Innovative Design Essentials: The Perfect Guide to Stamping Processes and Product Design

2023-02-28 12:46:30 No Comments Epower Metals News / Stamping knowledge

The secret to making beautiful stamping products: A complete guide to mastering stamping processes and product design knowledge.

1. Process classification of stamping products

Table of Contents

1. Process classification of stamping products
2. Blanking
- 1). Introduction to the shape and forming process of blanking products
3. Key points of blanking process related to product design and design examples
- 1). Classification, function, and structure of blanking products
- 2). Introduction to blanking die
4. Introduction to the shape and forming process of bending products
5. Introduction to Forming Process Form and Process
- 1). Classification and introduction of the molding process
- 2). Key points of forming process related to product design and design examples
6. Introduction to other stamping die structures

1). Basic process classification

According to its deformation properties, the stamping process can be divided into metal material separation and forming.
The separation process refers to the stamping process of the workpiece with the required shape and size after the stress of the deformed part of the billet reaches the tensile strength under the action of the punching force, and the billet breaks to separate.

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

20230228051347 43205 - Innovative Design Essentials: The Perfect Guide to Stamping Processes and Product Design

2). Category of separation operation

According to different deformation mechanisms, the separation process can be divided into blanking and trimming.

Blanking: refers to punching sheet metal along a certain curve or straight line with a die (including the following categories)

Trimming is a different processing method to re-process the section of the blanking part. Trimming deformation is a cutting mechanism, and the workpiece’s dimensional accuracy and section quality are better than those of the blanking part.

3). Type of forming process

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

2. Blanking

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

The form of blanking products. The cross-section of blanking products is divided into four forms: collapse angle, bright zone, fracture zone, and burr. These four forms are generated at different stages, with different parts and different stresses in the blanking process of products.

As shown in the figure above, one collapse angle: the height is about 8% T to 15% T;
Light band: height is about 15% T to 55% T;
Fault zone: height is about 35% T to 75% T; 4. Burr: height is about 5% T to 10% T.

(1) Elastic deformation stage

Stress analysis: some metal materials at the cutting edge are subject to shear force, and the force is smaller than the elastic limit. If the force disappears, the metal material will return to its original state.

State Description: the punch exerts pressure on the metal material, and the metal material is slightly squeezed into the cutting edge of the die.

(2) Plastic deformation stage

Stress analysis: the metal material stress gradually exceeds the elastic limit from the edge and center.

Status Description: The punch goes further into the metal material, and at this stage, the blanking parts have collapsed corners and bright bands.

(3) Shearing stage

Stress analysis: the stress of the part of the metal material near the die edge first reaches the metal material’s shear strength, which increases the metal material’s crack near the die edge. At this time, part of the punch edge metal material is still in the plastic deformation stage. With further penetration of the punch into the metal material, the metal material near the effectiveness reaches the shear strength, and cracks also occur. Then the two gaps overlap, and the metal material separates.

State Description: when the metal materials are separated, and the upper and lower cracks overlap, they tear each other to produce burrs.

3. Key points of blanking process related to product design and design examples

1). Classification, function, and structure of blanking products

Piercing

Function

It is used as a general through hole (lower requirements);
It is used as the bottom hole of the self-tapping tooth (the product design requires a high proportion of bright belts);
It is used as a high-precision shaft hole (no burr and few fracture zones are needed) (by mechanical deburring or die chamfering).

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

Blanking stamping

Function

It is used as a general shape (lower requirements);
It is used for butt joint laser welding assembly (no burr, large bright band, small gap of fracture zone);
Used as soft decoration support (curling or deburring required).

Note:

During product design, the connection of each straight line or curve of the blanking part should be appropriately rounded (Otherwise, the female die’s stress is concentrated and easily damaged);
Considering the die wire cutting processing technology, the minimum R angle of blanking parts or blanking parts shall not be less than R0.2.

Tongue cutting and curve cutting.

Function

It is used as a buckle;
Used as a limit;
Save the process, improve the utilization rate of metal materials, and combine the two methods of trimming and bending. (Disadvantage: the burr direction cannot be changed and must be opposite to the punch direction).

Note: The distance between the cut part and the bending part shall be large enough to meet the punch strength
Precautions for tongue cutting and curve cutting structure design:

(1) When cutting, the width of the punch shall be large enough. When designing the parts, ensure that the distance between the cut and bending parts is more than 5mm. Otherwise, the punch strength will be low, and the life of the mold will be affected.
(2) When designing the mold, the cutting part of the knife edge should ensure a straight edge of about 3mm to prevent the phenomenon of knife collapse. It is necessary to provide a gap on both sides of the punch to ensure the point is cut first and then bent.

Summary of product design considerations related to blanking.

