Common Defects in Bending Forming and Their Improvement Measures

 Common defects in punching

1 .1 Chip skipping

Several situations of skipping chips:

1Chip skipping during edge trimming:


Incomplete punching waste is difficult to effectively get stuck in the mold core, so it is easy to skip chips


Complicated trimming structure

2Small square hole, small round hole chip skipping


①The gap is too large

②There is more oil

③The material is too thin


① Tighten the gap

② Reduce oil consumption

③Add downward blowing to form a negative pressure under the waste

④Set the processing mode of the mold core (changed to PG processing) to improve the smoothness of the cutting-edge section

⑤ Redesign of the hedge substructure

⑥ Demagnetization must be done for iron product molds

3fine waste:

(common in torn molds, especially when there are heavy cuts, 821-220 molds)

4Chip skipping of large pieces of waste


①The gap is too large

②There is more oil

③The material is too thin


①Add ejector pin

② Add air

Complicate its shape (the side connected to the carry)

5Chip skipping in other situations:

reason :

Unreasonable design of process arrangement


Re-adjust the process arrangement (redesign the cutting tool)

Hazards of chipping :

① Defects such as mold marks and crushing

②For intermittent chip skipping, it is not easy to find, and it is easy to flow out defective products

③Difficult maintenance (often using special-shaped punches, cutting-edge draping, and other methods that damage the life of spare parts and the quality of product sections), resulting in low production efficiency and utilization rate

④ A large amount of manual selection is often required when defective products are produced

1 .2 Metal wire


① After being assembled into a finished product, it is very easy to cause a short circuit (SHORT), which is a serious defect that must be absolutely prohibited

②It is easy to produce mold marks on the product when it falls off

③When it happens, it takes a lot of manpower to rework it

Reason for output

Common in molds with tears, such as:

①It is torn downwards first and then flattened upwards, and the torn incision is squeezed twice to form a wire

②Tear in one direction and then bend in the opposite direction

③ When the tearing and bending are not in place at one time, the design is unreasonable


①The workstation behind the tear should be out of the way (discharge) to avoid tearing and being repeatedly slapped. (Preparation for drawing blanks)

②For the drawing, each station behind the torn preparation blank should avoid the torn gap, or redesign the carry to improve its deformation and prevent the seam section from being too long.

③The tearing direction should be consistent with the subsequent forming direction, and the one-time tearing and bending forming scheme should be used as much as possible.

④Secondary forming must be carried out at the tear, and the gap between the punch and the mold core passing through the gap and the tear should be reasonably designed >0.02.

1 .3 punch jump

Root cause: unreasonable structural design

①Straight knife face is too long (greater than 10mm)

②The unused one processed with PG, due to the influence of the pressure plate groove, the service strength is greatly weakened (820-380N1)

③The structural design is unreasonable when using PG processing, and the PG groove weakens it a lot. 822-173 U-shaped punch

④The part of the arc is too slender

⑤ Unreasonable sample delivery in the guide section of the punch that is too small (the processing method of the punch is not good)


① The straight knife surface of the punch for PG processing should be less than 10mm

②Length and width <33 should be designed and processed by the PG method

③For most U-shaped structure punches, PG processing should be used as much as possible, and a split structure should also be used to minimize the weakening of its strength. The difference between split and integral processing: because the split type can be used to feed in the direction perpendicular to the deep groove, it can reduce a lot of cutting.

When there are small parts with arc overhangs,

① Try to avoid when laying out samples (reconsider when laying out samples)

②When unavoidable, PG design should be adopted

When there is a slender punch (<diameter 1.0), the punch guide section and the working section should be separated (the length of the working section should be <2.0) (the ratio of length to diameter should not be greater than 3).

1 .4 gross head


① Poor design of knife connection: knife connection at the vertex of the arc or on the plane → none

② vertical cut

③The gap is unreasonable

④ Unreasonable selection of punch core material

⑤ The knife edge maintenance is not done properly


① The knife design does not improve

②Avoid connecting knives on arc vertices or planes to increase process escape (avoid vertical cutting)

③Use a reasonable daily blanking gap

④Try to use different materials for the punch and die core (punch V3, use ASP23 or SKD11 for the die core), and vice versa.

1.5 Dimensions of punching out of tolerance


①Material flipped

②Materials flow

③ Improper arrangement of forming and blanking process

④The influence of playing COIN

⑤Influence of chamfering angle

⑥ The forming design of the side of the blanking is unreasonable


①Forcibly press the material (except for those that do not allow indentation)

② As a limit to prevent material flow

③According to the test model to make corrections

④Adjust the arrangement of the forming and blanking process (without affecting the strength of the mold core)

⑤According to the experience value, correct the amount of variation caused by hitting COIN and knocking down corners, 0.05~0.10)

⑥ When it will not cause fine waste, add fine cutting, usually not)

⑦ Adjustment in the post-processing section

Principle: first make the forming size stable and then correct the blanking,

1.6 Press plate broken: (screw broken)

Reason: The pressure plate is not strong enough to withstand the stripping force, common design defects:

①The big punch has only one pressure plate

②The cantilever of the pressure plate is too long, A is too large, A ≦ 9mm is appropriate


① Increase the pressure plate

②Change the fixed punch of the pressure plate to the fixed punch of the key

③Change to the threaded crane (flange punch for Mini Din products)

1.7 Design error of connecting knife

1.7.1 Coarse positioning has no effect ① A>P+0.2 ② A<P

1.7.1 Vertical cutting when receiving the knife

1.7.1 The amount of precision cutting is too small

The order of receiving knives is reversed

2. Common bad design of bending forming

2.1 The angle is not in place

Cause Analysis:

