1. Stamping Scrap
1) Causes:
Poor quality of raw materials;
Improper installation, adjustment, or use of the stamping die;
The operator fails to correctly feed the strip along the positioning guides or fails to ensure the strip is fed at a consistent gap;
Prolonged use of the die leads to changes in clearance or wear of the working components and guide parts;
Prolonged impact and vibration loosen the fastening components, causing relative displacement among the die's mounting positions;
Operator negligence, failing to follow operating procedures.
2) Countermeasures:
Raw materials must conform to specified technical conditions (strictly inspect the specifications and grades of raw materials, and where conditions permit, conduct laboratory tests on workpieces with high dimensional accuracy and surface quality requirements).
All prescribed procedures in the process specification must be fully and strictly followed;
The press machines, dies, and other tooling equipment used must be ensured to operate under normal working conditions;
A strict inspection system must be established during the production process. The first piece of each stamping part must undergo comprehensive inspection, and production can only proceed after it passes inspection. At the same time, patrol inspections should be strengthened, and any unexpected issues should be handled promptly;
Adhere to the civilized production system. For example, workpieces and blanks must be transferred using appropriate work-positioning fixtures; otherwise, the surface of the workpiece may be dented or scratched, affecting its surface quality;
During the stamping process, the die cavity must be kept clean, the work area should be organized in an orderly manner, and finished workpieces should be stacked neatly.
2. Burrs on blanked parts
1) Causes:
The blanking clearance is too large, too small, or uneven;
The cutting edges of the working parts of the die have become dull;
Due to prolonged vibration and impact, the centerlines of the punch and die may shift, causing the axes to become misaligned, resulting in one-sided burrs.
2) Countermeasures:
Ensure the machining accuracy and assembly quality of the punch and die, ensure the perpendicularity of the punch, its resistance to lateral forces, and that the entire die has sufficient rigidity;
When installing the punch, it is essential to guarantee the correct clearance between the punch and die, and to secure the punch and die firmly in the die holder. The end faces of the upper and lower dies must be kept parallel to the worktable surface of the press;
The press must have good rigidity, minimal elastic deformation, and high precision in the guide rails, as well as high parallelism between the bolster plate and the slide;
The press must have sufficient blanking force;
Allowable burr height on the shear fracture surface of the blanked part
Blanked sheet thickness >0.3 >0.3-0.5 >0.5-1.0 >1.0-1.5 >1.5-2.0
Burr height for new die tryout ≤0.015 ≤0.02 ≤0.03 ≤0.04 ≤0.05
Allowable burr height during production ≤0.05 ≤0.08 ≤0.10 ≤0.13 ≤0.15
3. Warping deformation of blanked parts
1) Cause:
The action and reaction forces are not in the same line, creating a moment (when the punch-die clearance is excessive and the die cutting edge has a reverse taper, or when the contact area between the ejector and the workpiece is too small, warping deformation occurs).
2) Countermeasures:
Select a reasonable blanking clearance;
In the die structure, add a pressure plate (or support plate) so that the sheet material is in flat contact with the pressure plate and under a certain pressure;
Inspect the die cutting edge; if a reverse taper is found, the die cutting edge must be corrected and dressed appropriately;
If the blanking part has a complex shape with many internal holes, the shear force is uneven—increase the pressure plate force, press the strip tightly before blanking, or use a high-precision press for blanking;
Level the sheet material before blanking; if warping deformation still cannot be eliminated, re-level the blanked workpiece using a leveling die;
Regularly clean dirt from the die cavity, lubricate the surface of thin sheets, and provide oil/air vent holes in the die structure.
4. During blanking, the accuracy of the outer edge and internal holes of the blanked part decreases, and dimensions change.
1) Cause:
The position of locating pins, stop pins, etc. has changed or they are excessively worn;
Operator negligence during feeding causes left-right or front-back misalignment;
The strip material has low dimensional accuracy, being too narrow or too wide, making feeding difficult and preventing it from reaching the designated position; the strip may shift front-back within the guide plate, resulting in significant front-back positional deviation between the punched hole and the outer shape of the workpiece.
5. When bending parts, dimensions and shape are unqualified
1) Cause:
Springback of the material causes the product to be unqualified;
The locator has worn or deformed, causing inaccurate strip positioning; a new locator must be replaced;
In a bending die without guides, if the adjustment of the press slide's bottom dead center position is incorrect during press setup, it can also cause the bent part's shape and dimensions to be unqualified;
The die's material holding device is malfunctioning or not applying pressure at all; the holding force must be readjusted or the pressure spring replaced to restore normal operation.
