No abstract

Acrylonitrile butadiene styrene is used in applications where the surface appearance is important. Plates with surface texture, varying from highly smooth to very rough, were injection molded with various processing conditions. Morphology, roughness, and gloss of the surface were assessed in relation to the processing parameters. The surface characterization of the moldings was done using reflected differential interference contrast microscopy and laser microtopography. It was observed that the surface properties are affected by the molding parameters. The mold temperature, followed by the injection temperature and the holding pressure are the more influent parameters. The contribution of each parameter to the gloss varies with the type of surface, but for most of the surfaces the contribution of the mold temperature was above 90%. In general, higher values of these parameters improve the replication accuracy that reduce the roughness (and increase the gloss) of very smooth surfaces and cause the opposite effect in other surfaces. In surfaces with an anisotropic texture the gloss depends on the measuring direction. POLYM. ENG. SCI. 46:1394–1401, 2006. © 2006 Society of Plastics Engineers.

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.

To provide insight into the formation of shear-induced precursor structures, three apparently unrelated subjects are analyzed and discussed:  the saturation of crystallization from sheared polymer melts, evaluated with a new shear DTA instrument, the steady state in steady shear, and the entanglement−disentanglement transition. It is shown that the same large strains that saturate crystallization also lead to a reversible steady state in steady shear, where the viscosity of the sheared melt is constant in time, and their magnitude is only determined by the temperature of the sheared melt. Features of the melt morphology at this state are discussed, and their importance is highlighted to understand the possible mechanisms behind the formation of shear-induced precursors. Measurements of the reptation time for unsheared samples, and samples sheared up to the steady state, allowed the quantification of the entanglements loss during the transition between these two states (around 2/3), which is interpreted as an entanglement−disentanglement transition. The relevance of this result on assumptions of flow models, particularly the constant number of topological constraints, convective constraint release process, and chain stretch, is discussed.