Methods and systems are provided for heating a dielectric preform material. An exemplary method includes inserting the preform material into a microwave cavity along a longitudinal axis of the microwave cavity and supplying the microwave cavity with microwave power having a frequency that corresponds to an axial wavelength along the longitudinal axis of the microwave cavity. The axial wavelength is greater than a length of the preform material along the longitudinal axis. The method includes heating the preform material within the microwave cavity by the microwave power and determining temperatures of the preform material at one or more locations on a surface of the preform material. The method further includes adjusting, based on the temperatures of the preform material, the microwave frequency to achieve substantially uniform heating at least on a sidewall of the preform material along the longitudinal axis.