Plastic shrinkage refers to the percentage of the size of the plastic product at the molding temperature and the size after cooling to room temperature
it reflects the size reduction of plastic parts after they are taken out of the mold and cooled. The factors that affect the shrinkage of plastics are: plastic varieties, plastic injection mold conditions, mold structure, etc. Different polymer materials have different shrinkage. Secondly, the shrinkage of plastics is closely related to the shape of plastic parts, the complexity of internal structure and whether there are inserts
1：Types of plastics
According to statistics: so far, there are about 100 kinds of known plastic materials in the world. In the molding process of thermoplastic, there are many factors, such as volume change caused by crystallization, strong internal stress, large residual stress frozen in plastic parts, strong molecular orientation and so on. Therefore, compared with thermosetting plastics, the shrinkage rate is larger, the range of shrinkage rate is wide, and the directivity is obvious. In addition, the shrinkage rate after molding, annealing or humidity control treatment is generally larger than that of thermosetting plastics 。
2：Structural characteristics of plastics
The hardness and brittleness of body structure are higher. Plastics are two kinds of polymer structure, made of linear polymer is thermoplastic, made of body polymer is thermosetting plastics. With or without inserts, layout and quantity of inserts directly affect the material flow direction, density distribution and shrinkage resistance, so the characteristics of plastic parts have great influence on shrinkage size
3：Form, size and distribution of feeding port
The feed port is a channel from the runner to the mold cavity, which is the smallest and shortest part of the gating system. These factors directly affect the material flow direction, density distribution, packing feeding effect and molding time. When the cross-section of the direct feeding port and the feeding port is large, the shrinkage is small, but the directivity is large. When the width and length of the feeding port are short, the directivity is small
4：Injection mold conditions
Plastic Injection molding conditions mainly refer to the mold and plasticizing temperature of plastic parts, the pressure, speed and time of injection and packing.
When the mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystalline material, the shrinkage is larger because of the high crystallinity and large volume change. The mold temperature distribution is also related to the internal and external cooling and density uniformity of plastic parts, which directly affects the shrinkage and direction of each part.
Holding pressure and time also have a great influence on the contraction. The contraction is small but directional when the pressure is high and the time is long. High injection pressure, low viscosity difference of molten material, low interlaminar shear stress, large elastic rebound after demoulding, so shrinkage can be reduced appropriately