Die Stamping

Chapter One – What is Die Stamping?

Die stamping is a cold forming process that takes a strip of metal, also called a blank or tool steel, and cuts and shapes it using a single or series of dies to create a desired shape or profile. The force that is applied to the blank modifies and changes its geometry, which creates stress that makes the workpiece suitable for bending or shaping into complex forms. The parts produced can be exceptionally small or extremely large depending on the application.

The die stamping process, also known as pressing, includes a number of techniques such as punching, blanking, piercing, coining, and several other operations. Designs are required to be precise so that each punch produces optimal quality.

The dies in die stamping are specialized tools that have been customized to create a specific design, which can be simple common items or complex computer components. Dies can be designed to perform a single function or be part of a series of functions that happen in stages.

There are three common forms of die stamping manufacturing processes:

1. line: a single operation process
2. transfer: stamping completes several operations in one cycle
3. progressive: the most common and widely used

(These three processes will be further explained below in Chapter 3: Production Methods).

Chapter Two – Die Stamping Operations

Stamping dies perform two functions – cutting or forming, with some dies performing a combination of those functions. Each type of operation is designed to cause separation or plasticizing, giving it the ability to be shaped like plastic.

Forming dies are:

• Bending
• Flanging
• Drawing
• Stretching
• Coining
• Ironing

Cutting dies are:

• Shearing
• Blanking
• Trimming
• Notching
• Piercing

Below is a description of forming dies: Forming dies compress metals into specific shapes and are much like a stencil.

Bending -

Bending creates shapes that are similar to a L, U, or V. It is a plasticizing deformation that stresses the yield level below tensile strength over a single axis.

Flanging

Flanging is bending the workpiece along a curved axis. The two types are stretching and shrinking. Tension and compression are common in the flanging process, which is determined by the length of the tab. It can be produce curves or corners and requires a simple downward movement of the press.

Drawing

Drawing is a metal flow process that displaces of the surface of the workpiece with another shape with the same surface area. The reshaped metal maintains its thickness. The drawing direction is critical since it affects how the part can be moved, cut, and ejected.

A variation of drawing is deep drawing. It is non-directional, meaning the direction can be up, down, or vertical.

Stretching

The surface area of the workpiece is increased by tension and thinning. It produces a very smooth surface for painting and finishing. Dies use high pressure binding to stop the flow of the metal. In most cases, stretched metals are dent resistant.

Coining

A pattern is produced by squeezing the workpiece under extreme pressure, which reduces the thickness of the metal.

Ironing

Ironing is like coining. Its purpose is to reduce the wall thickness of the workpiece by squeezing it at a depth that is 30% of the workpiece‘s thickness. Ironing unifies wall thickness and increases its drawn vessel length.

Below is a description of cutting dies: Cutting, also referred to as shearing, is when a piece of metal is separated by applying force causing, the metal to fail.

Blanking

Blanking removes a portion of a metal strip along a specific contour line or shape. In very simple terms, it is cutting away one part of a strip from another. The cutaway part is the workpiece while the remainder is scrap, as seen in this diagram.

Shearing

Shearing produces a straight line cut and is used for parallel cuts, though angle cuts are possible. The diagram below shows a parallel cut.

Piercing

Piercing is similar to blanking. The difference between the two is that, in blanking, the piece punched out becomes the workable part. With piercing, the piece that is removed is scrap and what remains is the part. The punch dimensions determine the size of the removed part and the remaining hole. The diagram below is a simple presentation of the process.

Trimming

The perimeter edge of a form is cut away to conform with the desired profile. During the die stamping process, excess around a form, called flash, has to be trimmed using this process.

Notching

Notching can be used to assist in the bending or cornering processes. It is performed on the outside of the workpiece to create a specific profile.

The twelve dies that are described here are only a sampling of the many that are available. Consulting with a die stamping manufacturer can provide you with a complete selection of die types.

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Chapter Three – Production Methods

When choosing a die stamping method, the factors that define each of the processes are cost, time, and required geometric tolerances. The three common types of production are line, transfer, and progressive, which are described below.

