A crosshead assembly for forming extruded shapes from molten polymer. The assembly includes body sections, a mandrel, a choke ring, and a tip holder for admitting, turning, and accelerating molten polymer toward a novel rotatable die sub-assembly. The die sub-assembly includes a hub, axially-loaded duplex ball bearings, and a pulley for rotating the sub-assembly. The choke ring extends into the die sub-assembly, and a cylindrical seal element is disposed therebetween. A changeable extrusion tip is threaded onto the second cylindrical portion of the tip holder. A die in the sub-assembly is specific to the shape to be extruded. For forming a spiral ribbon and a skin layer on core material, the die opening is a circular central aperture and a radial slot. Multiple ribbons may be extruded simultaneously by using multiple slots.