Sheet metal stamping die problems and solutions

Training Content Overview

I. Common Defects and Causes of Blanked Parts

II. Common Defects and Causes of Bending Parts

III. Common Defects and Causes of Large Curved Deep-Drawn Parts

I. Common Defects and Causes of Blanked Parts
Blanking is a stamping process that uses dies to separate sheet metal.

Common defects in blanked parts include: burrs, surface warping, and dimensional deviations.

1. Burrs
During sheet metal blanking, burrs of varying degrees are generated. Generally, it is difficult to avoid them, but improving the manufacturability of the part and improving stamping conditions can reduce burrs.

The main causes of burrs are as follows:

1.1 Clearance
Excessive, insufficient, or uneven blanking clearance can all produce burrs. The following factors can cause excessive, insufficient, or uneven clearance:

1. Die manufacturing errors – machining of die parts does not conform to drawings, poor parallelism of the base plate, etc.

2. Die assembly errors – large clearance in the guide section, misalignment of the punch and die assembly, etc.

3. Poor press precision – such as excessive clearance in the press guide rails, poor parallelism between the bottom surface of the slide and the surface of the worktable, or poor perpendicularity between the slide stroke and the press table surface, poor worktable rigidity, resulting in deflection during punching, all of which can cause changes in clearance;

4. Installation errors – such as the upper and lower base plates of the die not being cleaned properly during installation, improper fastening methods for the upper die of large dies, misalignment of the upper and lower dies (especially dies without guide pillars), causing tilting of the working part;

5. Unreasonable die structure – insufficient rigidity of the die and working part, unbalanced punching force, etc.

6. Excessive bending of the steel plate – uneven steel plate.

1.2 Dull cutting edge Wear and dulling or chipping of the cutting edge can produce burrs. Factors affecting the dulling of the cutting edge include:

1. Poor material and surface treatment of the die punch and die, resulting in poor wear resistance;

2. Poor die structure and low rigidity, causing chipping;

3. Inadequate lubrication during operation, leading to rapid wear;

4. Failure to sharpen the cutting edge in a timely manner.

1.3 Improper blanking conditions
If the blank (including intermediate parts) does not make good contact with the punch or die, or during trimming and punching with an improper relative positioning height, the part height may be lower than the relative positioning height, causing the part shape to not conform to the cutting edge shape during the blanking process, resulting in burrs.

1.4 Improper die structure

1.5 Material not conforming to process specifications
Severely out-of-tolerance material thickness or use of the wrong material (e.g., incorrect steel grade) causes unreasonable relative clearance, resulting in burrs on the part.

1.6 Poor manufacturability of the part – Complex shapes with protruding or recessed sharp corners are prone to burr formation due to rapid wear.

Summary: Burrs not only cause stress concentration and cracking in subsequent deformation processes after blanking, but also make it difficult to delaminate the blank in later processes. Large burrs can easily cut hands; during welding, poor bonding between two steel plates can easily lead to weld penetration and weak welds; during riveting, they can easily create riveting gaps or cause riveting cracks.

Therefore, burrs exceeding the permissible range are extremely harmful. Existing burrs can be eliminated by filing, tumbling, electrolysis, chemical treatment, etc.

2. Warping and Unevenness of Parts When the material comes into contact with the punch and die, it is first stretched and bent, then sheared and torn. Due to the effects of drawing, bending, and lateral extrusion, the material unfolds into a wavy shape, resulting in warping.

The causes of warping are as follows:

2.1 Large Blanking Clearance If the clearance is too large, the tensile and bending forces on the part are large during the blanking process, easily causing warping. Improvements can be made by using a tight punch and blank holder during blanking, and by maintaining a sharp cutting edge.

2.2 Reverse Taper at the Die Opening: When the part passes through a small section, the outer periphery is compressed towards the center, causing bending.

2.3 Warping Due to Part Shape: When the part has a complex shape, the shear force around it is uneven, resulting in a force from the periphery towards the center, causing warping. The solution is to increase the blank holder force.

2.4 Warping Due to Internal Material Stress: The internal stress generated during rolling and winding of the material moves to the surface after blanking, causing warping. The solution is to level the material using a leveling machine during uncoiling.

2.5 Warping Due to Oil, Air, and Poor Contact: When oil, air, or other substances press on the part between the die and the part, or between parts themselves, warping will occur, especially with thin or soft materials. However, even oiling and the installation of venting holes can eliminate warping. Debris on the surface between the part and the die can also easily cause warping.

Poor contact surfaces during blanking can also cause warping.

