METHOD OF WELDING GENERALLY ROD-SHAPED STRUCTURES OF A FLUORINE-CONTAINING PLASTIC MATERIAL IN A BUTT WELD

摘要

The invention relates to a method of welding two generally rod-shaped structures (5, 6) of a fluorine-containing plastic material in a butt weld to form a closed-loop construction. The structures may comprise sealing or backup rings for use in high pressure swivels. The method comprises steps of: placing the end faces of the structures in a contacting relationship by supporting each structure in a clamping device (3, 4) at a distance from its end face, and applying heat to the end parts of the structures that project from the clamping device. The end faces are pressed together by moving the clamping devices (3, 4) towards each other such that a welding pressure of between 0.05 and 0.5 Nmm<-2>, perpendicular to the end faces, is transmitted from the clamping devices to the end faces via the end parts of the structures, wherein the distance of the heating element from the end parts is sufficient to substantially prevent the heated parts of the structures to contact the heating element (10). The heating element surrounds the end parts along a substantially closed contour, and may comprise two segments which can be hinged into an open or closed position to be removed from the completed weld. The heating element may be energized in accordance with a predetermined heating profile, wherein the temperature of the weld area is measured and the heating operation of the heat generating member is increased or decreased to transmit and absorb heat from and into the heat reservoir member such that the temperature of the weld area is maintained within a predetermined margin around the heating profile. By using the method according to the present invention a very uniform and non-deformed butt weld can be produced under accurate temperature control of the weld area. Because the heating element does not directly contact the inner parts a uniform temperature distribution throughout the sealing volume can be achieved without degradation of the fluorine containing plastic material.