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 air bend causes the bending line to compress, and the compression effect causes the bending line to extend longitudinally, and the metal at the bend is thickened. This is the compression/thickening effect of the empty bend.
The advantage of the hollow bend is that it can bend the side length when the solid bend cannot be performed, such as the upper side/side edge of the square tube and the simultaneous bending and finishing. The hollow bend can also bend the internal angle of R < 0.2t without causing the tube wall to break.
The disadvantage of the hollow bend is that when the square tube is synchronized with the upper side/side side, the forming force tends to exceed the critical point due to the simultaneous pressure of the upper and lower rolls, causing the edge to be unstable and concave, and also affecting the stability of the unit. Operation and molding quality. This is also a different feature of the rectangular tube and the tube when it is formed by air bending.
In practical applications, these two basic methods should be configured in a reasonable process position according to the product requirements in the square tube forming. Attention should be paid to the effect of the stretch/thinning effect of the solid bending forming and the compression/thickening effect of the air-bending forming on the product quality, otherwise the side tube will be concave when the square tube is formed, the longitudinal direction of the finished tube is warped, and it is difficult to correct straight.