Tensile stresses in the edges of injection moldings: Roles of packing pressure,machine compliance,and resin compression

The slow spontaneous development of cracks in the edges of injection moldings under “field” conditions has been observed for 30 years or more. While environmental stress cracking agents have long been implicated, the magnitude and distribution of the stresses associated with cracking have been obscure. The current study of these stresses involved polycarbonate as a model test material that was molded under systematically varied molding conditions. Surface tensile stresses, though rarely great enough alone to cause “dry” crazing or cracking were revealed through exposure to environmental stress crazing and cracking (ESC) agents. Using an old technique involving a set of calibrated ESC liquids, edge tensile stresses as great as 18 MPa were found in the edges of the moldings. Other, independent methods of stress assessment gave results in semiquantitative agreement with those of the ESC tests. Packing force, machine compliance, injection hold time, and mold flashing emerged as major variables either raising or mitigating stress levels. The root cause of the edge tensions is the mismatch in the times and pressures at which the skins and cores of moldings solidify. In short, skins quench at low pressure first, while cores solidify later during the packing stage. Upon release from the mold, elastic recovery of the core stretches the skin. More importantly, machine and mold compliances allow expansion of the part in the packing stage, during which certain areas of the skin are stretched. Solidifying the core during the packing preserves part of the skin extension as elastic strain. These effects are capable of outweighing the classical tendency of quenching to generate skin compression and core tension. A number of other effects, including release from the mold before the core has solidified, and flashing of the mold, have been found to limit the rise of skin tension.