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Three Conventional Profile Bending Processes

Profile bending is a process in which the moment acts on a metal profile workpiece (such as an aluminum profile) to cause plastic deformation of the material.
There are three common bending processes for metal profiles in the market: stretch bending, roll bending, and rotary bending.

Stretch Forming

The principle of stretch forming
The principle of stretch forming

Tensile bending is the most widely used profile bending process in the industry. Usually divided into three steps. First, the profiles are pre-stretched, then bent, and finally stretched. In production, it can be decided according to the actual situation whether to carry out the process of image supplementation, and the process flow is shown in figure right.

Read More: Stretch Bending & Forming Machine | Stretch Forming Presses

Three Steps in the Stretch Bending Process

  • Steps 1#: The specific operation is to first fix the two ends of the metal profile (such as an aluminum profile) on the clamping device, and then apply a large pre-tightening force to it, the pre-tightening force is slightly higher than the yield strength, roughly 1.1 times;
  • Steps 2#: Then start moving the clamping device or forming the die – the punch. When the metal profile contacts the punch, a forming moment is generated, which acts on the middle of the metal profile and divides the metal profile into two sections. As the clamping device or the punch continues to move, the torque increases, and the deformation of the profile increases. When the desired shape is reached, the movement stops;
  • Steps 3#: After the forming is completed, the profile is not taken out immediately, but the stretching force is increased, stretched again, and finally formed.

Read More: The Best Guide to The Stretch Forming Process

Three advantages of the metal profile stretch bending forming process

  • Advantages 1#: Compared with other bending processes that require multi-step bending with a relatively large arc, this process only requires one step;
  • Advantages 2#: Another important advantage is that the bounce is relatively small. Because during the bending process, there is always a pulling force, which makes the neutral layer in the center of the profile move inward, and even makes the entire section under the action of tensile stress, which can reduce the stress caused by bending. The difference between the inner and outer layers of the profile. Therefore, the resulting bending moment can reduce spring back;
  • Advantages 3#: Due to the existence of this tensile stress, the degree of wrinkling of the inner layer is reduced to a certain extent, and can even be avoided; but at the same time, the degree of indentation of the outer layer of the profile increases, which affects the bending quality of the profile.

Three important parameters in the profile stretch bending process

The bending radius, section shape and stretching amount of profile stretch bending are three important parameters in the whole process.

Since the bending radius and cross-sectional shape of the profile are often determined by the shape of the workpiece, determining the appropriate amount of stretching is the key to the entire process. The study found that the smaller the bending radius of the profile and the greater the stretching amount, the more obvious the wall thickness reduction and upper wall collapse.

Important parameters 1#: Tension

The determination of the stretching amount is affected by two factors, the forming limit, and the forming precision. When the amount of stretching is insufficient, there will be defects such as bottoming, poor mold clamping, and low forming accuracy, so it is necessary to maintain a certain amount of stretching.

Important Parameters 2#: Lower limit stretch

The amount of stretching must at least ensure the normal forming of the profile, that is, no demoulding and bottoming, which is the most basic requirement for bending forming. Therefore, the minimum amount of stretch required to ensure that the profile does not release from the mold and wrinkle at the bottom during stretching and bending is determined as the lower limit of stretching. The pre-stretching amount during the stretching and bending process of the profile should not be less than the lower limit stretching amount.

Important Parameters 3#: Upper limit stretch

After the amount of stretching meets the basic requirements of forming, it needs to be further increased, and the specific amount of increase should be determined according to the requirements of forming accuracy. The purpose of increasing the amount of stretching is to improve the forming accuracy of curved profiles and reduce spring back, but this is at the expense of sectional shape and wall thickness. When the amount of stretching is sufficient to move the neutral layer toward the bottom of the profile, increasing the amount of stretching has little effect on reducing spring back but will cause severe section deformation and wall thickness reduction. Therefore, the amount of stretch required to deflect the curved neutral layer to the bottom surface of the profile is determined as the upper limit stretch amount. In general, if there is no special requirement, the maximum value of stretching should not exceed the upper limit of stretching.

In summary

The amount of stretch is a key parameter in the aluminum profile stretch bending (aluminum profile bending) process. When formulating the process, the value of the stretching amount must be greater than the lower limit of the stretching amount. Since the rebound has a steep drop zone, when the wall thickness reduction requirements are not very strict and no supplementary stretching measures are taken, it can be considered to use as much as possible. The stretching amount close to the upper limit can obtain higher forming precision.

