When planning a plastic mold, after confirming the mold structure, you can plan the details of each part of the mold, that is, confirm the dimensions of each template and part, cavity and core dimensions, etc. This will involve primary planning parameters such as material shrinkage. Therefore, as long as the reduction rate of the molded plastic is grasped in detail, the dimensions of each part of the cavity can be confirmed. Even if the selected mold structure is correct, but the parameters used are incorrect, it is impossible to produce plastic parts of acceptable quality.

The characteristic of thermoplastics is that they swell after heating and shorten after cooling. Of course, the volume will shrink after pressing. In the injection molding process, the molten plastic is first injected into the mold cavity. After the filling is completed, the melt cools and solidifies. When the plastic part is removed from the mold, it appears shortened. This shortening is called shortening. Within a period of time from when the plastic part is taken out from the mold until it stabilizes, the dimensions will still show small changes. One kind of change is to continue shortening. This shortening is called post-shortening.

Another change is that some hygroscopic plastics swell due to moisture absorption. For example, when the moisture content of nylon 610 is 3%, the scale increase is 2%; when the moisture content of glass fiber reinforced nylon 66 is 40%, the scale increase is 0.3%. However, the first effect is shortened molding.