Injection molding has been widely used because of its versatility in fabricating complex shapes in a single process. Part warpage, however, is sometimes a problem with injection molding, hence many studies have been performed on understanding the mechanisms of warpage. Most of these studies have investigated simple injection molded shapes. In this report, the warpage generation mechanism was considered for a complex shape with the following results obtained:
1) Warpage was measured for injection molded plates with a variety of shapes. The greatest warpage was found in the plate with a rib.
2) The high warpage for the plate with the rib is due to large material property variations.
3) Very complex melt flow behavior, such as diverging flow, micro flow and accelerating flow, were observed in the vicinity of the rib. This complex melt flow was responsible for the strong location dependence in material properties near the rib.
4) Warpage in the parts with a complex shape is caused not only by the thermal-induced stresses but also by the flow-induced stresses.

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Heat sealing is currently a standard method for manufacturing plastic bags of polylactic acid film, which has high clarity and strength. However, the heat sealing of polylactic acid is difficult because its strength declines at high temperatures. Processing conditions such as the heat sealing temperature and pressure can greatly affect the mechanical properties of the heat sealed part. In this study, the relationship between the heat sealing temperature and the mechanical properties of heat-sealed polylactic acid film were examined through peel strength tests and tensile testing of samples with circular notches. Differential scanning calorimetry (DSC) and FT-IR imaging were used to investigate the dependence of crystallinity and molecular-orientation ratio (MOR) on the heat sealing temperature. These studies showed that the optimum heat seal temperature was 128°C, with a very narrow temperature window. Thus, precise control of the heat sealing temperature is required for polylactic acid plastic bag manufacture.

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