Bending Tool Selection
Bending tools are the primary equipment for sheet metal forming. With the development of industries such as construction machinery, automotive, shipbuilding, aerospace, electrical appliances, and architectural decoration, both domestic and international sheet metal processing companies have increasingly higher requirements for the forming accuracy and complexity of sheet metal products. Additionally, there is a growing demand for high surface quality, especially for stainless steel and aluminum sheet bending, where scratch-free processing is crucial. To avoid surface indentation during bending machine processing, the causes of surface indentation on sheet metal during bending are analyzed from four aspects: bending method, material hardness, lower die structure, and the selection of machine tool and mold accuracy.
Drawbacks of Traditional Bending
The lower die structure in traditional bending methods typically consists of V-shaped slots of varying sizes, used for bending different thicknesses of sheets. Points A and B are the points of force application on the sheet, and the fold usually appears at these points. During bending, the upper die descends, and the sheet slides along points A and B within the sliding range, denoted as L, which is the most susceptible area for surface indentation. If a protective film is used to prevent indentation, it can easily rupture with changing bending pressures. Once the protective film ruptures, surface indentations occur.
During bending, under the action of bending force, the sheet moves along the edge of the lower die V-shaped slot and experiences extrusion and sliding friction from the bending force. This results in significant surface indentation and stretching defects on the formed sheet metal, with the width of the defect zone being approximately 0.414 times the width of the V-shaped slot. This significantly affects the appearance quality of the product. To address the above-mentioned drawbacks of traditional bending, it is necessary to overcome the sheet movement during bending and the friction and extrusion forces exerted by the lower die V-shaped slot on the sheet.
Design of Indentation-Free Bending
Based on the principles mentioned above and considering the processing capabilities of current CNC bending machines in the market, an indentation-free mold structure has been developed.
1) Considering the company's sheet metal usage, the minimum bending material thickness for this mold is designed as 0.6mm, with a maximum bending thickness of 2mm for steel plates and 2.5mm for aluminum plates.
2) The bending length is designed by combining the characteristics of the lower die combination of CNC.
Translation:
2. Design Principles of Indentation-Free Bending
To address the drawbacks of traditional bending, engineers have utilized the principle of a lever and designed the lower die for bending to consist of separate left and right pieces that can rotate around their respective axes. When the upper die descends for bending, the left and right lower dies rotate around their respective rotation centers, forcing the material to rotate around the vertex of the upper die while in close contact with the working surface of the lower die. This generates the bending deformation and completes the bending process of the material. Since the working surface of the lower die is rotating, the force uniformly applied to the material surface through the working surface will not cause localized compression, avoiding surface indentation defects caused by compression. Additionally, as the material rotates around the vertex of the upper die during bending, there is no sliding on the working surface of the lower die, thereby avoiding surface abrasion defects on the part.
Indentation-Free Bending Technology and Mold Structure
Roller-Type Indentation-Free Mold
In the structure of a roller-type indentation-free mold, during the bending process of the metal sheet, the workpiece comes into contact with a hard alloy core rod on the lower die, and the core rod rotates along the direction of the sheet's movement. This transforms the contact between the workpiece and the lower die from traditional sliding friction to rolling friction, significantly reducing surface indentation and abrasion caused by compression. It effectively improves the appearance quality and precision of the workpiece. However, during the bending process, there is still compression force between the roller shaft of the lower die and the sheet, and bending indentation cannot be completely eliminated, especially for soft sheet materials.
Rotating Flip-Type Indentation-Free Mold
In the structure of a rotating flip-type indentation-free mold, as the upper die presses the workpiece into the lower die body, the workpiece comes into contact with the rotating flip plate. The flip plate rotates within the lower die body, and there is no relative sliding between the workpiece and the flip plate throughout the bending process. Moreover, the workpiece and the flip plate make face-to-face contact, eliminating surface indentation and abrasion caused by compression on the workpiece. This effectively improves the appearance quality and precision of the workpiece.
Other Indentation-Free Bending Techniques
In addition to the above-mentioned two types of indentation-free molds that can solve surface indentation during bending, other methods such as using anti-indentation rubber pads, single V-shaped lower die with anti-indentation rubber sleeves, block-shaped adhesive + AT lining, and hard rubber lower dies can also be used to address surface indentation during bending and meet product quality requirements. However, rubber pads, rubber sleeves, indentation-free molds, adhesive blocks, and hard rubber have a tendency to deform during the bending process and have a short service life, which can affect the dimensional accuracy of the bent workpiece. Therefore, they are only suitable for bending workpieces with low dimensional accuracy requirements.
