Device for heating plastic containers and resonator therefor

The invention relates to a device (1) for heating plastic preforms (10), comprising at least one microwave generator (2) that generates an alternating electromagnetic field in the form of microwaves, a microwave transmission unit (4) that transmits the microwaves generated by the microwave generator (2) to a resonator unit (6), and a conveying device that conveys the plastic preforms (10) to the resonator unit (6), wherein the resonator unit (6) comprises a resonator housing (8) that forms a receptacle space (10) for heating the plastic preforms (10), said housing having at least one inner wall (22a, 22b, 22c) facing the plastic preforms (10) and an injection area (12) through which the microwaves are introduced into the resonator unit (6), wherein the inner wall (22a, 22b, 22c) is annealed on the surface thereof facing the plastic preforms at least in some sections such that the annealing reduces power losses in the inner wall caused by the microwaves.