In this process, a mandrel—a metal rod or ball—is inserted into the tube while it is being bent, which creates a more precise bend with no wrinkles or kinks. For some applications, it is acceptable to have a wrinkled tube; particularly when the tube just has to function and quality is not a factor.
The process of mandrel bending provides a high quality tube because the amount of flow is improved, making the inside diameter of the tube consistent throughout, even at the bend; something that is not possible to do without using a mandrel. Mandrel bending is commonly used in exhaust applications in the automotive industry, pipe applications in the plumbing industry and instrument applications in the musical instrument manufacturing industry because the process improves the look of the bend and also creates a much tighter bend than is possible with empty bending. When the mandrel is inserted into the tube, it has to be strong enough to support the tube so that it won’t be deformed during bending, but also flexible enough to ensure that the tube is still properly bent.
Mandrel bending takes place during the rotary-draw tube-bending process. With rotary-draw bending, a tube’s bend is created by placing the tube around the form of a rotating bend. The tube is attached to the bend form, and as the form moves forward it is designed to bend the tube in the right places, specific to each tube. Mandrel bending occurs during this process by placing a mandrel inside the tube so that as the tube is bent, the mandrel bends with it, which helps keep the tube’s shape from deforming and provides a precise bend, free of kinks.
Mandrels are usually made of an aluminum-bronze alloy or steel. The aluminum-bronze type is used to bend harder materials such as stainless steel or titanium, while steel mandrels are typically used to bend softer materials such as copper or aluminum. There are different types of mandrels that meet various bending needs: common types include plug mandrels, form mandrels and ball mandrels. Plug mandrels are solid rods that are inserted into the tube; this type of mandrel is often used on tubes that have thicker walls, a larger radius and are going to be bent normally.
Form mandrels are similar to plug mandrels in that they are also a solid rod, but the tips of these mandrels are formed to fit a tube’s specific radius; essentially, it is a custom mandrel. Ball mandrels consist of ball bearings—which can be linked or unlinked—being inserted into the tube. These mandrels are used when a precise bend is required in a tube.