Troubleshooting: Blanking Die Scratches on Sheet Metal

With the development of the automobile industry, people have gained a deeper understanding of automobile manufacturing technology. In order to produce high-quality automobiles, advanced equipment and technology are essential.

In the past, blanking dies accounted for a small proportion of stamping dies used in the development of automobile panels. This was due to the high cost of investment in automobile stamping dies, which accounts for a significant portion of the development cost of the entire vehicle. To reduce the one-time investment in the early stages of development, automobile manufacturing enterprises often canceled the development of blanking dies and instead used square or rectangular blanks for forming. However, this led to an increase in material consumption.

With the recent sharp rise in steel prices, automobile manufacturers have gradually realized that improving the utilization rate of single vehicle materials is necessary to reduce the manufacturing cost of complete vehicles. As a result, large automobile manufacturers have introduced uncoiling and blanking lines and put uncoiling and blanking molds, and even swing shear molds, into operation. This effectively avoids the bumps and scratches that occur on sheet metal during uncoiling and blanking, improving the quality of sheet metal.

Furthermore, the use of automatic uncoiling and blanking can significantly reduce operator accidents related to sheet metal scratches. The most significant impact is that the application of uncoiling lines increases the production rate to 60SPM, greatly reducing the labor intensity of workers and improving production efficiency.

Description of the problem

To enhance product quality, material utilization, and production efficiency, our company has installed a 630-ton automatic uncoiling and blanking line. This newly constructed line features functions such as automatic uncoiling, leveling, continuous feeding, continuous shearing, and automatic stacking, as depicted in Figure 1.

process flow diagram of uncoiling and blanking line

Fig. 1 process flow diagram of uncoiling and blanking line

Currently, the new uncoiling line has a production rate of 30 to 60 strokes per minute, which is significantly higher than the previous line’s rate of 22 to 40 strokes per minute.

The increased production rate imposes higher demands on the accuracy of manufacturing and the efficient design of the die.

The blanking die must be able to accurately position and smoothly handle the sheet metal.

Its structure resembles that of a typical blanking die, with the upper die being identical. The only difference lies in the design of the lower die, which features a floating plate to transport the sheet metal, thereby reducing friction and preventing scratches on the edge of the punch.

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During the initial commissioning and production phase of the 630-ton uncoiling and blanking line, surface scratches on the sheet metal were a frequent occurrence, severely impacting the surface quality.

Due to limited investment in new blanking dies, the old dies had to be adapted for use in the new uncoiling line.

Currently, the primary issue is the scratching of the sheet metal, as depicted in Figure 2.

sheet scratch

Fig. 2 sheet scratch

Cause analysis of sheet metal scratch and deformation

The early stage debugging results of the blanking die revealed that the position of scratches on the sheet metal was constantly shifting and lacked a pattern, making it challenging to identify the root cause.

Over the next two months during mold commissioning, we thoroughly investigated all potential factors that could lead to scratches on the sheet.

After thorough on-site elimination and verification, we have finally determined the following four reasons as the cause.

Insert backrest cusp, contour high point

(1) The back of the cutting punch is not chamfered, and the insert is at a high point.

When the sheet metal is moving at high speed, it may oscillate and come into contact with the back of the cutting punch.

If the back of the cutting punch is not rounded, it may result in short-distance scratches on the sheet metal, as depicted in Figure 3.

fillet of insert knife back

Fig. 3 fillet of insert knife back

(2) High point in the guide wheel.

Most uncoiling and blanking dies will have a guide wheel on the feeding side to ensure smooth feeding and prevent the sheet from quickly hitting the high point.

The wheel in the guide wheel is secured with an open bolt that rotates along with the roller.

However, the open bolt is positioned higher than the roller and directly touches the plate, resulting in scratches on the plate, as depicted in the red circle in Figure 4.

the pin is higher than the roller

Fig. 4 the pin is higher than the roller

Sheet scratch caused by insufficient strength of feeding mechanism

(1) The floating plate used for feeding is insufficient in strength and prone to deform, leading to an uneven height of the feeding roller.

The standard thickness of the floating plate is between 16mm and 20mm.

Made of 45 steel, the material is not heat-treated, which results in low rigidity and hardness, making it easily deformable.

If the floating plate warps upwards at both ends (as depicted by the red circles in Fig. 5), the feeding height of some balls will be raised and may cause them to scrape against the plate.

