Morphological zones and orientation in injection‐molded polyethylene

Abstract

SynopsisThe properties of injection-molded plastics parts are affected by resin properties, molding geometry, and molding conditions. The thermomechanical history, which results from the interactions between resin properties and molding conditions, controls the development of microstructure within the article. In turn, the microstructure controls the ultimate behavior of injection-molded parts. Morphology and orientation and their distributions are among the most important microstructural characteristics affecting the ultimate mechanical and physical behavior of molded articles. This work summarizes some of the results of an extensive experimental and theoretical program to study the factors affecting the development of morphology and orientation in injection-molded polyethylene. In particular, various experimental techniques are employed to study the distribution of morphology and orientation in simple moldings and to relate these characteristics to resin properties and molding variables. Furthermore, an attempt is made to employ mathematical models to explain and predict some of the observed phenomena especially in relation to the distribution of morphological zones in the molding. It appears that model predictions are in good agreement with experimental results.

Influence of carbon nanotube–polymeric compatibilizer masterbatches on morphological, thermal, mechanical, and tribological properties of polyethylene

“…Conventional injection molding of polyethylene leads to skin-core structure since contact of hot polyethylene melt with the cold mold walls produces a temperature gradient and high stress for the melt, with resulting high orientation of polyethylene chains near the surface. In the skin layer, polyethylene chains are oriented in flow direction and form lamellas, while in the core the lamellas appear in the form of spherulitic crystals [42][43]. Four distinct layers have been characterized in an injection molded specimen of linear polyethylene [42].…”

“…In the skin layer, polyethylene chains are oriented in flow direction and form lamellas, while in the core the lamellas appear in the form of spherulitic crystals [42][43]. Four distinct layers have been characterized in an injection molded specimen of linear polyethylene [42]. Properties of the polymer, the processing parameters, and the geometry of the mold influence the formation of these layers and the orientation of polymer chains [42][43][44].…”

“…Four distinct layers have been characterized in an injection molded specimen of linear polyethylene [42]. Properties of the polymer, the processing parameters, and the geometry of the mold influence the formation of these layers and the orientation of polymer chains [42][43][44]. Moy and Kamal [45] found that maximum crystallinity of injection molded polyethylene occurs at the center, while maximum orientation occurs near the surface.…”

Anisotropic Elastic-Viscoplastic Properties at Finite Strains of Injection-Moulded Low-Density Polyethylene

“…3(b). A straight line is fitted to the marker displacements, and the slope of this fitted line provides an estimate of the engineering strain in the central part of the specimen, in this case 11 = du 1 /dX 1 .…”

“…However, between the skin and the bulk layers, adjacent layers of polymer melt flow at different velocities and generate shear stresses, which cause the polymer chains to be aligned in the direction of polymer flow. This produces a shear layer, which is highly oriented and is said to be in a state of frozen strain [8][9][10][11]. Hence, the injection-moulded material can be expected to be anisotropic and also inhomogenous in the thickness direction.…”