Abstract: In an apparatus for heating preshapes consisting of a thermoplastic, in particular a fiber-reinforced plastic, the preshape is fed, by a conveyor belt, to a heating apparatus equipped with bright infra-red radiators. To provide time-dependent metering of the thermal energy, the installation possesses an electronic control unit, equipped with a microprocessor, which unit generates a periodically increasing and decreasing electrical signal voltage, whose time-curve and amplitude can be varied in accordance with the heat uptake capacity of the preshape. The signal voltage additionally controls a feed unit, which in turn feeds to the heating apparatus, by means of a phase control system, a periodic electric current which in its time-curve and amplitude corresponds to the pre-programmed signal voltage.