Orientation in injection molded polystyrene

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Abstract
The ultimate properties of injection‐molded thermoplastics articles are controlled to a large extent by flow and heat transfer phenomena that take place during the injection‐molding process. In fact, the thermo‐mechanical history of the melt during the molding process leads to a non‐uniform distribution of many of the critical properties of the molding. Birefringence has been employed as an indirect measure of the distribution of frozen stresses or strains in amorphous polymers. The present study employs birefringence to study the development of frozen stresses in injection‐molded polystyrene. In general, orientation in the flow direction is much greater than the orientation in the transverse direction of the moldings. In the vicinity, of the gate, where mold filling is characterized by spreading radial flow of the melt, the hoop stresses (planar deformation) at the melt front give rise to high orientation in the transverse direction. It appears that relaxation phenomena are not very important during the filling stage; however, they become more, important in the packing and pressure holding stages. With the aid of the appropriate rheo‐optical relationship, it is shown that the distribution of frozen‐in orientation in injection‐molded polystyrene may be estimated on the basis of data relating to pressure variations during the filling stage.