Method of relieving stresses in extruded members having reinforcing bars

Abstract

Stresses are relieved in elongated extruded plastic members by passing electric currents through electrical conductors embedded in the reinforcing bars. The reinforcing bars are manufactured according to a pultrusion process in which 1/16 inch diameter MIG wire is introduced into a roving bundle of impregnated fibers and drawn through a curing and shaping die. The resulting product is a rigid reinforcing bar having a longitudinal wire extending its entire length. The reinforcing bars are cut to lengths corresponding to the desired lengths of the elongated members to be extruded. The elongated member is manufactured using an extrusion process in which molten plastic surrounds a plurality of reinforcing bars to form a continuously extruded member. The member is cooled, cut at the reinforcing bar joints, and prepared for electrical resistance heating. Electrical contacts are inserted into bores provided in the ends of each reinforcing bar and are placed in electrical contact with the embedded wire. An alternating current, fed from a source through an isolation transformer, passes through the embedded wires in the reinforcing bars to generate heat. The reinforcing bars are heated to a temperature in a predetermined temperature range between about 185 and 200.degree. F. and maintained at a temperature within that range for a predetermined period of time by turning on and off the power source. A temperature controller is provided for precisely controlling the rate of heating, and a modulating circuit is positioned in the supply line for modulating the electrical energy supplied to the bars. The heat generated at least partly melts the plastic surrounding the reinforcing bars, thereby relieving stresses in the elongated member. When fully stress-relieved, the elongated member is cut to final length.