Meaning
Systematic adjustments made to the physical dimensions of a mold cavity account for the expected volumetric contraction of the plastic material during cooling. Implementing cavity dimension correction ensures that the final molded part meets the specified tolerances. The correction requires calculations that incorporate the volumetric shrinkage of the specific polymer grade being molded.
Mold Sizing
Tool designers increase the machined dimensions of the cavity relative to the nominal part dimensions. This compensation must consider both the isotropic shrinkage of amorphous resins and the highly directional shrinkage of semi-crystalline polymers.
Shrinkage Variance
Actual shrinkage in the production run often departs from the nominal values provided on the material datasheet due to varying processing conditions. Factors such as injection pressure, melt temperature, mold temperature, and holding time directly influence the final part dimensions. If the hold pressure is too low, the part shrinks more than expected, leading to undersized components.
Conversely, high hold pressures pack more polymer into the cavity, reducing the final shrinkage and potentially creating ejected parts that are larger than the design specification.
Dimensional Control
Continuous monitoring of part dimensions allows the mold technician to adjust process parameters to keep the dimensions within tolerance. These adjustments must be within the processing window allowed by the material. If the required dimension cannot be achieved through process adjustments, the mold itself must undergo physical modifications.