(1) During product design, the connection of each straight line or curve of the blanking part shall be appropriately rounded (Cause: 1. The minimum R angle of ordinary wire cutting is 0.2, and the sharp corner is difficult to guarantee. 2. The stress of the concave die at the sharp corner is concentrated, and the die is easily damaged after stress.)
(2) Burr direction shall be marked during product design. Burr is essential for product assembly and operator safety. (Note: keep the burr direction, not the stamping direction)
(3) During punching design, due to the limitation of punch strength, the hole size should not be too small (generally greater than 0.5T; try not to make the hole diameter less than 0.8T)
(4) The metal material’s tensile strength should be less than 630MPa as far as possible when designing the product. Otherwise, the mold is difficult to manufacture. (When the product’s tensile strength is less than 630MPa, the mold metal material can be ordinary and relatively inexpensive mold steel, such as Cr12, Cr12MoV, SKD11, D2, etc. When the tensile strength of the product is more significant than 630MPa, the mold metal material should be unique and expensive mold steel, such as SKH-9)
(5) When the product design has unique requirements for the blanking section, the minimum acceptable value of each team must be indicated.
(6) When cutting, pay attention to the design of the trimming angle on the product to facilitate demolding and reduce the wear of the punch.

2). Introduction to blanking die

(1) Punching and blanking die

(2) Deburring die

(3) Side-punching die

4. Introduction to the shape and forming process of bending products

1). Form of bent 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 in the bending deformation zone to change, forming bending.
Bending stress analysis: when bending, the inner side of the metal material is subject to compressive stress, and the outer side is subject to tensile stress, and tensile stress plays a leading role, so the neutral layer of the metal material is the center of the metal material deviates from the inner side of the bending.
Neutral layer: about 0.255T from the inside of the metal material.
The outer fiber of the metal material is relatively moved due to the tensile stress of the metal material, and the shortage of the metal material is supplemented by the width direction

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

(1) The punch moves to contact the sheet metal (blank), and the bending moment is generated due to the different contact point forces between the punch and the die. Elastic deformation occurs under the bending moment, resulting in bending.
(2) As the punch continues to go down, the blank and the surface of the female die gradually close to contact, reducing the bending radius and bending force arm. The contact point between the blank and the female die moves from the two shoulders of the female die to the two slopes of the female die.
(3) As the punch continues to go down, both ends of the blank contact the punch bevel and begin to bend.
(4) In the flattening stage, as the gap between the male and female dies becomes smaller, the sheet metal is flattened between the male and female dies.
(5) In the correction stage when the stroke is over, the sheet metal shall be corrected to make the straight edge of the round corner fit with the punch to form the required shape.

3). Two types of problems (spring back and cracking) that are easy to occur in bending products

(1) Rebound:
The reason for spring back: the metal material is arranged by many layers of fibers, and the stress of each layer of fibers is different (the tensile stress of the outermost layer is the largest, the compressive stress of the innermost layer is the largest, and the magnitude of the two forces decreases toward the neutral layer). Therefore, after bending and forming, the stress of all fiber layers is not greater than the metal material’s elastic limit. Thus, the metal materials in the elastic deformation stage have the phenomenon of recovery.

The stress and strain of the neutral layer are zero;
The compressive stress of the neutral layer gradually increases toward the inside;
The tensile stress at the outer side of the neutral layer gradually increases.

When the stamping part is bent, most of the strain of the metal material layer enters the plastic deformation area, and these metal material layers will not rebound.
The strain of the metal material layer close to the neutral layer is still in the elastic deformation area, and these metal material layers will rebound after the external force disappears (the bending punch leaves the workpiece)

Factors affecting spring back:

The higher the elastic limit of the metal material, the greater the deformation stress required and the greater the spring back.
The smaller the relative bending radius R/T of the metal material, the more concentrated the stress, the smaller the proportion of elastic deformation, the smaller the spring back.

(2) Cracking
When the stress on some metal material layers of the workpiece during bending is greater than the tensile limit, the workpiece will crack. (The further away from the neutral layer, the greater the stress and strain of the metal material layer).
Method to avoid cracking: avoid the R angle inside the bending angle being too small when bending. (Generally, the R-value is not less than 0.5T).

4). Deformation characteristics of bending products

Due to the tensile stress on the outer fiber of the metal material, the metal material will move relatively, and the shortage of the metal material will be supplemented by the width and thickness direction, so the width and size of the fabric will be reduced.
The inner layer fiber of the metal material is subject to compressive stress, and the inner layer metal material moves towards the width direction, increasing the internal layer width of the fabric.
The above phenomenon is obvious when the width is less than 3 times the metal material thickness. The product design should avoid the width being less than 3 times the metal material thickness.

