All About Die Cutting Machines

Die cutting machines have a number of applications in the metalworking industry, such as forming, cutting and shaping metals. Using templates, or molds, the die is customized with the final product in mind. There are several traditional varieties of die cutters, including rotary, press, and flat bed die cutting machines, although newer forms have been gaining ground within the industry.

Rotary Die Cutting

Rotary die cutting machines feature a cylindrical anvil and die fabricated from a single piece of tool steel. As material is fed through the machine, a series of quick and accurate cuts modify the metal. The process is best suited for high volume projects or "kiss cutting,” in which punctures must be made through the material without piercing the fabrication liner.

In high volume production, Rotary die cutting machines can increase productivity while reducing

material waste. The machinery is well-suited for precision cutting at low tolerances and can also be used in conjunction with other processes, such as laminating and coating.

Press Die Cutting

Press die cutting machines range in size from compact personal models to large machine shop versions. They typically feature a cutting die that is raised and lowered upon the forming metal, which is supported by a flat table. The die's cutting action can be controlled by electric, hydraulic, pressurized, or manual sources. Depending upon the application, the die may cut and form a single piece of material or the material may be stacked to produce multiple copies at a time. 

Flatbed Die Cutting

A flatbed cutting machine uses varying degrees of hydraulic pressure to stamp shapes with a steel rule die. These machines are effective for making precision kiss-cuts, butt-cuts, and die-cuts to sheets and laminates.

The benefits of flatbed die cutting include lower tooling costs, greater tolerance on materials over 1/8 inch thickness, and high design flexibility. Flatbed die cutting machines are efficient for low volume orders, projects involving many different 

kinds of shapes, or applications in which no material curvature is needed.

Laser Die Cutting

Laser die cutting applies a non-thermal, fully focused beam to force material into custom shapes and sizes. The laser is typically computer-controlled and follows a pre-set CAD generated design, enabling the production of a large volume of uniform parts.

Laser die cutting is valuable for projects requiring accuracy and speed. It is also useful in creating quick initial prototypes and shaping tougher materials that would otherwise be unmanageable. It has a rapid turnaround time, and is well-suited for short run/high volume production.

Water Jet Die Cutting

Water jet die cutters fire highly pressurized streams of water that can move at almost two and a half times the speed of sound. The stream is released through a tiny opening (usually about 0.003 inches in diameter), but is able to cut through a wide variety of material.

Water jet die cutting is a relatively versatile and high precision cold cutting process. Since it uses a non-dulling cutter, maintenance costs are kept low. In addition, it produces few hazardous byproducts.

How to Metalize Plastic

Plastic parts can be coated with metal, a process called metallization, for both aesthetic and mechanical purposes. Visually, a metal coated piece of plastic features increased gloss and reflectivity. Other properties, such as abrasion resistance and electric conductivity, which are not innate characteristics of plastic, are often obtained through metallization. Metalized plastic components are used in similar applications as metal plated parts, but tend to be lower in weight and have higher corrosion resistance, although not in all cases. In addition, electrical conductivity can be controlled in metalized plastic components, and they are inexpensive to manufacture. To metalize a piece of plastic, several common methods are used: vacuum metallization, arc and flame spraying, or plating. It is also possible to metalize a transfer film, and use alternative methods to apply the film to the surface of the substrate.

Metallization Processes

Vacuum Metallization

Before the process can begin, the plastic component is washed and coated with a base coat, so that the metal layer is smooth and uniform. Next, a metal (typically aluminum) is evaporated in a vacuum chamber. The vapor then condenses onto the surface of the substrate, leaving a thin layer of metal coating. The entire process takes place within a vacuum chamber to prevent oxidation. This deposition process is also commonly called physical vapor deposition. Depending on the component’s application, a top coat may be applied after deposition to increase properties such as abrasion resistance. Metalized plastic components that receive their coats via this process are found in a range of applications, from automotive interior parts to certain types of foils.

