Influence of processing conditions on the weld line in doubly injection‐molded glassy polycarbonate and polystyrene: Microindentation hardness study

The microhardness ( H ) technique has been used to characterize the quality of the weld line in injection‐molded tensile bars from a two‐component machine in which both melt streams from the same material can be independently controlled. More specific, the influence of melt temperature and indentation location (closer or further from the sample edge parallel to the injection direction and across the weld line) has been followed on polycarbonate (PC) and polystyrene (PS) glassy samples. For both polymers at lower melt temperatures, a strong H decrease (between 15 and 50%) followed by a sharp increase in a narrow distance (around 0.10 mm), is observed. When the melt temperature increases up to 300°C (for PC) and 270°C (for PS), a much smaller H decrease is observed in the central part of the samples. However, closer to the tensile bar edges (2 mm) the weld line remains undetectable by microhardness measurements. The present results reveal that the processing temperature affects the broadening of the weld line through the conditions for effective mutual interdiffusion of chains from the two fronts coming from opposite sides. © 2005 Wiley Periodicals, Inc. Adv Polym Techn 24:14–20, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20028