Sheet metal forming processes primarily rely on the inherent plastic properties of metal materials to deform sheets into desired shapes. Typically targeting thin sheets, this manufacturing method involves a myriad of specialized techniques. Among these, bending processes are extensively applied, making sheet metal bending forming a prominent area of industrial research.
The quality of sheet metal bending processes directly impacts production schedules and economic outcomes for enterprises. However, sheet metal bending defects, that compromise product quality and lead to resource inefficiencies, often occur during the process. Therefore, this article focuses on identifying and discussing these common defects. It aims to develop effective preventive measures to ensure seamless sheet metal bending forming processes.
Sheet metal bending is primarily a processing method where external forces are applied to sheets to achieve curved shapes. Bending forming generally includes free bending and bottoming bending: Adjusting the upper die displacement of the equipment to a certain height, the sheet metal is bent according to the angle indicated on the drawing, which is free bending. Lowering the upper die displacement to the minimum, the required sheet metal is bent between the upper and lower dies of the equipment, which is bottoming bending. The upper and lower dies required for bottoming bending are shown in the Figure below.
Generally, the free bending method requires lower requirements for machine tools and dies, but the material products of bending forming exhibit diverse characteristics. Therefore, this process is one of the main applied techniques in sheet metal bending forming.
During sheet metal bending, various defects arise due to factors such as raw material characteristics, part properties, and operational processes. Common defects in sheet metal bending forming processes include noticeable issues like bending springback, bending cracks, and bending indentations. This article focuses on discussing these three defects and proposing corresponding solutions.
During the bending forming process, when the neutral layer of the sheet is subjected to pressure and undergoes elastic deformation, plastic deformation occurs both inside and outside the neutral layer. Upon removing the bending pressure after forming, the dimensions and shape of the bent part deviate to some extent from those of the die, resulting in springback. If springback occurs during sheet metal bending processing, it can adversely affect part accuracy and usability. Effective measures should be implemented to control this phenomenon.
During bending forming, as the sheet material bends, the bending surface undergoes tensile deformation. If this deformation exceeds the material's inherent tensile limit, bending cracks may occur. Due to stringent requirements on part bending angles and radii, bending cracks typically appear in the middle of the bending section or near the ends of the bending position, especially near the right angles of the bending line.
The causes of bending cracks can be attributed to several factors:
Poor section quality in parts obtained through punching or shearing, including hardening and burrs;
Directional fiber structure from raw material rolling leading to abnormal material physical and mechanical properties, causing cracks when the bending line direction aligns with the material rolling direction;
Increased plastic deformation of the bending surface when the bending radius is small; and issues stemming from improper selection or operation of bending processes.
Controlling these factors is essential to prevent bending cracks.
During bending forming, due to the pressure exerted by the lower dies and equipment on the bending machine, sheet metal parts inevitably exhibit varying degrees of indentations on their contact surfaces. Generally, there are no stringent requirements for the surface roughness of sheet metal parts, and minor indentations do not impact part performance. However, effective measures should be taken to prevent bending indentations if the parts are made from special materials or have high aesthetic requirements.
Understanding these common defects and their underlying causes is crucial for improving the quality and efficiency of sheet metal bending forming processes. By addressing these issues through appropriate measures and optimizations in material selection, tooling design, and process control, manufacturers can enhance product reliability and meet stringent industry standards.
To address springback in bending, the following measures should be implemented:
①When bending deformation, the greater the elastic deformation of the plate, the greater the amount of rebound caused by bending. Therefore, before bending, consider annealing the plate, particularly for materials prone to cold hardening. This step enhances the internal stress organization of the plate, which helps mitigate the rebound effect during processing.
②After forming through sheet metal bending, parts tend to recover due to the elastic deformation of the sheet metal. This results in an increase in bending inner diameter and angle beyond theoretical values. To counteract this, adjust the deformation size during bending to adequately compensate and reduce rebound.
③Use a small inner diameter upper die for multipoint bending. This method involves multiple bending deformations, effectively reducing or eliminating springback by distributing deformation stress more evenly across the material.
To prevent bending cracks, adopt the following measures:
①Adjust the mold gap uniformly to enhance the quality of blanking and cutting, ensuring smooth, straight sections free from burrs and cracks.
②Based on the direction of raw material rolling, use nesting software to arrange materials reasonably. If the bending direction aligns with the raw material grain direction, the minimum bending radius can be appropriately increased. If the bending direction is perpendicular to the raw material grain direction, the minimum bending radius can be appropriately decreased. The angle between the raw material grain direction and the bending line should generally be around 60°, with a minimum angle above 30°.
③If there are multiple bending edges, then at the intersections of bending edges, crack stop holes should be arranged under permissible conditions, and the size of the crack stop hole should be greater than or equal to the raw material thickness plus the bending inner diameter. When adding crack slots at right angles in bending lines, the width of the crack slot generally needs to be greater than twice the raw material thickness plus the bending inner diameter.
④Lubricate and maintain bending equipment according to specified intervals to reduce friction during the bending process.
To mitigate bending indentation, implement these measures:
①Lengthen the upper die and increase the radius of the die end's rounded corner.
②Increase the width of the lower die groove within permissible limits. This enlarges the sheet metal part's inner diameter after bending, reducing extrusion at the lower die groove.
③The lower die is designed as a roller V-shaped die, and when pressing the upper die, the plate is generally rotated in a driven way, so that the workpiece maintains full and comprehensive contact with the lower die, effectively avoiding the appearance of bending indentation.
④Add rubber skin or non-indentation film with a thickness of 0.1 ~ 0.5mm to the upper and lower die positions of the sheet material to avoid or reduce bending indentation.
During the application of sheet metal bending forming processes, common defects include springback, bending cracks, and bending marks. These process defects easily affect the quality and appearance of sheet metal products, leading to higher rates of non-conformance. To ensure the stability of sheet metal bending processes and improve product quality, effective measures must be taken to address these defects and enhance part conformity.
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