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 and compensate for this shrinkage.

Synthetic high polymer 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 certain shape under certain temperatures and pressures, and maintain their shape at room temperature.

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