Residual stresses in injection‐molded amorphous polymers

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Abstract
Nonisothermal flow of a polymer melt into a cold cavity and its rapid cooling give rise to the buildup of flow and thermal stresses in the molded article. In the present investigation the resultant residual stresses (RS) induced by these two sources were studied in two stages. First, the flow‐induced stresses were relaxed by proper heat treatment followed by quenching, resulting in only thermal stresses. The experimentally determined RS profiles in polysulfone and amorphous polyamide showed a parabolic shape and were correlated with the initial and final quenching temperatures, the glass transition temperature, and Biot Number. In the second stage, the combined effect of thermal‐ and flow‐induced stresses was studied using injection‐molded specimens prepared under a wide speptrum of molding conditions including melt and mold temperatures and injection rate and pressure. Results here indicated that the basic thermal‐induced parabolic RS profiles are altered by the flow‐induced stresses resulting in complicated profiles including local maxima and unbalanced RS. Finally, the tensile mechanical properties obatained for plaques molded under the various injection‐molding conditions were studied and correlated in part with the previously determined RS profiles. Results have shown that a property gradient exists as a function of distance from both the gate and surface of the molded plaque.