Predicting the Skin-Core Boundary Location in Injection Moldings

Abstract

Over the past few years a technique for relating the quality and properties of sub-components to those of real injection moldings has been developed. A major aspect of this is the correlation of microstructure in the two situations. This paper describes the computer prediction of structure development in injection molded polypropylene. This is done on a personal computer using a finite difference method and it is shown that accurate predictions can be made in relatively low computing time. The characteristic skin in polypropylene moldings is shown to be controlled by the filling phase and is dependent on the shear stress and temperature. The computer model is able to predict the onset of skin formation and hence the skin thickness. These predictions have been shown to agree well with experimental observations. The skin-core boundary which has been found to have a major effect on the mechanical properties of molded polypropylene has also been shown to lie between the no-flow isothermal and the maximum shear rate locus. This has important implications for computer simulations of the injection molding process.

Development of the skin layer in injection moulding: phenomenological model

“…The effect of the operative processing variables on the morphological state of the mouldings has been widely study in the last decades [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][22][23][24][25][26][27][28][29]. The influence of the material characteristics (e.g.…”

Manipulation of the Microstructure of Injection Moldings in a Special Mold and Their Mechanical Behavior

“…[12,23,24] This crystallographic form is favored by shearing and by the crystallization in a narrow temperature range. [25,26] It may result from a higher melt temperature (due to viscous dissipation) or a slower cooling rate due to the rotation action of the mold surface. This is mainly evident at position P3 that shows always-high concentration of b-phase spherulites.…”

“…[26] Figure 9 presents the dependence of the skin ratio on the rotation modes along the flow length, for thinner and thicker moldings.…”

Morphological features and mechanical behavior of one- and two-phase polymeric materials simulated by molecular dynamics

“…Necessarily, these factors determine the final properties of the molded part. Experimentally, the formation of the skin was found to be thermally controlled at high temperatures and governed by the shear stress level at lower temperatures [17,18].…”

“…Future work will improve on this concept and use a more accurate model. The thickness of the skin layer depends not only on the thermomechanical environment (cooling rate, stress fields) but also on the molecular weight of the chains, with higher molecular weights resulting in thicker skins [18].…”