Abstract: A core 6 is disposed inside of and spaced from a hollow plastic preform 1 subjected to external infrared radiation 4 including a first wavelength B.sub.1 that is absorbed by and heats the plastic, and a second wavelength B.sub.2 that propagates through the wall 2 of the preform. The propagated radiation 12 is converted into the first wavelength by the surface 7 of the core, and reemitted at 13 back towards the preform to heat its internal surface 8 to a higher temperature than its external surface 10. The preform may be axially rotated with the core stationary to implement uniform angular heating. Alternatively, the core may be irregularly configured and/or surface treated, with both the core and preform rotated in synchronism.