Residual thermal stresses in injection moldings of thermoplastics: A theoretical and experimental study

Abstract

Internal stresses in injection molded components, a principal cause of shrinkage and warpage, are predicted using a three‐dimensional numerical simulation of the residual stress development in moldings of polystyrene and high‐density polyethylene. These residual stresses are mainly frozen‐in thermal stresses due to inhomogeneous cooling, when surface layers stiffen sooner than the core region as in free quenching. Additional factors in injection molding are the effects of melt pressure history and mechanical restraints of the mold. Transient temperature and pressure fields from simulation of the injection molding cycle are used for calculating the developing normal stress distributions. Theoretical predictions are compared with measurements performed on injection molded flat plates using the layer removal method on rectangular specimens. The thermal stress development in the thinwalled moldings is analyzed using models that assume linear thermo‐elastic and linear thermo‐viscoelastic compressible behavior of the polymeric materials. Polymer crystallization effects on stresses are examined. Stresses are obtained implicitly using displacement formulations, and the governing equations are solved numerically using a finite element method. Results show that residual stress behavior can be represented reasonably well for both the amorphous and the semicrystalline polymer. Similarities in behavior between theory and experiment indicate that both material models provide satisfactory results, but the best predictions of large stresses developed at the wall surface are obtained with the thermo‐viscoelastic analysis.

Residual stresses determination in injection molded virgin and recycled HDPE blends: mechanical properties and morphology

“…One of the reasons of the ignorance of flow induced stresses is that they are at least one magnitude smaller than absolute value that of thermal residual stresses and a large part of flow stresses relax, since the relaxation is faster at the higher temperatures that prevail in filling stage [1,14,24]. However, they induce anisotropy of several properties of the final product; the most seen dimensional faults such as warpage and shrinkage are generally caused by thermal residual stresses [1,16,24]. As a consequence, in this study the effects of recycled material concentrations on thermal residual stresses along the flow direction were investigated.…”

“…They used thermo-rheologically simple viscoelastic material model to consider the stress relaxation effect and to describe the mechanical behavior according to the temperature change. Kamal et al [24] studied on the residual stresses of injection molded HDPE plates using models that assumed thermo elastic and thermo viscoelastic compressible behavior.…”

Estudio y modelización de las condiciones de proceso en el moldeo por inyección de blends ABS/PC procedentes de materiales reciclados

“…Kamal, Lai-fook y Hernández [64] efectúan un estudio pormenorizado de la generación de tensiones internas en el material inyectado, poniendo en evidencia la dependencia de este fenómeno de valores como temperatura del material y velocidad de enfriamiento del mismo.…”

“…• Presencia de fracturas o grietas superficiales (EF02_04) [64] . Este fenómeno surge cuando el nivel de tensiones producidas por elevadas velocidades de enfriamiento en la pieza superan en zonas puntuales la resistencia a fractura del material.…”

“…• Presencia de tensiones residuales (EF02_06) [63][64][65] . Debido al comportamiento del material durante su procesado, no es posible evitar que éste acumule tensiones internas inducidas por la velocidad de enfriamiento, las diferencias de flujo de calor a través de las caras del molde y la tensión de cizalla producida durante el llenado.…”