Wrinkle bending is a technique which has been used to custom-bend sections of pipe on a job site. Using an oxyacetylene flame, the technique consists of heating one or more narrow bands of the pipe wall at right angles to the pipe axis about two thirds of the way around the pipe circumference. When these bands are heated to a red heat, the section is bent to the desired curvature, either by hand or with some mechanical means. The force required depends on the diameter and wall thickness of the pipe. Usually one section is heated at a time and bent about 20o. Additional sections are heated and bent until the required bend is obtained. When done correctly, the pipe wall extends outward and does not restrict the bore. See PIPE WELDING.