In polymer injection molding the melt homogeneity in terms of thermal, optical and mechanical quality is of great importance to produce high quality parts. While the influence of optical (e.g. color mixing) and of mechanical quality (e.g. fiber or filler distribution) is evident, the role of the thermal quality is often less obvious. Nonetheless, keeping in mind that axial temperature gradients in the screw chamber lead to similar gradients in the cavity, resulting in residual stresses in the molded part. Moreover, the melt temperature has significant influence on important molding process parameters as injection pressure and cooling time. The aim of the presented work is to show the influence of process parameter variations on the temperature of the melt cushion in the screw chamber of a reciprocating screw plasticizing unit. The temperature is measured with an ultrasonic measurement system directly in the screw chamber, as well as with a thermographic imaging system when the melt is leaving the nozzle during an air shot. It will be shown that although the maximum melt temperature measure is in good accordance for both type of measurement systems, the temperature drop in the screw chamber towards the screw tip is underestimated by the thermal imaging system. Results are given for two different polymeric materials.