Stamping mould/molds: analysis of 17 reasons for the defects of stamping parts


1. Stamping waste

1) Reason:

The quality of raw materials is poor;

Improper installation, adjustment and use of the die;

The operator did not feed the strips correctly along the positioning or did not guarantee that the strips were fed in a certain gap;

Due to long-term use of the die, the gap changes or the working parts and guide parts wear out;

Due to the long time of impact and vibration of the punching mould, the fastening parts are loose, and the installation positions of the punching die are relatively changed;

The operator's negligence and failure to follow the operating procedures.

2) Countermeasures:

The raw materials must meet the prescribed technical conditions (strictly check the specifications and grades of the raw materials, and conduct laboratory inspections on workpieces with high dimensional          accuracy and surface quality requirements if conditions permit.);

All links stipulated in the process regulations should be fully and strictly abided by;

The presses and dies and other tooling equipment used should be guaranteed to work under normal working conditions;

A strict inspection system is established during the production process. The first stamping parts must be fully inspected, and production can only be put into production after passing the inspection. At the          same time, the inspection should be strengthened to deal with accidents in time;

Adhere to a civilized production system, such as the transfer of workpieces and blanks, must use appropriate workstations, otherwise the surface of the workpiece will be crushed and scratched and the               surface quality of the workpiece will be affected;

During the stamping process, it is necessary to ensure that the cavity of the mold is clean, and the orderly processed workpieces that must be organized in the workplace must be placed neatly.

2. Burr of blanking parts

1) Reason:

The blanking gap is too large, too small or uneven;

The cutting edge of the working part of the die becomes blunt;

The center line of the punch and the die changes due to long-term vibration and impact, and the axes do not coincide, resulting in single-sided burrs.

2) Countermeasures:

Ensure the processing accuracy and assembly quality of the male and female molds, ensure the verticality of the male mold and bear the side pressure, and the entire die must have sufficient rigidity;

When installing the punch, it is necessary to ensure the correct gap between the punch and the concave mold and make the punch and the concave mold be firmly installed on the mold fixing plate, and            the  end faces of the upper and lower molds and the work surface of the press must be parallel to each other;

The rigidity of the press is required to be good, the elastic deformation is small, the accuracy of the track and the parallelism of the backing plate and the slider are required to be high;

Require the press to have sufficient punching force;

The height of the allowable burr on the cut section of the blanking piece

Punching sheet thickness>0.3>0.3-0.5>0.5-1.0>1.0-1.5>1.5-2.0

Burr height of new test mold0.0150.

Glitch height allowed during production0.


3. Warping deformation of blanking parts

1) Reason:

There are clearance forces and reaction forces that do not produce moments on the same line. (When the gap between the convex and concave molds is too large and the cutting edge of the concave mold         has a reverse taper, or the contact area between the ejector and the workpiece is too small, warping will occur).

2) Countermeasures:

The blanking gap should be selected reasonably;

In the mold structure, the pressure plate (or support plate) should be in contact with the surface of the pressure plate and have a certain pressure;

Inspect the cutting edge of the die, if there is a back taper, the cutting edge of the die must be trimmed properly;

If the shape of the blanking part is complicated and the shear force is not uniform when the inner hole is large, the pressing force is increased, and the strip is pressed before blanking or a high-precision            press is used for blanking;

The plate should be leveled before punching. If the warping deformation still cannot be eliminated, the workpiece after punching can be leveled again through the leveling die;

Regularly remove the stolen goods in the mold cavity, lubricate the surface of the thin sheet, and have oil and gas holes in the mold structure.


4. When punching, the accuracy of the outer edge and inner hole of the punched part decreases and the size changes

1) Reason:

Locating pins, stop pins, etc. have changed or worn too much;

The operator's negligence and carelessness in feeding materials shift left and right back and forth;

The dimensional accuracy of the strip is low. It is too narrow and too wide to feed the material, making it difficult to deliver to the designated location. The strip will shift back and forth in the guide plate,           and the punched workpiece inner hole will have a large deviation from the front and rear position of the shape.


5. When the part is bent, the size and shape are unqualified

1) Reason:

The rebound of the material causes the product to be unqualified;

The locator is worn out and deformed, which makes the strip positioning inaccurate and must be replaced with a new locator;

In a non-guided bending die, when adjusting on the press, improper adjustment of the bottom dead center position of the press slider will also cause the shape and size of the bending part to be                      unqualified;

The pressing device of the mold fails or does not work at all. The pressing force must be re-adjusted or the pressure spring must be replaced to make it work properly.

