Corner Deformation of Injected Thermoplastic Parts

Abstract

During the cooling process in injection molding, heat fluxes in the mold are generally lower in inner regious of corners. This asymmetric cooling generates a corner deformation: the part angle is smaller than the mold one. This phenomenon is known to be the main cause of corner warpage in injection molding of thermoplastics.The second phenomenon leading to a corner warpage has firstly been described for thin compression molded SMC parts and is called "spring forward effect". Due to the large fiberlength / part-thickness ratio, most fibers in SMC parts are oriented in the planar direction leading to higher thermal expansion coefficients in the thickness direction as compared to those in the surface direction. When the part cools after polymer curing, a decrease of the enclosed angle in a corner occurs after mold extraction. Because of mold restraints for injection molded thermoplastic parts, the shrinkage in the thickness direction is much larger than the in-plane shrinkage and so the spring forward effect can appear.An experimental study and a two dimensional thermo-viscoelastic calculation of a polymer corner deformation in a mold performed on Abaqus code were carried out. We show that the spring forward effect is generally the major source of corner deformation for injected thermoplastic parts.

Deformation analysis of short glass fiber-reinforced polypropylene injection-molded plastic parts

“…Many studies have tested the deformation of unfilled thermoplastics. [14][15][16][17][18][19] Akay and Ozden 14 analyzed polycarbonate and acrylonitrile butadiene styrene and concluded that the examined specimens injection molded with a temperature difference between the two mold halves have resulted in the final part curving toward the hotter side of the mold. Tang et al 15 using acrylonitrile butadiene styrene plates for deformation analyses analyzed how melt temperature, filling time, packing pressure, and packing time affect warpage.…”

“…Ammar et al 17 used a specimen with four corners (Figure 1(b)) for their warpage measurements. It was concluded that two phenomena caused the deformation of the part: the first was the asymmetrical cooling and the second was the spring forward effect.…”

“…Figure1. Specimen geometries with their injection locations for deformation measurements used by (a) Jansen et al,16 (b) Ammar et al,17 and (c) Mlekusch 22. …”