The principle of strain controlled stretch bending
The principle of strain-controlled stretch bending

Strain Controlled Tensile Bending Process

In addition to the stress-controlled process mentioned above, there is also a stretch-bending process controlled by strain, as shown in the left diagram of the principle.
The profiles are fixed at both ends of the tool, and as both parts of the tool are turned, they both eventually reach a certain strain. Theoretical studies show that this method has higher forming accuracy than the stress-controlled stretch-bending process.Read More: Metal Forming in Wiki

Read More: Profile Bending Machine: 5 Things Before Buy, 6 Steps to Bend

Rotary Bending

Rotary bending of 3 conventional profile bending processes
Figure 1: Rotary bending of 3 conventional profile bending processes. 1-Round bending die; 2- chuck; 3- side pressure roller; 4- profile; 5- tension device (guide block)

Rotary bending is a typical and widely used metal profile bending process. The metal profiles produced by the rotary bending method are of high quality. The principle of the test device is shown in Figure 1.

The operation method of the rotary bending process is to clamp one end of the metal profile in the chuck of the bending die, and the side pressure wheel presses it against the bending die and rotates around the bending die during work, and the chuck rotates with the metal profile, so Metal profiles can be bent into desired shapes.

Read More: Rotary Draw Bending: Basics, Benefit, 8 Key Knowledge

Process characteristics

The application of mandrel during bending
Figure 2: The application of mandrel during bending

The bending die is the power device in the rotary bending process. Due to the restriction of the profile by the chuck fixed on the bending die, the deformation zone is the part in contact with the bending die. If the bending die is inclined, the profile can be bent into a three-dimensional article. Compared with other bending processes, the spring-back of the product after bending around the bending die is larger.
Adding an opposite force along the axial direction at the end of the profile can effectively reduce the amount of spring back. It can be seen from Figure 1 that this process has certain requirements for the length of the metal profile because the profile must be sufficiently long when it is fixed, which limits its application to a certain extent.

For the common defects such as wrinkling, collapse, and cross-sectional deformation of hollow profiles during bending, the degree of defects can be reduced by supporting the profile with a core rod whose cross-section is similar to the hollow part of the profile during the bending process, as shown in Figure 2.

Read More: Profile Bending Machine: 5 Things Before Buy, 6 Steps to Bend

Advantages of Rotary Bending

Rotary bending has some advantages over other methods. The most advantageous feature is the simplicity of adjustment. Changes in the bend angle can be made simply by shimming or grinding the height of the assembly. Doing so takes very little time, and time is money.

Rotary benders can bend as much as 120 degrees and are well-suited to bending high-strength material. One company in Sweden has successfully created two 90-degree return bends in steel with a yield strength of 980 megapascals. This translates into steel that by U.S. standards has a yield strength of more than 142,000 pounds per square inch (PSI)—five times stronger than low-carbon steel. Attempting to make such a bend in a conventional wipe-bending operation most certainly would be impossible.

Unlike conventional wipe bending, rotary benders require much lower forces to create the bend. Anywhere from a 40 percent to 80 percent reduction in force can be expected. This makes this method ideal for producing long, heavy-gauge, large parts, such as trucks and semi-frame rails.

You can expect less hole distortion in rotary bending. Consider a hole that is pierced in a flat blank and later bent into a vertical wall. During conventional bending, this hole can be subjected to a great deal of tension, which causes the hole to distort. Because rocker benders fold the metal around the punch, hole distortion is eliminated.

Rotary benders can be used to bend up or down. They also can be placed on cam slides.

Disadvantages of Rotary Bending

Despite the many advantages, rotary benders do have some disadvantages. First, they can be quite expensive; however, consider the advantages of the reduction in downtime and frustration. Overall, they often pay for themselves in a short period of time.

Also consider that you most likely will not need an external pad, which reduces die cost. Often the true cost of designing and building a conventional wipe-bending die is much greater than the rocker bender. Don’t confuse cost with value. In my opinion, rotary benders are worth every penny.

Because these benders have moving parts, there is a risk of galling up and failing to rotate. This can be prevented by periodically cleaning and lubricating them.

Remember that rotary benders can be used for straight-line bending only. Avoid using them to bend special-shaped trim lines that do not allow for simultaneous punch contact. Angled corners are not good candidates for rocker benders.

Rotary Draw Bending For Pipe Bending

One of the most versatile and common methods to bend pipe and tube is rotary draw bending. The radius of such bends is often described as, for example, “2D.” A 2D bend is one whose center-line radius is equal to two times the outside diameter of the pipe to be bent.

Rotary draw bending involves clamping on the outside diameter of a pipe and drawing it over a form whose radius matches the desired bend radius.

Rotary draw bending often employs an internal supporting mandrel and a wiper die to prevent wrinkling on the inside wall of a tight bend. Some rotary drawing machines can perform both push bending and rotary bending with a single tooling setup.

Trouble Shooting Guide

Tube bending is a matter of handling a handful of procedures and variables. The bending process can cause the material to get thicker where the metal is under compression and thinner where it is under tension. Too much thickening can result in wrinkles and too much thinning will ultimately result in failure.

