Bending of Square and Rectangular Tubes: Solid Bend vs. Hollow Bend

Solid Bend:

Solid bending, as the name suggests, involves compact bending where the internal and external rolls compress the tube billet in both directions.

Advantages of solid bending include minimal rebound, precise shaping, and accurate formation of internal angles as long as the roll profile is precise.

However, solid bending has its drawbacks. Firstly, it induces stretching/thinning effects. Solid bending causes stretching at the bend, shortening the longitudinal length of the bend line. Secondly, the metal at the bend becomes thinner due to stretching.

Hollow Bend:

Hollow bending is achieved by unilateral contact between the external roll and the outer wall of the tube billet, forming a bending moment to bend the strip. Hollow bending compresses the bend line, resulting in longitudinal extension due to compression effects. This process thickens the metal at the bend, known as the compression/thickening effect of hollow bending.

The advantages of hollow bending include the ability to perform edge bends when solid bending is not feasible, such as synchronous bending and finishing of the upper and side edges of square and rectangular tubes. Hollow bending also allows bending of internal angles with R<0.2t without causing wall fracture.

However, hollow bending has its drawbacks. When synchronously bending the upper and side edges of square and rectangular tubes, the simultaneous pressure from the upper and lower rolls can exceed the critical point, leading to instability and concavity at the edges. This can also affect the stability and quality of the forming process, distinguishing it from the hollow bending process of round tubes.

In practical applications, both methods are essential in the forming of square and rectangular tubes and should be positioned in the process according to product requirements. Attention should be paid to the stretching/thinning effects of solid bending and the compression/thickening effects of hollow bending on product quality. Failure to do so may result in concavity at the edges, longitudinal curvature of the finished square and rectangular tubes, and difficulties in straightening.