In the past, generally, spirally wound tubes formed from plastics have been made from soft rubber-like plastics or from rigid plastics that have been heated and softened and then wound into a spiral tubular configuration.
In U.S. Pat. No. 4,209,043, Julian M. Menzel, a new approach is described according to which a unique ribbed ribbon is used. The ribbon is shaped so that a spirally wound tube can be produced with interlocking edges of the ribbon formed by the shape of the ribs on the ribbon itself.
Also known is a machine for spirally winding such a strip to produce an elongated tubular article. In the operation of this machine, a differential pressure or force is applied to the parts of the strip being brought into contact with one another for interlocking the edges of the ribbon to form a spiral joint. These parts are fed under differential force so that the diameter of the tube can be controlled according to the extent of the difference in the force applied to the two interengaging edges. Also, the method of interengaging the two edges of the tube which are to be locked together is such that the application of this differential force forces these interengaging edges firmly together to give an effective joint.
However, it is possible to wind such strip on a mandrel. But whether the machine described above is used to produce the tube, or whether a mandrel is used, it is only possible to maintain control over relatively very minor changes in diameter that can occur during the winding process.
It is an object of the present invention to provide certain improvements to the form of the strip which will allow greater changes in diameter to occur during the actual winding of the strip into a tube, and such changes in diameter can be relatively rapid or quite slow, to give approximately a range of from 1.degree. of taper to 20.degree. or more of taper.
It is a further object of the invention to so construct the spiral joint that, even when made of a very rigid plastic for a tube of considerable rigidity, either with ribs as an outer part of the configuration, or the ribs as an inner part of the configuration, or both, the strip can readily be spirally wound into a tube, yet have a spiral seam of each integrity that it will seal into a watertight joint with or without the need for using a sealing compound.
This special feature is achieved by using a particular type of snap lock continuous joint which is so designed that when snapped into place, by whatever means, for example by means of a pair of pinch rollers in one type of the above-described tube forming machines, the spiral joint will seal and yet still allow itself to be manipulated into a larger or smaller diameter anywhere along the length of the tube by simply applying a circumferential twisting moment, applied either within the confines of the above-described machine which is winding the tube, or even after the tube has been wound.