Line Production

Line dies are used for low-volume part production, or for very large parts that do not fit on a single press. The workpiece is moved from station to station, where a single feature is added at each station. With combination dies, a single pressing performs a variety of operations in one stroke.

Advantages:

1. Faster production – Multiple cuts can be made from several dies.
2. Positioning of blank – Loading and repositioning of the blank is easy. It can be turned, flipped, and shifted with little effort.
3. Complex geometries – Produces complex geometries without the need of special calculations or adjustments.
4. Handling of dies – Dies are lighter and less expensive to handle.
5. Tooling – Tooling is smaller and conveniently accessible.

Disadvantages:

1. Machine limitations – Not all presses have the capability of loading combination dies.
2. Slow production – Unlike progressive die stamping, line die processing produces one part at time making it slower and more time consuming.
3. Turnaround times –Turnaround times and volume of production are both low.
4. Costs – Machines have to be maintained and controlled by an operator, increasing labor costs when and several machines are needed to complete a process.

Transfer Production

Transfer dies use the same concept as line dies, but they have multiple dies that are timed together. There is an evenly spaced distance, or pitch, on a single press. Parts move between presses automatically on side-by-side mounted rails or are moved manually. When one cycle is completed, the workpiece is grabbed and transferred to the next die.

Advantages:

1. Multiple motions – Two and three-axis motions can be performed during a single cycle. Three-axis motion lifts the workpiece for the next operation.
2. Part placement – Using gauges or locators, each part is automatically positioned perfectly for each operation.
3. Faster production – Large parts are rotated, turned, and positioned easily, and moved rapidly from station to station.
4. Computerization – Servo drive transfers program the types of parts, press speeds, and length of press strokes.
5. Turnaround times – High volumes of parts are completed with less handling, lower waste, and decreased labor costs.

Disadvantages:

1. Technical planning – Transfer die stamping requires highly sophisticated and technical equipment monitoring. The process has to be carefully planned, tested, and adjusted to ensure quality.
2. Cost – Expertise for planning and design is expensive and time consuming. In general, the overall process is more expensive than progressive stamping.
3. Destacking – A specially designed destacking mechanism is necessary to control the flow of blanks and time their insertion.
4. Process regulation – Production happens quickly, making it impossible to check the status of dies. Die protection sensors are a necessity.
5. Restrictions of process – In the two-axis process, workpieces slide from die to die, which slows down production.

Progressive Dies

Progressive die stamping has several dies that are activated together. The metal strip, as seen below, is fed through, producing a continuous stream of parts. The stress on the metal is distributed evenly over multiple operations. The equal distance between them is called the progression.

Advantages:

1. Volume – Produces large numbers of parts very quickly. It has the potential to produce seven or eight parts per minute, up to 1500 per hour.
2. Labor – It operates automatically, unattended or monitored.
3. Equipment – One machine can produce all of the parts.
4. Die configuration – All die stations are mounted on a single die. Parts are produced together in a single pressing.
5. Speed – Progressive dies are faster and run on less expensive equipment.

Disadvantages:

1. Technical considerations – A complicated set of variables and calculations are necessary to determine and synchronize feed speed to protect the die and precisely time the feeding of the coil to be sure it is fed at a constant rate.
2. Cost – It is more expensive than line or transfer die stampings. The calculations, multiple elements, and equipment are expensive and require significant expertise.
3. Equipment costs – Equipment is very heavy and cumbersome.
4. Maintenance - Damage to a single station requires removal of the whole system and changeover, which can lead to days of delay.

Compound Die Stamping

In compound die stamping, strips of steel are fed through a compound die that cuts or punches out a part in a single stroke. A knock-out ejects the part, and the steel strip continues to feed through the die. The process can produce parts in a few seconds at over 1000 per hour, which reduces labor costs and lead times.

The compound die stamping process eliminates the need for multiple dies that increase the cost of stamping. Using a single die ensures consistency, accuracy, flatness, and dimensional stability. The choice of compound die stamping is due to its ability to lower costs and reduce waste, a major concern for modern manufacturing.

