Flexible Bending Spring

internal spring bending
internal springs are just a spring made from flattened steel with a taper at one end (which is inserted into the pipe) while the other end incorporates an eye. With the spring within the pipe is prevented from collapsing inwards as it is bent. Internal bending springs are commonly used for (pvc pipe from 12 to 25 mm) external diameter (larger spring § are available but are not recommended). A specific spring is required for each size of pipe being bent.
bending with the help of internal spring:

internal spring
before you start, tie a length of cord to eye on the end of the spring, this will allow the spring to be removed
 from the pipe.
insert the spring into the pipe so that it is positioned evenly across the area of the required bend. If the spring is a light fit in the pipe, rotate the spring anticlockwise as is it inserted - this will cause the spring to `wind-up' so that it will have a slightly smaller overall diameter. If the pipe has been cut before bending, the end of may have become slightly reduced, don't try to force the spring, but carefully remove the burr so that the spring will freely enter. Some pipe cutters incorporate a vee shaped blade which can be used to remove these burrs.
with the spring correctly positioned, the usually method with pipe upto 15mm is to place the center of the bend across the knee and slowly pull both ends to form the band. Slightly over bend the, pipe, then reverse the pipe from the spring and make it easier to remove the spring. For pipe larger than 20mm, some people prefer not to bend around the knee due to the force which needs to be applied. In these cases (or for pipe under 20mm is necessary) the pipe can be bend around anything fairly solid with a curved surface greater than about 100mm (4 inch) radius.
if possible, offer up the bend and adjust it if necessary to suit the installation.
when happy with the bend, remove the spring from pipe, using the cord and/or rotating the spring anticlockwise as it is withdrawn if necessary.