Summary of stamping process and product design knowledge

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1 Process classification of stamping products

1. Basic process classification

According to its deformation properties, the stamping process can be divided into two categories: material separation and forming.

The separation process refers to the stamping process in which the blank is broken and separated after the stress of the deformed part reaches the tensile strength under the action of the stamping force, so as to obtain the workpiece of the required shape and size.

The forming process refers to the stamping process in which the stress of the deformed part of the blank reaches the yield point under the action of the punching force, but does not reach the tensile strength, so that the blank is plastically deformed without fracture and separation, thereby obtaining a workpiece of the required shape and size. .

2. Types of separation process

According to their different deformation mechanisms, the separation process is divided into two categories: punching and repairing.

Punching: Refers to punching a sheet with a die along a certain curve or straight line (including the following categories)

Refurbishment is a separate processing method for reprocessing the section of the blanked part. The refurbishment deformation is a cutting mechanism, and the dimensional accuracy and cross-section quality of the workpiece are better than those of the blanked part.

3. Types of molding process

There are many forming processes, including: bending, deep drawing, flanging, bulging and extrusion processes. (details as follows:)

2.Punching

1. Introduction to the shape and forming process of blanking products

The shape of the blanking product. The section of the blanking product is divided into: collapse angle, bright zone, fracture zone, and burr. These four forms are produced in different stages, different parts, and under different stresses during the blanking process of the product.

As shown in the figure above, 1. Slump angle: height is approximately equal to 8%T to 15%T; 2. Bright band: height is approximately equal to 15%T to 55%T; 3. Fault zone: height is approximately equal to 35%T to 75%T; 4. Glitch: the height is approximately equal to 5%T to 10%T

1) Elastic deformation stage

Stress analysis: The material at the cutting edge is subjected to shear force, and the magnitude of the force is less than the elastic limit. If the force disappears, the material returns to its original state.

State description: The punch exerts pressure on the material, and the material slightly squeezes into the cutting edge of the die.

2) Plastic deformation stage

Stress analysis: the material is stressed from the side to the center, and gradually exceeds the elastic limit

State description: The punch goes deeper into the material, and at this stage, the blanking part produces a collapsed angle and a bright band

3) Shearing stage

Stress Analysis: The partial stress of the material close to the cutting edge of the die first reaches the shear strength of the material, which increases the cracks generated by the material next to the cutting edge of the die. At this time, the material at the cutting edge of the punch is still in the plastic deformation stage. As the punch penetrates further into the material, the material near the punch also reaches the shear strength, and cracks are also generated. Afterwards, the two cracks overlap and the material separates.

Status description: The material is separated, and when the upper and lower cracks overlap, they tear each other and produce burrs

3.Key points and design examples of punching technology related to product design

1. Classification, function and structure of blanking products

piercing

Function 1. Used as a general via hole (lower requirements); 2. Used as a self-tapping bottom hole (the product design requires a higher proportion of bright bands); 3. Used as a high-precision shaft hole (requires no burrs, less fractured belts) ) (by mechanical deburring or mold inversion)

Note: When designing the punching hole, due to the limitation of the strength of the punch, the size of the hole should not be too small (generally greater than 0.5T)

Blanking stamping

Function 1. Used as a general shape (lower requirements); 2. Used as a butt joint laser welding assembly (no burrs, large bright bands, small gaps in the fracture zone); 3. Used as a soft decoration bracket (requires curling or Deburring)

Note: 1. When designing the product, the joints of the straight lines or curves of the blanking parts should have appropriate rounded corners. (Otherwise, the stress of the die will be concentrated and easily damaged); 2. Considering the processing technology of the die wire cutting, the blanking parts Or the minimum R angle of blanking parts should not be less than R0.2.

Function 1. Used as a buckle; 2. Used as a limit; 3. Saves the process, improves the utilization rate of materials, and combines the two processes of trimming and bending into one. (Disadvantage: The direction of the burr cannot be changed, it must be opposite to the direction of the punch)

Note: It is required that the distance between the cut part and the bending part is large enough to meet the strength of the punch.

Points for attention in structural design of tongue cutting and bending:

• 1) The width of the punch should be large enough when cutting, and the distance between the cutting part and the bending part should be more than 5mm when designing the part, otherwise the strength of the punch will be low, which will affect the life of the mold.
• 2) When designing the mold, the cutting part of the knife edge should ensure a straight edge of about 3mm to prevent the knife from collapsing. There must be a break on both sides of the punch, so as to ensure that it is cut first and then bent.

