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Mandrel bending pipe and tubing: 4 types & features, 5 benefits, how to avoid wrinkles

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HOW TO AVOID WRINKLES WITH MANDREL BEND PIPE AND TUBE
4 Types of Mandrel Bending: Plug mandrels, Formed mandrels, Ball mandrels, and Ball mandrels with low steps. 4 Features of Mandrel Bending. 5 Benefits: Time savings, Cost savings, Material cost, and space savings, Labor savings, Accuracy. Top 8 Reasons for Tube Wrinkling, and How to avoid wrinkles? 
This article will explain these contents one by one.

What is mandrel bending?

Pipe and tube bending applications range from very simple to very complex. An ideal bend doesn’t require a mandrel. However, not all bending situations are ideal, so many of the more complex bends require that the pipe and tube be supported by an internal mandrel.

A mandrel's purpose is to keep the tube or pipe from collapsing and wrinkling during the bending process.
the die of mandrel bending

Mandrel bending is a method of rotary draw bending, a solid mandrel(up to five segmented, donut-shaped, balls that are just slightly smaller than the ID of the pipe/tube that is being bent) shaft is inserted into the pipe/tube to provide internal support during bending, its held in a fixed position and holds it firmly in the groove of the bend die.

The picture right shows the components needed to produce a mandrel bend (except the metal tube or pipe itself.) After clamping the tube or pipe with the clamp die, the bend dies body rotates up to 180 degrees supported internally by the mandrel and externally by a pressure die and a wiper die.

Four Types of Mandrel Bending

  1. Plug mandrel: Plug mandrel: These mandrels are like rigid rods and are used mainly for heavy-wall tubes and larger radii, but they are not used to create critical high-angle bends. It is used quite often in international pipe size (IPS) applications without a wiper.
  2. Formed mandrels: Formed mandrels: A formed-tip mandrel is a variation of a plug mandrel, but it has a radius cut into the tip to match the radius of the bend. The formed end gives more support to the inside of the tube. A formed-tip mandrel generally is used with heavier-wall tubing and an average radius. These types of mandrels are intended for similar applications to rigid mandrels, but the shaped part of the mandrel bends better to support the outer radius.
  3. Ball mandrels: This type of mandrel is used to create critical bends with high accuracy.
  4. Ball mandrels with low steps (balls are closer to each other): There are several types of ball mandrels that can be used for different thicknesses. The smaller the wall thickness of the pipe, the greater the number of balls and the shorter the distance between them so that they can support more points of the pipe wall.
    Thin-wall or close-pitch mandrels are used for thinner walls (wall factor of 70 or more) and tight radii. Ultrathin-wall or ultra close-pitch mandrels are for the thinnest walls (wall factor of 200 or more) and the tightest radii commonly found in aircraft applications.
    Using a thin-wall or ultrathin-wall mandrel in place of a standard-wall mandrel is not recommended. Thin-wall and ultrathin-wall mandrels typically are weaker by design, and attempting to bend a tube with a heavier wall likely will break the mandrel.

Ball mandrel

The most common type of mandrel in use today is the standard ball mandrel. It is used in applications with normal wall thickness (wall factor from 15 to 70), average radius, and in which a plug mandrel isn’t sufficient. To determine the wall factor, use this formula:

Wall factor = Outside diameter/Wall thickness

Generally, most ball mandrels flex in all planes. Single-plane mandrels still are available but not commonly used. Ball mandrels usually are used with wiper dies.

Mandrel Materials

Using a mandrel made from the proper material for the tubing is important to prevent galling inside the tube and to prolong the life of the mandrel.
Mandrels typically are made from an aluminum-bronze alloy or tool steel with hard chrome plating. An aluminum-bronze combination is used to bend stainless steel, titanium, INCONEL®, and other hard materials. Chromed tool steel mandrels shouldn’t be used to bend stainless steel, but are useful for most other hard materials and also for mild steel, steel, copper, aluminum, and other soft materials. There are also some situations in which chromed tool steel mandrels can be used with more success than aluminum-bronze for bending titanium and INCONEL.
Other coatings and materials are used in special cases.

Mandrel Placement

Proper placement of the mandrel is critical. The front end of the mandrel shank should be set just past tangent. Bending off the mandrel balls causes the bend quality to suffer. The balls do not support the tube—that’s what the shank is designed for.

Three conditions that require mandrel bending

  1. Tighter bend radii;
  2. Thinner-wall tubes;
  3. Exotic, harder-to-bend materials.

Four Features of Mandrel Bending

  1. Mandrel bending is the ideal choice for bending thin tubing without any deformities on an extremely tight radius.
  2. Mandrels help prevent spring back. The tendency of a metal to return to its original shape, spring back, can be excessive when a mandrel is not used.
  3. The mandrel bending prevents damage such as distortion and perfects the tube bend without wrinkling or breaking the tube at the bending point, such as distortion, collapsing, rippling, and flattening, and gives the technician greater control over the pipe/tube’s ovality or roundness.
  4. Mandrel bends are held within the same ASME wall thickness and ovality specs to which fitting manufacturers are held. With a mandrel tool supporting the pipe’s interior, technicians actually have more control to make accurate, repeatable bends.

