Summary and solution of common problems of stamping die

1、 Information from waste
The scrap is essentially the reverse image of the formed hole. That is, the same part with opposite position. By checking the scrap, you can judge whether the upper and lower die clearance is correct. If the gap is too large, the scrap will have rough and undulating fracture surface and a narrow bright zone area. The larger the gap, the greater the angle between the fracture surface and the bright zone area. If the gap is too small, the scrap will present a small angle fracture surface and a wide bright zone area.
Excessive gap forms holes with large curls and edge tears, making the profile slightly with a thin edge protruding. Too small gaps form slight crimping and large angle tearing, resulting in the profile more or less perpendicular to the material surface.
An ideal scrap should have a reasonable collapse angle and a uniform bright band. This can keep the punching pressure to a minimum and form a clean round hole with few burrs. From this point of view, extending the die life by increasing the gap is at the expense of the finished hole quality.
2、 Selection of die clearance
The clearance of the die is related to the type and thickness of the material to be stamped. Unreasonable clearance can cause the following problems:
(1) If the gap is too large, the burr of the stamped workpiece is relatively large, and the stamping quality is poor. If the clearance is too small, although the quality of punching is good, the wear of the die is relatively serious, which greatly reduces the service life of the die, and is easy to cause the breaking of the punch.
(2) Too large or too small clearance is easy to produce adhesion on the punch material, which will cause material entrainment during stamping. Too small gap is easy to form a vacuum between the bottom surface of the punch and the sheet metal, resulting in scrap rebound.
(3) Reasonable clearance can prolong the life of the die, have a good discharging effect, reduce burr and flanging, keep the plate clean, keep the hole diameter consistent and will not scratch the plate, reduce the number of sharpening, keep the plate straight and accurate punching positioning.
3、 How to improve the service life of moulds
For users, improving the service life of the die can greatly reduce the stamping cost. The factors affecting the service life of the die are as follows:
1. Type and thickness of materials;
2. Whether a reasonable lower die clearance is selected;
3. The structural form of the mold;
4. Whether the material is well lubricated during stamping;
5. Whether the mold has undergone special surface treatment;
6. Such as titanium plating, carbon titanium nitride;
7. Neutralization of upper and lower turrets;
8. Reasonable use of adjusting shims;
9. Whether the inclined edge mold is properly used;
10. Whether the mold base of the machine tool is worn;
4、 Problems needing attention in punching holes with special dimensions
(1) Minimum hole punch φ 0.8—— φ 1.6 Use special punch for punching in the range.
(2) When punching thick plates, please use a larger mold than the hole diameter. Note: At this time, if a die of normal size is used, the punch thread will be damaged.
5、 Die grinding
1. The importance of die grinding
Regular sharpening of dies is the guarantee of the consistency of punching quality. Regular sharpening of the mould can not only improve the service life of the mould but also the service life of the machine. It is necessary to grasp the correct sharpening time.
2. The specific features of the mold that need to be sharpened
There is no strict number of blows to determine whether to grind the die. It mainly depends on the sharpness of the blade. It is mainly determined by the following three factors:
(1) Check the fillet of the edge. If the fillet radius reaches R0.1 mm (the maximum R value shall not exceed 0.25 mm), it needs to be sharpened.
(2) Check the punching quality. Is there any large burr?
(3) The need for sharpening is judged by the noise of machine stamping. If the noise of the same die is abnormal during stamping, the punch is blunt and needs to be sharpened.
Note: If the edge of the edge becomes round or the back of the edge is rough, the edge grinding should also be considered.
3. Method of sharpening
There are many ways to grind the edge of the mold, which can be achieved by using a special edge grinder or a surface grinder. The frequency of punch and lower die grinding is generally 4:1. Please adjust the die height after grinding.
(1) The harm of incorrect grinding method: incorrect grinding will aggravate the rapid damage of the mold edge, resulting in a greatly reduced number of blows per grinding.
(2) The benefits of correct sharpening method: regularly sharpen the mold, and the quality and accuracy of punching can be kept stable. The edge of the mold is damaged slowly and has a longer service life.
4. Grinding rules
The following factors should be considered when grinding the die:
(1) The sharpness of the edge should be considered when the size of the edge fillet is R0.1-0.25mm.
(2) The surface of grinding wheel shall be cleaned.
(3) A loose, coarse and soft grinding wheel is recommended. For example, WA46KV
(4) The grinding amount (cutting amount) per time should not exceed 0.013mm. Excessive grinding amount will cause overheating of the mold surface, which is equivalent to annealing treatment, and the mold will become soft, greatly reducing the life of the mold.
(5) Sufficient coolant must be added during sharpening.
(6) During grinding, ensure that the punch and lower die are fixed stably, and use special tooling fixtures.
(7) The grinding amount of the die is certain. If it reaches this value, the punch will be scrapped. If it continues to be used, it is easy to cause damage to the mold and machine, and the gain outweighs the loss.
