First, match the spring to the bore. A spring that is too small lets the wall collapse inward at the bend; one that is too large will not go in. The spring is a size-specific tool, not a universal one, which is why we supply them alongside the pipe.

Second, position the spring so that its middle sits where the middle of the bend will be. A spring that stops short of the bend leaves the last part of the curve unsupported, and that is exactly where the flattening appears.

Third, bend progressively over the knee or a former, easing the pipe round rather than forcing it in one movement. PVC will take a considerable bend at ambient temperature if the deformation is spread along the curve, and will kink if it is concentrated.

Fourth, over-bend slightly and let it relax. The material springs back, so a bend made exactly to angle will open up a few degrees once the spring is withdrawn.

The mistake that ruins the job is bending a pipe that is cold from an unheated store on a winter morning. Below about ten degrees the material is far less forgiving, and the same technique that worked in summer will kink it.