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Effect of an obstacle during processing on the weld line of injection-molded glassy polystyrene: Microhardness study



The microhardness (H) technique is used to characterize the quality of the weld line in injection-molded glassy polystyrene by means of a cylindrical obstacle. In particular, the effect of the indentation location (closer or further from the obstacle edge parallel to the injection direction and across the weld line), both on the surface and in the bulk, was examined. Only for surface measurements close to the obstacle (up to 10 mm) a well-pronounced decrease in H (∼30%), followed by a sharp increase in a narrow distance (0.20–0.25 mm), was observed. For the bulk measurements on the same location a slight decrease in H was detected. Additional H measurements made up to 60 mm from the obstacle for both cases showed that the weld line remains undetectable. The results obtained reveal that the presence of a cylindrical obstacle causes the formation of a weld line on and near the surface only at distances not exceeding the obstacle diameter. At larger distances, because of the effective mutual interdiffusion of polymer chains, the two parallel fronts coming from the two sides of the obstacle developed a homogeneous material without any weld line according to the microhardness test. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3362–3367, 2004