Advantages:

1. Efficiency - Compound dies cut complicated parts in a single stroke avoiding the need for multiple dies.
2. Cost-Effectiveness - Compound die stamping manufactures parts quickly, saving time and money.
3. Speed -Compound die stamping produces parts in seconds and can produce over 1000 parts in an hour.
4. Repeatability -Using a single die in compound die stamping ensures that every part has the same dimensions and configuration.

Disadvantages:

1. Costly Tool Development - Tool development requires time and cost.
2. Unsuited for Small Runs - The cost of tool development makes it unsuitable for small part runs.
3. Post Process Finishing - The force of compound die stamping requires extensive after-process finishing, such as deburring, clamping, and polishing.

Lubricants

Regardless of the production process, die stamping requires the use of lubricants for:

• Protection of tools and dies
• Providing hydrodynamic film to prevent surface abrasions
• Assisting material flow
• Preventing rips, tears, and wrinkles
• Reductioning friction

Punching dies forcefully against a metal strip creates friction that can cause scratches, burn the piece, or damage the die. A lubricant forms a layer on the metal workpiece to protect it and reduce the damage to the die, decreasing defect rates.

The three methods for applying lubricant are drip, spray, and roller.

Manufacturers use lubricants made from plant, animal, and mineral oils in addition to graphite, soap, and acrylic ones. Modern lubricants are synthetic and do not contain any oil.

Chapter Four – Types of Die Stamping Presses

There are four types of die stamping presses: mechanical, hydraulic, servo, and pneumatic. They get their names from the mechanism they use to create their force. Each type is further divided into C-frame and straight side, where C-frame has three open sides and straight side has two. The ram or slide, where the upper die is mounted and applies force, can have double or single connectors.

The picture below is a straight side press, which has four to eight guideways. They can handle off centered loads and protect against deflections.

Stamping Press Terminology

Stamping press manufacturers have their own language to describe the operation of their equipment, while individual companies may have proprietary terms. The diagram below is a complete list of terms for a die stamping press.

Below is a sampling of stamping terms from Sutherland Presses Auto Stamping located in Malibu, CA. A full listing of their die stamping terminology is located at their website - https://www.sutherlandpresses.com/news/press-terminology

• Capacity – the tonnage of pressure the slide can produce
• Continuous on Demand – meaning the press runs in continuous mode
• "Counter Balance" – a system that equalizes the weight of the upper slide
• Daylight – opening in a hydraulic press between the slide and bolster
• Die Blocks – a safety measures inserted when working on the press
• Eccentric – a disk used on an eccentric press to drive attachments
• Flywheel – a wheel that provides rotational energy to prevent excessive or sudden speed changes
• Gibs – guides that ensure proper sliding fit between two machine parts

When speaking to a die casting company, it is beneficial to have a knowledge of the vocabulary to be able understand the lingo.

Die Stamping Presses

Hydraulic and pneumatic die stamping presses are the most common, though mechanical presses are still the mainstay of the industry. Each type of machine uses a different process to perform the same functions using dissimilar kinds of force. In some models, hydraulic and pneumatic methods are combined. Motor presses are a recent development being tested and explored by larger producers.

Pneumatic Stamping Press

A pneumatic press uses air pressure for the down stroke of the ram and springs for its upstroke. A cylinder is filled with air, when actuated by the controller, to expand and create pressure. At the completion of the cycle, the air is released, and the ram goes back to the top.

Benefits:

1. Moving parts – Fewer moving parts enables pneumatic machines to reach full ram velocity quickly and require little maintenance.
2. Precision – Ram pressure is uniform with low deflection. Since it reaches velocity rapidly, it has an increased flow rate.
3. Fast stroke cycles – Stroke speeds can be as high as 400 strokes per minute (spm) without the need for extra framing.
4. Automation – Can be fitted with robotics and special transfer units.

Hydraulic Stamping Press

Hydraulic presses provide force using static pressure over a finite and small area. They use pressurized incompressible fluid in a cylinder or cylinders to drive the ram. They are used for metal forming, shallow stretching, and bending. There are three parts to a hydraulic press: machine, power system, and control system.

