The goal of the heating phase in thermoforming is to obtain a uniform through-thickness temperature distribution as quickly as possible. Although the importance of the heating phase in thermoforming is clear, trial and error methods to determine the heating parameters are still common practice in industry. The current study addresses this issue by means of combined experimental and modelling efforts. The heating characteristics of two different thermoforming installations (halogen and ceramic heater elements) are experimentally determined by means of a silicone sheet with embedded thermocouples. The measurements allow to characterize the through-thickness temperature differences as a function of time. Lowering the emitted radiative power, decreases these temperature differences but also in-creases the required heating time. The experimental time-temperature data was implemented in a finite difference script to construct a framework of each thermoforming installation. In this script, heat transfer through radiation, convection and conduction are taken into account during the heating and pre-forming equilibration time. Thermal properties of the silicone sheet and various thermoplastic sheets were measured using DSC and MTPS methods. Based on the material properties and the target forming temperatures, the optimal heating settings of a thermoplastic sheet are determined. The methodology has been validated in industry by means of different case studies. Moreover the developed scripts allow to gain knowledge on the through-thickness temperature distribution as a function of time. The results of this study lead to a more comprehensive understanding of the contact free heating of thermoplastic sheets and to the op-timization of the heating step which lowers the start-up time and cost.