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If the two ends of the floating plate collapse downwards (as depicted by the red circles in Fig. 5), the conveyance height of the plate will be lower than the height of the punch edge, causing collisions to occur.

Fig. 5 blanking die

(2) Six to ten leading blocks, depicted by the six yellow circles in Figure 5, will be placed on the floating plate and joined together via welding.

Typically, the welded regions will experience local deformation, potentially altering the height at which the ball is fed and leading to scratches on the plate if the ball is too high.

Ball scratch

(1) Causes of ball manufacturing error:

At present, the accuracy error in domestic ball manufacturing is significant. Field experience has shown that when the height error of a ball exceeds 0.1mm, it results in noticeable scratches on sheet metal and is considered unacceptable in terms of quality.

(2) Material reason of ball

The ball is prone to rusting in high humidity environments, and once rusted, it can cause direct scratching on sheet metal.

(3) The production environment of the blanking die is poor.

Powders and iron filings can enter the ball, causing it to fail to rotate normally. This results in sliding friction between the plate and ball, leading to scratches on the plate.

Scratch of support arm

(1) The material of the support arm is too rigid, while the sheet material is too flexible.

During high-speed transportation, the sheet metal comes into contact with the support arm, causing scratches on the sheet metal.

(2) The spring at the base of the support arm is overly stiff.

When the sheet is conveyed to the support arm, the excessive spring strength and large lifting angle of the support arm cause the sheet to fall through a steep arc and experience rapid friction with the ball, resulting in noticeable friction marks, as depicted in Figure 6.

material supporting arm

Fig. 6 material supporting arm

Solution to sheet scratch

According to the original structure of the old mold, after many debugging scheme comparisons, we have determined the final solution, as shown in Fig. 7.

Fig. 7 layout of support arm

(1) Chamfer and polish all punch backs to avoid edge scratches.

(2) Replace all dead balls.

Before replacing, make sure to verify that the replacement ball is identical to the original ball in terms of size and specifications.

Please note that there is a height discrepancy of 0.5mm between the purchased ball and the original ball.

Due to the poor precision of the purchased ball, the height difference causes difficulties in on-site debugging.

Continuous padding and adjustment may be necessary to achieve the same height as the original ball.

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(3) Reinstall the support arm.

The arm support component on the lower die has been misplaced.

The model for the arm support component is identified by referencing the digital model, and new parts are procured and installed.

The spring at the base of the newly procured arm support component was found to be too stiff, and a more suitable spring with the appropriate elasticity was selected.

When the copper arm support component pushes up the sheet metal, there is no significant dip, resulting in improved feeding stability. Inspection of the sheet metal after commissioning shows no signs of scratching.

(4) Add a thin pad or polyurethane under the ball.

The lower die’s floating plate is 16mm thick and has been in use for 6 years.

Through the platform inspection, local deformations have been detected, leading to unequal ball height. To balance the deformations of the floating plate, a thin pad or polyurethane must be added.

The height and placement of the polyurethane have been confirmed through on-site testing, which effectively prevents the high-speed running sheet metal from jumping or scratching, as depicted in Figure 8.

layout of polyurethane block

Fig. 8 layout of polyurethane block


The issue of sheet scratching in the blanking die process can be effectively prevented during the early design stage.

To start with, it’s crucial to have a well-designed layout for the uncoiling and blanking process.

The insert design should aim to minimize sharp corners as much as possible.

The insert’s back should be rounded and smoothed using a polishing machine, not just sandpaper. Sanding alone may result in scratches if the roughness is not up to standard.

Additionally, it is recommended to enhance the strength and stiffness of the floating plate.

The material support frame should have an integral structure for added support.

If a steel plate is used, its thickness should be 25mm and tempered to 30~40HRC to prevent scratches from deformation caused by differences in height among parts.

Finally, a more efficient roller or brush should be used instead of a ball.

While the ball support is a point of contact, a roller or brush provides surface contact, reducing the pressure on the sheet metal.

Placing a support arm in the appropriate location will also significantly reduce the impact scratching caused by the sudden acceleration of the sheet metal.

In conclusion, an appropriate plan can completely eliminate the issue of sheet scratching in the uncoiling and blanking die process. This will improve the surface quality of the blanking sheet, and minimize economic loss caused by scratching.

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