Arc and Flame Spraying

In basic flame spraying, a hand-held device is used to spray a layer of metallic coating on the substrate. In flame spraying, the primary force behind deposition is a combustion flame, driven by oxygen and gas. Metallic powder is heated and melted, as a combustion flame accelerates the mixture and releases it as spray. This process has a high deposition rate and creates very thick layers, but the coatings tend to be porous and somewhat rough. Due to the nature of the application process, coatings can be applied to specific areas of components, which is useful when working with complex or unusually shaped components. The process is relatively easy and requires minimal training.

Arc spraying is similar to flame spraying, but the power source is different. Instead of depending on a combustion flame, arc spraying derives its energy from an electric arc. Two wires, composed of the metallic coating material and carrying DC electric current, touch together at their tips—the energy that releases when the two wires touch heats and melts the wire, while a stream of gas deposits the molten metal onto the surface of the substrate, creating a metal coat. Like flame spraying, the resulting coating typically suffers from high porosity.

Electroless Plating and Electroplating

Plating is typically divided into two categories, depending on the presence of electric current. In electroless plating, electric current is not used; in electroplating, electric current is used. Both processes tend to be more effective than vacuum metallization at producing metallic coats with strong adhesion, although plating tends to be more dangerous.

Electroless plating is often used to deposit nickel or copper metal onto plastic substrates. First, the surface of the plastic is etched away using an oxidizing solution. Because the surface becomes extremely susceptible to hydrogen bonding as a result of the oxidizing solution, typically increases during coating application. Coating occurs when the plastic component (post-etching) is immersed in a solution containing metallic (nickel or copper) ions, which then bond to the plastic surface as a metallic coating.

In order for electroplating (or electrolytic plating) to be successful, the plastic surface must first be rendered conductive, which can be achieved through basic electroless plating. Once the plastic surface is conductive, the substrate is immersed in a solution. In the solution are metallic salts, connected to a positive source of current (cathode). An anodic (negatively charged) conductor is also placed in the bath, which creates an electrical circuit in conjunction with the positively charged salts. The metallic salts are electrically attracted to the substrate, where they create a metallic coat. As this process happens, the anodic conductor, typically made of the same type of metal as the metallic salts, dissolves into the solution and replaces the source of metallic salts, which is depleted during deposition.

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How To Prevent Wrinkling During Deep Drawing

In the deep drawing process, a punch pushes a sheet metal blank into a die cavity, resulting in a contoured part. A part is said to be deep-drawn if the depth of the part is at least half of its diameter. Otherwise, it is simply called general stamping.

Deep draw stamping is a widely used process that produces a range of household items, such as soup cans, battery casings, fire extinguishers, and even the kitchen sink. A deep draw process may have one or more drawing operations, depending on the complexity of the part. 

                                       

 

Wrinkling and Deep Drawing Operations

One of the primary defects that occurs in deep drawing operations is the wrinkling of sheet metal material, generally in the wall or flange of the part. The flange of the blank undergoes radial drawing stress and tangential compressive stress during the stamping process, which sometimes results in wrinkles. Wrinkling is preventable if the deep drawing system and stamped part are designed properly.

 

 

 

 

Causes Of Wrinkling In Deep Drawn Parts

Several factors can cause wrinkles in deep drawn parts, including: 

• Blank holder pressure

• Die cavity depth and radius

• Friction between the blank, blank holder, punch and die cavity

• Clearances between the blank, blank holder, punch and die cavity

• Blank shape and thickness

• Final part geometry

• Punch speed

Other factors, such as die temperature and the metal alloy of the blank, can also affect the drawing process. A variation in any of these factors influences the potential for wrinkling or cracking in the deep-drawn part.

The blank holder, as the name implies, holds the edges of the sheet metal blank in place against the top of the die while the punch forces the sheet metal into the die cavity—the sheet metal deforms into the proper shape, instead of simply being pulled into the die cavity.

The blank holder, however, does not hold the edges of the blank rigidly in place. If this were the case, tearing could occur in the cup wall. The blank holder allows the blank to slide somewhat by providing frictional force between the blank holder and the blank itself. Blank holder force can be applied hydraulically with pressure feedback, by using an air or nitrogen cushion, or a numerically controlled hydraulic cushion.

The greater the die cavity depth, the more blank material has to be pulled down into the die cavity and the greater the risk of wrinkling in the walls and flange of the part. The maximum die cavity depth is a balance between the onset of wrinkling and the onset of fracture, neither of which is desirable.

