Shrinkage
Shrinkage is a significant factor in thermoforming large precision parts and allowances must be made for it in the design of the part. Shrinkage takes place in three basic forms: mold shrinkage, after-mold shrinkage, and in-service shrinkage and expansion.
Mold Shrinkage:
When a thermoplastic material is heated and formed to a mold, shrinkage of the material occurs during the cooling cycle. The dimensions of the formed part after its surface reaches a temperature at which it can be de-molded are slightly less than the dimensions when the part was first formed. This difference is referred to as mold shrinkage; it is usually expressed in terms of inch per inch per °F (millimeter per millimeter per °C). It varies with processing and design factors as well as with different materials.
Shrinkage is less critical with male drape forming than straight vacuum forming. With male drape forming, the material shrinks onto the rigid mold as it cools, retarding the shrinkage. Conversely, with straight female vacuum forming, the material shrinks away from the mold against the negligible resistance of the outer air with nothing to retard the shrinkage. Although this phenomenon improves final part dimensions, it requires molds with proper draft angles in order to extract the part from the mold.
After-mold Shrinkage:
After de-molding, the part will shrink due to heat loss from the part as it cools to room temperature. The part continues to shrink as the hot center or core of the plastic cools. This shrinkage ceases when temperature equilibrium is reached in the cooled material.
In-service Shrinkage and Expansion:
This is the normal contraction or expansion of an object caused by changes in temperature and humidity. It is considered a significant factor only where tolerances are extremely critical or where the formed plastic is rigidly fastened to a material with a markedly different coefficient of expansion. Each type of thermoplastic material has a different coefficient of thermal contraction or expansion. For applications involving precise specifications, you must conduct actual end user testing.
Figure: In-service Shrinkage and Expansion
