a plastic bobbin consisting of a hollow cone defined by a thin - walled sheath includes an inner skeletal frame of longitudinally - spaced circumferential ribs and radially - spaced longitudinal ribs defining an inner support surface for a mandrel with a sheath having segments of varying wall thickness and the outer surface having a constant taper with respect to the bobbin axis . some of the longitudinally - extending ribs may be partial ribs defined in each of the segments .