Abstract: A hollow heating head for insertion into an open, spouted, top end closure of a carton comprises an interior chamber with slot-form outlets for hot air supplied to the chamber. The slots include a U-shaped, narrow slot, or its equivalent, extending generally horizontally around two lateral vertical walls and an end vertical wall of the head. Each limb of the U-shape includes horizontal, upper and lower sections and a vertical intermediate section. The base of the U-shape is adjacent the lower sections, is at substantially the same level as the lower sections and is of a shallow V-shape in a vertical plane. The slots also include several vertically extending slots above each of the base and the lower sections of the U-shaped slot.