Injection Molding of Polystyrene with Spiral Test Mold

II. Mechanical Properties of Spiral Moldings

Abstract

Effects of molding conditions on mechanical properties of polystyrene (flexural, tensile, impact and stress cracking properties) were studied using the long spiral injection moldings which had been obtained in the course of evaluation of the flow property in the spiral flow mold cavity. The test pieces were cut from straight part in the pseudoellipse spiral moldings parallel and perpendicular to the direction of flow.
1) From the mechanical characteristic values parallel and perpendicular to the direction of flow, Q_// and Q_⊥, the ratio Q_///_Q⊥ was calculated as a measure of the mechanical anisotropy, and from the mechanical characteristic values in the part near the gate and the other end, Q₀ and Q₁, the ratio Q₀/Q₁ was calculated as a measure of mechanical inhomogeneity. The values of Q_///Q_⊥ in the part near the gate and Q₀/Q₁ parallel to the direction of flow were magnified in such a long moldings as spiral, so that these proved to be a good measure to evaluate the materials and to select its optimum molding conditions.
2) Mechanical properties of polystyrene especially impact strength were much affected by the molding conditions and the location in the moldings, and the value of Q_///_Q⊥ in the part near the gate or Q₀/Q₁ parallel to the direction of flow was extremely great, and became greater under higher injection pressure and at lower melt temperature.
3) These mechanical properties were correlated intimately with the structure of the moldings observed by polarizing microscope or with degree of orientation in the moldings measured by optical birefringence.