(8) After sharpening, the edge shall be treated with oilstone to remove excessively sharp edges.
(9) Clean, demagnetize and oil the blade after grinding.
Note: The grinding amount of the die mainly depends on the thickness of the pressed plate.
6、 Pay attention to the punch before use
1. Storage
(1) Clean the inside and outside of the upper mold sleeve with a clean rag.
(2) Be careful not to scratch or dent the surface when storing.
(3) Oil to prevent rust.
2. Preparation before use
(1) Thoroughly clean the upper mold sleeve before use.
(2) Check the surface for scratches and dents. If any, remove it with an oilstone.
(3) Oil inside and outside.
3. Precautions when installing the punch on the upper die sleeve
(1) Clean the punch and oil its long handle.
(2) Insert the punch into the bottom of the upper die sleeve on the large station mold without force. Do not use nylon hammer. During installation, the punch cannot be fixed by tightening the bolts on the upper die sleeve, and the bolts can only be tightened after the punch is correctly positioned.
4. Install the upper mold assembly into the turret
If you want to extend the service life of the mold, the gap between the outer diameter of the upper mold sleeve and the turret hole should be as small as possible. So please perform the following procedures carefully.
(1) Clean and oil the keyway and inner diameter of the turret hole.
(2) Adjust the keyway of the upper mold guide sleeve to match the key of the turret hole.
(3) Insert the upper mold sleeve straight into the tower hole, be careful not to tilt. The upper mold guide sleeve should slide into the turret hole by its own weight.
(4) If the upper mold sleeve inclines to one side, use soft material tools such as nylon hammer to gently knock it right. Repeat knocking until the upper mold guide sleeve slides into the correct position by its own weight.
Note: Do not use force on the outer diameter of the upper die guide sleeve, only use force on the top of the punch. Do not hit the top of the upper mold sleeve to avoid damaging the turret hole and shortening the service life of individual stations.
7、 Maintenance of mold
If the punch is bitten by the material and cannot be taken out, please check according to the following items.
1. Re-grinding of punch and lower die. A die with a sharp edge can produce a beautiful cutting section. If the edge is blunt, additional punching pressure is required. Moreover, the rough section of the workpiece produces great resistance, causing the punch to be bitten by the material.
2. Die clearance. If the gap of the die is not properly selected relative to the plate thickness, the punch needs a large demoulding force when separating from the material. If the punch is bitten by the material for this reason, please replace the lower die with reasonable clearance.
3. The status of the processed material. When the material is dirty or has dirt, the dirt attaches to the mold, which makes the punch bite by the material and cannot be processed.
4. Materials with deformation. After punching the hole, the warped material will clamp the punch and make the punch bite. Please flatten the warped material before processing.
5. Excessive use of springs. It will cause spring fatigue. Please always check the performance of the spring.
8、 Oiling
The oil quantity and oil filling times depend on the conditions of processing materials. For rust-free and scale-free materials such as cold-rolled steel plate and corrosion-resistant steel plate, the mold shall be filled with oil. The oil filling points are the guide sleeve, oil injection port, the contact surface between the tool body and the guide sleeve, and the lower mold. Light engine oil is used for oil.
For materials with rust and scale, the rust powder will be sucked between the punch and the guide sleeve during processing, and dirt will be generated, which will make the punch unable to slide freely in the guide sleeve. In this case, if oil is applied, the rust scale will be more easily stained. Therefore, when flushing this material, the oil should be cleaned on the contrary, and the oil should be disassembled once a month, and the dirt on the punch and lower die should be removed with steam (diesel) oil, and then cleaned before reassembly. This will ensure that the mold has good lubrication performance.

9、 Common problems and solutions in the use of moulds
Problem 1. The plate comes out of the clamping jaw
reason
resolvent
Incomplete mold unloading
1. Use a punch with slope
2. Apply lubricant on the plate
3. Adopt heavy-duty die
Problem 2: The mold is severely worn
reason
resolvent
Unreasonable mold clearance (too small)
Increase mold clearance
The upper and lower mold bases are not aligned
1. Station adjustment, upper and lower mold alignment
2. Horizontal adjustment of turret
Failure to replace worn die guide components and turret inserts in time
replace
Punch overheating
1. Add lubricant to the sheet
2. Ensure lubrication between punch and lower die
3. Use multiple sets of molds with the same specification and size in the same program
Improper grinding method will lead to annealing of the die, which will lead to increased wear
1. Use soft abrasive wheel
2. Clean the grinding wheel frequently
3. Small bite
4. Enough coolant
Step punching
1. Increase step
2. Adopt bridge step punching
Problem 3. Punch with material and punch adhesion
reason
resolvent
Unreasonable mold clearance (too small)
Increase mold clearance
Passivation of punch edge
Timely sharpening
Poor lubrication
Improve lubrication conditions
Problem 4. Scrap rebound
reason
resolvent
Lower mold problem
Use bulletproof material to lower the mold
10% reduction in clearance for small diameter holes
Diameter greater than 50.00 mm, gap enlarged
Increase scratches on the edge side of the concave die
Punch
Increase the injection depth
Install the unloading polyurethane ejector bar
Use oblique edge
Problem 5. Difficulty in unloading
reason
resolvent
Unreasonable mold clearance (too small)
Increase mold clearance
Punch wear
Timely sharpening
Spring fatigue
Replace the spring
Punch adhesion
Remove adhesion
Problem 6. Stamping noise
reason
resolvent
Difficulty in discharging
Increase the clearance of the lower die and provide good lubrication
Increase discharge force
Discharging plate with soft surface
There is a problem with the support of the plate on the workbench and in the turret
Use spherical support mold
Reduce working size
Increase working thickness
Sheet thickness
Use inclined edge punch
10、 Precautions for using special molding tools
1. The stroke of the sliders of different models of machines is different, so pay attention to the adjustment of the closing height of the forming mould.
