Meaning
Mathematical method used to sum the individual variations in material shrinkage, mold machining tolerances, and process fluctuations determines the total expected dimensional variance of an assembled plastic component. Performing a shrink stack-up calculation prevents assembly failures by ensuring that mating parts fit together despite variations in the molding run. This calculation is essential when designing multi-component assemblies made of different polymer materials.
Tolerance Aggregation
Individual variations must be added together to find the worst-case scenario or the statistical distribution of the assembly dimensions. This step prevents the mistake of designing components that cannot be assembled because the variances accumulated during the manufacturing of each individual part.
Mold Design
Tool designers use the results of the calculation to establish the tolerances for the mold cavity and core inserts. This practice ensures that the mold is manufactured with sufficient precision to accommodate the natural shrinkage variation of the specified resin.
Production Analysis
Running production trials provides the empirical data required to refine the statistical model and confirm that the dimensions will remain within specifications under routine operating conditions. If the actual shrinkage of the molded parts exceeds the predicted limits, the process parameters must be adjusted to bring the dimensions back into the planned range, or the mold inserts must be modified to alter the cavity size before full-scale production begins.