3. Dimensional Accuracy Defects

3.1 Manufacturing defects in die cutting edge dimensions

3.2 Springback during blanking, or inconsistencies between the shape of the part from the previous process and the support surface shape of the die's working part in the next process, causing deformation of the part during blanking and elastic recovery after blanking, thus affecting dimensional accuracy.

3.3 Poor sheet shape.

3.4 For multi-process parts, improper adjustment or wear of fillets in the previous process can disrupt the principle of equal volume during deformation, leading to changes in dimensions after blanking.

3.5 Poor positioning during operation, or a poorly designed positioning mechanism, can cause the blank to shift during blanking. Defects in the sheared part (such as rhomboidity or missing edges) can also cause inaccurate positioning, all of which can lead to dimensional defects.

3.6 Incorrect punching sequence.

II. Common Defects and Causes of Bending Parts
Common defects in bending parts include: shape and size discrepancies, bending cracks, surface scratches, deflection, and twisting.

1. Shape and Size Discrepancies
The main causes are springback and improper positioning. Besides taking measures to reduce springback, improving the reliability of blank positioning is also crucial. The following two measures are commonly used:

1.1 Pressing the Blank
Using air cushions, rubber, or springs to generate clamping force, the sheet metal is pressed firmly before bending begins. To achieve this, the ejection height of the pressure plate or pressure rod should be slightly higher than the die plane.

1.2 Reliable Positioning Methods
The main positioning methods for blanks are based on the external shape and based on the hole. External shape positioning is convenient but has poor accuracy. Hole positioning is less convenient and has a narrower range of applications, but it is accurate and reliable. Under specific conditions, initial positioning using the external shape can roughly control the blank within a certain range, while final positioning using the hole positions combines the advantages of both methods, resulting in positioning that is both accurate and convenient.

2. Bending Cracks
The factors influencing crack formation are multifaceted, primarily including the following:

2.1 Poor material plasticity.

2.2 The angle between the bending line and the rolling direction of the sheet does not meet specifications. During layout, for unidirectional V-bending, the bending line should be perpendicular to the rolling direction; for bidirectional bending, the bending line should ideally be at a 45-degree angle to the rolling direction.

2.3 Insufficient bending radius.

2.4 Poor quality of the blank shearing and punching cross-section-burrs, cracks.

2.5 Worn or insufficient clearance of the punch and die fillet radii-increased feeding resistance.

2.6 Insufficient lubrication – High friction

2.7 Severely out-of-tolerance material thickness – Difficulty in feeding

2.8 Poor pickling quality

3. Surface scratches (scratches)
The main causes of surface scratches are improper material selection for the working parts of the mold, low heat treatment hardness, wear and poor surface finish of the die corners, poor surface quality of the bent blank (rust, scars, etc.), out-of-tolerance material thickness, unreasonable process selection, and lack of lubrication.

4. Deflection and torsion

III. Common Defects and Causes of Large Curved Deep-Drawn Parts

1. Characteristics of Deep-Drawing of Large Curved Parts

1.1 Deformation Characteristics
The deformation characteristics of large curved parts are: the periphery is deep-drawn, while the interior has bulging components. The surface shape is supplemented by the material outside the blank holder, while the interior relies on material extension to meet the bulging requirements. Simultaneously, due to varying drawing depths and complex shapes, the deformation is unevenly distributed around the periphery. Therefore, controlling the material flow direction and velocity is extremely important. Large curved parts are prone to localized wrinkling and cracking.

1.2 Sufficiently Stable Blank Holder Force Large curved surface parts require not only a certain drawing force but also a sufficiently stable blank holder force during the drawing process. These parts are often spatial curved surfaces with large contour dimensions and deep depths, thus requiring both large deformation forces and blank holder forces. On ordinary single-action presses with air cushions, the blank holder force is only about 1/6 of the nominal tonnage, and the blank holder force is also unstable, making it difficult to meet the process requirements of such parts. Therefore, in mass production, the drawing of such parts is carried out on double-action presses. Double-action presses have two slides for drawing and blank holder force, namely the inner slide and the outer slide. The blank holder force can reach 40%-50% or more of the total drawing force, which can meet the requirements of uneven deformation distribution around the part, and the blank holder force is stable, making it easy to obtain drawn parts with good rigidity.

1.3 Drawing Parts Must Have Sufficient Rigidity These parts are mostly used as the shell of machinery, requiring sufficient rigidity (to prevent vibration and noise during use) and dimensional stability (to ensure welding and assembly quality). This requires that the material be subjected to uniform tensile stress (ideally biaxial tensile stress) throughout the deep drawing process, exceeding the yield strength but below the ultimate tensile strength, minimizing the elastic recovery of the part, preventing shape distortion, and avoiding breakage.

