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Distortion During Bending of Pipes and Profiles and Their Prevention

cold pipe bending machine

When pipes and profiles are bent, it is inevitable that there will be distortion in the cross-sectional shape. The outer side of the neutral layer experiences thinning due to tangential tensile stress, while the inner side of the neutral layer undergoes thickening due to tangential compressive stress. In some cases, compression instability may occur, resulting in complete destruction of the cross-sectional shape. Unlike solid plates, where the effects of radial compressive stress are less pronounced, bending pipes and profiles can cause significant changes in the cross-sectional shape. This can lead to circular pipes becoming elliptical and square pipes developing inwardly concave shapes.

Effects of Distortion

Distortion in shape can lead to a decrease in the cross-sectional area and an increase in fluid flow resistance. Thinning of the pipe wall thickness can reduce its ability to withstand internal pressure. Transition variations in the cross-sectional shape can affect the functional effectiveness of pipes and profiles in structures. Therefore, various measures are often taken to prevent cross-sectional distortion during bending.

Two Methods to Reduce Wall Thickness

One method to reduce wall thickness thinning is to lower the tensile deformation on the outer side of the neutral layer.

  • For example, using local resistance heating can decrease the deformation resistance of the metal material on the inner side of the neutral layer, causing more deformation to concentrate in the compressed region and thereby reducing the stress level in the tensile region.
  • Another approach is to change the stress state in the deformation zone by increasing the compressive stress component. For instance, switching from bending to pushing can significantly overcome the defect of wall thickness transition.

Four ways to prevent deformation of bending pipe profiles

Effective Methods to Prevent Cross-Sectional Distortion:

Mandrel Bending

Mandrel bending, also known as core bar bending, involves using a mandrel or core bar to support the cross-section during bending. Various shapes of mandrels can be employed, depending on the specific bending process. This method helps prevent detrimental deformations and maintains the desired cross-sectional shape.

Support the section with a mandrel in the bending deformation zone to prevent harmful deformation. In the various bending processes, mandrels of different shapes are used.

When bending and bending, a rigid mandrel is often used. The head of mandrel is hemispherical or other curved surface shape. When bending, the mandrel is in the bending deformation zone (the position where the straight line segment meets the bending segment), and with the transfer of the bending deformation zone (during bending and bending), the mandrel is gradually drawn out from the tube. Sometimes multi-section flexible mandrels are also used. This mandrel is made of a multi-segment mandrel device. Each segment uses a structure similar to a universal joint shaft, which can be rotated arbitrarily within a certain range. During the bending process, the flexible mandrel can be bent freely with the deformation of the tube blank. Very convenient, and the effect of preventing section distortion is also good. This mandrel can be removed after bending.

Filling with Granular Media

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Another approach is to fill the interior of the bent pipe with granular media, fluid media, elastic media, or low-melting-point metals. This technique serves as an alternative to mandrel bending, providing support and preventing cross-sectional distortion. These filling materials can be easily removed after the bending process without affecting the product’s performance.

Filling the curved tube with granular medium, fluid medium, elastic medium or metal with low melting point can also replace the mandrel and prevent distortion of the cross-sectional shape. These filling substances can be taken out after bending and deformation without affecting the performance of the product. This method is relatively easy and widely used. It is mostly used in small and medium batch production. The way to prevent the distortion of the section shape is to fill the groove-shaped profile with polyurethane rubber strips. play a supporting role. During the bending process, the polyurethane rubber strip deforms along with the profile. In order to ensure a certain effect of preventing the deformation of the cross-sectional shape, filling materials with sufficient hardness should be selected. When the strength of the bent pipe or profile material is high, the hardness of the filling medium should be appropriately increased.

Local Heating

Applying local heating can prevent collapse caused by radial compressive stress on the web. Utilizing this method in non-die bending processes maintains the metal materials on both sides of the bending deformation zone in a stiffer state, thereby preventing collapse deformation and cross-sectional distortion.


Profile bending machine for bending steel pipes

Tooling techniques involve designing molds or dies that match the pipe or profile’s cross-sectional shape, reducing pressure on the contact surface and inhibiting twisting of the cross-section. Post-bending correction processes, such as hydraulic correction using high-pressure liquid or steel ball correction, can also be employed to achieve the desired shape of the bent pipe or profile.


Distortion in the cross-sectional shape during bending poses challenges to the performance and functionality of pipes and profiles. By employing techniques such as mandrel bending, filling with granular media, local heating, and tooling, manufacturers can effectively prevent cross-sectional distortion. These methods ensure that the desired cross-sectional shape is maintained, resulting in high-quality bent pipes and profiles with optimal functionality.