Warpage and its prediction in injection-molded parts

In injection molding, the pol5uner is cooled from the melt temperature to a temperature below the solidification, which is accompanied by a reduction of pressure in a mold before the molded item is ejected from the mold. Since the ejected molded part is still hot, its cooling to room temperature occurs under atmospheric pressure. As a result of temperature and pressure changes, the cooled polymer shrinks according to the PVT (pressure-specific volume-temperature) relationship or the thermal expansion coefficient of the polymer. The linear mold shrinkage is 0.1-0.8% for amorphous polymers and 1.0-2.0% for crystalline polymers. For polypropylene, it is about 1.7%. Shrinkage affects the dimensional accuracy and dimensional stability of molded items. 

For an L-shaped molded part, its flanks come closer to each other because of the local shrinkage at the comer. This is called the comer effect. The inner warpage of box-shaped parts is also due to this corner effect. For a center-gated disk, if the inner shrinkage is larger than the outer one, the disk will twist, otherwise it will dome. These phenomena are called buckling. 

Matsuoka, T., Takabatake, J., Koiwai, A., Inoue, Y., Yamamoto, S., and Takahashi, H. (1991) Integrated simulation to predict warpage of injection molded parts. Polym. Eng. Sci-, 31,1043-1050. 

Matsuoka, T. (1995) Fiber orientation prediction in injection molding, in Polypropylene Structure, Blends and Composites, Chap. 3, (ed. J. Karger-Kocsis), Chapman Hall, London, pp. 113-141. 

Folgar, F. and Tucker 111, C.L. (1984) Orientation behavior of fibers in concentrated suspensions. /. Reinforced Plast. Composites, 3, 98-119. 

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Warpage and its prediction in injection-molded parts FACTORS OF WARPAGE... 

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