Summary of product design points related to blanking

1) When designing the product, the joints of the straight lines or curves of the blanking parts should have appropriate rounded corners.

Reason:

• 1. The minimum R angle of ordinary wire cutting is 0.2, and the sharp corners are not easy to guarantee.
• 2. The die at the sharp corners Stress concentration, the mold is easily damaged after being stressed.

2) The direction of the burr should be marked when designing the product. The burr is very important to the safety of product assembly and operating staff. (Note: the direction of the burr is marked, not the direction of punching)

3) When designing the punching hole, due to the limitation of the strength of the punch, the size of the hole should not be too small (generally greater than 0.5T, try not to make the diameter of the hole smaller than 0.8T)

4) When designing the product, the tensile strength of the material should be less than 630MPa as much as possible, otherwise the mold will be difficult to manufacture. (When the tensile strength of the product is less than 630MPa, the mold material can be selected from ordinary relatively cheap mold steel, such as: Cr12, Cr12MoV, SKD11, D2, etc. When the tensile strength of the product is greater than 630MPa, the mold material should be selected from special more expensive mold steel, such as SKH-9)

5) When the product design has special requirements for the punching section, the minimum acceptable value of each section must be marked.

6) When cutting, pay attention to designing the trimming angle on the product to facilitate demoulding, thereby reducing the wear of the punch.

2. Brief introduction of punching die

• 1) punching, blanking die
• 2) Deburring mold
• 3) Side punching die

4. Introduction of bending product shape and forming process

1. Shape of curved products

Bending forming mechanism: The stress on the metal material is greater than the elastic limit (yield strength) but less than the fracture limit (tensile strength), causing the curvature of the sheet to change in the bending deformation zone, forming a bend.

Stress analysis of bending: when bending, the inner side of the material is subjected to compressive stress and the outer side is subjected to tensile stress, and the tensile stress plays a dominant role, so the neutral layer of the material is the center of the material that is biased towards the inner side of the bending.

Neutral layer: about 0.255T from the inner side of the material

The outer fiber of the material moves relative to the material due to the tensile stress, and the insufficiency of the material is supplemented by the width direction

2. Bending process (take V curve as an example):

1) The movement of the punch and the contact sheet (blank) produce a bending moment due to the different contact point forces of the convex and concave molds, and elastic deformation occurs under the action of the bending moment, resulting in bending.

2) As the punch continues to move downward, the blank and the surface of the die gradually come into contact, so that the bending radius and the bending arm are reduced accordingly, and the contact point between the blank and the die moves from the two shoulders of the die to the two slopes of the die.

3) As the punch continues to go down, both ends of the blank contact the slope of the punch and begin to bend.

4) In the flattening stage, as the gap between the punch and die continues to decrease, the sheet is flattened between the punch and die.

5) In the correction stage, when the stroke is over, the sheet is corrected so that the rounded corners and straight edges fit the punch to form the desired shape.

3. Two types of problems that are prone to occur in bent products (rebound, cracking)

1) Rebound:

The reason for springback: the material is composed of many layers of fibers, and the stress of each layer of fibers is different, (the outermost layer has the largest tensile stress, the innermost layer has the largest compressive stress, the magnitude of the two forces Decrease toward the neutral layer), so after bending, not all the fiber layers are stressed greater than the elastic limit of the material, so the material in the elastic deformation stage has a recovery phenomenon

1) The stress and strain of the neutral layer are zero

2) The compressive stress of the neutral layer gradually increases towards the inside

3) The tensile stress of the neutral layer gradually increases outward

1) When the stamping part is bent, the strain of most of the material layers enters the plastic deformation area, and these material layers do not spring back.

2) The strain of the material layer closer to the neutral layer is still in the elastic deformation region, and these material layers will spring back after the external force disappears (the bending punch leaves the workpiece)

Factors affecting rebound:

(1) The higher the elastic limit of the material, the greater the required deformation stress and the greater the rebound

(2) The smaller the relative bending radius R/T of the material, the more concentrated the stress, the smaller the proportion of elastic deformation, and the smaller the rebound

2) cracking

When the stress on part of the material layer of the workpiece is greater than the tensile limit during bending, the workpiece will crack. (The farther the material layer is from the neutral layer, the greater the stress and strain)

Ways to avoid cracking: When bending, the R angle inside the corner is too small. (generally the R value is not less than 0.5T)

4. Deformation characteristics of bending products

(1) Due to the tensile stress of the outer fiber of the material, the material moves relatively, and the deficiency of the material is supplemented by the width and thickness directions, so the width of the material is reduced.

