shown in fig4 this snap like action results from the junctions of the groove sides with the floors and interconnecting bands functioning as toggle joints after the tube has been removed from the forming machines it may be flexed to a bent position and thereafter retain such configuration during this operation individual groove walls on the outside of the bend snap often in a random fashion from mutually convergent to divergent configurations in fig5 for example the outside wall portions of groove walls 40 have so snapped while their inside curved portion 41 have retained their mutually convergent orientation additional flexing of the tube causes groove walls 43 also to snap with toggle like action to an open mutually divergent configuration in other cases as shown in fig2 the snap action of the groove walls along the convex side of the bent flexible zone is random here for example it is seen that groove walls 45 have snapped open while walls 46 have not it thus is difficult to predict the exact order of the progression of toggle action we thus see that a flexible tube is provided of the type having minimal spring back tendencies which has a substantially smooth and unruffled peripheral surface that may be.