2) Measures to reduce springback:
Select stamping materials with a high elastic modulus, low yield point, and stable mechanical properties;
Add a sizing operation, using sizing bending instead of free bending;
Annealing the material before bending to pre-soften the cold-work hardened material before forming;
If shape deformation occurs during stamping and is difficult to eliminate, replace or rework the angle of the punch and die, and set the clearance between the punch and die to the minimum material thickness;
Increase the contact area between the die and the workpiece, and reduce the contact area between the punch and the workpiece;
Use the over-bending method to reduce the effect of springback.
6. Cracks occurring at the bend area of bent parts
1) Countermeasures:
Remove burrs on the outer side of the bend zone, as burrs cause stress concentration in this area and reduce the bending deformation; remove burrs from this area;
Place the side with burrs on the inside of the bending zone;
When bending a workpiece, it is best to make the bending direction perpendicular to the fiber direction (rolling direction) of the material;
The bending radius should not be too small; under the premise of acceptable quality, the corner radius should be increased as much as possible;
The surface of the bending blank should be smooth and free of obvious protrusions and scars;
During bending, an intermediate annealing process should be adopted to eliminate internal stress, and softened bending is less prone to cracking;
For large bending parts, lubricant must be applied during bending to reduce friction during the bending process.
7. Offset of bending parts during the bending process
1) Causes:
During the bending process, when the blank slides along the surface of the die, it is subject to frictional resistance. If the frictional resistance on both sides of the blank differs significantly, the blank will shift toward the side with greater frictional resistance.
2) Countermeasures:
For bending parts with asymmetric shapes, use symmetrical bending (for single-sided bending parts, bend two pieces symmetrically and then cut them apart).
Add an elastic blank holder on the bending die to press the blank during bending and prevent movement.
Use inner hole and outer contour positioning to ensure accurate positioning.
8. Surface scratches on bending parts
1) Causes and countermeasures:
When continuously bending soft materials such as copper and aluminum alloys, metal particles or dregs tend to adhere to the working surface, causing significant scratches on the part. In such cases, carefully analyze the shape of the working area and the lubricant condition to prevent particles and dregs from forming on the blank, which could lead to scratches.
When the bending direction is parallel to the rolling direction of the material, cracks may appear on the part surface, reducing surface quality. When bending at more than two locations, ensure that the bending direction is at an angle to the rolling direction as much as possible.
When the burr side is used as the outer surface during bending, the part is prone to cracks and scratches. Therefore, during bending, the burr side should be used as the inner surface of the bend.
If the die radius is too small, impact marks may appear at the bending area. Polishing the die and increasing the die radius can prevent scratches on the bending part.
The clearance between the punch and die should not be too small, as excessive tightness can cause thinning and scratches. During stamping, always monitor changes in die clearance.
When the punch enters the die too deeply, it can cause surface scratching on the part. Therefore, while ensuring that springback is not affected, the depth of punch entry into the die should be appropriately reduced.
To meet the precision requirements of the workpiece, bending dies with bottom pressure pads are often used. During bending, the springs on the pressure pad, locating pin holes, support plates, and ejection holes can all create indentations on the part, so adjustments should be made accordingly.
9. Position of holes in the blank changes during bending
1) Causes:
Incorrect hole position dimensions (due to tensile thinning during bending);
Holes are not concentric (insufficient bending height, blank slippage, springback, or surface undulation on the bending plane);
The bending line is not parallel to the centerline of the two holes, and areas where the bending height is less than the minimum bending height exhibit an outward flaring shape after bending;
Holes located near the bending line are prone to deformation.
2) Countermeasures:
For incorrect hole position dimensions, strictly control the bending radius, bending angle, and material thickness; trim the neutral axis of the material, and ensure the punch entry depth into the die and the punch-die clearance are properly and uniformly adjusted;
Measures for addressing misaligned holes;
Ensure the correct bending height on both left and right sides;
Repair worn locating pins and locating plates;
Reduce springback to ensure parallelism and flatness of the two bent surfaces;
Change the process sequence by performing bending and correction before punching.
Countermeasures for the outward flaring (open-mouth) shape defect
During bending, ensure the minimum bending height H (H≥R+2t, where t is material thickness and R is bending radius);
Modify the shape of the part, removing the portion that is less than the minimum bending height when it does not affect functionality.
