1. Dimensional shrinkage of plastic parts due to thermal expansion and contraction, elastic recovery upon demolding, and plastic deformation causes the parts to shrink after cooling to room temperature. Therefore, the mold design must account for this and provide compensation.

Synthetic high molecular materials. They, along with synthetic rubber and synthetic fibers, constitute the three indispensable synthetic materials in today's daily life. Specifically, plastics are materials primarily composed of synthetic resins, which can be molded into a specific shape under certain temperatures and pressures, and maintain their shape at room temperature.

2. During directional forming, the molecules align in the direction, resulting in anisotropic molded parts. Along the material flow direction (i.e., parallel direction), there is greater shrinkage and higher strength, whereas at right angles to the material flow direction (i.e., perpendicular direction), shrinkage is smaller and strength is lower. Additionally, due to uneven density and filler distribution across different parts of the molded part during forming, shrinkage is also uneven. This uneven shrinkage can lead to warping, deformation, and cracking in the molded parts, especially during extrusion and injection molding, where directional effects are more pronounced. Therefore, when designing molds, it is advisable to consider the shrinkage directionality according to the shape of the molded part and the flow direction of the material to select an appropriate shrinkage rate.