ABSTRACT
I. INTRODUCTION At a sufficiently low temperature, every polymer acts like a hard glassy materiaL To process such a polymer into a fiber or a film or a molded article, it is necessary to beat it to a temperature at wbicb it either softens (if amorphous) or melts (if crystalline) and at which it can undergo a shaping operation. Because the shear viscosity of flexible chain polymers decreases as the temperature increases, polymer processing is done at high temperatures. The particular temperature picked is based on a number of considerations. The temperature has to be high enough that the polymer flows easily, but not so high that an excessive amount of heat has to removed when the plastic part is cooled back to room temperature. since this represents a penalty both in terms of energy consumption and processing time. ln addition, the material must not thermally degrade at the chosen temperature. Besides temperature, the operating pressure and the rate of shear are also important. ln injection molding, for instance, very high pressures are employed, especially in the packing stage. This results in an increase in the viscosity. During mold filling, on the other hand. the shear rate in the melt delivery system is typically about 1000 sec 1, while at the mold gate it may be as high as 105 sec -1• Under these conditions, the polymer may exhibit marked shear thinning, and the viscosity at high rates of shear may be several orders of magnitude smaller than the viscosity at low rates of shear. It is, therefore, necessary to measure the shear viscosity, called the apparent viscosity, as a function of temperature, pressure, and the shear rate.
