time with the effect of shrinking force, The concave problem is thus generated. And the slower the surface hardening rate, the easier it is to produce concave, such as PP material, and the easier it is to produce shrinkage holes.

Therefore, after the injection parts are out of the mold early, it is necessary to cool them properly, so that the surface of the injection parts maintains a certain hardness, so that it is not easy to produce concave. However, if the concave problem is more serious, moderate cooling will not be able to eliminate, it is necessary to take the method of freezing water chilling, so that the surface of the injection parts can be rapidly hardened to prevent concave, but the internal shrinkage holes will still exist. Such as PP surface soft materials, due to the effect of vacuum and shrinkage force, injection parts will also have the possibility of concave, but the degree of concave has been greatly reduced.

At the same time of taking the above measures, if the method of extending the injection time to replace the cooling time is used, the improvement of the surface concave and even the internal shrinkage hole will be better.

In solving the problem of shrinkage holes, because the mold temperature is too low will increase the degree of shrinkage holes, so the mold is best to use machine water cooling, do not use frozen water, if necessary, the mold temperature will rise some more, such as injection PC material mold temperature rise to 100 degrees, the improvement effect of shrinkage holes will be better. But if in order to solve the problem of shrinkage, the mold temperature can not be increased, but need to reduce some.

Finally, sometimes the above method may not be able to completely solve the problem, but there has been a great improvement, if we must completely solve the problem of surface concave, the appropriate amount of anti-shrinkage agent is also a last resort. Of course, transparent parts can not do so.

If there are still shrinkage marks on the surface of thick wall parts, or encounter plastic parts such as deviated walls, then the introduction of gas-assisted injection molding will be solved.

Gas assisted injection molding is a novel plastic molding technology by introducing high pressure gas into the thick wall of the parts, producing a hollow section inside the injection parts, completely filling the process, realizing gas pressure retention and eliminating the shrinkage marks of the products.

The traditional injection molding process can not be thick wall and thin wall together molding, and the residual stress of the parts is large, easy to warpage deformation, and the surface has shrinkage marks. The newly developed gas-assisted technology successfully produces thick wall and off-wall products by hollowing out the interior of the thick wall, and the appearance of the product has excellent surface properties and low internal stress. Light weight and high strength.

The gas-assisted product structure and mold design has been successfully developed, including pouring system, intake mode and airway 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 exclusion technology, gas-assisted process special material technology.

TV, home appliances, automobiles, furniture, daily necessities, office supplies, toys and other plastic molding has opened up a new field of application, gas-assisted injection molding technology is especially suitable for tubular products, thick wall, off-wall (parts of different thickness sections) and large flat structure parts.

Gas auxiliary device: including nitrogen gas generation and pressurization system, pressure control unit and air intake element. The gas-assisted process can be fully connected with the traditional injection molding process (injection molding machine).

Reduce product weight (material saving) can be 40% higher, shorten the molding cycle (time saving up to 30%, eliminate shrinkage marks, improve the yield; Reduce injection pressure up to 60%, small tonnage injection molding machine can be used to produce large parts, reduce operating costs; The life of the mold is extended, the manufacturing cost is reduced, and more stable structures such as thick roots, thick ribs and connecting plates can also be used, which increases the freedom of mold design.

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