2. Common Defects and Cause Analysis
Common defects in large curved deep-drawn parts include: cracks and fractures, wrinkles and creases, unclear edges, poor stiffness, surface scratches, surface roughness, and slip lines.

2.1 Cracks and Fractures
Cracks and fractures are mainly caused by the tensile stress on the blank exceeding the ultimate tensile strength in certain areas. Specific causes include:

2.1.1 The stamping performance of the material does not meet the process requirements.

2.1.2 Sheet thickness exceeding tolerance – When the sheet thickness exceeds the upper tolerance, areas with small gaps may jam during feeding, making stamping deformation difficult, and the material may break due to difficulty passing through the die at that location. 2.1.3 When the sheet metal thickness exceeds the lower tolerance, the material becomes thinner, increasing the compressive stress per unit area in the cross-section. Alternatively, due to the thinner material, resistance decreases, leading to excessive sheet metal flowing into the die and causing wrinkles to form first. In this case, the material is difficult to flow and may tear.

2.1.3 Poor material surface quality – scratches cause stress concentration, and increased corrosion increases resistance.

2.1.4 Excessive feeding resistance on the blank holder surface – large blank shape, small blank holder groove clearance, excessively small die corner radius, excessively deep outer slide adjustment, excessively high drawing ribs, and poor surface finish of the blank holder surface and die corner radius.

2.1.5 Excessive local drawing depth, exceeding the material deformation limit.

2.1.6 During operation, misalignment of the blank can cause one side to have excessive blank weight and the other side to have insufficient blank weight. The side with excessive weight weight will have difficulty feeding, causing cracking; the side with insufficient weight weight will have excessive blank weight feeding, easily causing wrinkles, which will then make feeding difficult and lead to breakage.

2.1.7 Failure to apply lubricant according to process specifications increases resistance, causing feeding difficulties and cracking.

2.1.8 Improper die installation or poor press precision causes misalignment of the clearance, resulting in uneven feeding resistance.

2.2 Wrinkles and Creases
Wrinkles are mainly caused by instability due to localized pressure on the blank and uneven material flow leading to localized material accumulation. Specifically, the following aspects are considered:

2.2.1 Poor stamping processability of the part, improper determination of the stamping direction and blank holder shape, making it difficult to control the material flow rate and causing wrinkles.

2.2.2 Insufficient feeding resistance at the blank holder surface leads to excessive feeding and wrinkling. This can be addressed by adjusting the outer slide pressure or changing the local shape of the drawn blank to increase the blank holder area, or by adding drawing ribs to increase feeding resistance.

2.2.3 Poor contact at the blank holder surface, in severe cases resulting in a loose inner surface and a tight outer surface. After the material passes through the tight outer area, the blank holder loses its clamping function, causing excessive feeding and wrinkles. At this point, the blank holder surface needs to be re-ground to ensure full contact, allowing for slight tightness on the inside and looseness on the outside.

2.2.4 Excessive lubrication.

2.2.5 Improper adjustment of the outer slide block, causing tilting, resulting in uneven pressure on the blank holder surface, and wrinkling easily occurs in loose areas.

2.3 Unclear Edges From the outside, the parts should have clear edges. If the press pressure is insufficient, during deep drawing, there will not be enough pressure to hold the material in place at the end of the deformation process, resulting in unclear edges. Additionally, poor die guidance, uneven clearance between working parts, incorrect installation of the punch and die (tilting), and poor press parallelism can also cause unclear edges.

2.4 Poor Rigidity Besides poor part manufacturability, the main reason for poor rigidity is insufficient feeding resistance on the blank holder surface, leading to insufficient plastic deformation of the material. In this case, consider adding draw beads or changing round draw beads to chamfered draw beads to increase feeding resistance. This is also why parts drawn by a single-action press have lower rigidity than those drawn by a double-action press.

2.5 Surface Scratches (Drawing Marks)
Surface scratches are usually caused by the following: insufficient smoothness of the die corners, which scratches the material during drawing and may cause material to adhere to the die, forming scratches; dirt falling into the die or unclean drawing oil can also scratch the part surface; if the blank holder is composed of inserts, poor insert bonding can also cause scratches; if the process filler is too small, scratches passing through the die opening are not cut off.

2.6 Surface Roughness and Slip Lines
Surface roughness is caused by excessively large grain size in the material itself.