Molding Shrinkage and Warpage for Reinforced Poly(ethylene Terephthalate)

Abstract

The shrinkage values, their variation and anisotropy were observed under various molding conditions and for various composites of reinforced poly(ethylene terephthalate). Then the relationship between shrinkage and warpage is discussed with reference to box-shaped and plate-shaped molded-articles. It was found that the shrinkage rates at two axes are respectively comparable to their thermal expansions between room temperature and mold temperature. The shrinkage of the transverse direction is highly sensitive to variation in the injection pressure and the distance from the gate. The warpage was very small if the differential shrinkage was smaller than 0.3%. It is considered that differential shrinkage larger than 0.3% exceeds the tolerable strain as internal stress. Formless or lesser anisotropy fillers are more effective than flaky fillers in lowering the inward inclining warpage of a box-shaped article. As for a three dimensional part, lowering the shrinkage in the isotropic state is believed to be most important in avoiding warpage.