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Three Conventional Profile Bending Processes (1) – Stretch Forming

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

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