Thermoplastic refers to plastics that have the characteristics of softening when heated and hardening when cooled. Most of the plastics we use in our daily lives belong to this category. When heated, they soften and flow, and when cooled, they harden. This process is reversible and can be repeated.
1. The variety of plastics. During the thermoplastic forming process, due to the volume change caused by crystallization, strong internal stresses, large residual stresses in the frozen plastic parts, and strong molecular orientation, the shrinkage rate is higher than that of thermosetting plastics. The shrinkage range is wide with obvious directionality. In addition, the shrinkage after forming, annealing, or moisture adjustment treatment is generally larger than that of thermosetting plastics.
2. The characteristics of plastic parts. During molding, the molten material comes into contact with the surface of the cavity and immediately cools to form a low-density solid outer shell. Due to the poor thermal conductivity of plastics, the inner layer of the plastic part cools slowly and forms a high-density solid layer with large shrinkage. Therefore, the thicker the wall, the slower the cooling, and the thicker the high-density layer is, the greater the shrinkage will be. In addition, the presence, layout, and number of inserts directly affect the direction of material flow, density distribution, and shrinkage resistance. Therefore, the characteristics of plastic parts have a greater impact on the shrinkage size and directionality.
3. The form, size, and distribution of the feed port directly affect the flow direction of thermoplastic materials, density distribution, pressure-holding shrinkage compensation, and forming time. A direct feed port or a feed port with a large cross-sectional area (especially with a thick cross-section) will result in smaller shrinkage but greater directionality, while a wide feed port with a short length will result in less directionality. Shrinkage will be greater when closer to the feed port or parallel to the material flow direction.
4. Molding conditions. A high mold temperature, slow cooling, high density, and high shrinkage are associated with a higher degree of crystallization, which leads to greater volume changes, leading to larger shrinkage. The temperature distribution of the mold and the uniformity of cooling inside and outside the plastic part are also related, directly affecting the size and directionality of shrinkage in various parts. In addition, maintaining pressure and time also have a significant impact on shrinkage. The higher the pressure and longer the time, the smaller the shrinkage but the greater the directionality.
High injection pressure, low viscosity of the molten material, low interlayer shear stress, and high rebound after demolding can all reduce shrinkage. Higher temperature materials have higher shrinkage but less directionality. Therefore, adjusting molding factors such as mold temperature, pressure, injection speed, and cooling time during molding can also properly change the shrinkage of plastic parts.
When designing molds, the shrinkage rate of each part of the plastic part is determined based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the form, size, and distribution of the feed port, and then the cavity size is calculated based on experience.