Method and apparatus for heating plastic preforms

A method for heating plastic preforms, wherein the plastic preforms are transported along a predetermined transport path and are heated during this transport, wherein at least one first heating section with at least one first heating element as well as at least one second heating section with at least one second heating element is arranged along the transport path, wherein by a first temperature measuring device an actual temperature of the plastic preforms is measured in or before a start region of the transport path, and as a function of this actual temperature of the plastic preforms the at least one first heating element is controlled in such a way that the plastic preforms reach a predetermined target temperature after heating by this at least one first heating element, wherein a material constant of the plastic preforms is determined, wherein in this determination of the material constant, which considers a mass of the plastic preforms, an absorption coefficient of the plastic preforms and a specific thermal capacity of the plastic preforms, is taken into consideration, wherein the material constant is taken into consideration during the heating of the plastic preforms.