Dimensional Reduction
A volumetric contraction occurs as a molten polymer or metal cools and solidifies within a forming cavity. This physical change, termed mold shrinkage, dictates the final dimensions of the molded component and must be compensated for during tool design. If the contraction is not accurately estimated, the finished sensor housing or connector will fall outside the specified mechanical tolerances.
It varies significantly across different material families.
Material Density
Amorphous plastics exhibit lower contraction rates because their molecular chains remain disorganized as they solidify. Semi-crystalline resins, such as polyamide or polyphenylene sulfide, undergo a substantial volume reduction as highly ordered crystalline structures form. This density increase results in higher contraction rates that demand precise control of cooling rates.
Process Factor
Injection pressure and mold temperature influence the extent of the contraction. High injection pressure packs more material into the cavity, reducing the final volume loss. Long holding times prevent the backflow of material before the gate freezes, ensuring more consistent part dimensions.
Tolerance Compensation
Mold designers scale the cavity dimensions upward using a material-specific contraction factor to ensure the final product meets the blueprint requirements. This factor is verified by measuring sample parts from a trial run with calibrated micrometers. The adjustment ensures that sensitive internal electronics fit perfectly within the protective shell.