Bending tools are the primary equipment for sheet metal forming. With the development of industries such as construction machinery, automotive, shipbuilding, aerospace, electrical appliances, and architectural decoration, both domestic and international sheet metal processing companies have increasingly higher requirements for the forming accuracy and complexity of sheet metal products. Additionally, there is a growing demand for high surface quality, especially for stainless steel and aluminum sheet bending, where scratch-free processing is crucial. To avoid surface indentation during bending machine processing, the causes of surface indentation on sheet metal during bending are analyzed from four aspects: bending method, material hardness, lower die structure, and the selection of machine tool and mold accuracy.
Drawbacks of Traditional Bending
The lower die structure in traditional bending methods typically consists of V-shaped slots of varying sizes, used for bending different thicknesses of sheets. Points A and B are the points of force application on the sheet, and the fold usually appears at these points. During bending, the upper die descends, and the sheet slides along points A and B within the sliding range, denoted as L, which is the most susceptible area for surface indentation. If a protective film is used to prevent indentation, it can easily rupture with changing bending pressures. Once the protective film ruptures, surface indentations occur.
During bending, under the action of bending force, the sheet moves along the edge of the lower die V-shaped slot and experiences extrusion and sliding friction from the bending force. This results in significant surface indentation and stretching defects on the formed sheet metal, with the width of the defect zone being approximately 0.414 times the width of the V-shaped slot. This significantly affects the appearance quality of the product. To address the above-mentioned drawbacks of traditional bending, it is necessary to overcome the sheet movement during bending and the friction and extrusion forces exerted by the lower die V-shaped slot on the sheet.
Design of Indentation-Free Bending
Based on the principles mentioned above and considering the processing capabilities of current CNC bending machines in the market, an indentation-free mold structure has been developed.
1) Considering the company's sheet metal usage, the minimum bending material thickness for this mold is designed as 0.6mm, with a maximum bending thickness of 2mm for steel plates and 2.5mm for aluminum plates.
2) The bending length is designed by combining the characteristics of the lower die combination of CNC.
Translation:
2. Design Principles of Indentation-Free Bending
To address the drawbacks of traditional bending, engineers have utilized the principle of a lever and designed the lower die for bending to consist of separate left and right pieces that can rotate around their respective axes. When the upper die descends for bending, the left and right lower dies rotate around their respective rotation centers, forcing the material to rotate around the vertex of the upper die while in close contact with the working surface of the lower die. This generates the bending deformation and completes the bending process of the material. Since the working surface of the lower die is rotating, the force uniformly applied to the material surface through the working surface will not cause localized compression, avoiding surface indentation defects caused by compression. Additionally, as the material rotates around the vertex of the upper die during bending, there is no sliding on the working surface of the lower die, thereby avoiding surface abrasion defects on the part.
Indentation-Free Bending Technology and Mold Structure
Roller-Type Indentation-Free Mold
In the structure of a roller-type indentation-free mold, during the bending process of the metal sheet, the workpiece comes into contact with a hard alloy core rod on the lower die, and the core rod rotates along the direction of the sheet's movement. This transforms the contact between the workpiece and the lower die from traditional sliding friction to rolling friction, significantly reducing surface indentation and abrasion caused by compression. It effectively improves the appearance quality and precision of the workpiece. However, during the bending process, there is still compression force between the roller shaft of the lower die and the sheet, and bending indentation cannot be completely eliminated, especially for soft sheet materials.
Rotating Flip-Type Indentation-Free Mold
In the structure of a rotating flip-type indentation-free mold, as the upper die presses the workpiece into the lower die body, the workpiece comes into contact with the rotating flip plate. The flip plate rotates within the lower die body, and there is no relative sliding between the workpiece and the flip plate throughout the bending process. Moreover, the workpiece and the flip plate make face-to-face contact, eliminating surface indentation and abrasion caused by compression on the workpiece. This effectively improves the appearance quality and precision of the workpiece.
Other Indentation-Free Bending Techniques
In addition to the above-mentioned two types of indentation-free molds that can solve surface indentation during bending, other methods such as using anti-indentation rubber pads, single V-shaped lower die with anti-indentation rubber sleeves, block-shaped adhesive + AT lining, and hard rubber lower dies can also be used to address surface indentation during bending and meet product quality requirements. However, rubber pads, rubber sleeves, indentation-free molds, adhesive blocks, and hard rubber have a tendency to deform during the bending process and have a short service life, which can affect the dimensional accuracy of the bent workpiece. Therefore, they are only suitable for bending workpieces with low dimensional accuracy requirements.