2. It is necessary to ensure that the molding is sufficient, so it needs to be carefully adjusted. The adjustment amount should not exceed 0.15 mm each time. If the adjustment amount is too large, it is easy to cause damage to the machine and mold.
3. For stretch forming, please select light spring components to prevent the tearing of the sheet or the difficulty of unloading due to uneven deformation.
4. Spherical support mould shall be installed around the forming mould to prevent the sheet from tilting.
5. The forming position should be as far away from the clamp as possible.
6. The forming process is best implemented at the end of the processing program.
7. Make sure that the plates are well lubricated.
8. Pay attention to the displacement of special molding tools when ordering. If the distance between the two molding tools is relatively close, please be sure to communicate with our salesmen.
9. Because the forming tool needs a long unloading time, it is necessary to use a low speed during the forming process, preferably with a delay.

11、 Precautions for using rectangular cutter
1. The step distance should be as large as possible, greater than 80% of the whole tool length.
2. It is better to realize jumping step by programming.
3. It is recommended to select the die with oblique edge.
12、 How to punch without exceeding the nominal force of the machine
During the production process, round holes larger than 114.3mm in diameter need to be punched. Such a large hole will exceed the upper limit of the nominal force of the machine, especially for materials with high shear strength. This problem can be solved by punching large holes with multiple times. Cutting along the circumference of a large circle with a small size mold can reduce the punching force by half or more. Most of the molds you already have can do this.
13、 A Simple Method of Punching Large Round Hole
The mold of this convex lens can be made into the radius size you need. If the hole diameter exceeds the nominal force of the press, we recommend using (A) scheme. Use this mold to punch out the circumference of the circle. If the hole diameter can be punched within the nominal force range of the press, a radial mold and a convex lens mold can punch the required hole in four times without rotating the mold (B)
14、 Finally, it forms downward
When selecting the forming die, the downward forming operation should be avoided, because it will take up too much vertical space and lead to additional flattening or bending process. Downward forming may also fall into the lower die and then be pulled out of the turret. However, if downward forming is the only process option, it should be taken as the last step of processing the sheet.
15、 Prevent material distortion
If you need to punch a large number of holes in the sheet and the sheet cannot be kept flat, the cause may be the accumulation of stamping stress. When punching a hole, the material around the hole is pulled downward, which increases the tensile stress on the upper surface of the plate. The downward movement also causes the increase of the compressive stress on the lower surface of the plate. For a small number of holes punched, the result is not obvious, but with the increase of the number of holes punched, the tensile stress and compressive stress will increase exponentially until the plate is deformed.
One way to eliminate this deformation is to punch every other hole, and then return to punch the remaining holes. Although this produces the same stress on the plate, it disintegrates the accumulation of tensile stress/compressive stress caused by continuous punching one after another in the same direction. This also makes the first batch of holes share the partial deformation effect of the second batch of holes.
16、 If your stainless steel flanging is deformed
Apply high-quality forming lubricant to the material before flanging, which can better separate the material from the mold and move smoothly on the surface of the lower mold during forming. This will give the material a better chance to distribute the stress generated by bending and stretching, and prevent the deformation on the edge of the forming flanging hole and the wear on the bottom of the flanging hole.
17、 Some suggestions for overcoming the difficulty of unloading
1. Use a punch with a fine core of rubber particles.
2. Increase the lower die clearance.
3. Check the fatigue degree of the spring.
4. Use heavy-duty molds.
5. Oblique blade die shall be used appropriately.
6. Lubricate the sheet.
7. The large station mold needs to be installed with polyurethane discharge head.
18、 The main reason for scrap rebound
1. Sharpness of the blade. The larger the fillet of the blade, the easier it is to cause scrap rebound.
2. The modulus of penetration of the mold. When stamping the die at each station, the requirements for the input modulus are certain. The input modulus is small, which is easy to cause scrap rebound.
3. Whether the clearance of the mold is reasonable. Unreasonable mold clearance is easy to cause scrap rebound.
4. Whether there is oil stain on the surface of the processed plate.