1. The dimensional shrinkage of plastic parts due to thermal expansion and contraction, elastic recovery upon demolding, and plastic deformation results in the parts shrinking after cooling to room temperature. Therefore, the mold design must account for and compensate for this shrinkage.

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

2. During directional shaping, molecules align in the direction, causing the molded part to exhibit anisotropy. Along the direction of material flow (i.e., parallel direction), there is greater shrinkage and higher strength, whereas perpendicular to the 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 shaping, shrinkage is also uneven. This uneven shrinkage can lead to warping, deformation, and cracking in the molded part, particularly during extrusion and injection molding, where the directional effect is more pronounced. Therefore, mold design should consider the shrinkage directionality, selecting an appropriate shrinkage rate based on the shape of the molded part and the direction of material flow.