ABSTRACT
During various processing operations, polymeric melts frequently flow through juncture regions—areas with a sudden or gradual change in cross section (contraction or expansion). In injection molding such regions are usually encountered at the entrance to runners, gates, and cavities, and within the cavities themselves due to possible ribs, bosses, obstacles, or dimensional nonuniformities. Although this situation in many respects resembles flow through fittings, considered in fluid mechanics, the problem of polymer melt flow through junctures is much more complicated, especially when evaluating stresses and pressure losses. The major complication arises due to the viscoelastic nature of polymeric melts exhibiting highly non-Newtonian flow behavior accompanied by significant elasticity. Moreover, the nonisothermal character of the flow in most polymer processing adds another dimension of complexity. For Newtonian flows in classical fluid mechanics, various experimental charts and tables have been compiled to evaluate the pressure required to push the fluid through various fittings [1]. Such a generalized approach to polymeric fluids is extremely difficult and does not seem to be amenable at the present time, although these difficult problems are receiving extensive attention both theoretically and experimentally. In particular, the theoretical work includes numerical simulation of polymeric flow in juncture regions in terms of various constitutive models. Despite considerable effort by investigators, the progress has been slow, and more time will be 138needed before the results can be used in practical applications. On the other hand, the trend in experimental work includes establishing some semiempirical correlations (master curves) for different geometries useful for industrial applications. Due to limitations of both the numerical simulations and the experimental work, however, there has been little common ground between these two approaches. Obviously, to verify the predictive capability of existing constitutive models for describing real experiments, we need extensive comparisons between simulated results and experimental data. Thus, the main scope of this chapter is to give an up-to-date account of the developments in the area of juncture-type flow problems along with some implications for mold design.
