Pour me a mandrel

Bending tubes, pipes, and other forms with low-melting-point alloys

By Charles C. Gaver Jr.

September 25, 2003

To reduce weight and cost for all types of products, design engineers often specify tubes and pipes with thinner walls instead of the previously used heavier-walled tubes and pipes. Shorter tube or pipe lengths also achieve the same objectives, but usually they require sharper or more complex bends. These designs make the tube bender's task more difficult.

A low-melting-point alloy is heated and poured into a length of tube.

To reduce weight and cost for all types of products, design engineers often specify tubes and pipes with thinner walls instead of the previously used heavier-walled tubes and pipes. Shorter tube or pipe lengths also achieve the same objectives, but usually they require sharper or more complex bends (see Figure 1). These designs make the tube bender's task more difficult.

The use of a recyclable low-melting-point alloy as a mandrel may be helpful in achieving these more difficult configurations. Low-melting-point alloys have the advantage of conforming to any tube or pipe diameter, as well as any irregular shape, because they expand upon cooling rather than shrink. Because the alloys completely fill the tube or pipe during bending, the tube can be bent smoothly without kinking or wrinkling.

One low-melting-point alloy traditionally used to bend tube and pipe up to 21/2 inches in diameter melts at 158 degrees F. A stronger alloy that melts at 255 degrees F is used for larger-diameter tube and pipe. The recent addition of a special grain refiner causes them to solidify, or freeze, with a close-packed, fine grain structure, which strengthens the alloy. Although the 158-degree alloy is recommended for bending tube and pipe less than 21/2 in. in diameter, this strengthening characteristic does make it possible to use this alloy on diameters somewhat larger than 21/2 in.

The 158-degree alloy is safer to use than the 255-degree grade, because it can be melted out of the tube or pipe in a hot water bath, so less costly release agents can be used with it.

Bending With 158-degree Alloy

The following steps are recommended for bending tube and pipe up to 21/2 in. in diameter:

Prep

1. Make sure that the tube is fully annealed.
2. Clean interior of the tube thoroughly with a water-soluble cleaning product. Rinse and dry thoroughly.
3. Coat the inside of the tube with a silicone release agent, mineral oil, nondetergent motor oil (most motor oils contain detergents that can cause the tube bending alloy [TBA] to stick to the side of the tube), or similar release agent.
4. Remove excessive coating, making sure that the entire inside is coated.

Install the mandrel

5. Plug one end of the tube securely.
6. Fill the tube with 158-degree-F TBA, pouring down the side of the tilted tube to prevent air pockets. Tubes 1/4 in. in diameter and smaller should be filled while they are in hot water or otherwise heated to prevent the alloy from solidifying before the tubes are completely filled. (158-degree-F TBA weighs 0.339 lbs./cubic in., and the weight required to fill a tube can be easily estimated.)
7. Immediately put the filled tube into circulating cold water or other quick-chilling medium. Allow the tube to cool until it reaches approximately 70 degrees F. This may take 15 minutes for a 1-in. tube, longer for larger tubes.
8. Remove the plug.
9. Rewarm the tube and alloy to 95 degrees F.

Bend the tube

10. Bend the filled tube with slow, uniform pressure over a forming block or in a bending machine to the desired shape (see Figure 2).
11. Immerse the formed tube in boiling water. Tilt and shake the tube to remove the alloy (see Figure 3).
12. Flush the tube with a suitable cleaner to remove oil film and any remaining alloy. If necessary, use a tight-fitting pull-through to complete cleaning.

This method also may be used for tubes larger than 21/2 in. diameter, but care should be taken when following steps 1 through 10 to bend the tube only partially into its final shape. Then, after performing step 11, steps 1 through 10 should be repeated until the desired shape is attained, finishing the process with steps 11 and 12. A more convenient method for bending tube and pipe larger than 21/2 in. diameter is discussed next.

Bending With 255-degree Alloy

Prep

1. Make sure that the tube is fully annealed.
2. Clean interior of the tube thoroughly with a water-soluble cleaning product. Rinse and dry thoroughly.
3. Coat the inside of the tube with a high-temperature silicone release agent, oil that can accommodate 255-degree-F temperatures, or similar release agent.
4. Remove excessive coating, making sure that the entire inside is coated.

Install the mandrel

5. Plug one end of the tube securely.
6. Fill the tube with 255-degree-F TBA, pouring down the side of the tilted tube to prevent air pockets. (The 255-degree-F TBA weighs 0.380 lbs./cubic in., and the weight required to fill a tube can be easily estimated.)
7. Allow the tube to cool until it and the alloy reach approximately 70 degrees F. This may take more than 30 minutes.
8. Remove the plug.
9. Rewarm the tube and alloy to 95 degrees F.

Bend the tube

10. Bend the filled tube with slow, uniform pressure over a forming block or in a bending machine to the desired shape (see Figure 2).
11. Immerse the formed tube in a hot oil bath or an oven. Tilt and shake the tube to remove the alloy (see Figure 3).
12. Flush the tube with a suitable cleaner to remove oil film and any remaining alloy. If necessary, use a tight-fitting pull-through to complete cleaning.

Bending Solid Forms

Rolled or extruded solid shapes also can be bent and formed using the same basic procedure, except that the entire form is encapsulated in a solid block of TBA. Leaving the formed sections in the TBA block for several hours will minimize springback.

Tips

When melting and holding the 158-degree-F alloy, using a cover of 1 in. or more of water will prevent the metal from oxidizing and causing metal loss and possibly casting problems. If the local water is hard (basic), it is important that the water be acidified slightly. Acetic acid (white vinegar) normally is the safest and most cost-effective way to acidify the water, resulting in a clean, shiny metal bath.

If the tube bending alloy is properly handled, very little loss of the alloy should occur as it is melted, poured into the tube, remelted, and poured out of the tube. It is possible that prolonged reuse will cause the melting point of the alloy to increase. If this does occur, the alloy manufacturer usually can restore the alloy to the proper melting point by refining or adding missing elements.