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Stamped Parts Defects and The Preventive Measures

1. Causes of waste generation:

A. Poor quality of raw materials;

B. Improper installation, adjustment, and use of stamping dies;

C. The operator did not feed the strip correctly along the positioning or did not ensure that the strip was fed with a certain gap;

D. Due to long-term use, the punching die may experience changes in clearance or wear on its working and guiding parts;

E. Due to prolonged impact and vibration, the fastening parts of the die become loose, causing relative changes in the installation positions of the die;

F. The operator's negligence resulted in not following the operating procedures.

2. Main measures to prevent waste:

A. Raw materials must comply with the specified technical conditions (strictly inspect the specifications and grades of raw materials, and if conditions permit, conduct laboratory tests on workpieces with high dimensional accuracy and surface quality requirements.)

B. All aspects specified in the process specifications should be strictly followed comprehensively;

C. The tooling equipment such as presses and dies used should be ensured to work in normal working conditions;

D. Establish a strict inspection system during the production process, and the first piece of stamped parts must be thoroughly inspected. Only after passing the inspection can they be put into production. At the same time, strengthen inspections and handle accidents in a timely manner;

E. Adhere to the civilized production system, such as using appropriate workstation equipment for the transportation of workpieces and blanks, otherwise it will crush and scratch the surface of the workpiece, affecting the surface quality of the workpiece;

F. During the stamping process, it is necessary to ensure the cleanliness of the mold cavity, and the workplace should be organized in an orderly manner. The processed workpieces should be placed neatly.

3. Causes of burrs on punched parts

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

The cutting edge of the working part of the stamping die becomes dull;

Due to long-term vibration and impact, the center lines of the convex and concave molds change and do not coincide, resulting in single-sided burrs.

countermeasure

Ensure the machining accuracy and assembly quality of the convex and concave molds, ensure the perpendicularity of the convex molds and their ability to withstand lateral pressure, and ensure that the entire die has sufficient rigidity;

When installing the convex mold, it is necessary to ensure the correct clearance between the convex and concave molds and firmly install them on the mold fixing plate. The end faces of the upper and lower molds should be kept parallel to the worktable of the press.

The rigidity of the press machine should be good, the elastic deformation should be small, the accuracy of the track and the parallelism between the pad and the slider should be high;

The pressure machine is required to have sufficient punching force.

The allowable height of burrs on the cutting section of punched parts

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

New trial mold burr height ≤ 0.015 ≤ 0.02 ≤ 0.03 ≤ 0.04 ≤ 0.05

The allowable burr height during production is ≤ 0.05 ≤ 0.08 ≤ 0.10 ≤ 0.13 ≤ 0.15

4. Reasons for warping and deformation of punched parts:

There is a gap force and a reaction force that do not produce a torque on the same line. When the clearance between the convex and concave molds is too large and the concave mold edge has a reverse taper, or when the contact area between the ejector and the workpiece is too small, warping deformation occurs

measures:

The punching gap should be selected reasonably;

On the mold structure, a pressing plate (or support plate) should be added to ensure that the plate is in contact with the pressing plate and has a certain level of pressure;

If a reverse taper is found on the concave die edge during inspection, the die edge must be adjusted appropriately.

If the uneven shear force increases the pressing force due to the complex shape of the punched part and the large number of inner holes, the strip material should be compressed before punching or a high-precision press machine should be used for punching.

The board should be leveled before punching. If the warping deformation cannot be eliminated, the punched workpiece can be leveled again through a leveling die.

Regularly remove dirt from the mold cavity, lubricate the surface of the sheet metal, and provide oil vent holes on the mold structure.

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

reason:

The position of positioning pins, blocking pins, etc. has changed or worn out too much;

The operator's negligence and carelessness caused the material to deviate left, right, front, and back during feeding;

The dimensional accuracy of the strip material is low, and it is difficult to feed it to the designated location due to being too narrow or too wide. The strip material will deviate back and forth in the guide plate, resulting in a large deviation between the inner hole of the workpiece punched out and the position of the outer shape.

6. Reasons for non-conforming dimensions and shapes when parts are bent

A. The rebound of the material causes the product to be unqualified

Measures to reduce rebound:

Choose stamping materials with high elastic modulus and small yield point, which have stable mechanical properties;

◆ Add calibration process and use calibration bending instead of free bending;

Before bending, the material should be annealed to soften the cold work hardened material before bending into shape;

If shape deformation occurs during the stamping process and is difficult to eliminate; Then the slope between the convex and concave molds should be replaced or adjusted, and the gap between the convex and concave molds should be made 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.

