The Virtual Bending And Actual Bending Of Square Tubes

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The hollow bend and solid bend are two basic bending processes for cold forming of square section tubes. The core difference lies in whether the inner wall of the tube blank is provided with support and compaction during the forming process. Together, they constitute the core forming process of square section tubes from strip steel/round tube blanks to square cross-section.

1. True bend (compaction bend)

A true bend refers to "bending under compaction conditions", which is a forming method where both the outer and inner forming rollers come into contact with the inner and outer walls of the tube billet simultaneously, and apply bidirectional compression and compaction.

• Operating process: The forming station is equipped with both the outer roller and the inner roller. When the tube billet passes through, the outer and inner rollers respectively adhere to the outer and inner sides of the tube wall, and simultaneously apply pressure to completely constrain and compact the bending area, forcing the material to complete the angle bending according to the roller shape. The deformation zone is always under the rigid constraint of the mold.
• Core deformation effect: The material on the outer side of the bending area is subjected to tension, while the material on the inner side is subjected to compression. The overall effect is a tensile thinning effect - the longitudinal length of the bending line slightly shortens, and the tube wall at the bending angle will thin due to tension.

2. Empty Bend (Suspension Bend)

An empty bend refers to "a bend where the inner wall is suspended". It is a process where only the outer roller contacts the outer wall of the tube billet, and there is no supporting roller on the inner wall. The shape is formed through the indirect deformation by bending moment.

• Operating process: At the forming station, only forming rollers are arranged on the outer side of the tube billet, while there are no rollers on the inner side. After the outer roller exerts pressure on the tube wall, the bending moment is formed based on the cross-sectional stiffness of the tube billet, causing plastic deformation in the bent area. The inner material is in a "suspended" state of free deformation.
• Core deformation effect: The material at the bent area is compressed as a whole, resulting in a compression thickening effect - the longitudinal length of the bent line slightly elongates, and at the bending angle, the metal will be thickened due to compression accumulation.

The two processes are not in a substitutive relationship but are complementary core technologies in the production of cold-formed square profile tubes. Their value lies in:

1. Meeting all specifications of forming requirements:

True bending solves the stability problem of thick-walled and high-precision square profile tubes; Empty bending solves the forming difficulties of small R-angles, thin walls, and complex cross-sections, filling the technological gap that cannot be achieved by true bending.

2. Balancing precision and efficiency:

True bending ensures the accuracy of dimensions and angles, while Empty bending supports multi-pass synchronous bending and edge finishing, enhancing the production efficiency and product complexity of the unit.

3. Adapting to different material characteristics:

For materials with poor plasticity and prone to cracking, Empty bending can avoid the risk of fracture caused by reduction in thickness due to stretching in true bending; For thick-walled high-strength materials, True bending can control the rebound and ensure the forming stability.

In actual cold bending machines, the two are usually used in combination: the first few passes complete the main angle forming through actual bending, while the latter passes achieve the section refinement and edge length correction through empty bending.

Main applications of the actual bending process:

• The main forming process for thick-walled square and rectangular tubes and high-strength steel square and rectangular tubes;
• Structural square and rectangular tubes with strict requirements for dimensional accuracy and angle tolerance (such as in construction machinery, building steel structures, and automotive frames);
• Batch production of standard-sized square and rectangular tubes with common R-angles (R ≥ 0.5t).

Main applications of the hollow bend:

• Forming of thin-walled square and rectangular tubes, as well as small-sized square and rectangular tubes;
• Decorative and furniture-grade square and rectangular tubes with extremely small inner angles R;
• Multi-pass finishing and edge correction of special-shaped and non-standard square and rectangular tubes;
• Avoiding cracking caused by actual bending and stretching of stainless steel and alloy steel square and rectangular tubes with poor plasticity.

In actual production and procurement selection processes, the following priorities can be used for judgment:

1. Based on wall thickness

 For specifications with a wall thickness of ≥ 4mm or those with a high proportion of thick sections, solid bend technology is preferred to ensure stability; for thin-walled pipes with a wall thickness of ≤ 2mm, hollow bend is preferred to avoid excessive thinning.

2. Based on the required inner angle R value:

For sharp-angle square or rectangular pipes with an R value less than 0.2t, hollow bend is the only option; for conventional R values (0.5t - 2t), solid bend is preferred to ensure accuracy.

3. Based on precision requirements

For structural load-bearing and high-precision assembly scenarios, solid bend technology is selected; for decorative, protective covers, and other non-load-bearing scenarios, hollow bend technology can be chosen.

 4. Based on material properties:

Carbon steel and low-alloy high-strength steel are suitable for solid bend; stainless steel, high-alloy steel, etc., with low plasticity, are preferred for hollow bend.

5. Based on the complexity of the cross-section:

Standard square and rectangular cross-sections are mainly processed by solid bend; complex cross-sections with irregular shapes and multiple bends require the combination of hollow bend processes.

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