A method of heating a thermoplastic board by heat conduction in a heating system which has at least a first surface contacting tool and a second surface containing tool spaced from the first tool, at least one of which is heatable. The thermoplastic board is placed into the heating system between the two surface contacting tools, with an auxiliary sheet being placed between the contact surface of the respective heatable surface contacting tool and the surface of the thermoplastic board facing it. The space between the two surface contacting tools is then reduced to a predefined gap measurement, and the heatable surface contacting tool or tools is heated.