(2) Due to the compressive stress of the inner layer fibers of the material, the inner layer material moves to the width direction, resulting in an increase in the width of the inner layer of the material.

(3) When the width is less than 3 times the material thickness, the above phenomenon is obvious, and the product design should avoid the situation that the width is less than 3 times the material thickness.

5. Key points and design examples of bending process related to product design

(1) The fillet radius of the bent part should not be smaller than the minimum bending radius to avoid cracks; but it should not be too large, otherwise the rebound will be large due to incomplete deformation. (Generally, the minimum bending radius R>=0.5T)

Notice:

1) When designing the product, the bending R angle should be avoided to be too small, otherwise it will easily cause stress concentration.

2) The R angle dimension must be marked on the inside. (Specific reason: the workpiece is close to the punch when bending, and the R angle of the punch determines the R angle of the workpiece, and it is easy to control and adjust.)

(2) The length of the bending edge of the bending part should not be too small, otherwise the length of the support of the mold to the material is too small during the bending, it is not easy to obtain parts with accurate shape, and the bending part is often easy to fall out. H>R+2T.

Note: When designing the product, avoid bending the straight edge too small, otherwise it will easily cause outward fall, and it is difficult to control the verticality.

(3) The bending part should not be bent at the sudden change in the width of the part to avoid tearing. If it must be bent at the sudden change in width, the process groove should be designed in advance.

(4) Since the blank will more or less slip during bending, the process hole should be designed as much as possible during product design.

6. Brief introduction of bending die

5.Molding process form and process introduction

1. Classification and introduction of molding process

Forming mechanism: The stress on the metal material is greater than the elastic limit (yield strength) but less than the fracture limit (tensile strength), and the deformation mode desired by the designer is produced within the plastic deformation range.

Forming process classification: 1. Deep drawing 2. Extrusion 3. Flanging 4. Flipping (pumping) 5. Shrinking and flaring

2. Key points of molding process related to product design and design examples

1) Squeeze

There are three functions of extrusion convex hull:

(1) Used as a self-locating pin between two parts

Notice:

a. When the boss is used as a positioning pin, the diameter of the boss needs to be strictly controlled. Generally, the diameter tolerance of the boss can be controlled at about +/- 0.04mm

b. Since the convex hull is extruded, the sides of the convex hull are all bright bands;

(2) Used as a limit of the movement mechanism

(3) Used as a bump for projection welding

Attention points and punch size of convex hull design:

Principles: 1) It is necessary to ensure that there is sufficient material connection between the convex hull and the matrix, otherwise the convex hull is easy to fall off. 2) When used as projection welding, the bump diameter D>= 2t+0.7, and greater than 1.8mm.

Bump height H>=(0.4t+0.25), and greater than 0.5mm

The design dimensions of the convex hull limit height are as shown in the figure below

Note: When marking the size of the convex hull, only the size of the convex part can be controlled, and the size of the concave part cannot be controlled.

Extrusion Convex Die Structure: The size of the die determines the diameter of the convex hull. The thimble and extrusion punch together determine the height of the convex hull. Note: When marking the size of the convex hull, only the size of the convex part can be controlled, and the size of the concave part cannot be controlled.

2) pumping hole

The pumping hole has two functions:

a) Used as rivet connection parts (including punching riveting and turning riveting);

Advantages: rivets can be omitted, saving costs.

Disadvantages: Cannot withstand large pull-off force or shear force.

Hole punching and riveting: it acts as a fixed connection.

Pulling hole turning riveting: it acts as a rotating shaft.

b) Used as a connecting nut

Points for attention in hole design and punch size:

Principles: a) Sufficient material flow must be ensured (ie, pumping feasibility must be calculated).

b) When used as a turning riveting, the outer diameter of the extraction hole (dimension standard outer diameter) must be controlled.