Adopting the method of "overcorrection" to reduce the impact of rebound.

B. The locator has worn and deformed, causing inaccurate positioning of the strip material, and a new locator must be replaced.

C. In an unguided bending die, improper adjustment of the bottom dead center position of the press slide during adjustment on the press can also cause the shape and size of the bent part to be non-conforming.

D. The pressing device of the mold fails or does not function at all, and the pressing force must be readjusted or the pressure spring replaced to make it work properly.

7. Cracks occur at the bending part of the bent part (the internal stress in the bending deformation zone exceeds the strength limit of the material)

A. Eliminate burrs on the outer side of the curved area, which can cause stress concentration in that area,

◆ Reduce bending deformation

◆ Remove burrs in this area

Place the side with burrs on the inside of the curved area.

B. It is best to bend the workpiece in a direction perpendicular to the fiber direction (rolling direction) of the material.

C. The bending radius should not be too small, and the fillet radius should be increased as much as possible within the allowable quality.

D. The surface of the bent blank should be smooth, without obvious protrusions or scars.

E. During bending, an intermediate annealing process is used to eliminate internal stress, and the softened bending rarely produces cracks.

F. When bending large curved parts, it is necessary to apply lubricant to reduce friction during the bending process.

8. Deviation of bent parts during bending process

When the blank slides along the surface of the concave mold during the bending process, it will experience frictional resistance. If the frictional resistance on both sides of the blank is significantly different, the blank will shift towards the side with greater frictional resistance.

measures:

◆ Asymmetric curved parts are formed by symmetrical bending (single-sided curved parts are formed by bending two pieces symmetrically and then cutting open)

Add an elastic pressing device on the bending die to press the billet and prevent it from moving during bending.

Using internal holes and external positioning forms to ensure accurate positioning.

9. Reasons and solutions for surface scratches on curved parts.

A. When continuous bending is carried out on soft materials such as copper and aluminum alloys, metal particles or slag are prone to adhere to the surface of the working part, causing significant scratches on the workpiece. At this time, it is necessary to carefully analyze and study the shape and lubricating oil of the working part to ensure that the blank does not have particles or slag, which may cause scratches.

B. When the bending direction is parallel to the rolling direction of the material, cracks will appear on the surface of the workpiece, resulting in a decrease in the surface quality of the workpiece. When bending in two or more parts, it is necessary to ensure that the bending direction has a certain angle with the rolling direction as much as possible.

C. When the burr surface is used as the outer surface for bending, the workpiece is prone to cracking and scratching; Therefore, when bending, the burr surface should be used as the inner surface of the bend.

D. The radius of the concave mold fillet is too small, and there are impact marks on the curved part. Polishing the concave mold and increasing the fillet radius of the concave mold can prevent scratches on bent parts.

E. The gap between the convex and concave molds should not be too small, as a small gap can cause thinning and scratching. During the stamping process, it is necessary to constantly check the changes in the clearance of the mold.

F. When the depth of the convex mold entering the concave mold is too large, it will cause scratches on the surface of the part. Therefore, while ensuring that it is not affected by rebound, the depth of the convex mold entering the concave mold should be appropriately reduced.

G. In order to meet the accuracy requirements of the parts, bending molds are often used for bottom pressing. During bending, the springs, positioning pin holes, support plates, and return holes on the pressing plate will be pressed into indentations, so adjustments should be made.

10.What is the reason for the change in the position of the blank hole during bending? How to eliminate,

A. The position and size of the hole are incorrect (thinning due to bending and tension)

measures:

Strictly control the bending radius, bending angle, and material thickness; Adjust the neutral layer of the material and ensure that the depth of the convex mold entering the concave mold and the convex and concave molds are appropriately uniform.

B. Reasons for non concentric holes

◆ Insufficient bending height

◆ The blank slides

◆ Rebound

Unending phenomenon on curved plane

measures:

Ensure that the left and right bending heights are correct.

Correct the worn positioning pins and plates,

Reduce rebound to ensure the parallelism and flatness of the two curved surfaces.

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

C. The part where the bending curve and the centerline of the two holes are not parallel and the bending height is less than the minimum bending height shows an outward opening shape after bending.

measures:

When bending, the minimum bending height H (H ≥ R+2t t material thickness R bending radius) should be ensured

Change the shape of the processed parts and remove the part below the minimum bending height without affecting their use.

D. Holes near curved lines are prone to deformation.

measures:

When designing bent parts, it is necessary to ensure that the distance X from the bent part to the hole edge is greater than a certain value X ≥ (1.5-2.0) t t and the thickness of the bent sheet metal

Design an auxiliary hole at the bending point to absorb bending deformation stress, which can prevent deformation of holes near the bending curve. Generally, the scheme of bending first and punching later is adopted.

