Introduce
Mandrels are a commonly used tool when bending pipes. Proper use of the mandrel can help prevent some of the most common problems and problems you might encounter when bending a tube.
tube bending machine 
bent profile 
pipe bending machine
However, if the positioning is not correct, the mandrel can cause its own problems. Before you start bending, it is important to understand how to bend a pipe. But first, it’s important to understand why a mandrel is used when bending a pipe.
Why do we use mandrels?
The most basic reason to use a mandrel when bending a pipe is to support it. The mandrel provides support throughout the bend radius and secures the pipe firmly to the bend mold slot.
One of the most important issues to prevent by using a mandrel is springback. The tendency of springbok or metal to return to its original shape when bending a tube can be a serious problem. A mandrel is the easiest way to prevent rebound.
How the mandrel bends the pipe
The first step in mandrel bending is the same as any bending, and tools must be set up. The correct tool settings are essential for the correct execution of bends. Improper tooling can cause wrinkling, kinking, buckling, bulging, and tube collapse. In fact, the first step in solving any pipe bending problem is to check the tool settings.
With the machine ready and the tools set up, you can start. When using a mandrel, first pull the tube onto the mandrel and keep it in a fixed position. However, this position is very important.
For example, when the mandrel is placed too far in the pipe, it cannot travel far enough to generate the necessary pressure inside the elbow to compress the material. Your bend may start well, but once the bend advances more than 20 degrees or so, the material starts to push backwards. This, in turn, creates ripples or waves. The corrugations are formed between the mandrel and the curved mold and continuously flatten. After removing the tube from the bending mold, you will see a large retaining ring .
For example, when the mandrel is placed too far in the pipe, it cannot travel far enough to generate the necessary pressure inside the elbow to compress the material. Your bend may start well, but once the bend advances more than 20 degrees or so, the material starts to push backwards. This, in turn, creates ripples or waves. The corrugations are formed between the mandrel and the curved mold and continuously flatten. After removing the tube from the bending mold, you will see a large retaining ring (point A in the image below).
Moving the mandrel too far can also cause serious problems with bending. Mainly, when the mandrel is too forward, it may cause a bulge to form at the curved end.
These types of problems become more severe when bending or bending thin-walled tubes. The mandrel and other tools must be set up correctly.
Photos of profile pipe bending machines
profile tube bending machine profile pipe bending machine
