Precise Shrinkage & Attainable Long Term Part Tolerances

The Tolerance/Shrinkage/Cycle time analysis begins with the identification of factors, which with their variability affect the tolerances of part dimensions; for the computation more than thirty variables are taken into consideration.  They include the type, size, geometry and location of gate(s); wall thickness and its distribution in the product; fiber/molecular orientation; geometry of gated section of part; variability of material, mold temperature, machine, molding and inspection environment; distance of the critical dimension from the gate, and the geometry of the part section at the important dimension.  The analysis results are the attainable, long term part tolerances, minimum cycle time and optimum holding time, required material and machine control, cavity dimensions based on computed shrinkage, and maximum tolerances of cavity/core dimensions.  The following table shows the required and computed attainable tolerances and computed shrinkage for selected part dimensions under molding plant-specific (AS IS) and improved (TIGHTENED) molding and material control.