What Affect Part Shrinkage, and Cause of Shrinkage

Instead of relating shrinkage to processing parameters, we consider some fundamental factors that affect shrinkage. These factors are volumetric, shrinkage, crystalline content, stress relaxation and orientation.

Good Explanation relation between Plastic Shrinkage and Injection parameter we can read below.
Plastic materials are injection-molded in a thick melt. The thickness of the melt varies with the temperature of the plastic. When plastic is heated, molecules expand. In this expanded (or melt condition) the molecules can be compressed, distorted, or rearranged. The injection pressure plus the mold geometry produces a distortion and compression of the molecules. The molecules along the mold wall, at corners, at the gate and at junctions which go from thick to thinner walls, experience the greatest pressures. These pressures cause distortion in the molecule structure and are called molded-in stresses.

Cooling of the plastic part also effect shrinkage. Plastic are good insulators; thus, when they cool on the outside, the inside tends to cool at a slower rate. The difference in cooling and flow of the material as it fills the mold forms a three layer flow pattern from the center of the part to the mold wall. The portion of the plastic closest to the wall quickly freezes into an amorphous pattern. This first layer insulates and slows the cooling of the plastic. The plastic molecular structure directly behind the frozen wall cools more slowly.

Injection pressure can also distort molecular structure and effect part shrinkage. The molecules form twisted ribbons growing out from the wall side toward the center of the part. Molecules toward the center of mold cavity are insulated by the first two layers. At the center the cooling is especially slow which reduces molecular distortion. Figure 6-31 illustrates this three layer principle of differential cooling from the injection mold cavity wall.

The cooling and shrinkage together result in a reduction of the volume of the plastic filling the mold cavity. Crystalline molecular areas shrink significantly more then amorphous areas. A crystalline plastic, such as Acetal, will shrink 0.005 inches per inch upon cooling in the mold. The areas undergoing compacting and molded in stress will not return completely to the original crystalline structure. However, over time, the material strives to relieve the stress areas and continues to shrink. Within 24 hours 0.0005 of an inch will return; another 0.0005 within one week. (http://online.sfsu.edu/)

Cause of Shrinkage
Shrinkage of plastic components is driven by the volumetric change of the material as it cools from the melt state to sold. Despite the apparent simplicity of this statement it is important to note that the relationship between volumetric shrinkage and the linear shrinkage of the component is affected by mold restraint, crystallinity and orientation.