What is the shape of the welded square pipe

　　There are two main methods for forming welded square tubes: solid bending and hollow bending.

　　1. solid bending

　　It means that the bending is compacted, and the inner and outer rollers and the inner and outer walls of the tube blank are biaxially compacted during the solid bending.

　　1) The advantage of solid bending is that the rebound is small, the forming is accurate, and as long as the roll type is accurate, the R formed at the inner corner is more accurate.

　　2) The disadvantage of solid bending is the stretching/thinning effect. First, the solid bend causes the bend to be stretched, and the stretch effect shortens the longitudinal length of the bend line. Second, the metal at the bend bend becomes thinner due to stretching.

　　2. hollow bending

　　The hollow bend is formed by the unidirectional contact between the outer roller and the outer wall of the tube blank to form a bending moment to bend the strip. The hollow bend causes the bending line to compress, and the compression effect causes the bending line to longitudinally elongate, and the metal at the bend is thickened. This is the compression/thickening effect of the empty bend.

　　1) The advantage of the empty bend is that the side length can be bent when the solid bend cannot be performed, for example, the upper side/side side of the square tube is synchronously bent and finished. The hollow bend can also bend the internal angle of R < 0.2t without causing the tube wall to break.

　　2) The disadvantage of the empty bend is that when the upper side/side side is synchronized, the forming force easily exceeds the critical point due to the simultaneous pressure of the upper roll and the lower roll, causing the edge to be unstable and concave, and also affecting the stable operation of the unit, and molding quality. This is also a different feature of the rectangular.