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

 

Among commonly used raw materials, due to fast cooling rate, shrinkage problem of PC materials is the most difficult to solve, shrinkage and shrinkage problems of PP materials are also more difficult to deal with.
Therefore, when encountering a serious shrinkage problem of thick and large parts, some unconventional injection molding techniques need to be adopted, otherwise it will be difficult to solve problem.
First of all, under premise of ensuring that injection molded parts are not deformed, method of shortening cooling time as much as possible is adopted to make injection molded parts come out of mold early at high temperatures. At this time, temperature of outer layer of injection molded part is still high, and skin is not too hardened, so temperature difference between inside and outside is relatively small, which is conducive to the overall shrinkage, thereby reducing concentrated shrinkage inside injection molded part.
Since the overall shrinkage of injection molded part is constant, the more overall shrinkage, the smaller concentrated shrinkage, degree of internal shrinkage and surface shrinkage can therefore be reduced.
Shrinkage problem occurs because mold surface heats up and cooling capacity decreases. Surface of injection molded part that has just been solidified is still soft. Internal shrinkage cavity that has not been completely eliminated has formed a vacuum, causing surface of injection molded part to compress inward under pressure of atmospheric pressure. At the same time, shrinkage force is added to cause shrinkage problem. And the slower surface hardening speed, the easier it is to produce shrinkage, such as PP material, and vice versa.
Therefore, after injection molded part is ejected from mold early, it must be properly cooled to keep surface of injection molded part to a certain degree of hardness, so that it is not easy to produce shrinkage. However, if shrinkage problem is more serious and moderate cooling cannot be eliminated, it is necessary to take method of freezing water chilling to quickly harden surface of injection molded part to prevent shrinkage, but internal shrinkage holes will still exist. For materials with a softer surface like PP, due to effect of vacuum and shrinking force, injection molded parts may still have shrinkage, but degree of shrinkage has been greatly reduced.
While taking above measures, if method of extending injection time instead of cooling time is used, improvement of surface shrinkage and even internal shrinkage will be better.
When solving shrinkage problem, because mold temperature is too low, it will increase degree of shrinkage. Therefore, it is best to cool mold with machine water. Do not use frozen water. If necessary, mold temperature should be increased. For example, when mold temperature is raised to 100 degrees when injecting PC material, shrinkage cavity improvement effect will be better. But if it is to solve shrinkage problem, mold temperature cannot be raised, but needs to be lowered.
Finally, sometimes above methods may not completely solve problem, but it has been greatly improved. If 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, introduction of gas-assisted injection molding will be resolved.
Gas-assisted injection molding is a novel plastic molding technology that introduces high-pressure gas into thick-walled part of part to produce a hollow section inside injection part, complete filling process, realize gas pressure retention, and eliminate product sink marks. Traditional injection molding process cannot combine thick and thin walls to form, parts have high residual stress, are easy to warp and deform, and sometimes have sink marks on the surface.
Newly developed gas-assisted technology successfully produces thick-walled and partial-walled products by hollowing out thick-walled interior, with excellent appearance and surface properties and low internal stress, low internal stress, light weight and high strength.
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, partial walls (parts composed of sections with different thicknesses) and large flat structural parts.
Gas auxiliary equipment: including nitrogen generation and booster system, pressure control unit and air intake components. Gas-assisted process can be fully integrated with traditional injection process (injection molding machine).
Reduce weight of product (saving materials) by 40%, shorten molding cycle (save time by 30%), eliminate sink marks, and increase yield; reduce injection pressure by 60%, and small tonnage injection molding machines can be used to produce large parts, reducing operating costs ; mold life is extended, manufacturing costs are reduced, more stable structures such as thick roots, thick ribs, and connecting plates can also be used, which increases freedom of mold design.