Rebound is one of the most common and headache inducing quality issues in bending and forming processing. The board is forced to undergo plastic deformation during bending, but once the pressure is released, the elastic stress inside the material will be released, and the bending angle will rebound in the original direction. Although the program was set to 90 degrees, the actual folding turned out to be 92 degrees or even larger, with dimensions exceeding tolerances, assembly difficulties, and high rework costs. This article focuses on the issue of springback, explaining its causes and discussing the solutions commonly used by bending machine manufacturers.
1、 How does rebound occur
The essence of rebound is the result of the combined action of material elastic-plastic deformation. During bending, the outer side of the sheet is subjected to tension and the inner side is subjected to compression. When the stress exceeds the yield strength, plastic deformation occurs; But at the moment of unloading, the elastic part will recover first, and the angle will rebound accordingly. The magnitude of rebound is mainly influenced by three factors.
One is the material itself. The higher the yield strength and the lower the elastic modulus, the more obvious the rebound. The elastic modulus of stainless steel, high-strength steel, and aluminum alloy is relatively low, and the rebound amount is usually greater than that of ordinary carbon steel. This is also why the original parameters do not work when changing a batch of materials.
The second is the process parameters. The bending inner corner radius, lower die V-groove width, holding time, and bending speed all affect rebound. If the V-groove is too wide, the holding time is insufficient, and the plastic deformation is not sufficient, the rebound will be too large; On the contrary, the higher the proportion of plastic deformation, the smaller the rebound.
The third is the equipment status. The deflection of machine tools under stress, mold wear, and temperature drift can all cause unstable rebound performance. The same batch of materials can be folded accurately in the morning and run at different angles in the afternoon, often not due to parameter tuning errors, but rather due to equipment thermal drift.
2、 Common methods for solving rebound
Several effective methods have been developed in the industry for rebound, with the core idea being to make plastic deformation more complete and rebound amount predictable.
Compensation for cornering. This is the most direct approach. Since the board will bounce back, let's fold it a little more first. For example, the target is 90 °, carbon steel is usually bent 1-2 °, and stainless steel is bent 3-5 °. The specific compensation amount needs to be calibrated through trial bending. Modern CNC equipment can directly write the compensation value into the program, saving the trouble of repeated machine adjustments during mass production.
Increase the holding time. At the moment of bending in place, the internal stress of the material is still continuously released. Appropriately extending the holding time, usually 2-5 seconds, allows the stress to be fully released, which can significantly reduce rebound fluctuations and make the angle more stable.
Optimize the mold. Choosing narrower V-grooves and smaller inner corner radii can increase the proportion of plastic deformation and reduce rebound at the root. But pay attention to matching with the thickness of the board, as narrow slots can easily cause cracking.
Numerical control compensation and real-time detection. High end bending forming equipment will be equipped with a deflection compensation system and real-time angle detection. During the bending process, the pressing depth will be automatically corrected according to the actual angle, and the rebound error will be controlled within a very small range. This is also the key to ensuring consistency in automated production lines.
3、 Rebound Control in Glass Hot Bending Forming
The rebound problem not only exists in metal bending, but also in glass hot bending forming. 3D curved glass is formed by its own weight or mold pressure after being heated to the softening temperature during hot bending. If the molding temperature is not sufficient, the heating is uneven, or the holding time is insufficient, the glass will rebound in the plane direction after cooling, resulting in substandard curvature and poor bonding, directly affecting the yield of subsequent bonding processes.
The key to solving the problem of glass hot bending rebound lies in three points: firstly, temperature control should be precise, the temperature in the forming area should be uniform, and local overheating or undercooling should be avoided; Secondly, the holding time should be sufficient to allow the glass to fully shape on the mold; Thirdly, mold design should consider springback and reserve compensation. In addition, thickness detection and monitoring before discharge are also necessary to detect abnormalities in a timely manner and avoid batch defects.
Taking the 3D curved glass hot bending forming machine as an example, it adopts dual high-frequency heating with a forming temperature of 600-850 ℃. The parameters can be set and displayed on the touch screen, and the mold has thickness detection and monitoring before discharging. The purpose of these configurations is to control rebound and dimensional deviation within a reasonable range, ensuring consistency in mass production.
4、 What to look for when choosing a bending forming machine manufacturer
Whether the rebound can be controlled well or not, the equipment is only the foundation, but also tests the manufacturer's process accumulation and engineering capabilities. When choosing a bending machine manufacturer, it is recommended to focus on three points.
Take a look at the device accuracy configuration. Whether it has the ability of numerical control compensation, real-time detection, temperature closed-loop control, etc., directly determines the stability of rebound control and also determines the upper limit of product yield.
Secondly, evaluate the process validation capability. Good manufacturers will provide trial bending and mold testing services, calibrate process parameters according to your material and product requirements, rather than just selling the equipment. The precipitation of process data is often more valuable than the equipment itself.
Check the engineering service capability three times. Supporting services such as product development, hardware testing, and manufacturing automation can help you solve process problems in advance before mass production, shortening the cycle from sampling to mass production.
V. Conclusion
Rebound is an issue that cannot be bypassed in bending forming, but it is not unsolvable. By understanding the causes of rebound, utilizing methods such as over bending compensation, pressure holding, mold optimization, and numerical control compensation, combined with the manufacturer's process support, it is entirely possible to stabilize the angle accuracy within the ideal range. Choosing the right bending machine manufacturer is equivalent to solving half of the rebound problem.
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