the formation of tubular objects by spirally winding and seaming a relatively rigid strip of plastics material . a sealing flap along one longitudinal edge of the strip , and an engaging surface along the other edge positioned close to but separate from the interlocking portions of these longitudinal edges . the provision of these sealing flaps and engaging surfaces permit the interlocking portions at the edges to be designed independent of the ability of these interlocking portions to create a fluid seal . this results in optimum operation of these interlocking portions to , for example , permit relatively rapid changes in the diameter of the tube as it is formed , as well as subsequent to its formation .