The machine operator needs to look at clamping pressure when bending. If there is not enough pressure, the tube will slip during the bending process. If too much pressure is used, then that will cause the tube to collapse if a mandrel isn’t used and to wrinkle if a mandrel is used. The problem occurs when metal flows into areas where it isn’t supposed to go. Successful bending comes down to proper material containment while at the same time reducing drag. It is critical to know every characteristic of the tubing: shape, size, wall thickness, tolerances, yield strength, tensile strength, and ductility. This information will help assess if the material will be able to be formed to the desired bend radius.

Roll Bending

ending method is also one of the widely used bending processes. There are various forms such as three-roll bending, four-roll bending, and six-roll bending. The principle is shown in Figure 1.

Work principle

The principle of roll bending
Figure 1: The principle of roll bending

Taking three-roll bending as an example, the specific operation method of this process is to place the profile between the rollers arranged in a zigzag, and the friction force generated when the rollers rotate drives the profile to move and bend it into a certain shape. During the process, the rollers rotate back and forth multiple times to obtain uniform plastic deformation and bending radius of the curved profile.

The relative position between the rollers determines the difference in the bending moment and also determines the curvature of the bent profile.

Read More: Profile Bending Machine: 5 Things Before Buy, 6 Steps to Bend

3-roll and 4-roll bending rolls

The principle of four-roll bending is the same as that of three-roll bending, but compared with three-roll bending, the existence of the fourth roll improves the bending accuracy of profiles and reduces the occurrence of section deformation, so it is more suitable for bending thin-walled profiles.

For the bending of profiles with special-shaped sections, the contour of the roller can be processed to be consistent with the shape of the contact part of the profile, which is conducive to supporting the profile during bending, ensuring that the section of the profile does not deform during bending, and improving the bending quality. The type roller is shown in the bottom right of Figure 2.

Pros and Cons

PBA CNC Aluminium Profile Bending Machine Curve The Aluminium U Channel

Compared with the above two methods, different mold requirements are required when obtaining profiles with different bending angles and curvatures. This process has an obvious advantage, that is, only the relative position between the rollers needs to be adjusted, which saves time, and Production costs are reduced. This process also has its disadvantages, that is, the profile will have straight sides after bending, that is, there is a section at the front and rear ends of the profile that cannot be bent, and the length of this section depends on the distance from the center roller to the two side rollers. The shortcoming of reducing the wheelbase limits the application of this process to a certain extent.

Curve the Angle Iron With The PBH-300 Steel Profile Bending Machine

Six steps of profile bending

  • Steps 1#: Loading bending metal profiles
  • Steps 2#: The right lower roller up to the top roller movement achieves the pinching position
  • Steps 3#: Pre-bending between the top roller and the left roller
  • Steps 4#: Working phase
  • Steps 5#: Pre-bending between the top roller and the right roller
  • Steps 6#: Unloading of the metal profile

Main bending parameters

There are two main process parameters of the roll bending method, the relative position between the rolls and the bending time.

The longer the bending time, the more uniform the plastic deformation and curvature radius of each part of the obtained workpiece, and tend to a certain value, and the fixed value is determined by the amount of pressing or pressing.

Laser Bending

Laser bending forming is a flexible forming technology that uses laser heating to realize workpieces. It is a new technology for bending metal profiles. Its basic principle is to use high-energy laser beams to scan the surface of metal pipes, and the thermal expansion of materials in the heating area will cause materials to accumulate. After the material is cooled, the heat-affected zone is the wedge-shaped compression deformation zone shown in Figure 1. The material in this area is shortened along the axial direction. take shape. Read More: Laser

The principle of laser bending
The principle of laser bending

Working principle

Corresponding to the heating and cooling stages, the laser bending forming of the pipe also undergoes two deformation stages reverse bending and forward bending respectively. After the laser scanning is finished, a large axial compression plastic is formed between the heating surface and the back material, and the resistance strain is poor. In this way, between the laser-irradiated side and the back side of the tube, due to different transverse shrinkages, there is a strain difference, which is manifested as the axial contraction of the scanning area on the laser-irradiated side and the axial elongation of the back side, so the tube There is a positive bend towards the laser beam. Therefore, the final intrinsic mechanism of the process is the temperature gradient mechanism.

Process parameters of laser bending forming

The process parameters that affect the laser bending of metal tubular parts mainly include laser power, scanning angle, scanning times, spot diameter, and scanning speed. Within a certain range of values, increase the laser power, increase the scanning angle, increase the scanning times, and reduce the scanning speed. Any speed can increase the bending angle; increasing the spot diameter will decrease the bending angle.

Applications and deficiencies

Three-dimensional bending can be easily achieved by heating a specific area with a laser. This process also has some shortcomings, such as the difficulty in bending multi-cavity profiles, and the processing time is too long. Although the process is a heat-affected process, it does not have a detrimental effect on the mechanical properties of the material after processing.