Apparatus for forming internally grooved bells on plastic tubes, having a mandrel comprising coaxial front and rear mandrel sections spaced apart to form a gap, angularly spaced supporting pins joining internal flanges on the elements across the gap, two sets of alternating core sections in the gap, one set having inwardly converging sides and the other having inwardly diverging sides, and an expanding mechanism comprising an actuator shaft rotatably supported in the mandrel, two rotary actuating plates mounted on the shaft on opposite sides of the core sections, and two guide plates mounted in the mandrel in fixed positions. The guide plates have a plurality of spaced radial guide slots, and the actuating plates have a plurality of inclined cam slots that extend radially outwardly across the guide slots. Follower pins on the core sections project into the cam slots and guide slots to be cammed and guided thereby as the actuating plates are rotated back and forth. In a functionally similar second embodiment, the guide plates are tied together in an assembly that supports the front mandrel section, and the shaft is shapted to receive and locate the actuating plates, and to permit easy disassembly for service.