Volume 2, Issue 4

A sound scientific basis for understanding what happens during the processing of materials, and how that stage affects the service performance of the structure, is an essential prerequisite to the development of efficient low cost fabrication technology. This paper considers the phenomena – chemical, thermal, thermodynamic and rheological – which occur during processing and reviews the information which we have available today as a basis on which to build a more comprehensive study and data base.

A finite element program is developed for simulating the non-isothermal flow of non-Newtonian liquids in a multi-cavity mold with convergent runners and gates of any arbitrary cross-sectional shape. The program is used to investigate the problem of balancing multi-cavity molds. Results of simulating an actual mold filling show that balancing can be achieved by using a straight runner together with convergent gates having successively increasing angles of convergence. In addition, a strategy for approximating those angles of convergence is suggested.

The effect of high molecular mass (MM) tail in the molecular mass distribution (MMD) of polypropylene (PP) on injection molding has been studied using injection molded tensile bars from a commercial PP resin and two samples of the above resin deprived of the high MM tail in different degrees by controlled degradation during extruder reaction with an added processing aid named Promax. The central portions of the tensile bars were studied by polarizing microscopy of microtomed sections parallel and perpendicular to the melt flow direction, and by density and birefringence profiles across the thickness of the bar. Mechanical properties and acoustic emission behavior in stretching of these injection molded tensile bars are also discussed. The experimental results show that by stripping off the high MM tail in the MMD the injection molded tensile bar has smaller degree of crystallization, less frozen orientation, improved uniformity across the thickness and the thickness of the skin part is greatly reduced. Better quality of the fabricated articles is to be expected by using Promax with PP for injection molding.

Basic studies of flow visualization for elastomers and plastics in an internal mixer are described. A modified laboratory mixer with transparent front and trasverse windows was used to observe parallel and perpendicular motions. Shearing, stretching and tearing motions were observed especially for elastomers. Various rotor designs were used. These indicate different flow fields along the rotor axes, and the interchange of materials between rotors. We describe the different flow fields observed for elastomers and plastics creasted by various rotor designs.

The viscous and especially the elastic properties of polymer melts can be altered by a previously imposed deformation history. This phenomenon, usually termed as shear modification, is caused by the reversible physical disentanglement of the polymer molecule chains. The shearing unit presented in this paper was used to systematically investigate the processing parameters governing shear modification. A set of rheological tests was carried out with the shear modified sample material and the expected changes in the rheological behaviour were detected. The relative change of the storage modulus G’, measured using a Rheometrics On-line Rheometer system, was taken as a measure for the degree of modification and was found to correlate with the specific energy dissipated during sample preparation. The effects of shear modification on product properties were investigated for the tubular film-blowing process. Changes in the optical and mechanical properties of films which can be attributed to the previously imposed shear modification process were found.

A critical discussion is presented of the influence of chemical degradation mechanisms on the mechanical properties, especially stress relaxation, of vulcanized rubber. The response of the vulcanizate is interpreted in terms of the kinetic theory of rubber elasticity. Different chemical mechanisms are explored. The influence of diluent is interpreted in terms of the kinetic theory of rubber elasticity. Different chemical mechanisms are explored. The influence of diluent is considered. Comparisons are made to experiments on polyisoprenes and polyurethanes.

A model for simulation of flow in an intermeshing kneading disc region of corotating twin screw extruder is presented. This is based on hydrodynamic lubrication theory for a Newtonian fluid. Numerical procedures are used to compute the pressure distribution and flow field in the kneading disc region. The influence of staggering angle, nip clearance, screw speed and throughput rate are investigated.