The heating of polymeric semi-finished parts is a crucial step in the thermoforming process. The efficiency of the heating stage is influenced by several factors, with the choice of the heating method being one of them. The most prominent and economically most common selection is infrared radiation heating due to its simplicity and fairly short cycle times. Yet, the efficiency of this choice of heating for polymer thermoforming is relatively low, accounting to fewer than 20% in most cases. There are several ways to improve the heat absorption of plastic sheets. One of them is to decrease the optical transmission of the radiation with the help of inorganic filler materials. Those fillers do not only alter the optical properties, but also increase the thermal conductivity and decrease the specific enthalpy of the polymer which should lead to a more uniform temperature distribution in the semi-finished part during the heating step. This study shall reveal the influence of the concentration of a talc filler in a polypropylene matrix on the heating prior to thermoforming. The used material is PP with a talc content of 0, 10, 20 and 30% extruded to sheets of varying thickness. Three thermocouples are soldered onto the sheets from both sides. Using quartz heaters, the sheets are heated from one side only, to study the effect of the filler on the thorough heating of the material. The results of the investigations show, that with increasing amount of the talc filler, the time needed to reach the designated forming temperature decreases. Furthermore, the temperature drop from one side to the other relatively to the actual temperature also decreases when talc is added. This leads to the conclusion, that inorganic fillers can improve the efficiency of infrared heating and lead to a more homogeneous temperature distribution.