In the process of raising elongated thermoplastic preforms to molding temperature by programming heat from vertically adjacent heaters at different temperatures into vertical zones of rotating preforms advancing before the heaters, the improvement of preventing heat waves emanating from one heater toward an oppositely disposed preform zone from overlapping into an area through which waves are issuing from the next adjacent heater. The apparatus includes blind members between immediately adjacent heaters extending toward the preforms to concentrate heat onto the oppositely disposed preform zone.