1) The bending height is less than the minimum bending height

2) The gap is unreasonable

3) No shaping station

4) The pressing material is not accurate (arc)

5) Frame wear

6) Punch rigidity is too poor

7) No relief angle


1) Additional shaping station (secondary bending station) (oblique, slider, pendulum…)

2) Adjust the bending gap

3) Function of playing Coin (when allowed): reduce material thickness === reduce minimum bending height (rounded corners, material thickness)

4) Improve pressing material:

a. Increase Lifter area

b. grinding reinforcement

c. Change strong spring

d. dead center

e. The punch is formed into a stripping shape (bottom dead center stacked dead)

5) Add template entry

6) Lower die plus punch block

7) The back angle of the mold kernel flash

2 .2 Surface brushing


1) The gap is too small

2) There is an affinity between the punch material and the part material

3) Poor lubrication

4) Punch surface finish is not enough


1) Adjust the gap to 0~0.01

2) Change the material of the punch (related to the material of the part: WC, B, Tic, C, ceramics)

3) Proper refueling

4) Polish the surface of the punch during the production trial

Suggestions on the arrangement of the bending process for post-plating formed products:

1) Using two bending forming process

2) Bending and shaping station is adopted

3) The punch adopts the insert structure as much as possible

4) The consideration and type of refueling during production must be strictly controlled

5) When the stamping process is difficult to solve, consult with the manufacturing and production engineers to solve it together

2.3 The size is not in place


1) The angle is not in place

2) Defective unfolded length

3) Poor fillet R size

4) Deformed

5) Impact of COIN

6) The position of the bending line is wrong

7) The bending force is not enough


1) Correct the angle first

2) Whether there is any error in the position of the CHK bending line

3) CHK unfolded length

4) Is there any deformation in CHK bending?

5) Whether there is a COIN (whether it will affect the size)

6) After eliminating all the above defects, decide whether to correct the blanking

* Tips for mold trial and mold repair: adjust the bending fillet (mold core) to fine-tune the bending forming size within a certain range.

7) Spring force is not enough to provide bending force. (It is especially obvious when the material is thick and the bending line is long.) Sometimes it is not enough to achieve the required bending force by using a stripping plate (there is a gap of 0.10 between S02A and P01A), and a dead punch can be added.

2.4 Deformation (not including deformation with insufficient flash)


1) The pressing material is not good

2) The bending gap is too small (the effect on small bending is obvious)

3) The breaking difference of the bending punch is too large


1) Improve pressing

2) Properly loosen the gap (using secondary bending or adding a shaping station to make the angle in place

3) Reduce the punch break to (6+0.05~0.10) <0.5

This type of improvement has been extremely large, accounting for about 30% of the previous total improvement

The role of the gap:

Reshape the bending fillet at the bottom dead center, plasticize deformation, and reduce spring back

It is convenient to adjust the bending gap during production (displaceable)

2 .5 Dimensional instability

⑴The material has changed

⑵The bending line is unstable


1) The punch has no entry, the frame mouth or the punch is worn

2) Shaking off the bending die core (especially the floating bending die core)

3) The positioning section of the floating bending die is short

4) Floating mold core or frame wear


1) Enlarge the die core and increase the positioning section

2) Add sub, sink into the lower backing plate

2.6 Packing and bending cracks


1) The fillet of the bending die is smaller than the minimum bending fillet

2) The bending angle is less than 90 ∘ (the outer elongation is greater than the elongation of the material)

3) Performance variation of materials

Points to note when designing the mold:

1) Whether the CHK bending fillet is smaller than the minimum bending fillet

2) The rounded corners of the bending die should not be too small

3) Do not make the second bending (or shaping mold core) with the second bending (or shaping mold core) for the first bending in two bendings or (one bending + shaping); when the hardness of the material is low, the bending fillet is small, the first bending is very easy to shape and wrap the material, and the first bending die will not wrap the material if the back corner does not flash.

2.7 Advantages and disadvantages of several typical bending and shaping mechanisms

Bevel shaping

Advantages: simple structure, low cost, small mold space required

Disadvantages: The effect is limited, and there are the following problems at the same time:

The stress point is not near the bending line

1) Generate an arc

When there are holes on the bending surface and other conditions that reduce its strength, the plastic yield point will not be at the desired bending line, but at the weakest point

2) Form hair head

Scope of application:

1) Occasions where the size and angle requirements are not high

2) When the material thickness and bending line are not large

3) When the bending height is not greater than 5mm

Design points: α<20 ∘ L value should be reasonable

Side push slider shaping

Advantages: stable. Reliable, can adjust the bending angle to less than 90 ∘ , applicable to the widest range of mechanisms in the actual design

Disadvantages: complex structure, many parts, high cost, large mold space, and limited use occasions.

Scope of application: It is widely used in bending and shaping stations when the size and angle requirements are high or the angle requirements are 90 ∘ . As long as the mold structure space permits, it can be boldly used.

Design Points:

1) Prevent the slider from jumping out

a. Press with a pressure plate

b. Press with a guide plate

c. With T-slot

2) α1<α2

3) H should not be too high, it is better to use a spacer when the template is 25 OR30 thick

4) The stroke should be sufficient and cannot interfere with the formed part of the product, L1>L2, otherwise: the product (material strip) will interfere with the slider when it goes up

5) The adjustment amount should be appropriate. Principles: When designing: when adjusting to the innermost position, the minimum gap between the slider and the bending die core should not be less than 0.5, otherwise: when the adjustment is appropriate, the slider or the die core will be damaged.

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