The bulk temperature is a very important parameter for the processing of rubber compounds. Currently, standard injection molding simulation is taking into account just shear heating, while elongational heating is neglected by most available software tools. A new viscous model for the prediction of temperature changes in rubber compounds flowing through conical dies and runner segments was deduced where elongational heating is considered as well. Shear viscosity was directly measured on a Capillary Rheometer and elongational viscosity was calculated from the inlet-pressure drops using a new approach. 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 die angle bulk temperature measurements in the purged material were performed. 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 showed good correlation with the model results with an average error being <5%. POLYM. ENG. SCI., 55:701–709, 2015. © 2014 Society of Plastics Engineers