Plastic web to be fed into opposite dies for forming articles must be heated and softened by heater means in front of the forming machine, even if said machine is combined with an extruder supplying said plastic web. The invention provides a radiant heater adapted to be sensitively responsive to manual or automatic control so as to more suitably regulate a temperature of the plastic web in comparison with the conventional heater comprising quartz tubes each having a coiled nichrome wire. The heater of the invention comprises an extended thin nichrome electrical resistance heater element plate (30) arranged in a zig-zag on a support plate (11) and insulating plate (13).