Benefits:

1. Weight of parts – Parts produced have a light-weight structure with strong rigidity.
2. Mold or die – Only one mold is needed to complete forming.
3. Strength – Parts have increased fatigue resistant and exceptional strength.
4. Cost – It is a cost effective method that significantly lowers the costs of individual parts compared to other stamping methods.
5. Stroke – Delivers a shorter stroke with maximum tonnage throughout the stroke.

Servomotor Stamping Presses

Until recently, the only way to increase tonnage was by building bigger motors. Press manufacturers have removed motors, clutches, and flywheels and replaced them with servomotors that can supply energy at a specific location to offer better control of the ram.

Servo presses enable operators to program the dwell time at the bottom of each stroke, allowing the workpiece to settle in perfectly before forming. This step significantly adds to the lifespan of the die. Programming the dwell also permits advanced in-die functions, such as heating the metal prior to forming. Heating prevents tough materials like stainless steel from tearing during a deep draw. Programmable functions also enable the use of water-soluble lubrication instead of oil-based lubrication, eliminating the time-consuming and environmentally troublesome oil-removal step downstream in the process. These features and more make servo forming an attractive alternative to mechanical presses.

Benefits:

1. Flexibility – Ram motion can be controlled throughout its stroke. It is possible to always know the position of the ram. The stroke can be matched to fit the application.
2. Speed – The speed can be set to the needs of production and the application.
3. Forming – Progressive forming can be accomplished with one die.
4. Designing – Engineers are able to see when fractures will occur and make the proper adjustments.
5. Space – The machines are small and take up less manufacturing floor space.

Mechanical Press

All mechanical presses produce force by stored energy from a flywheel. Punches can be 5 mm up to 500 mm at stroke speeds of 20 to 1500 spm. They are categorized by their type of drive, which can be single gear, double gear, double action, linked, or eccentric geared.

Energy from the flywheel is released using one of the drive types. When it makes a complete turn, it consumes energy, slowing it down by 10 to 15 percent at each turn. The consumed energy is restored by an electric motor.

Benefits:

1. Speed – They run at a higher production rate producing more parts per minute efficiently with superior quality.
2. Consistency – The tonnage at the bottom of a stroke is consistent.
3. Tonnage – They can vary in size from 20 tons to 1600 tons with the ability to supply substantial force.
   
   

   

   
   
4. Accuracy – Larger, more complex parts that are thinner and made of stronger material can be produced as well as complete assemblies.
5. Larger materials – The large bed size allows the processing of parts up to 24 feet.

Chapter Five – Choosing Metals for Die Stamping

There are a variety of factors to consider when choosing a metal for die stamping, which include its mechanical characteristics, lubricant, press speed and capacity, magnetic properties, and the type of steel used to make the die. Both ferrous and nonferrous metals are used in die stamping, with aluminum being the most used for its strength, weight, and corrosion resistance.

There are two major considerations that need to be examined when choosing a metal – ductility and tensile strength. Ductility is the crucial ability of a metal to be shaped and formed without cracking, tearing, or breaking. Tensile strength is the resistance of a metal to breaking under tension and pressure. Both factors, are the measures used to determine the feasibility of a metal for die stamping.

Tensile testing:

Tensile testing is the simplest means of determining how a sample will react when pulled apart: aka, determining its breaking point when external force is applied. The tests give designers and developers a material analyses report to predict how a metal will react in the intended application. The image below shows a diagram of the test. Tensile strength reports include a metals MPa, or megapascals. The MPa for 1090 mild steel is a yield strength of 247 and ultimate tensile strength of 841 with a density of 7.58, while aluminum has a MPa yield strength of 241 and ultimate tensile strength of 300 with a density of 2.7.

Benefits include:

• Achieving lean manufacturing
• The safety of materials, components, and products
• Providing design data
• Compliance with industry standards
• Product quality and consistency

Ductility testing:

Ductility refers to a metal's ability to change shape without breaking, which can be seen in the diagram below.

There are four factors that determine a metal's ductility: elongation percentage, tensile strength, yield strength, and hardness.

Elongation percentage:

Elongation percentage is a measure of how much a metal can be stretched inside a specified boundary, which is normally two inches. A metal with a 38% elongation will stretch 38% of its length before it fractures when stretched over two inches.

