No abstract

Material functions of two engineering plastics [a poly(phenylene ether) and a polycether imide)] were characterized, including the shear viscosity, first normal stress coeffkient, storage and loss moduli, growth and relaxation of shear stress, and first normal stress coeffkient and relaxation moduli. The oscillatory shear and relaxation moduli data were employed to determine the temperature-dependent parameters of Wagner model. Various material functions, which were determined on the basis of this model in conjunction with the fitted parameters agreed reasonably well with the experimental results. The reported data and parameters should facilitate a better understanding of the processability characteristics of these two engineering plastics.

The gapwise density distributions of the injection molded specimens of two engineering thermoplastics, i.e., poly(pheny1ene ether) and poly(ether imide), were characterized employing the density gradient column technique. The samples were molded using a 40t Van Dorn injection molding machine. The effects of the thermal history on the density distribution of unconstrained quenched specimens were also investigated. In addition, various material properties, such as pressure-volume-temperature, isothermal contraction, and pressure induced densification behavior were characterized for the two resins employed in this study. The moldings of the two resins exhibited different trends in their density distributions. These findings were explained in terms of the competing effects of cooling rate and the pressure history experienced by the engineering plastic resins during the molding cycle. The data collected were also used as input to mathematical modeling of density distributions in injection molded articles, which is reported in Part I1 of this article.

Specimens of two engineering plastics i.e., poly(ether imide), PEI, and poly(2,6dimethyl-1,4-phenylene ether), PPE, were injection molded employing a 40t Van Dorn injection molding machine and industrial practices. The mold and melt temperatures and the injection speed were varied in a limited range which furnished acceptable samples. The density, birefringence, residual stress distributions, flexure and tensile properties, and crack development of the injection molded specimens were studied. Vacuum compression molded samples were also prepared to investigate the role played by the cooling rate in shaping microstructural distributions. The results revealed significant differences in the development of microstructure of the molded specimens of the two resins, which was related to rheology and molding conditions on one hand and to development of cracks and ultimate properties on the other hand.

SYNOPSISMathematical models were developed to predict the various microstructural properties, including birefringence, residual stress, and density distributions, in the freely quenched compression molded samples as well as in the injection molded samples. To model the birefringence distribution in the injection molded samples, the BKZ type integral constitutive equation was employed to account for the nonisothermal stress relaxation, which takes place during the cooling stage of the molding cycle. The predicted birefringence agreed well with the experimental data near the mold walls. The residual stress distribution was modeled by the existing thermoelastic theory. The residual thermal stress distribution in the freely quenched samples was predicted very well by the model. However, the predicted residual thermal stresses in the injection molded samples were much larger than the measured ones.A phenomenological model to predict the density distribution in injection molded sample is proposed by including the effects of both cooling rate and the pressure on the density development. The predicted results agreed well with the experimental data. I NTRO DUCT IONThe various mechanical and optical properties of injection molded articles are strongly affected by the thermo-mechanical history to which they are exposed during the molding cycle and the resulting microstructure.' Various aspects of the microstructure, including orientation distribution, density distributions, and residual stress distributions, can be predicted with mathematical models of the injection molding process in conjunction with realistic material proper tie^.^-^ The most recent literature regarding the simulation of injection molding process was recently reviewed by Kamal et a1.,6 where a comprehensive model was also presented. However, there were very few attempts to model density distributions in the injection molded specimens,'0J1 despite the fact that density is known to correlate