Process skills for shrinkage problem of thick-walled injection molded parts

Introduction:

The shrinkage problem of plastic parts (surface shrinkage and internal shrinkage cavity) is caused by insufficient melt glue replenishment when the thicker part is cooled. We often encounter the situation that the shrinkage problem cannot be solved no matter how the pressure is increased, the water inlet is increased, and the injection time is prolonged.

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

Therefore, when encountering the serious shrinkage problem of thick and large parts, it is necessary to adopt some unconventional injection molding techniques, 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 problem of injection molding.

 

Check under the premise of ensuring:

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 allow the injection molded parts to be released early at high temperature. At this time, the temperature of the outer layer of the injection molded part is still 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 be reduced.

 

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

 

The problem of shrinkage occurs because the surface of the mold heats up and the cooling capacity decreases. The surface of the injection molded part that has just solidified is still soft (unlike the surface of the PC part that is hard after it is released from the mold, and it is easy to produce shrinkage cavities), and the internal parts that have not been completely eliminated Due to the formation of a vacuum in the shrinkage cavity, the surface of the injection molded part is compressed inwardly under the pressure of atmospheric pressure, and at the same time, the shrinkage force is added, so the problem of shrinkage occurs. Moreover, the slower the surface hardening speed, the easier it is to produce shrinkage cavities, such as PP materials, and vice versa, the easier it is to produce shrinkage cavities.

 

Therefore, after the injection molded part is released from the mold in advance, it should be properly cooled to keep the surface of the injection molded part with a certain hardness so that it is not easy to produce shrinkage. However, if the problem of shrinkage is serious, moderate cooling will not eliminate it. It is necessary to adopt the method of chilling with freezing water to quickly harden the surface of the injection molded part to prevent shrinkage, but internal shrinkage will still exist. For materials with a soft surface such as PP, due to the effect of vacuum and shrinkage force, the 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 prolonging the injection time instead of the cooling time is adopted, the improvement of the surface shrinkage and even the internal shrinkage cavity will be better.

 

When solving the problem of shrinkage cavity, because the mold temperature is too low, the degree of shrinkage will be aggravated. Therefore, it is best to cool the mold with machine water instead of freezing water. If necessary, the mold temperature should be raised. When the temperature rises to 100 degrees, the improvement effect of shrinkage cavity will be better. But in order to solve the shrinkage problem, the mold temperature cannot be raised, but needs to be lowered.

 

Finally, sometimes the above methods may not be able to 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-shrinkage agent is also a last resort. Of course, transparent parts cannot do this.

 

Gas-assisted injection molding:

If sink marks still exist on the surface of thick-walled parts, or plastic parts such as partial walls are encountered, the introduction of gas-assisted injection molding will solve the problem.

 

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 part, complete the filling process, realize gas pressure retention, and eliminate product sink marks.

 

The traditional injection molding process cannot combine thick wall and thin wall together, and the residual stress of the part is large, easy to warp and deform, and sometimes there are sink marks on the surface. The newly developed gas-assisted technology has successfully produced thick-walled and partially-walled products by hollowing out the inside of the thick-walled products, and the products have excellent appearance and surface properties and low internal stress. Lightweight and strong.

 

Gas-assisted product structure and mold design have been successfully developed, including gating system, air intake method and air channel distribution design technology, gas-assisted injection molding process design technology, gas-assisted injection molding process design technology, computer simulation technology for gas-assisted injection molding process, gas-assisted injection molding products Defect diagnosis and elimination technology, special material technology for gas-assisted process.

 

TV sets, home appliances, automobiles, furniture, daily necessities, office supplies, toys, etc. have opened up new application fields for plastic molding. pieces) and large flat structural parts.

 

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

Conclusion:

Reduce product weight (save material) by 40%, shorten molding cycle (save time by 30%, eliminate sink marks, improve yield; reduce injection pressure by 60%, use small tonnage injection molding machines to produce large parts, and 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 be used, which increases the freedom of mold design.

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