Method and apparatus for expanding and conferring a cup shape to the terminal junction segment of bi-axially oriented pipes made of thermoplastic material

Abstract

A method and apparatus for conferring a cup shape to the terminal junction segment of pipes that are bi-axially oriented longitudinally and circumferentially and hence very sensitive to diameter and length reduction through heat. The apparatus includes a furnace which heats the segment to a differentiated temperature, increasing towards the end of the segment such that the inner diameter of the terminal segment progressively drops down to a controlled value as temperature increases (whilst length of the pipe is simultaneously reduced, with a corresponding increase in thickness of the wall of the terminal segment). Preferably then, in an appropriate station, an additional heating is executed to a plastic deformation temperature suited to obtain a correct preliminary dilation of the terminal segment, upon introducing a rigid element which acts as inner contrast, thereby inhibiting any retraction thereof. Thus, the segment undergoes a thickening of the dilated wall both during the introduction of the rigid element, and during its extraction therefrom. Such terminal segment, dilated and heated, is then easily and rapidly fitted onto a calibration expander, itself also heated to such a temperature as to limit heat absorption from the segment. Again the terminal segment thickens and takes its shape and, after the possible additional heating to force the terminal segment to adhere spontaneously and perfectly to the underlying expander. Finally, at least the outer surface of the terminal segment, thus shaped, is cooled.