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Computer simulation packages have had success in predicting filling behavior in extremely complicated geometries. However, few packages offer the possibility of predicting the location and strength of weld lines. This work examines the sensitivity of Moldflow's weld‐line prediction algorithm to variations in material properties and processing conditions. Qualitatively, the algorithm correctly predicted the effects of changes in viscosity, density, and PVT relationship on weld‐line strength of a poly(methyl methacrylate). The algorithm is also successful in predicting the influence of variations in the injection time on weld line strength. However, the algorithm predictions for changes in the mold and die temperatures were at odds with the experiment. An attempt was made to correlate Moldflow's computed viscosities with the experimentally measured reduction in the strength in the weld‐line area. It was shown that a one‐to‐one relationship existed between these two quantities. Whereas the potential of using the viscosity to predict weld‐line strength has been demonstrated, further refinement of this new concept is needed and its validity for other systems has to be established.

The Hamiltonian Bracket Formulation combined with the Avrami equation was used to develop a model for the flow-induced crystallization of polymers. The amorphous part of the chain was modeled as a non-linear Hookean dumbbell and the crystalline part as a rigid rod. Evolution equations for the configuration tensors of the two phases and for the degree of crystallinity were obtained and were subsequently solved for the transient and cessation case under different flow conditions. The model has a number of parameters that can be obtained (with the exception of one) from Theological measurements and quiescent crystallization experiments. The model predicts the variation in the degree of crystallinity and the induction time as a function of the polymer properties and the processing conditions. The model can also be used to predict the development of birefringence. The predictions of the model are in general agreement with the experimental observations.

SynopsisAn analysis is presented of the effects of external flow kinematics on the so-called local flow in seeded, flow-induced crystallization and orientation. The flow field around a growing crystal or nucleation seed is modelled by the Stokes flow equations past a prolate ellipsoid of high aspect ratio. Exact solutions for various flow kinematics, worked out elsewhere by the singularity method, are applied here to the analysis of local gradients. The results show that along the symmetry axis of the spheroid, the extensional gradients which result for various free-stream velocity fields are primarily the result of the constant-velocity free-stream component. However, free-stream, extensional flow can significantly enhance the region of such high gradients. Along the symmetry plane of the spheroid, primarily shearing gradients result, with small extensional gradients occurring when the free-stream flow has extensional components. Results of chain extension and birefringence calculations are also presented and discussed.A key feature in the dynamics of the crystallization process is the difference in influence of the macroscopic flow kinematics on the primary nucleation and continuous growth steps. In the former case, it has been borne out by a number of studies that nucleation of fibrous crystals is related to the presence of regions of extensional flow, while continued growth of an already nucleated or seeded fiber can occur in predominantly shearing flows.2 Mackley3 gave an explanation for this observation by introducing the idea of "local flow." He pointed out that though extensional flow is necessary to nucleate a fiber, subsequent growth will be more influenced by the local flow around the tip of the fiber than by the macroscopic velocity field existing in the main flow. To show the effectiveness