The shrinkage of plastic parts, are due to the thicker parts of the cooling melt when the lack of supplements caused by defects we often encounter any case of increased pressure, increase the water inlet, to extend the injection time, the problem of shrinkage can not be resolved Case.
Of the commonly used raw materials, due to the cooling rate, PC material shrinkage problem can be described as the most difficult to solve, PP material shrinkage and shrinkage problems are more difficult to deal with.
Therefore, when encountering the shrinkage problem of thicker and heavier pieces, some unconventional injection molding techniques need to be adopted or it is difficult to solve the problem.
First of all, under the premise of ensuring the injection molding is not deformed, take the shortened cooling time as far as possible to let the injection molded parts die early at high temperature. At this time, the temperature of the outer layer of the injection molded parts is still high and the skin is not over hardened The temperature difference between inside and outside is relatively not too large, so that it will help the overall shrinkage, thereby reducing the internal shrinkage of injection molded parts.
As the overall amount of shrinkage of the injection molded parts is constant, the more the overall shrinkage is, the smaller the amount of shrinkage is. The shrinkage of the internal shrinkage cavity and the surface is therefore reduced.
The problem of shrinkage cavity is due to the mold surface heating, cooling capacity decreased, the just solidified injection molding surface is still soft, not completely eliminate the internal shrinkage due to the formation of a vacuum, resulting in injection molding surface under the pressure of atmospheric pressure to Within the compression, combined with the role of contractility, shrinkage problems so produced.And the slower the surface hardening easier to produce shrinkage, such as PP material, on the contrary the more prone to shrinkage.
Therefore, the injection molded parts early after the mold, to be properly cooled, so that the surface of the injection molded parts to maintain a certain hardness, making it difficult to produce shrinkage, but if the problem is more serious indentation, moderate cooling will not be eliminated, it is necessary Chilled water quenching method to make the surface of the injection molded parts can be quickly hardened to prevent shrinkage, but the internal shrinkage will exist.Such as PP soft surface material, due to the role of vacuum and shrinkage, there will be injection molded parts Decreasing the possibility of depression, but the extent of depression has been greatly reduced.
In taking the above measures at the same time, if the extension of the injection time to replace the cooling time method, the surface shrinkage and even the improvement of internal shrinkage will be better.
In solving the problem of shrinkage cavity, due to the mold temperature is too low will increase the degree of shrinkage, so the best mold with machine water cooling, do not use frozen water, if necessary, the mold temperature will then rise some, for example, injection mold PC material will die Temperature rise to 100 degrees, the impact of shrinkage will be better, but if it is to solve the problem of shrinkage concave 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 has been greatly improved, if we must solve the problem of shrinking the surface, the appropriate amount of anti-shrink agent is a last resort, of course, the transparent You can not do that.
If thick-walled parts still exist shrink marks on the surface, or encounter plastic parts such as partial wall, then the introduction of gas-assisted injection molding will be resolved.
Gas-assisted injection molding is a novel plastic molding technique that introduces high-pressure gas into the thick-walled parts of a workpiece and creates a hollow cross-section inside the injection-molded material to completely fill the gas and to eliminate the indentation of the product. Process can not be combined with the thick-walled and thin-wall molding, and the workpiece residual stress, easy to warp deformation, shrink marks when the surface.
The newly developed gas-assisted technology succeeded in producing thick-walled and partially-walled products by hollowing-out the thick-walled interior, and the products have excellent surface appearance, low internal stress, light weight and high strength.
TV sets, home appliances, automobiles, furniture, daily necessities, office supplies, toys and other plastic forming opened up new areas of application, gas-assisted injection molding technology is particularly applicable to pipe-like products, thick walls, partial wall Pieces) and large flat structural parts.
Gas Assist: Includes Nitrogen Generation and Pressurization System, Pressure Control Unit, and Air Inlet Gas Assist Process Fully interfaces with conventional injection molding processes (injection molding machines).
Reduce the weight of the product (save material) can be 40% higher, shorten the molding cycle (saving up to 30%, eliminate sink mark, improve the yield; reduce injection pressure up to 60%, small tonnage injection molding machine can be used to produce large parts, reduce operating costs ; Die life extension, lower manufacturing costs, but also can be used, such as coarse roots, thick tendons, connecting plates and other more stable structure, an increase of mold design freedom.