Cavitation Counts Chosen against a Volume Forecast Nobody Guarantees
Select initial tool cavitation against guaranteed first-year order volumes rather than unbacked sales forecasts to prevent balance sheet tooling write-downs.
Resin Thermal Shrink
Meaning
Physical contractions occur when a polymer cools from its molten state to room temperature, resulting in a part that is smaller than the mold cavity. Accounting for resin thermal shrink is a primary task for mold designers, who must oversize the cavities to ensure the finished part meets the target dimensions. The amount of shrinkage is determined by the molecular structure of the plastic and the conditions under which it is processed.
Shrinkage Variable
Semi-crystalline polymers like nylon or polyethylene exhibit higher levels of shrinkage because their molecules pack together in tightly ordered structures as they cool. Amorphous polymers like polystyrene have more random structures and shrink significantly less. Designers must also consider that shrinkage is often non-uniform, with more contraction occurring in the direction of the flow than across it.
Control Factor
Processing variables such as the hold pressure and the cooling time can be used to fine-tune the final dimensions of a part. Increasing the pressure during the packing stage forces more material into the cavity, which helps to offset the natural shrinkage of the resin. A longer cooling time ensures that the part is stable before it is ejected, which prevents further deformation after it leaves the tool.
Prediction Limit
Accurate prediction of shrinkage is difficult for complex geometries where the wall thickness varies or where the part is constrained by the tool. While data sheets provide a range for the material, the actual shrinkage can vary depending on the specific machine and the mold design. The limit of predictable shrinkage is reached when the part geometry is so complex that internal stresses cause unpredictable warping.