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Three causes of defects in the metal bending process

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The lightweight structures can reduce energy consumption and alleviate the increasingly severe energy crisis. Therefore, in recent years, the lightweight design of automobile structures has attracted more and more attention. Extruded aluminum profiles are increasingly used in lightweight structural design. Different from ordinary civil building materials, extruded aluminum profiles are generally used in a bent state because the structural design of automobiles must take into account factors such as structure, aerodynamics, and aesthetics. In the bending process of aluminum profiles, especially thin-walled hollow aluminum profiles, various defects such as section deformation and spring back will appear, which have become obstacles to its large-scale application.

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Causes 1#: Springback phenomenon during bending

Like sheet/plate metal bending, spring back during bending of extrusions is a common phenomenon in bending forming, and it is also the most important factor affecting forming accuracy. It is difficult to completely avoid it through existing processes. It is mainly manifested as overall unloading spring back, trimming spring back, and partial unloading spring back,

Two Causes of Springback

There are two main reasons for the rebound:

  1. When the surface metal of the inner and outer edges of the profile enters a plastic state while the center of the profile is still in an elastic state, or the inner and outer edges of the surface are both in a state of tension (compression) stress and the stress is inconsistent, the profile will rebound after unloading;
  2. Metal plastic forming is always accompanied by elastic deformation, so when the profile is bent, even if all the inner and outer metals enter the plastic state, after unloading, the elastic deformation part of the total deformation will recover immediately, and spring back will also occur.

How to reduce the impact of the rebound

The result of spring back is manifested in the change of bending curvature and angle. This change causes an error between the actual shape and size of the profile after bending and the shape and size required by the design. Sometimes this error is relatively large, so when designing the mold, it is necessary to consider the impact caused by the spring back of the profile after bending.

Three Causes Of Defects In The Metal Bending Process
Appearance defects that are prone to appear during the bending process

a) wrinkling; b) torsion; c) section deformation; d) rupture, e) collapse

Many factors affect the spring back, and it is difficult to accurately calculate the amount of spring back in theory. Therefore, in the actual bending process, to reduce the impact of spring back on the bending accuracy, the amount of grinding is usually reserved in the mold design. , and then adopt the method of repeated mold trial, mold repair, and manual correction after forming, so that the produced profiles can reach the required precision. When the amount of spring back exceeds the allowable tolerance, it becomes a forming defect. Therefore, spring back has always been an important factor affecting and restricting mold design and product quality.

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Causes 2#: Bending capacity of profiles

In addition to spring back, some appearance defects during the bending process also affect the bending forming of the profile. The generation of these defects is related to the bending ability of the profile. Bending ability refers to the difficulty of processing a profile into a curved piece with no defects, a certain curvature, and a bending radius, which is determined by the geometric size, bending degree, and material properties of the profile. However, profiles, especially hollow and semi-hollow profiles, have many defects that are easy to produce during the bending process, which seriously restricts the application of the product. How to effectively solve these problems is an important aspect of perfecting the bending process.

These defects are mainly manifested in wrinkling, torsion, deformation of the cross-section, the inconsistent wall thickness of the outer and inner layers of the bent profile, cracking of the outer layer, the collapse of the inner layer, etc. The specific forms are shown in the figure above.

The extent of these defects can be mitigated or even avoided by effective process control methods. To minimize the degree of wrinkling, collapse, and section deformation that are easy to occur when hollow and semi-hollow profiles are bent, most of the methods used in the industry use a core rod with a cross-section similar to the hollow part of the profile during the bending process. The inside of the profile is used to support the deformed part of the profile, or it is bent after the hollow profile is filled with filler. Generally used fillers are sand, paraffin, low melting point metals or alloys,, and other organic materials.

Causes 3#: Residual Stress

In addition to the above defects, the bent profile has other deficiencies. For example, the outer layer of the bent part will be subject to residual tensile stress, and the inner layer will be subject to residual compressive stress. The existence of these residual stresses will affect the use and subsequent processing of the bent profile. adverse effects. For example, during the welding process, due to the influence of heat, the bending accuracy of the profile will change due to the release of residual stress.

Final words

Therefore, for bending processing, it is hoped that through a certain process, the above-mentioned defects and deficiencies should be avoided as much as possible, to reduce the problems and adverse effects of bending profiles during use.

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