Measures for holes near the bending line that are prone to deformation
When designing bent parts, ensure the distance X from the bend to the hole edge is greater than a certain value: X≥(1.5–2.0)t, where t is the thickness of the bent sheet metal.
Design an auxiliary hole at the bend area to absorb bending deformation stress, which can prevent deformation of holes near the bend line. The preferred approach is to bend first and then punch the hole.
10. After bending, the bend area exhibits obvious thinning
1) Countermeasures:
When the bend radius is too small relative to the sheet thickness (r/t>3 for right-angle bending), generally increase the bend radius;
Multi-angle bending increases thinning at the bend area. To reduce thinning, use a single-angle, multi-step pressing and bending method whenever possible;
When using a sharp punch, excessive penetration of the punch into the die significantly reduces the thickness at the bend area.
11. Wrinkling occurs on the flange of drawn parts during the drawing process
1) Causes:
The blank holder force on the flange area is too low to resist excessive tangential compressive stress, leading to tangential deformation and, consequently, wrinkling once stability is lost. Thinner materials are also more prone to wrinkling.
2) Countermeasures:
Increase the blank holder force and appropriately increase the material thickness.
12. Causes and Prevention of Wall Cracking in Deep-Drawn Parts
1) Causes:
The radial tensile stress on the material during deep drawing is too high;
The die corner radius is too small;
Poor lubrication during deep drawing;
The raw material has poor plasticity.
2) Countermeasures:
Reduce the blank holder force;
Increase the die corner radius;
Use lubricant correctly;
Select materials with better formability or add an intermediate annealing process.
13. Bottom of drawn part is torn
1) Cause:
The die radius is too small, causing the material to be in a cutting state.
2) Countermeasure:
(Generally occurs at the initial stage of deep drawing) Increase the die radius, ensure a smooth transition, and maintain a low surface roughness, typically Ra<0.2µm。 14.拉深零件边缘高低不平及有褶皱 1）原因： 毛坯与凸凹模中心不合或材料厚度不均匀，以及凹模圆角半径和凸凹模间隙不均匀(凹模圆角半径太大，在拉深的最后阶段脱离了压边圈，使尚未越过圆角的材料压边圈压不到起皱后被拉入凹模形成口缘褶皱。 2）对策： 冲模重新定位，校正凹模圆角半径和凸凹模间隙使其大小均匀后再投入生产(减少凹模圆角半径或采用弧形压边圈装置即可消除褶皱)。 15.锥形零件或半球形零件拉深时腰部起皱 1）原因： 在拉深开始时大部分材料处于悬空状态，加之压边力太小，凹模圆角半径又太大或者使用的润滑剂太多。使得径向拉应力变小使得材料在切向压应力的作用下失去稳定而起皱。 2）对策： 增大压边力或采用压延筋结构，减小凹模圆角半径或使材料厚度稍微加大。 16.拉深件表面产生拉痕的原因及预防措施 1）原因及对策： 凸模或凹模表面有尖利的压伤，致使工件表面相应的产生拉痕，此时应将压伤表面进行修磨或抛光即可； 凸凹模间隙过小或者间隙不均匀，使其在啦深时工件表面被刮伤，此时应修整凸凹模间隙直至合适为止； 凹模圆角表面粗糙，拉深时工件表面被刮伤，此时应将凹模圆角半径进行修磨打光； 冲压时由于冲模工作表面或材料表面不清洁而混进杂物从而压伤了工件表面，因此在拉料时一定要始终保持凸凹模表面的清洁，坯料拉深前一定要擦拭； 当凸凹模硬度低时，其表面附有金属废屑后，也使得拉深工件表面产生拉痕，因此除了增加凸凹模表面的硬度外在拉深时还要时常检查凸凹模表面即使清除其遗留下的金属废屑； 润滑剂质量差，也会使拉深工件表面粗糙度加大，这时应使用适合于拉深工艺使用的润滑剂，必要时应将润滑剂过滤后再使用。以防止杂质混入而损伤工件表面。 17.拉深件拉深直壁部分不平整 1）原因及对策： 凸模上没有设计和制造出通气孔，使其表面因压缩空气而变形，出现不平整现象，此时必须增加通气孔； 材料的回弹作用也会使拉深工件表面不平，最后应增加整形工序； 凸凹模间隙过大致使拉深难以被拉平，此时必须将间隙调整均匀。