Dynamic Cavity Pressure Integral Correlation with Part Dimensions in Injection Moulding
Dynamic cavity pressure curve integration over hold time governs volumetric polymer density and predicts final part dimensions before tool ejection occurs.
Polycarbonate Shrinkage
Meaning
Dimensional change in molded polycarbonate parts is characterized by a low and highly uniform contraction rate due to the rigid, amorphous molecular structure of the polymer. This predictable behaviour means polycarbonate shrinkage is typically less than one percent, making the material highly suitable for precision optical, electronic, and mechanical housings. The uniform contraction minimizes internal stresses and results in parts that closely match the mold cavity dimensions.
Processing Effect
Mold temperature and holding pressure are the primary process variables that dictate the final dimensions of the part. High holding pressure forces more material into the cavity, which directly reduces polycarbonate shrinkage and limits the formation of sink marks. If the mold temperature is set too low, the polymer freezes quickly, locking in internal stresses that can cause premature part failure under mechanical load.
Dimensional Control
Precision tooling must be machined to account for the specific contraction of the selected polycarbonate grade. Unfilled grades shrink almost isotropically, whereas glass-filled variants shrink less but show greater directional variation. Tooling designers must separate these directional contraction values to avoid warpage in parts with complex geometries or varying wall thicknesses.
Annealing Procedure
Post-moulding heat treatment can be used to relieve the molded-in stresses caused by rapid cooling. Placing parts in an oven below the glass transition temperature allows the polymer chains to relax, which can cause a small amount of additional polycarbonate shrinkage but greatly improves the impact resistance and environmental stress crack resistance of the final product. This secondary process must be carefully monitored because excessive temperatures can cause part deformation, while insufficient time in the oven fails to relieve the residual stresses that lead to premature stress cracking when the part is exposed to chemicals.