a gas fed catalytic heater is employed as a heat source in the thermoforming of shaped articles from a sheet of thermoplastic material . the thermoplastic sheet is fed from a continuous supply roll past an array of gas catalytic heaters in a heating section in order to soften the sheet prior to entering a mold for shaping the articles . a closed loop temperature control system is provided for controlling the pressure and hence the volume of gas entering the catalytic heaters and thus the emitted infrared radiation to automatically maintain the temperature of the thermoplastic sheet at a predetermined set value . the control system includes a temperature sensor for measuring the temperature of the sheet , a temperature controller for comparing the actual temperature of the sheet against a preset temperature and for producing an output signal that is proportional to the difference temperature and a gas modulating valve which is responsive to the output signal for adjusting the flow of gas to the catalytic heaters .