Five Benefits

  1. Time savings: When comparing a welded 90° angle to a pipe bent at the same angle, mandrel bending reduces the number of fabrication hours, as well as the time required to inspect the finished piece. Using CNC-controlled bending machines, manufacturers can save up to 75% of the time it would take to hand-make each weld.
  2. Cost savings: There can be savings of more than 50% of costs by bending stainless steel instead of welding it, especially due to the tedious visual inspection that certain welding codes require. Upon completion of the welding process, if NDE is required, welds must be x-rayed, costing hundreds of dollars per x-ray. For a pipe with multiple welds, inspection costs add up quickly.
  3. Material cost and space savings: Not only does bending reduce wasted material because it only requires the original workpieces, but it also saves precious space in your shop as there is no need to store any raw welding material other than the pipe or tube itself.
  4. Labor savings: By implementing bending technology, onsite labor to install your piping system can be greatly reduced which significantly reduces the risk of safety and/or production issues onsite for the owner/GC.
  5. Accuracy: Pipe bending facilitates the creation of custom, non-isometric angles and allows engineers enhanced flexibility when constructing complex geometries. The bending equipment creates highly accurate, reproducible results while the mandrel supports the pipe to eliminate distortion. Bending is also more consistent and will reduce any imperfections that are often found in the welding process.

Mandrels are a commonly used piece of tooling when bending pipe and tube. Used correctly, mandrels can help prevent some of the most common problems and issues you can run into when pipe bending.

However, if not positioned correctly, mandrels can also cause problems of their own. It is vital that you understand how to mandrel bend a pipe before you begin bending.

Top 8 Reasons for Tube Wrinkling

  1. Tube slipping in clamp die
  2. The mandrel is not far enough forward
  3. Wiper die not seated properly in bend die
  4. Wiper die worn or of improper fit
  5. Too much clearance between mandrel and tube
  6. Not enough pressure on the pressure die
  7. An improper or excessive amount of lubrication
  8. If the mandrel and wiper die are in proper locations, check the inboard pressure on the pressure die. You may need to apply more pressure on the pressure die to hold the tube into the die. Adjust in slowly until you have no wrinkles. Lastly, check mandrel fit.

How to avoid wrinkles with mandrel bend pipe and tube

A CNC mandrel pipe and tube bending machine is designed to avoid the issue of wrinkling along the inside of pipes and tubing. The mandrel stays in a fixed position as the outer radius of the tube experiences pressure. This pressure causes the pipe to stretch into the desired bend.

Set your tooling

The first step in mandrel bending is the same as any bend, you must set your tooling. Correct tooling setting is essential to performing the bend correctly. Improperly set tooling can result in wrinkles, kinks, buckling, bulging, and tube collapse. In fact, the first step in troubleshooting any pipe bending problem should be checking the tooling setup.

Held and fixed mandrel

Once your machine is ready and your tooling set, you can now begin. When using a mandrel, the tube is first pulled over the mandrel, which is held in a fixed position. That position, however, is very important.

The mandrel is placed too far back in the pipe

When the mandrel is placed too far back in the pipe, it is not far enough forward to generate the necessary pressure on the inside of the bend to compress the material. Your bend may begin well enough, but once the bend has progressed past 20 degrees or so, the material begins to push back. This, in turn, forms a ripple or a wave.

The ripple is forming and being flattened continually between the mandrel and the bend die. When the pipe is removed from the bend die, you will observe a large buckle.

Moving the mandrel too far forward

Moving the mandrel too far forward can also result in serious problems with the bend. Mainly, when the mandrel is too far forward, it can cause a bump to form at the end of the bend.

Notes

These types of issues can become worse when making tight bends or when bending thin-walled tubing. It is essential that your mandrel, as well as your other tooling, is set correctly.

There is a wide range of reasons why you may be seeing wrinkles from faulty tube bending, so you may have to experiment to determine the actual issue. One of the first things that people typically do is to increase the tube pressure. It stands to reason that, with more pressure, there won’t be room for the tube to wrinkle as it should remain smooth throughout the draw.

However, if you increase the pressure and still experience wrinkling, then you will have to take additional factors into consideration. Checking the mandrel insertion rate, the amount of lubrication in use, and the condition of the wiper die may also eliminate the issue.

Or, you may find that you are using the wrong mandrel for the type of metal you are trying to bend. Always remember to use a soft mandrel when bending hard tubing and a hard mandrel when bending soft tubing.