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 m oldings under ''field'' conditions has been observed for 30 years or m ore. While environmental stress cracking agents have long been implica ted, the magnitude and distribution of the stresses associated with cr acking have been obscure. The current study of these stresses involved polycarbonate as a model test material that was molded under systemat ically varied molding conditions. Surface tensile stresses, though rar ely great enough alone to cause ''dry'' crazing or cracking were revea led through exposure to environmental stress crazing and cracking (ESC ) agents. Using an old technique involving a set of calibrated ESC liq uids, 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. pa cking force, machine compliance, injection hold time, and mold flashin g emerged as major variables either raising or mitigating stress level s. The root cause of the edge tensions is the mismatch in the times an d pressures at which the skins and cores of moldings solidify. In shor t, skins quench at low pressure first, while cores solidify later duri ng the packing stage. Upon release from the mold, elastic recovery of the core stretches the skin. More importantly, machine and mold compli ances allow expansion of the part in the packing stage, during which c ertain areas of the skin are stretched. Solidifying the core during th e packing preserves part of the skin extension as elastic strain. Thes e effects are capable of outweighing the classical tendency of quenchi ng to generate skin compression and core tension. A number of other ef fects, including release from the mold before the core has solidified, and flashing of the mold, have been found to limit the rise of skin t ension.