Metal Stamping and Die Design: The Ultimate Guide

Basic knowledge of the metal stamping process

1.1 Characteristics and applications of metal stamping process

Stamp definition

What is stamping?

The definition of stamping: at normal temperature, a metallic (or non-metallic) sheet is pressed in a stamping press by a stamping die to cause separation or plastic deformation, thus obtaining parts with a certain shape, size and performance.

From the stamping concept:

(1) Stamping is carried out at room temperature, that is, it does not require heating, which is why it is called cold stamping.

(2) The stamping processing objects are all sheets, so it is also called sheet metal stamping.

(3) Stamping is done by equipment and molds. Three elements are needed: punch (equipment), mold and raw material.

(4) Stamping is one of the basic forms of plastic deformation.

Features and applications of metal stamping

(1) High productivity, simple operation and easy mechanization and automation.

(2) High dimensional accuracy and good interchangeability.

(3) The material utilization rate is high, generally up to 70% ~ 85% and some up to 95%.

(4) Parts with complex shapes are available that are difficult to machine or cannot be machined by other machining methods, such as thin-shell deep drawing parts.

(5) Parts with light weight, good rigidity and high strength can be obtained.

(6) No heating is required, energy can be saved, and the surface quality is good.

(7) When mass is produced, the cost of the product is low.

It can be seen that stamping can combine high quality, high efficiency, low energy consumption and low cost, which is incomparable with other processing methods.

Therefore, the application of stamping is very extensive. For example, in the automobile and tractor industries, stamped parts account for 60% to 70%, instruments and gauges account for 60 to 70%, and include other stainless steel tableware in daily life.

From thin electronic components, instrument pointers to tough car covers and beams, as well as aircraft skins, stamping is necessary.

Disadvantages of stamping

The mold manufacturing cycle is long and the cost is high. Because it uses traditional processing methods and means and traditional mold materials

However, with the advent of advanced mold processing technology and non-traditional mold materials, this deficiency can be gradually overcome.

As:

• Mold making from low melting point alloy materials
• Mold manufacturing using rapid prototyping
• Economic mold

In short, the mold industry is the basic industry of a country, the level of mold design and manufacturing has become a measure of the level of product manufacturing in a country.

Developed countries attach great importance to mold development.

Japan believes that “mold is the driving force to enter a rich society”;

Germany: “the emperor in the metal processing industry”;

Romania: “the mold is Golden Touch”; mold is considered a stone in the international industry.

However, the molds here also include molds, forging dies, pressure casting dies, rubber molds, food molds, building materials molds, etc., but currently cold dies and plastic molds are the most widely used, each accounting for about 40%.

1.2 Classification of the stamping process

1. Classified by deformation properties

• Separation process
• Training process

（1）Separation process

During stamping, the material to be processed is deformed by external forces.

When the shear stress of the material in the deformation zone reaches the shear strength of the material, the material is sheared and separated to form a part of a certain shape and size.

The separation process mainly includes cutting, punching, shearing, notching, slicing, etc.

The separation process is given below:

Separation occurs, but does not change the shape of the space.

Table 1-1 Separation process

（2）Metal forming process

During stamping, the material under the action of external force, the equivalent stress of the material in the deformation zone reaches the yield strength σs of the material, but does not reach the strength limit σb, so that the material only plastically deforms, thus obtaining pieces of a certain shape and size.

The forming process mainly includes bending, deep drawing, turning, shrinking, bulging, etc.

The formation process is shown below:

Only change the shape of the blank, no separation occurs.

Table 1-2 Training process

2. According to the nature of the deformation zone

• Formation of stretching type: The maximum principal stress of the deformation zone is tensile stress, and the failure mode is tensile cracking, which is characterized by thickness thinning.
• Compression-type formation: The maximum principal stress of the deformation zone is compressive stress, which is characterized by thickening of thickness and the failure form is wrinkling.

3. According to the basic deformation method

• Suppression
• Doubling
• Deep drawing
• Forming

4. According to the combination of processes

• Single process stamping
• Composite stamping
• Progressive stamping

1.3 Stamping material

Basic requirements for foil stamping

• Meet performance requirements
• Meet the requirements of the stamping process

Satisfying performance requirements is the first, and meeting the requirements of the stamping process as much as possible while meeting the performance requirements.

1.3.1 Process requirements for stamping materials

1.Stamping forming performance

Stamping performance refers to the ability of the sheet to adapt to the stamping process.

Two types of instability:

• Tensile instability – local narrowing or fracture under tensile stress;
• Compression instability – instability wrinkles under compressive stress.

The former is like the throttling phenomenon in the tensile test of low carbon steel, and the latter is the pressure bar instability phenomenon.

Thus, there is a formation limit, which is divided into a global formation limit and a local formation limit. The higher the forming limit, the better the press forming performance.

How to measure sheet metal stamping performance?

(1) Cracking resistance refers to the ability of a sheet to resist damage during deformation.

(2) Pastability refers to the ability of the sheet to adapt to the shape of the mold during the press forming process.

(3) Moldability refers to the ability of a part to maintain its shape in the mold after demolding.

The stamping properties of the sheet can be measured by the mechanical properties of the sheet. Mechanical properties can be obtained through experiments.

Sheet metal forming performance test method:

• Direct test: The actual stamping process is simulated directly using special equipment.
• Indirect test: The general performance of the material is obtained through stretching, shearing, hardness tests, metallographic tests, etc., using general equipment.