11. After the part is bent, there is a significant thinning at the bent part

A. The bending radius is too small relative to the plate thickness value (r/t>3 for right angle bending), and generally, increasing the bending radius is adopted

B. Multi angle bending causes the bending area to become thinner and larger. In order to reduce thinning, single angle multi process bending methods should be used as much as possible.

C. When using a pointed convex mold, if the convex mold enters the concave mold too deeply, the thickness of the bent part will be significantly reduced.

12. Causes and prevention of wrinkling of the flange of deep drawn parts during the deep drawing process

Reason: The edge pressure on the flange is too small to resist excessive tangential compressive stress; And it causes tangential deformation, resulting in the formation of wrinkles after losing stability. Thin materials are also prone to wrinkle formation

measures:

◆ Increase the edging force of the edging ring and appropriately increase the thickness of the material.

13. Reasons and prevention of wall cracking in deep drawn parts

reason:

A. The radial tensile stress borne by the material during deep drawing is too high;

B. The radius of the concave mold fillet is too small,

C. Poor lubrication during deep drawing;

D. The plasticity of raw materials is poor.

measures:

Reduce the edge pressure;

◆ Increase the fillet radius of the concave mold

◆ Proper use of lubricants

Choose materials with good performance or add annealing processes in the workshop.

14. The bottom of the deep drawn part is cracked by pulling

The reason is that the fillet radius of the concave mold is too small, causing the material to be in a cutting state. (usually occurring in the initial stage of deep drawing) Increase the fillet radius of the concave die and make it smooth. The surface roughness of the transition should be small, generally Ra<0.2 μ m

15. The edges of the deep drawn parts are uneven and wrinkled.

Reason: The blank does not match the center of the convex concave die or the material thickness is uneven, as well as the uneven radius of the concave die corner and the gap between the convex concave die (the concave die corner radius is too large, which separates from the edge ring in the final stage of deep drawing, causing the edge ring of the material that has not yet crossed the corner to be unable to be pressed and wrinkled, and then pulled into the concave die to form mouth edge wrinkles)

measures:

Repositioning the punching die, correcting the fillet radius of the concave die and the gap between the convex and concave dies to make them uniform before putting them into production (reducing the fillet radius of the concave die or using an arc-shaped edge pressing ring device can eliminate wrinkles)

16. Reasons and preventive measures for wrinkling of the waist during deep drawing of conical or hemispherical parts;

Reason: At the beginning of deep drawing, most of the material is in a suspended state, and the edge pressure is too small, the radius of the concave die corner is too large, or too much lubricant is used. Reducing the radial tensile stress causes the material to lose stability and wrinkle under the action of tangential compressive stress.

measures:

Increase the edge pressure or use a rolled rib structure, reduce the radius of the concave die corner, or slightly increase the material thickness.

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

reason:

A. If there are sharp pressure marks on the surface of the convex or concave mold, causing corresponding tensile marks on the surface of the workpiece, the pressure damaged surface should be ground or polished.

B. The clearance between the convex and concave molds is too small or uneven, causing scratches on the surface of the workpiece when it is deep. At this time, the clearance between the convex and concave molds should be adjusted until it is suitable.

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

D. During stamping, impurities may mix into the working surface or material surface of the die, causing damage to the workpiece surface. Therefore, it is necessary to always maintain the cleanliness of the convex and concave die surfaces during material drawing, and wipe the blank before deep drawing;

E. When the hardness of the convex and concave molds is low, the presence of metal shavings on their surfaces also causes stretch marks on the surface of the deep drawn workpiece. Therefore, in addition to increasing the hardness of the convex and concave mold surfaces, it is necessary to regularly check the surface of the convex and concave molds during deep drawing and even remove the remaining metal shavings.

F. Poor quality lubricants can also increase the surface roughness of deep drawn workpieces. In this case, lubricants suitable for deep drawing processes should be used, and if necessary, the lubricant should be filtered before use. To prevent impurities from entering and damaging the surface of the workpiece.

18.What is the reason for the unevenness of the straight wall of the deep drawn part?

reason:

A. There are no ventilation holes designed and manufactured on the convex mold, causing the surface to deform due to compressed air and become uneven. In this case, ventilation holes must be added.

B. The rebound effect of the material can also make the surface of the deep drawn workpiece uneven, and finally, a shaping process should be added.

C. The clearance between the convex and concave molds is too large to be flattened during deep drawing, and it is necessary to adjust the clearance evenly.