THE ASSESSMENT OF ORIENTATION, STRESS AND DENSITY DISTRIBUTIONS IN INJECTION-MOLDED AMORPHOUS POLYMERS BY OPTICAL TECHNIQUES

The injection-molding process leads to complicated spatial distributions of properties, in particular in the thickness direction. With the aid of optical measurements the complete state of an injection-molded amorphous polymer can be quantified with high resolution in terms of molecular orientation, residual stresses and density distributions. Although this statement holds in principle, in practice the attractiveness of optical techniques varies from polymer to polymer and the property in question. In this review the possibilities and limitations of optical measurements will be illustrated with examples of experimental results leading to an improved understanding of the molding process. The basis for the use of optical techniques is the stress-optical behavior of the polymer. This, however, depends on the type of polymer, PS, PC and PMMA representing three very different cases. The density is linked to the average refractive index, whereas the deviatoric part of the refractive index contains an orientation and a stress contribution. A generalized stress-optical rule is necessary to link the refractive index to the stress histories calculated by simulation programs. The generalized stress-optical rule contains a memory function in order to account for the time-dependence of the molecular orientation and two limiting linear stress-optical coefficients representing glassy and rubbery behavior. By systematic quenching experiments and sectioning of samples the various contributions to birefringence are identified. Besides the classical picture of molecular orientation due to flow in the filling and packing stage, an additional source for orientation is identified, i.e. transient internal stresses due to temperature- and pressure-induced shrinkage differences. Relaxation of internal stresses during and after cooling also leads to molecular orientation. Residual stresses in injection-molded samples are mainly pressure induced. The stress level is lower than in free quenching. Residual stresses can be relieved by the appropriate sectioning of the samples. The gapwise density distribution is derived from the gradients in the average refractive index determined by a quantitative Schlieren-optical set-up. In quenched samples a competition between residual stresses and cooling rate determines the gapwise distribution. In injection-molded samples the pressure history is the dominating parameter. A local maximum is always found close to the surface. With the pseudo-compressibility concept the heights of the maxima are overpredicted. In contrast to the peak, the density in the core varies strongly with distance from the gate.