Tensile strength:

Tensile strength is the amount of stress a metal can withstand. The higher the tensile strength, the more stress it will be able to handle.

Yield strength:

This is the measure of the amount of force necessary to shape and deform a metal. When a metal is deformed, it goes through two changes – elastic and plastic. Elastic deformation can happen when it bends under its own weight, while plastic deformation is when a metal is processed and permanently changed.

Hardness:

The hardness of a metal is expressed using the Rockwell hardness scale. It is a measure of a metal's penetrability, which is tested by applying weight until the metal is penetrated.

Metals Used in Die Stamping

Any type of metal can be used in the stamping process, all of which are either ferrous or nonferrous. Ferrous metals contain iron while nonferrous metals do not. Steel is the perfect example of a ferrous metal since it is made from iron ore. Aluminum has no iron and is made from raw aluminum. With a few exceptions, ferrous metals are magnetic, and nonferrous are not.

Since nonferrous metals do not contain iron, they are not susceptible to rust or oxidation. Nonferrous metals used in stamping are aluminum, bronze, brass, gold, silver, tin, and copper. Of the nonferrous metals, aluminum is the most used due to its strength, lightness, and resistance to corrosion.

Of the ferrous metals, steel is the most used in stamping due to its strength and durability.

Steel in Die Stamping

The main element of steel is carbon, which is an extremely hard and durable substance. The higher the carbon content of steel, the harder it will be. Stamped steel is highly desirable due to its longevity and durability. In order to increase its resilience, steel is normally alloyed to enhance its resistance to rust. The most common alloys for steel are chromium and nickel.

Stainless Steel in Die Stamping

Another form of steel for stamping is stainless steel, another ferrous metal. The combinations of alloys, mostly chromium and nickel, in stainless steel determines its grade. Each grade has properties and characteristics that make them ideal for a wide variety of applications. Stainless steel grade 316 is ideal for marine applications while stainless steel grade 404 is used for chemical and food processing.

Typical grades of stainless steel used for stamping are 301, 302, 304 & 304L, 316 & 316L, 321, 410, and 18-8.

Aluminum

Aluminum is a nonferrous metal that is used in stamping due to its lightweight, strength, and resistance to rust and corrosion. In most cases, aluminum is not used in its pure form but is alloyed with other methods to enhance its strength and to increase some of its other properties and characteristics.

The formability of aluminum makes it the perfect metal for stamping since it can be shaped and formed into any configuration.

Copper in Die Stamping

Copper, like aluminum, is a nonferrous metal that is easily formed and can quickly be shaped into one piece of seamless components. It is a low maintenance metal that is highly resistant to corrosion and has naturally hygienic properties for use in medical, food, and beverage production. Though pure copper is used in stamping, it is often alloyed to enhance its durability and strength. Its high ductility makes it an ideal metal for the stamping process.

Brass in Die Stamping

Brass is a copper alloy that is a combination of copper and zinc. The percentages of each metal determine the grade of brass and its ductility. Brass has a very smooth and silky surface that can be easily shaped, resists to corrosion, and has exceptional conductivity. Another factor related to the choice of brass is its appearance and excellent aesthetic value.

Of the various grades and types of brass, C26000 tends to be the most popular due to its exceptional resistance to corrosion. The hardness of brass is determined by the percentage of zinc it contains.

Specialty Metals in Die Stamping

There is a very broad spectrum of metals that fall into the category of specialty metals, which are designed to withstand extreme environmental conditions without corroding, degrading, or becoming brittle. Included in this group are various types of titanium and nickel based alloys. The diversity and range of these types of metals makes it difficult to describe their characteristics. They are engineered to fit the conditions for which they are being produced.

There are two commonalities that specialty methods share, which are corrosion and heat resistance. Part of the engineering of specialty metals includes the enhancement of the base metal's strength, durability, and resistance to impact and physical harm.

