Tight Radius Offset Bend

Published: July 31, 2019
by Charles Busada

Tight –radius bends are a problem to all tube benders. On a tight radius, we can run into excessive flattening, cracking of the outside wall, or buckling in the inside wall (most commonly). The problem is intensified in transparent, thermoplastics benders, for we do not have the option of using internal mandrels as in metal-tube benders and all imperfections are visible.

This week we had two challenges on making tight-radius bends. Such bends are a problem for two reasons: The first is the probability of crimping of the inside wall. In all bends, but particularly thermoplastics’ bends, the forming process creates strains. In the bending process, the inner arc of the bend gets compressed while the outer arc is stretched and thinned. Without tight controls and careful bending, the bend will kink on the inside radius. The Second is related to the first. As the molecular density is greater on the inside than the outside radius of the bend we can experience “spring back”. In 90° bends, for example, the compressive forces on the inside are less than the tensile forces on the outside wall. Bends will tend to spring back.

But on U bends we have the opposite effect. Bends tend to spring forward. The movement of the bend depends on OD, wall, and radius. Even after bending a U bend can move. Think of it as a huge thermometer of a coiled spring.

And, with this project, we had a large OD, a very tight radius, a 180° angle, and uneven tangents.

Concerning radii, benders use the terminology of 1D, 2D, and 3D. “D” does not stand for “dimensional” but for “diameter.” In thermoplastics bending, generally, our lower limit is 3D ( so that the center-line radius of the bend is minimally 3x the outside diameter of the tube.

So, in making 6” ID Bend on a 15” Radius we had a challenge to the max. We have some techniques and tricks and used them all. In the end, what is most important to us is clarity. So here is the result. We and our customer were quite happy with it.

We are currently working on a 1.5D bend of 4” Schedule 40 Pipe.
To be continued . . .

A newly fabricated bend on the cooling blocks
Measuring a small, tight-radius u bend with a ruler

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