A base sheet is spanned over a lower roller, and an upper roller pulls a cover sheet along a path and presses it against the base sheet at the lower roller. A preheater housing along the path upstream of the rollers is formed with a chamber open toward the path. A radiant heater in the chamber is directed transversely at the path so as to heat the cover sheet passing along the path. A sensor along the path adjacent the chamber detects atmosphere around the chamber and generates an output when the detected atmosphere indicates an unsafe hot condition of the cover sheet. A shutter can shift between a retracted position offset from the chamber and an advanced position closing the chamber. A controller connected to the sensor shifts the shutter from the retracted position to the advanced position when the output is generated by the sensor.