Note: The mold can control both the inner and outer diameters of the pumping hole, the punch controls the inner diameter; the die controls the outer diameter, but not at the same time. That is, each part can only control one value.

c) When used as a nut, the inner diameter of the pumping hole (dimension standard inner diameter) must be controlled.

d) When used as a nut, it must be ensured that the thickness of the thinned straight edge is greater than 1.3 times the thread pitch.

e) When it is used as a nut and has strength requirements, it must be ensured that the minimum height of the straight edge after drilling the hole is greater than 3 times the thread pitch.

Pumping hole feasibility calculation:

Hole Hole: A stamping process in which the material is turned into a side flange along the circumference of the inner hole.

Hole turning coefficient: the ratio of the diameter of the pre-punched hole to the diameter of the straight edge after turning the hole (the larger the hole turning coefficient, the smaller the degree of deformation)

Factors affecting the turning hole coefficient:

a) The plasticity of the material, the better the plasticity, the smaller the hole turning coefficient.

b) The relative diameter D/t of the pre-punched hole, the smaller the D/t, the smaller the hole turning coefficient.

c) Hole processing method. (If the turning hole is higher, it is not easy to crack when the burr is located on the inside; when it is located on the outside, it is necessary to increase the guide surface process and then drill the hole.)

d) The form of the hole punch. (The spherical punch can reduce the turning coefficient and increase the degree of deformation.)

In theory, it is necessary to judge whether the pumping process is feasible according to the pumping coefficient (this method needs to determine too many factors, which is time-consuming and labor-intensive). In general, it can be judged according to the proportional relationship between pre-punching and material thickness. When the relative diameter D/t of the pre-punched hole is greater than 1, it is generally considered feasible.

Calculation of pre-punched hole size:

Principle: The principle of constant volume before and after turning the hole.

AB=/T

Pre-punched hole diameter d=D-2*AB

Generally, the thickness of the material becomes thinner after turning the hole, and the thinning coefficient is between 0.45 and 0.9.

The thinning factor refers to the ratio of EF to the thickness T of the raw material

It is generally believed that when d>=T, drilling is feasible (empirical value, detailed judgment can refer to drilling coefficient)

Hole-drawing mold structure

Hole punching punch structure: a) When a parabolic punch is used, the turning quality is higher because of the excessive arc. (The structure is as follows)

Note: When the arc radius is different,

The extrusion effect of the punch on the material is not the same. Because the small arc punch is too small, the instantaneous extrusion force on the material is large, so the deformation of the material is also large. Therefore, under the same conditions, the small arc punch is used to turn the hole. Higher.

b) One-shot forming punch without pre-punching.

Note: The size of the piercing hole is consistent with the size of the pre-punched hole in the two formings (A=a, B=b). The one-time punching and turning structure is only suitable for the case where the turning burrs are on the outside.

3) Concave flanging

Flanging is the process of turning the material into a sideways short side along the contour curve.

a) Concave flanging (elongated flanging): the deformation is similar to that of a hole.

b) The thinning rate ranges between 0.9 and 1 (the most severely deformed area is at the highest end face)

Feasibility judgment of concave flanging:

a) Expanded size

b) Judgment

End arc length L1 before flanging

End arc length L2 after flanging

When the deformation rate K of the end surface is greater than the elongation rate of the raw material, cracking will occur

During product design, the values ​​of R, r, and h can be adjusted so that the deformation rate of the end face meets the design requirements without cracking.

4) Convex flanging

a) Convex flanging (compression flanging): The deformation property belongs to compression molding.

b) Expanded dimensions of the convex flange

6 Brief introduction to other stamping die structures

1. Rolling mold structure (method 1)

Steps: 1. Roll one-eighth of a circle, 2. Curve upwards obliquely at 80 degrees, 3. Push down to form a circle.

2. Rolling mold structure (method 2)

Steps: 1. Roll a quarter circle, 2. Use the slider to push sideways.

3. Flatten the mold structure (flatten the outer edge)

Steps: 1. Blanking; 2. Upward bending 90 degrees; 3. Pressing down 70 degrees (the size of the punch R is twice the thickness of the material minus 0.3) 4. Flattening

4. Flattening mold structure (inner hole flattening)

Steps: 1. Blanking; 2. Upward bending 90 degrees; 3. Pressing down 70 degrees (the size of the punch R is twice the thickness of the material minus 0.3) 4. Flattening

5. Deep drawing structure