this embodiment a cascade regulating unit with two control circuits which operate independently of each other is realized for regulating the exterior temperature 75 of the preform 1 the target value 74 and the output value 77 of the pyrometer 76 are here fed to the difference former 78 as well as to a difference former 17 whose output value 18 is conducted to a regulating unit 19 which in the embodiment represented is realized as a pid regulating unit an output magnitude 20 of the regulating unit 19 is fed to an amplifier 21 an output magnitude 22 of the amplifier 21 and the output magnitude 89 of the amplifier 90 are compiled in the region of a difference former 23 an output magnitude 24 of the difference former 23 serves in this regulatory variant as an input signal for the delimiter 93 it is additionally possible to outfit the regulating unit 80 with a monitoring system 25 which issues a warning signal especially when the cooling output of the blowers 58 68 has reached its minimal or its maximal value and if in this case the regulation of the exterior temperature 75 of the preforms 1 takes place exclusively through the heating elements 57 as an alternative to realizing the regulator 19 as a pid regulator it is also possible to undertake a realization as a pi regulator or for example as a pid ti regulating unit it is especially possible with the regulatory variant in accordance with fig1 to undertake a compensation through the regulating unit 80 in the event of negative regulatory deviations and to implement the equilibration with the aid of the regulating unit 19 in the event of positive.