2) Measures to reduce rebound:

Select stamping materials with a large elastic modulus and a small yield point and stable mechanical properties;

Increase the correction process, and use correction bending instead of free bending;

The material should be annealed before bending, so that the cold-work hardened material is pre-softened and then bent into shape;

If the shape deformation occurs during the stamping process and it is difficult to eliminate it; the inclination of the punch and the die should be replaced or trimmed, and the gap between the punch and            the die should be equal to the minimum material thickness;

Increase the contact area between the concave mold and the workpiece, and reduce the contact area between the convex mold and the workpiece;

Adopt "overcorrection" methods to reduce the impact of rebound

6. Cracks occur in the bending part of the bending part

1) Countermeasures:

Eliminate burrs on the outside of the bending area, which will cause stress concentration in the area and reduce the amount of bending deformation; remove the burrs in this area;

The side with burrs is placed on the inner side of the bending area;

When bending the workpiece, it is best to make the bending direction and the fiber direction of the material (rolling direction) perpendicular;

The bending radius should not be too small, and the fillet radius should be enlarged as far as possible if the quality permits;

The surface of the curved blank should be smooth, without obvious protrusions and scars;

The intermediate annealing process is adopted during bending to eliminate internal stress, and the softened bending rarely produces cracks;

When bending, large bending parts must be coated with lubricant to reduce friction during bending.


7. The deviation of the bending part during the bending process

1) Reason:

When the blank slides along the surface of the die during the bending process, it will experience frictional resistance. If the frictional resistance on both sides of the blank is large, the blank will shift to the           side with the larger frictional resistance.

2) Countermeasures:

The curved parts with asymmetrical shapes are formed by symmetrical bending (single-sided curved parts are cut symmetrically after two parts are bent).

Adding an elastic pressing device to the bending die so that the blank can be pressed to prevent movement during bending;

Use the inner hole and shape positioning form to make the positioning accurate.


8. Surface scratches on curved parts

1) Reasons and countermeasures:

When performing continuous bending operations on soft materials such as copper and aluminum alloys, metal particles or scum are easy to adhere to the surface of the working part, causing larger       scratches on the part. At this time, the shape and lubricating oil of the working part should be carefully analyzed and studied. In such circumstances, it is best not to have particles and dross on the blank, and even scratches;

When the bending direction is parallel to the rolling direction of the material, cracks will occur on the surface of the workpiece, which will reduce the surface quality of the workpiece. When bending at two or more places, ensure that the bending direction has a certain angle with the rolling direction as much as possible;

When the burr surface is used as the outer surface for bending, the workpiece is prone to cracks and scratches; therefore, the burr surface should be used as the curved inner surface when bending;

The corner radius of the concave mold is too small, and impact marks appear on the curved part. Polish the concave mold and increase the radius of the corner of the concave mold to avoid scratches on the bent parts;

The gap between the convex and concave molds should not be too small. Too small a gap will cause thinning and scratches. In the stamping process, always check the change of the mold gap;

When the depth of the punch into the concave mold is too large, the surface of the part will be scratched. Therefore, while ensuring that it is not affected by springback, the depth of the punch into the concave mold should be appropriately reduced;

In order to make the parts meet the requirements of accuracy, the bending die that presses the material at the bottom is often used. When bending, the spring on the press plate, the positioning pin hole, the pallet and the return hole will all be pressed into indentations, so adjustments should be made. .


9. The position of the blank hole changes when bending

1) Reason:

The position and size of the hole are wrong, (bending and tensioning becomes thinner);

The holes are not concentric (the bending height is not enough, the blank slips, springs back, and there are undulations on the bending plane);

The bending line and the center lines of the two holes are not parallel, and the part where the bending height is less than the minimum bending height shows an outward opening shape after bending;

The holes near the bending line are prone to deformation.

2) Countermeasures:

The position and size of the hole are not strictly controlled for the bending radius, bending angle and material thickness; the neutral layer of the material is trimmed and the depth of the punch into the              concave mold and the convex and concave molds are properly uniform;

Measures for different reasons of disagreement;

Ensure that the left and right bending heights are correct;

Correct the worn positioning pins and positioning plates;

Reduce springback to ensure the parallelism and flatness of the two curved surfaces;

Change the process route, first bend and correct before punching.

Shows countermeasures to open the mouth shape

When bending, ensure the minimum bending height H (HR+2tt material thickness R bending radius);

Change the shape of the processed parts, and remove the part less than the minimum bending height without affecting the use.

