Bending die is the main processing equipment of sheet metal forming, with the development of engineering machinery, automobiles, ships, aerospace, electrical instrumentation and building decoration industries, the processing enterprises at home and abroad have more and more requirements for forming precision and shape complexity of sheet metal products, and the requirement for surface quality of the sheet metal parts is more and more high. Especially the bending processing for stainless steel plate, the aluminum plate is also requiring no scratches.

In order to avoid the bending marks on the workpiece surface during the bending process for press brakes machine, we will analyze the causes of the bending marks on the surface of the sheet metal parts from four aspects, which including the bending method, material hardness, concave die structure, press brake selection and die precision.

Defects of traditional bending

Lower Die Structure of Traditional Bending Method

Fig.1

Fig. 1 shows the lower die structure under the traditional bending method, which usually has a V-shaped groove of varying size, used for bending different thickness of the plate. Under the force of bending, the plate along the lower die V-groove edge movement and bear the squeeze and friction generated by the bending force, so that the surface of the formed sheet metal has distinct bending marks defect, the width of the defect place is about 0.414 times the width of V-Groove, seriously affecting the appearance of the product quality.

In order to solve the defects in traditional bending, it is necessary to overcome the plate movement and friction and extrusion pressure generated by the lower die V-groove in the bending process.

Mark-free bending technology and die structure

Roller-type mark-free die

Roller-type non-Indentation die

Fig.2

The structure of the roller-type mark-free die is shown in Fig. 2, when bending the sheet metal, the workpiece in the bending process contact with the cemented carbide mandrel on the lower die, and the mandrel will rotate follow the moving direction of the plate, so that after the workpiece in contact with the lower die, the friction changed from the traditional sliding friction into rolling friction, which greatly reduces the bending marks and abrasion caused by extrusion on the workpiece surface, and effectively improves the appearance quality and precision of the workpiece.

However, in the bending process, the lower die roller and the plate still exists squeeze pressure, cannot completely avoid bending marks, especially when bending the soft metal plate.

Plate rotate-turning mark-free die

Plate rotate-turning non-indentation die

Fig.3

The structure of Plate rotate-turning mark-free die as shown in Fig. 3, in the process of pressing the workpiece into the lower mold by the top punch, as the workpiece contact with the rotation surface, rotating the turning plate in the lower die. There is no relative slide during the whole bending process of the workpiece, meanwhile, it is the surface contact for the workpiece and rotating-turning plate. It eliminates the marks and abrasion caused by extrusion on the workpiece surface, which effectively improves the appearance quality and precision of the workpiece.

Other mark-free bending technology

Block polyurethane die

Fig.4

In addition to the above two kinds of mark-free mold structure can reduce the bending marks on the surface of metal plate during bending process, anti-marks mat, single v concave die pressure-proof rubber sleeve, block polyurethane+AT pad lining, hard rubber concave die can also be used to prevent bending marks on the workpiece surface, to meet the product quality requirements, as shown in Figure 4.

But the rubber cushion, the rubber sleeve, the excellent strength rubber block and the hard rubber are easy to deform in the bending process, the service life is short, affects the dimension precision of the bending workpiece, only applies to the workpiece bending which do not require high the dimension accuracy.

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