This is the best information for die stamping on the internet.
Here is what you will learn:
- What is die stamping?
- The types of die stamping (progressive vs. transfer)
- Metals used in die stamping
- And much more.
Chapter One – What is Die Stamping?
Die stamping is a cold forming process that takes a sheet of metal, referred to as 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 very 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:
- line: a single operation process
- transfer: stamping completes several operations in one cycle
- 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 doing 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:
Cutting dies are:
Below is a description of forming dies: Forming dies compress metals into specific shapes and are much like a stencil.
Bending creates shapes that are similar to a L, U, or V. It is a plasticizing deformation that stresses the yield level but below tensile strength over a single axis.
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 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 direction of drawing is critical since it affects how the part can move, be cut, and ejected.
A variation of drawing is deep drawing, which is non-directional meaning the direction can be up, down, or vertical.
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.
A pattern is produced by squeezing the workpiece under extreme pressure, which reduces the thickness of the metal.
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 is when a piece of metal is separated by applying force to cause the metal to fail and is referred to as shearing.
Blanking removes a portion of a metal sheet along a specific contour line or shape. In very simple terms, it is cutting away one part of a sheet from another. The cutaway part is the workpiece while the remainder is scrap as seen in this diagram.
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 is similar to blanking. The difference between the two is that the piece punched out in blanking is the actual 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.
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 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.
Chapter Three – Production Methods
When choosing a die stamping method, the factors that define each of the processes is cost, time, and required geometric tolerances. The three common types of production are line, transfer, and progressive, which are described below.
Line dies are used for low volume part production or 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.
- Faster production – Multiple cuts can be made from several dies.
- Positioning of blank – Loading and repositioning of the blank is easy. It can be turned, flipped, and shifted with little effort.
- Complex geometries – Produces complex geometries without the need of special calculations or adjustments.
- Handling of dies – Dies are lighter and less expensive to handle.
- Tooling – Tooling is smaller and conveniently accessible.
- Machine limitations – Not all presses have the capability of loading combination dies.
- Slow production – Unlike progressive die stamping, line die processing produces one part at time making it slower and time consuming.
- Turnaround times –Turnaround times and volume of production are very low and slow.
- Costs – Machines have to be maintained and controlled by a person, and several machines are needed to complete a process increasing labor costs.
Transfer dies use the same concept as line dies but have multiple dies that are timed together with an evenly spaced pitch, distance between dies, on a single press. Parts move between presses on side by side mounted rails. When one cycle is completed, the workpiece is electronically grabbed and transferred to the next die.
- Multiple motions – Two and three axis motions can be performed during a single cycle, which can be seen in the above diagram with the different colors representing each axis. Three axis motion lifts the workpiece for the next operation.
- Part placement – Part placement is automatic with the use of gauges or locators that position the part perfectly for each operation.
- Faster production – Large parts are rotated, turned, and positioned easily, while being moved rapidly from station to station.
- Computerization – Servo drive transfers can be added to program the types of parts, press speeds, and length of press strokes.
- Turnaround times – High volumes of parts are completed with less handling, lower waste, and decreased labor costs.
- 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.
- Cost – Expertise for planning and design is expensive and time consuming. In general, the overall process is more expensive than progressive stamping.
- Destacking – A specially designed destacking mechanism is necessary to control the flow of blanks and time their insertion.
- Process regulation – Production happens quickly making it impossible to check the status of dies. Die protection sensors are a necessity.
- Restrictions of process – In the two axis process, workpieces slide from die to die, which slows down production.
Progressive die stamping has several lined up dies, which 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 with an equal distance between them called the progression.
- 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.
- Labor – It operates automatically unattended or monitored.
- Equipment – One machine can produce all of the parts.
- Die configuration – All die stations are mounted on a single die. Parts are produced together in a single pressing.
- Faster – Progressive dies are faster and run on less expensive equipment.
- 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.
- Cost – It is more expensive than line or transfer die stampings. The calculations, multiple elements, and equipment are expensive and require significant expertise.
- Equipment costs – Equipment is very heavy and cumbersome.
- Maintenance - Damage to a single station requires the removal of the whole system and change over, which can lead to days of delay.
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
- To assist material flow
- Prevent rips, tears, and wrinkles
- Reduction of friction
Punching dies with force against a metal sheet creates friction that causes scratches, can burn the piece, and damage the die. A lubricant forms a layer on the metal workpiece to protect it and reduces the damage to the die, decreasing defect rates.
The three methods for applying lubricant are dripping, spray, and roller.
The image below is for a coil lubricator that applies the lubricant prior to the metal coil entering the forming machine. It applies a controlled amount of lubricant using a reservoir and electric pump that is attached to the two rollers.
Manufacturers use lubricants made from plant, animal, and mineral oils as well as 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 sided 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.
As can be seen in this picture, C frame die stamping press is open on three sides and manually operated. The open design allows for deflections for off center loads.
Stamping Press Terminology
Stamping press manufacturers have their own language to describe the operation of their equipment, while individual companies may have proprietary terms. 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. The diagram below is a complete list of terms for a die stamping press.