(1) Direct test method

As Conical Cup Test (GB/T 15825.6-2008)

(2) Indirect test method

Such as tensile test of low carbon steel, etc.

Mechanical indicators affecting press forming properties

(1) Total elongation δ and uniform elongation δb

δ is good → Allows large plastic deformation

(2) Yield rate σs /σb

σs/σb is small → good crack resistance, shape fixation and good moldability

(3) Modulus of elasticity E

Large modulus of elasticity E → good shape

(4) Hardening index n

n is big → not easy to break

(5) Plastic deformation ratio γ

γ = εb /εt is large → Good crack resistance

(6) Specific anisotropy coefficient of plastic deformation

Δγ =(γ0 +γ90 – 2γ45 )/2 is large → The more different the anisotropy

• Chemical composition requirements: Different content of some elements in steel will result in different plasticity and brittleness of the material.
• Requirements for metallographic structure: Different metallographic structures can lead to different mechanical properties such as strength and plasticity.
• Requirements for surface quality: Requires a smooth surface without oxide scale, cracks, scratches and other defects.
• Requirements for material thickness tolerance: A certain thickness corresponds to a certain mold gap, and the material thickness tolerance must comply with the national standard.

1.3.2 Common stamping materials and cutting methods

A．Common stamping materials

• Sheet metal: ferrous metal; non-ferrous metal
• Non-metallic sheet: rubber plate, rubber sheet, plastic plate, etc.

Related Reading: Ferrous vs Non-Ferrous Metals

Sheet metal specifications: steel strip, steel plate, cut steel strip, etc.

Size range of steel sheets and strips (GB/T708-2006)

1) The nominal thickness of steel plate and steel strip (including steel strip cutting) is between 0.3mm and 4.0mm, and steel plate and steel with nominal thickness below 1mm have any size in multiples of 0.05 mm; the nominal thickness is above 1mm. Steel and steel sheets are available in any size in multiples of 0.1 mm.

2) The nominal width of steel plates and strips is between 600mm and 2050mm, and there are any sizes in multiples of 10mm.

3) The nominal length of the steel plate is between 1000mm and 6000mm, and any size in multiples of 50.

4) According to the buyer's needs, steel sheets and strips of other sizes may be supplied through negotiation between the supplier and the buyer.

1. Stamping material suppression method

(1) Cutting machine cutting

(2) Disc cutting

(3) Other cutting methods

• laser cutter
• Plasma Cutting Machine
• High Pressure Water Cutting Machine
• Wire EDM Machine
• Electric drilling, etc.

1.4 Stamping equipment

Stamping equipment selection principle:

• The nature of the stamping process: separation or molding process
• Pressing force: open, closed
• Mold structure
• Mold closing height, contour size
• Production batch
• Production cost
• Product quality
• Match existing workshop equipment conditions

Type of stamping equipment:

• According to the different driving forces of the slider, there are mechanical presses, hydraulic presses and pneumatic presses;
• According to the bed frame, there are open and closed presses;
• According to the number of sliders, there are single-action presses (one slider), double-action presses (two sliders), etc.;
• According to the number of connecting rods, there are single-point press (one connecting rod), two-point press (two connecting rods), four-point press (four connecting rods), etc.
• ……

Working principle and main components of the crank

(1) Operating mechanism

Crank and link mechanism: The crank mechanism is composed of a crankshaft, a connecting rod and a slider. The connecting rod length can be adjusted to suit different mold sizes.

(2) Transmission system

Motor, belt, flywheel, gear, etc.

(3) Operating system

Air distribution system, clutches, brakes, electrical control boxes, etc.

(4) Support parts

Body: open, closed

(5) Auxiliary system

Pneumatic system, lubrication system

(6) Annex

Press model and technical parameters

(1) Model

1) Forging machine type:

• J – mechanical press
• D Forging Machine
• Y hydraulic press
• A – cutting machine
• Z – automatic press
• W Bending Bending Press
• Hammer C
• T-other

2) JB23-63A Crank Code Description

• J–press machine category
• Variant B design code
• 2nd press column
• 3 levels of press
• 63 – nominal pressure
• A – Improved design number
• “63T Open Double Column Tilting Modified Second Press”

(2) Technical parameters

• Nominal pressure F
• Nominal pressure stroke SF
• Sliding stroke: top dead center → bottom dead center
• Slider stroke times n: top dead center → bottom dead center → top dead center
• Press pressure height
• Countertop Plate and Slider Bottom Size
• Throat depth C
• Mold hole size

Basic theory of the stamping process

2.1 Basic concept of plastic forming

• Elastic and elastic deformation
• Plasticity and plastic conformation
• Plasticity Index
• Deformation resistance
• Internal strength and stress

Different materials have different plasticities under the same deformation condition, and the same material will have different plasticities under different deformation conditions.

2.2 Plastic forming mechanics

• state of stress
• voltage state
• Yield criterion (plastic condition)
• Stress-strain relationship during plastic deformation

3 main voltage states:

9 main types of stress states:

2.3 Basic law of plastic conformation

• Labor Hardening Law
• Unloading elastic recovery law
• Law of least resistance
• Plastic deformation volume invariance law

4 types of metal stamping process

Now, let's dive into the four different stamping processes below.

• Metal Stamping and Die Design: Suppression
• Metal Stamping and Die Design: Bending
• Metal Stamping and Die Design: Deep Stamping
• Metal Stamping and Die Design: Forming