The problem of shrinkage of plastic parts
is caused by insufficient filling of the melt when it is cooled in a thicker place. We often encounter no matter how to increase the pressure, increase the water inlet and prolong the injection time. There is no way to solve the problem of shrinkage. Among the commonly used raw materials, the shrinkage of the PC material is the most difficult to solve due to its fast cooling rate, and the shrinkage and shrinkage of the PP material is also difficult to solve. Therefore, when encountering the serious shrinkage problem of thick and large parts, it is necessary to take some unconventional injection molding techniques, otherwise it is difficult to solve the problem.
First of all, in the case of ensuring that the injection molded part is not deformed, the method of minimizing the cooling time is adopted to allow the injection molded part to be prematurely released at a 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 advantageous for 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 is, the smaller the concentrated shrinkage is, and the degree of internal shrinkage and surface shrinkage is thus reduced.
The problem of shrinkage and concavation is caused by the temperature rise of the mold surface and the cooling capacity. The surface of the newly-formed injection molded part is still soft. The internal shrinkage hole that has not been completely eliminated forms a vacuum, which causes the surface of the injection molded part to be pressed under atmospheric pressure. Internal compression, coupled with the effect of shrinkage force, the shrinkage problem arises. Moreover, the slower the surface hardening speed, the more likely it is to cause dents, such as PP material, and the more easily the shrinkage holes are produced.
Therefore, after the injection molded part is prematurely released, it is appropriately cooled to maintain a certain hardness on the surface of the injection molded part, so that it is less likely to cause shrinkage. However, if the problem of shrinkage is serious, moderate cooling will not be eliminated. It is necessary to adopt a method of freezing water to make the surface of the molded part harden quickly to prevent shrinkage, but internal shrinkage holes 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 have the possibility of shrinkage, but the degree of shrinkage has been greatly reduced.
At the same time as taking the above measures, if the method of extending the glue injection time is used instead of the cooling time, the surface shrinkage and even the internal shrinkage cavity improvement will be better.
When solving the shrinkage hole problem, the mold cavity temperature is too low, which will increase the degree of shrinkage. Therefore, the mold is preferably cooled by machine water. Do not use frozen water. If necessary, increase the mold temperature, for example, when molding PC material. When the temperature rises to 100 degrees, the improvement effect of the shrinkage hole will be better. However, if it is to solve the problem of shrinkage, the mold temperature can not be raised, but it needs to be reduced.
Finally, sometimes the above methods may not completely solve the problem, but there has been a great improvement. If the problem of surface shrinkage must be completely solved, it is a last resort to have a proper amount of anti-shrinkage agent. Of course, transparent parts can't do this.
If the surface of the thick-walled part still has a sink mark, or if a plastic part such as a partial wall is 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, creates a hollow section inside the injection molded part, completes the filling process, achieves gas holding pressure, and eliminates product shrinkage. The traditional injection molding process cannot combine the thick wall and the thin wall together, and the residual stress of the workpiece is large, it is easy to warp and deform, and there is a shrinkage on the surface.
The newly developed gas-assisted technology successfully produced thick-walled and siding products by hollowing out the thick-walled interior, and the surface appearance of the product was excellent and the internal stress was low. Lightweight and strong. Home appliances, automobiles, furniture, daily necessities, office supplies, etc. have opened up new applications for plastic molding. Gas-assisted injection molding technology is particularly suitable for pipe-like products, thick-walled, partial walls (pieces of different thickness sections) and large flat Structural parts.
Reduce the weight of the product (saving material) up to 40%, shorten the molding cycle (saving time by 30%, eliminating sink marks, improving the yield; reducing the injection pressure by 60%, using small tonnage injection molding machines to produce large parts, reducing operating costs The mold life is prolonged, the manufacturing cost is reduced, and a more stable structure such as a thick root, a thick rib, a connecting plate, etc., can be used, which increases the degree of freedom in mold design.
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