The bulk temperature is an important parameter for processing of rubber compounds being crucial for the cross linking behavior and influencing various material properties. In rubber injection molding reduction of cure time by means of shear heating is state-of-the-art. Devices at the injection molding machine which generates coupled shear and elongational flow are in practical use but up to now available software tools for injection molding simulation neglect the effect of elongational heating. In this work as engineering approach a new viscous model for the prediction of temperature changes in rubber compounds flowing through conical dies and runner segments was deduced and experimentally validated. It describes the coupled shear and elongational flow in conical dies in cylindrical coordinates and allows predicting the potential for reduction of cure time taking into account shear and elongational heating. The calculations are carried out iteratively using shear viscosity data directly measured on a capillary rheometer as well as elongational viscosity calculated from the inlet-pressure drop of the same measurements. To verify the theoretical calculations experiments on a rubber injection molding machine were carried out by means of a specially designed mold. Varying injection speed and the angle of the conical dies bulk temperature measurements in the purged material were performed. By far the highest contribution to heating is caused by elongation as the polymer mass flows into a die. This was found to account for approx. 80 percent of heat introduced into a NBR compound, compared to just 10 percent caused by shear at the die walls. The measured bulk temperature rise showed a dependence on the flow rate and the cone angle with the flow rate being dominant. The results of the temperature measurements of the bulk material showed good correlation with the model results with an average error being lower than 5%. Keywords: Injection molding, Rubber, Cure time