Two grades of 5 mm thick polypropylene (PP) sheets, one having linear polymer chains (PP-TF-1) and the other having long-chain branches (PP-TF-2), were drape formed, using moulds of different parameters. Single-sided heating of sheet was found to be suitable only at lower depths of draw. Double-sided heating gave good part shape conformance at all depths of draw. PP-TF-2 was found to have lower crystallinity than PP-TF-1, suggesting that it would have a lower sagging tendency and would give parts with more uniform wall thickness distribution. This was confirmed in formability studies. At higher depths of draw, an increase in draft angle was found to give more uniform wall thickness distribution, maintaining reasonable values of wall thickness. At lower depths of draw, draft angle was found to have no influence on product quality. Irrespective of material characteristics and other mould parameters, only moulds with generous values of corner radii gave strong corners.
High impact polystyrene sheets (HIPS) of 1.5 mm and 2.5 mm thicknesses were thermoformed on moulds of different parameters at sheet heating temperatures of 130 C and 140 C. Formability studies indicated no difference in wall thickness distribution at different sheet temperatures. Due to non-uniform stretching of the sheet, wall thickness distribution along the slant length was found to have three regions. In the regions near the mould corner and clamp point, wall thickness was found to decrease with increase in distance from the corner, in these zones. The decrease near the corner was much greater and sharper than that near the clamp point. The decrease in both regions was found to increase at higher depths of draw and decrease at higher draft angles. In the intermediate portion of the slant length, no significant variation in wall thickness was observed.
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