In thermoforming, a polymer sheet is heated by means of IR radiation. This method allows a fast and position dependent temperature distribution across the thermoplastic sheet. In principle, this heating pattern can be optimized according to the occurring strains and deformations during the forming step of the process. The feasible resolution of this temperature distribution is however linked to the size, type and geometrical configuration of the heater elements in the heater banks of the thermoforming equipment. Up to now the link between these restrictions and the feasible temperature resolution in a thermoplastic sheet was not yet described in literature. A first order finite difference model was developed to describe the transient temperature distribution in plane and across the thickness of the thermoplastic sheet. The model takes into account the relevant material properties, applied IR heat flux and convective environment boundary conditions. In order to capture the actual machine specific heating conditions, a single experimental measurement was used to update the model parameters. The obtained model was successfully validated against 17 different heating configurations. This model was subsequently used to determine the actually feasible in plane temperature resolution of a thermoplastic sheet. This result was compared to the optimal temperature field proposed by commercial software solutions. Since this does not take the actual heating configuration of the thermoforming equipment into account, the proposed optimal temperature fields were identified as non-realistic. Additionally, a user friendly link between the developed finite difference model and Microsoft excel was made to allow the user to generate the required input data for the heater banks. The effect of different heating zones and broken heater elements on the final sheet temperature can thus be simulated as well.