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How to reduce metal spring back in profile stretch forming?

profile stretch press

For parts with a large bending radius, if the common bending method is used, the deformation is close to elastic deformation. Therefore, the desired shape and size cannot be obtained. For such metal profiles, the stretch forming process should be used.

Working principle

Stretch bending process

The metal profile stretch forming process is to add an axial tension before the sheet is bent, and its value makes the stress in the blank section slightly greater than the yield point of the material, and then bends while the tension is acting. Sometimes in order to improve the accuracy, the pulling force is increased at the end to carry out supplementary pulling.

This is because the stretch forming process can produce plastic deformation of the metal profile by applying axial tension, thereby increasing its bending capacity and stability. By pre-applying axial tension, metal profiles can be bent more easily and avoid uneven stress distribution and surface defects during bending. Increased pulling force for supplementary pulling can further improve accuracy and quality. This process can improve the deformability of metal profiles so that they can be used in more complex and fine shapes while avoiding defects such as uneven stress distribution and surface scratches during bending.

Reduce metal spring back

When the profile is stretched and bent, spring back is an inevitable phenomenon, and some measures can be taken to reduce its magnitude. Among them, using a larger elongation is a common method, which can adjust the stress state of the material during the stretching process, thereby reducing the spring back value during the forming process. This can further improve the shape and flatness of profiles, making curved surfaces smoother, more symmetrical, and more precise.

To reduce the rebound value, a larger elongation is used when stretching. Stretch first to obtain about 1% elevation rate, bend along the punch in the stretched state, and then stretch and shape.

Specifically, the material is generally stretched in a stretched state to achieve an elongation rate of about 1%. Then the punch is bent in the stretched state, that is, the blank is bent on the punch under the action of tension. At this time, the stress state inside the blank is relatively stable, thereby reducing the amount of spring back of the blank. Finally, supplementary stretching and shaping can be carried out under the action of a larger tension to further reduce the rebound range.

Stretch Forming Process of Aluminum Alloy Automobile Roof Rack【Stretch Forming Machine】

It should be noted that in the process of profile stretch bending, corresponding adjustments need to be made according to the characteristics of specific materials, bending radius, equipment conditions, and other factors. In addition, sufficient experiments and tests are required to determine the best combination of parameters to achieve the effect of reducing spring back.

Top 6 tips to reduce metal spring back in profile stretch forming

Material Selection

Choose materials with lower elastic modulus (Young’s modulus) and higher ductility. These materials are less prone to springback. Aluminum alloys, for example, are commonly used in stretch forming due to their favorable properties.

Bend Allowance

Adjust the bend allowance based on the material’s behavior and springback characteristics. This requires knowledge of the material’s mechanical properties and the expected amount of springback for a given bend angle and radius.

Tooling Design

  • Use Smoother Radii: Avoid sharp bends or radii that are too small, as they can increase springback. Larger radii help distribute the deformation more evenly.
  • Minimize Friction: Ensure that the forming die surface has a low coefficient of friction. Proper lubrication or coatings on the die can help reduce friction during forming, which can impact springback.
  • Control Die Gap: Maintain consistent die gap throughout the forming process. Variations in die gap can lead to uneven deformation and increased springback.

Forming Process Parameters

  • Stretching Speed: Optimize the stretching speed to ensure uniform deformation. Rapid or uneven stretching can contribute to springback.
  • Hold Time: If applicable, consider introducing a hold time at the maximum stretch to allow the material to “settle” before releasing the force.
  • Stretching Force: Control the stretching force to avoid overstretching, which can increase springback.

Post-Forming Treatments

  • Heat Treatment: Depending on the material and the level of springback, heat treatment (annealing) may be used to relieve residual stresses and minimize springback. However, this might not be suitable for all materials.
  • Strain Relief: Some materials may benefit from a controlled stress-relief process after forming to reduce springback.

Simulation and Iterative Testing

Use computer simulations and physical prototypes to understand the material behavior and springback tendencies for a specific profile. This allows for iterative adjustments to the process parameters and tooling design to achieve better results.

Final words

The metal profile stretch bending process can make the metal profile have higher bending performance and stability by applying axial tension and bending control, while improving the precision and quality of the final formed part. It is a practical metal sheet forming technology, which has the advantages of low cost and high processing precision, so it is widely used in metal processing, especially for the forming of some materials with complex shape requirements.