Chapter Seven – Microstamping

Microstamping is the production of parts, barely visible to the human eye, that have dimensions of a fraction of a millimeter. Micro-stamped parts require extremely precise technical processing with tight tolerances and exceptionally accurate dimensions. These miniature parts are pressure formed at microscopic sizes and contain even smaller components using line, transfer, or progressive die stamping techniques.

Microstamping compared to regular die stamping:

1. Process - Parts are formed in one stroke of the stamping press.
   
   

   

   
   
2. Technical requirements - Dies are specially designed for a single operation.
   
   

   

   
   
3. Cost – The technology and expertise to design dies costs between $5000 and $30,000. The more complex the design, the higher the cost.
4. Lead times – The complex nature of producing dies takes months to produce and configure.
   
   

   

   
   
5. Equipment – Presses and other equipment are the same as in regular die stamping.
   
   

   

   
   
6. Tolerances – Precision stamping produces tolerances of +/- .0005"
7. Metals – Beryllium copper, phosphor bronze, and brass. The tensile strength of metals has to be precision controlled to ensure quality and proper performance.
   
   

   

   
   

   

   
   

   

   
   
8. Dimensions – Dimensions within 5 mm, thicknesses of 0.1 mm, and diameters of 0.1 mm.

Microstamping products

The microstamping industry is constantly faced with new challenges to design and develop smaller and more precise parts. Listed below are some recent developments.

1. Rivetless Nutplate – Fastener for use in the aerospace industry.
   
   

   

   
   
2. Micro lumbar retractor – Micro Lumbar Discectomy at 1.57 in (40 mm).
   
   

   

   
   
3. Micro USB Breakout Board – Breakout board with USB Micro-B connector.
   
   

   

   
   
4. Ammunition Cartridges - Ammunition cartridges are normally made from brass. In an innovative development, ammunition cartridges are now being made from stainless steel, which makes the casings much lighter, offering a critical military advantage. The casings are tapered with a bottom shaped to form a rivet after which a primer base is attached. Using a specially proprietary process, the casings are formed under high pressure and come in caliber cartridges of 4.6, 5.56, and 7.62.

Chapter Eight – Simulation

One of the difficulties with the die stamping process is its rigidity. Once a die is cast or a product is made, there is little opportunity to reverse engineer or correct the process. New auto sim software allows designers to run a simulation in one continuous process to reduce iterations and validate designs before sending them on to manufacturing.

Reducing Flaws in Die Stamping:

Simulation software is programmed to calculate the steps in the die stamping process. It helps developers predict possible flaws and errors in designs such as those listed below.

1. Necking -

   Tensile failure that occurs by overstitching metal, creating a smile or elongation caused by stretching a metal to its max threshold.

   

   
   

2. Splits -

   a tear or rip caused by too much stretching; happens after necking.

   

   
   

3. Springback -

   a geometric change in a part at the end of the forming process. The effects of springback can be seen in the image below.

   

   
   

4. Cracking -

   a result of excessive cold working or strain hardening.

   

   
   

Benefits:

AutoForm and Stamping Simulation technology can predict and correct complex die stamping problems. The image below presents a solution to resolve a springback problem.

1. Examining the total process – Engineers can simulate the entire process, including each of the operations of drawing, flanging, or coining.
   
   

   

   
   
2. Tool design – Complete design and analysis of tools.
3. Repeatability – Once a design is made, engineers can refine and analyze it down to the finest detail.
4. Complete imaging – The software produces 2D and 3D images, multi-axis machining, CNC programming, and areas for maintenance.
5. Formed parts – The software provides an image of the completed part for close evaluation and determination of any flaws.
   
   

   

   
   

Conclusion

• Die stamping uses operations that include flanging, piercing, blanking, coining, and shearing.
• Die stamping is a method for cutting and forming metal into a specified shape.
• There are three types of die stamping production methods: line, transfer, and progressive. with progressive being the most used.
• Ferrous and nonferrous metals are used in die stamping. Metals should be tested for their ductility and tensile strength.
• The fastest-growing form of die stamping is microstamping, which produces miniature precision parts with exact tolerances.
• There are four types of die stamping machines: hydraulic, pneumatic, mechanical, and servomotor.

Leading Manufacturers and Suppliers