Stretch bending is one of the most widely used profile bending processes, especially those with large bending radius. Because the workpiece is often in elastic deformation, it is easy to rebound to the initial state, so it is difficult to use ordinary bending method, so it is necessary to select the stretch bending process.
1. The forming process of stretch bending
Stretch bending is usually divided into three steps, as shown in Fig. 7.14. Firstly, the profile is pre stretched to eliminate the elastic deformation and enter the plastic deformation state; then, the die is used to bend to make it reach the required shape; finally, the profile is pulled. In production, it can be decided whether there is supplementary drawing process according to the actual situation. The specific operation is to fix both ends of the profile on the clamping device, and then apply a relatively large pre stretching force to it, which is slightly higher than the yield strength, about 1.1 times of it; then start to move the clamping device or forming die punch, when the profile touches the punch, the A forming torque is generated and acts on the middle part of the profile, and the profile is divided into two sections of forming area. With the continuous movement of the clamping device or punch, the torque is increasing, and the deformation degree of the profile is also increasing. When the required shape is reached, the movement stops. After forming, the profile is not taken out immediately, but the drawing force is increased and it is stretched again to form finally.
7.14 stretch bending principle
Compared with other bending processes, the distribution of tangential stress is different in stretch bending. Both bending and rolling bending belong to single bending. The stress state of the inner layer is compressed, the outer layer is tensile and the neutral layer is unchanged. As shown in Fig. 7.15, in the process of stretch bending, the tensile stress of the outer layer increases when the tensile force P is applied along the bending direction, and the compressive stress in the inner layer begins to decrease, and then the tension begins. When the tensile force P causes the tensile stress of point a at the innermost edge of the material to exceed the yield point σ s, after removing the tensile force P, the workpiece can basically keep the shape obtained during stretch bending, and the springback is very small.
2. Characteristics of stretch bending
(l) Only one step is needed to reach the bending radius. The bending angle can reach 180. And other bending methods need multi-step process to bend a larger radian.
The comparison of bending stress www.aitmy.com
Figure 7.15 comparison of bending stresses
(2) The springback is relatively small. Because in the bending process, there is always a stretching force, which makes the neutral layer in the center of the profile move to the inside, and even makes the whole section under the tensile stress. This can reduce the bending moment caused by the different stresses in the internal and external layers of the profile, so as to reduce the springback.
(3) Because the tensile force is applied to the material, the wrinkle degree of the inner layer is reduced to a certain extent, and even the wrinkle can be avoided, so the forming precision is high.
(4) But at the same time, because of the existence of the tensile force, the inner concave degree of the outer layer of the profile increases, which affects the bending quality of the profile.
(5) Stretch bending processing is suitable for workpieces with large length and large bending radius. When the bending radius is less than 10 mm, the outer layer is easy to crack.