- Capacity – the tonnage of pressure the slide can produce
- Continuous on Demand – the press runs in continuous mode
- Counterbalance "Counter Balance" – equalizes the weight of the upper slide
- Daylight – opening in a hydraulic press between the slide and bolster
- Die Blocks – are a safety measure inserted when working on the press
- Eccentric – used on an eccentric press to drive attachments
- Flywheel – provides rotational energy to prevent excessive or sudden speed changes
- Gibs – 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 Presses
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.
- Moving parts – Fewer moving parts enables pneumatic machines to reach full ram velocity quickly and require little maintenance.
- Precision – Ram pressure is uniform with low deflection. Since it reaches velocity rapidly, it has an increased flow rate.
- Fast stroke cycles – Stroke speeds can be as high as 400 strokes per minute (spm) without the need for extra framing.
- Automation – Can be fitted with robotics and special transfer units.
Hydraulic Stamping Presses
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.
- Weight of parts – Parts produced have a light weight structure with strong rigidity.
- Mold or die – Only one mold is needed to complete forming.
- Strength – Parts have increased fatigue resistant and exceptional strength.
- Cost – It is a cost effective method that significantly lowers the costs of individual parts compared to other stamping methods.
- 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 offering 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 adds significantly 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.
- 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.
- Speed – The speed can be set to the needs of production and the application.
- Forming – Progressive forming can be accomplished with one die.
- Designing – Engineers are able to see when fractures will occur and make the proper adjustments.
- Space – The machines are small and take up less manufacturing floor space.
Mechanical Stamping Presses
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.
- Speed – They run at a higher production rate producing more parts per minute efficiently with superior quality.
- Consistency – The tonnage at the bottom of a stroke is consistent.
- Tonnage – They can vary in size from 20 tons to 1600 tons with the ability to supply substantial force.
- Accuracy – Larger more complex parts that are thinner made of stronger material can be produced as well as complete assemblies.
- Larger materials – The large bed size allows the processing of parts up to 24 feet.
Chapter Five – Metals Used in Die Stamping
There are a variety of factors to consider when choosing a metal for die stamping, which include its mechanical characteristics, proper lubricant, press speed and capacity, its 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 ability of a metal to be shaped and formed without cracking, tearing, or breaking and is a major factor. Tensile strength is the resistance of a metal to breaking under tension and pressure. Both factors, ductility and tensile strength, are the measures used to determine the feasibility of a metal for die stamping.
Tensile testing is the simplest means of determining how a sample will react when pulled apart or 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 two forms of testing equipment and 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.
The benefits of testing metals:
- Achieving lean manufacturing
- The safety of materials, components, and products
- Provides design data
- Compliance with industry standards
- Product quality and consistency
Ductility refers to a metals ability to change shape without breaking, which can be seen in the diagram below.
There are four factors that determine a metals 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.
The hardness of a metal is expressed using the Rockwell hardness scale. It is a measure of a metals penetrability, which is tested by applying weight until the metal is penetrated.
Chapter Six – Microstampings
Microstamping is the production of parts that are barely visible to the human eye with dimensions that are 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 containing even smaller components using line, transfer, or progressive die stamping techniques.
Microstamping compared to regular die stamping:
- Process - Parts are formed in one stroke of the stamping press.
- Technical requirements - Dies are specially designed for a single operation
- Cost – The technology and expertise to design dies costs between $5000 and $30,000. The more complex the design the higher the cost.
- Lead times – The complex nature of producing dies takes months to produce and configure.
- Equipment – Presses and other equipment are the same as is used in the regular die stamping.
- Tolerances – Precision stamping produces tolerances of +/- .0005"
- Metals – Beryllium copper, phosphor bronze, and brass. The tensile strength of metals has to be precision controlled to ensure quality and proper performance.
- Dimensions – Dimensions within 5 mm, thicknesses of 0.1 mm, and diameters of 0.1 mm.
The microstamping industry is constantly faced with new challenges to design and develop smaller and more precision parts. Listed below are some recent developments.
- Rivetless nutplate – Fastener for use in the aerospace industry.
- Micro lumbar retractor – Micro Lumbar Discectomy at 40 mm or 1.57 in.
- Micro USB Breakout Board – Breakout board with USB Micro-B connector
Chapter Seven – 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.
Reduction of flaws:
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.
Tensile failure that occurs by overstitching metal creating a smile or elongation caused by stretching a metal to its max threshold.
Splits is a tear or rip caused by too much stretching and happens after necking.
Springback is a geometric change in a part at the end of the forming process. The effects of springback can be seen in the image below./
Cracking comes from excessive cold working or strain hardening, which can be seen in this coin.
AutoForm and Stamping Simulation technology can predict and correct complex die stamping problems. The image below is from simulation software that presents a solution to resolve a springback problem.
- Examine the total process – Engineers can simulate the entire process including each of the operations such as drawing, flanging, or coining.
- Tool design – Complete design and analysis of tools.
- Repeatability – Once a design is made, engineers can refine and analyze it down to the finest detail.
- Complete imaging – The software produces 2D and 3D images, multi-axis machining, CNC programming, and areas for maintenance.
- Formed parts – The software provides an image of the completed part for close evaluation and determination of any flaws.
- 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 into die stamping. Metals should be tested for their ductility and tensile strength.
- The fastest growing form of die stamping is microstamping that produces miniature precision parts with exact tolerances.
- There are four types of die stamping machines: hydraulic, pneumatic, mechanical, and servomotor.