The holes near the bending line are prone to deformation measures

When designing bent parts, ensure that the distance X from the bent part to the hole edge is greater than a certain value X(1.52.0)tt the thickness of the bent sheet;

Designing an auxiliary hole in the bending part to absorb the bending deformation stress can prevent the deformation of the hole adjacent to the bending line. Generally, the solution of first bending and            then punching is adopted.


10. After the part is bent, the bending part has obvious thinning

1) Countermeasures:

The bending radius is too small relative to the thickness of the plate (r/t>3 right-angle bending). Generally, the bending radius is increased;

Multi-angle bending makes the bending part thinner and larger. In order to reduce the thinning, try to use a single-angle multi-process bending method;

When a sharp-angled punch is used, the punch enters the concave mold too deeply, which significantly reduces the thickness of the bent part.


11. The flange of the drawn part wrinkles during the drawing process

1) Reason:

The blank holder force at the flange is too small to resist excessive tangential compressive stress; it causes tangential deformation, which causes wrinkles after losing stability. Thinner materials are easier           to   form wrinkles.

2) Countermeasures:

Increase the blank holder force of the blank holder and appropriately increase the thickness of the material.


12. The reason and prevention of the wall of the drawing part being cracked

1) Reason:

The radial tensile stress that the material bears during deep drawing is too large;

The corner radius of the concave mold is too small;

Poor deep drawing and lubrication;

The raw material has poor plasticity.

2) Countermeasures:

Reduce the blank holder force;

Increase the fillet radius of the die;

Use lubricants correctly;

Choose materials with better quality or increase the annealing process between workshops.


13. The bottom of the deep drawing is cracked

1) Reason:

The fillet radius of the die is too small, which makes the material in a state of being cut.

2) Countermeasures:

(Usually occurs in the initial stage of deep drawing) increase the fillet radius of the die, and make it smooth and the surface roughness should be small, generally Ra<0.2µm.


14. The edge of the drawn part is uneven and wrinkled

1) Reason:

The blank and the center of the convex and concave die are not aligned or the thickness of the material is uneven, and the corner radius of the die and the gap between the convex and concave die are not uniform. The material blanking ring is pulled into the die to form the edge folds after it is not pressed into the wrinkle.

2) Countermeasures:

Reposition the die, correct the corner radius of the die and the gap between the convex and concave die to make the size uniform before putting it into production (reducing the corner radius of the die or using an arc-shaped blank holder device can eliminate wrinkles).


15. Waist wrinkles when tapered or hemispherical parts are drawn

1) Reason:

At the beginning of deep drawing, most of the materials are in a suspended state. In addition, the blank holder force is too small, the corner radius of the die is too large, or too much lubricant is used. Making the radial tensile stress smaller makes the material lose stability and wrinkle under the action of tangential compressive stress.

2) Countermeasures:

Increase the blank holder force or adopt a rolling rib structure, reduce the radius of the die fillet or increase the material thickness slightly.


16. Reasons and preventive measures for drawing marks on the surface of deep-drawn parts

1) Reasons and countermeasures:

There are sharp crush marks on the surface of the convex mold or the concave mold, resulting in corresponding pull marks on the surface of the workpiece. At this time, the crushed surface should be ground or polished;

The gap between the convex and concave molds is too small or uneven, so that the surface of the workpiece is scratched when it is deep. At this time, the gap between the convex and concave molds should be trimmed until it is suitable;

The rounded surface of the die is rough, and the surface of the workpiece is scratched during deep drawing. At this time, the rounded radius of the die should be polished;

When stamping, the working surface of the die or the surface of the material is not clean and the surface of the workpiece is mixed with impurities, so the surface of the workpiece must be kept clean when drawing, and the blank must be wiped before drawing;

When the hardness of the convex and concave die is low, the metal scraps attached to the surface will also cause the drawing marks on the surface of the deep drawing. Therefore, in addition to increasing the hardness of the convex and concave die surface, check the surface of the convex and concave die from time to time even if it is removed Scrap metal underneath;

The poor quality of the lubricant will also increase the surface roughness of the deep-drawn workpiece. At this time, a lubricant suitable for the deep-drawing process should be used, and the lubricant should be filtered before use if necessary. To prevent impurities from mixing in and damaging the surface of the workpiece.


17. The part of the straight wall of the deep drawing part is not flat

1) Reasons and countermeasures:

There is no ventilation hole designed and manufactured on the punch, so that the surface is deformed by compressed air and becomes uneven. At this time, ventilation holes must be added;

The rebound effect of the material will also make the surface of the drawn workpiece uneven, and finally, a shaping process should be added;

The gap between the convex and concave molds is too large to make the drawing difficult to be flattened. At this time, the gap must be adjusted evenly.