apparatus for heating a plastic sheet uses a high dielectric constant material to shape an rf electric field to be either concentrated or uniform as desired depending upon the object to be formed from the sheet . electrodes can be annular with a circular or elliptical cross - section or be plates or wires , and can be disposed in the hdcm or exterior of the hdcm . a process for heating a plastic material comprises generating an rf field and shaping the field near the material using high dielectric constant members .