5). Bending process points and design examples related to product design

The fillet radius of bending parts should not be less than the minimum bending radius to avoid cracks; However, it should not be too large. Otherwise, the spring back will be significant due to incomplete deformation (Generally, the minimum bending radius R>=0.5T)
be careful:

During product design, the bending R angle should be avoided as being too small. Otherwise, it is easy to cause stress concentration.
The R angle dimension must be marked on the inside. (Specific reason: when bending, the workpiece is close to the punch, and the R angle of the hole determines the R angle of the workpiece, which is easy to control and adjust.)

The bending length of bent parts should be manageable. Otherwise, the support length of the die to the metal material is too small when bending, it is not easy to get the parts with accurate shape, and bent legs often easy to fall out H>R+2T.

Note: During product design, avoid bending the straight edge too small. Otherwise, it is easy to cause outward fall, and it isn’t easy to control perpendicularity.
The bent part shall not be bent at the abrupt change of part width to avoid tearing. If it is necessary to bend at the abrupt change of width, the process groove shall be designed in advance.
Since the blank will slip more or less during bending, the process hole should be designed as far as possible during product design.

5. Introduction to Forming Process Form and Process

1). Classification and introduction of the 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), which produces the deformation mode desired by the designer within the plastic deformation range.

Forming process classification: 1. deep drawing 2. extrusion 3. flanging 4. flanging (hole drawing) 5. necking and flaring

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

Extrusion
There are three functions of extruding convex hull:

Used as a self-locating pin between two parts.

Be careful:

a. When the boss is used as a locating pin, the diameter of the boss needs to be strictly controlled. Generally, the diameter tolerance of the boss can be held at about+/- 0.04mm;
b. Since the convex hull is extruded, the side of the convex hull is all bright.

Used as the limit of the motion mechanism.

Used as a bump for projection welding.

Attention points of convex hull design and punch size:

Principle:

Ensure that there is a sufficient metal material connection between the convex hull and the parent body. Otherwise, the convex hull is easy to fall off.

When used as projection welding, the diameter of projection point D>=2t+0.7 and is greater than 1.8mm.

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

The design dimension of the convex hull limit height is shown in the figure below:

Note: When marking the size of the convex hull, only the size of the convex parts, not the size of the concave parts, can be controlled.

Die structure of extrusion bulge: the die’s size determines the bulge’s diameter. The ejector pin and the extrusion punch together determine the height of the bulge. Note: When marking the size of the convex hull, only the size of the convex parts, not the size of the concave parts, can be controlled.

Hole drawing
Hole drawing has two functions:
a) Used as a rivet connection part (including punch riveting and turn riveting).
Advantages: rivets can be omitted to save cost. Disadvantages: It can not bear great pulling force or shearing force.
Punching and riveting: used for fixing and connecting.

Hole pulling and riveting: it plays the role of rotating shaft.

b) Used as a coupling nut.

Points for attention in hole drawing design and punch size:

Principle:

Sufficient metal material flow must be ensured (i.e., the feasibility of hole drawing must be calculated).
When used as a turning rivet, the outer diameter of the hole must be controlled (the outer diameter of the dimension mark).

Note: The mold can control the inner and outer diameter of the hole, and the punch can control the inner diameter; The die controls the outer diameter, but not simultaneously. That is, each part can only hold one value.

When used as a nut, the inner diameter of the hole must be controlled.

When used as a nut, it must be ensured that the thickness of the thinner straight edge after hole extraction is greater than 1.3 times the thread pitch.

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

Hole drawing feasibility calculation:

Hole drawing: the stamping process of turning the metal material into a side flange along the inner hole. Flanging coefficient: the ratio of the pre-punched diameter to the pitch diameter of the straight edge after flanging (the more significant the flanging coefficient, the smaller the deformation degree)

Factors affecting the flanging coefficient:

The plasticity of the metal material, the better the plasticity, and the smaller the flanging coefficient.
The relative diameter of the pre-punched hole D/t; the smaller the D/t, the smaller the flanging coefficient.
Hole processing method. (If the flanging is high, it is not easy to crack when the burr is located at the inner side; if it is located at the outer side, it is necessary to add the guiding surface process and then extract the hole.)
Form of flanging punch. (The spherical punch can reduce the flanging coefficient and increase the degree of deformation.)

Theoretically, it is necessary to judge whether the hole-drawing process is feasible according to the hole-drawing coefficient (this method needs to determine too many factors, which is time-consuming and laborious). Generally, it can be judged according to the proportion between the pre-punched hole and the metal 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 volume before and after flanging shall not change.

AB={H*EF-(π/4-1)*EF*EF}/T

Pre-punching hole diameter d ＝ D-2 * AB Generally, the thickness of the metal material becomes thinner after flanging, and the thinning coefficient is between 0.45 and 0.9.
Thinning coefficient refers to the ratio of EF to raw metal material thickness T.

It is generally believed that when d>=T, hole drawing is feasible (empirical value, guide to hole drawing coefficient for detailed judgment).