The radii degrees of the punch and die cavity edges control the flow of blank material into the die cavity. Wrinkling in the cup wall can occur if the radii of the punch and die cavity edges are too large. If the radii are too small, the blank is prone to tearing because of the high stresses.

Methods for Preventing Wrinkling in Deep Drawn Parts: Using a Blank Holder

The simplest method for eliminating wrinkling in deep-drawn parts is using a blank holder. In most deep drawing processes, a constant blank holder pressure is applied throughout the entire drawing action.

Variable blank holder pressure, however, has been employed with some success. A pneumatic or hydraulic blank holder cushion can vary the blank holder pressure linearly over the stroke of the machine. This provides some increase in the allowable die cavity depth.

A numerically controlled (NC) die cushion can be used to provide a variable blank holder pressure over the course of drawing action. In an optimal blank holder pressure force profile, the initial force is large so as to provide initial deformations.

The cushion drops off to pull material into the die cavity, and then slowly increases back up to ensure strain hardening in the drawn part. An NC die cushion can dramatically increase the allowable die cavity depth while preventing both wrinkling and cracking.

Methods for Preventing Wrinkling in Deep Drawn Parts: Die Cavity Design

The design of the punch and die cavity can be optimized to reduce the probability of wrinkling. Choosing a flange radius that is just large enough to prevent cracking can minimize the potential for wrinkles. Additionally, considering minimizing the part complexity and any asymmetry can also help. Incorporating a multi-step drawing process offers a variety of advantages in preventing wrinkling in deep-drawn parts.

Designing the blank geometry to minimize excess material can reduce the potential for wrinkling. The sheet metal blank has an inherent grain structure, so the stresses can vary depending on the design of the die and the orientation of the grain. Adjusting the grain in an asymmetrical design to minimize the compound of grain stresses and the general stresses of the deep draw process is something to take into consideration.

Other Factors To Consider 

Surface conditions of each component can be tailored to improve overall performance. Lubricants reduce the friction between the blank and the punch and die cavity and can be liquid (wet) or films (dry). Generally, they are applied to the blank before drawing.

Today, dry films are gaining acceptance because they reduce the need for part washing after fabrication. While lubricants can facilitate the metal flow into the die cavity, consider increasing the blank holding force to account for the reduced friction.

In the past, trial and error and operator experience optimized part and die design. Today, computer aided design and finite element modeling are used to create part and die designs and to simulate the deep drawing process, significantly reducing the costs of tooling and labor in the design process.

Shear (sheet metal)

Types 

Alligator shear 

Bench shear

A bench shear

Shearing machine can cut flat bar up to 6mm.

A bench shear, also known as a lever shear, is a bench mounted shear with a compound mechanism to increase the mechanical advantage. It is usually used for cutting rough shapes out of medium sized pieces of sheet metal, but cannot do delicate work. For the small shear, it mostly designed for a wide field of applications. Light weight and easy efficient operation, yet very sturdy in construction. The cutting blades fitted are carefully and accurately ground to give easy, clean quick cuts, and free of burrs. These special features help the operators save a great deal of their energy. But some shearing machines can cut sheet bar and flat bar up to 10mm. It is electrically welded together to make it a sturdy stable unit capable to withstand highest stresses due to heavy duty usage. The footplates are reinforced with bracing angles so that they give firm stability to the shear. The machine is provided with section knives with sliding blades which can be adjusted by hand to make 90 cuts on angels and T-sections of different sizes as well as with openings for cutting round and square bars.

 Guillotine

The machine used is called a squaring shear, power shear, or guillotine. The machine may be foot powered (or less commonly hand powered), or mechanically powered. It works by first clamping the material with a ram. A moving blade then comes down across a fixed blade to shear the material. For larger shears the moving blade may be set on an angle or "rocked" in order to shear the material progressively from one side to the other; this angle is referred to as the shear angle. This decreases the amount of force required, but increases the stroke. A 5 degree shear angle decreases the force by about 20%. The amount of energy used is still the same. The moving blade may also be inclined 0.5 to 2.5°, this angle is called the rake angle, to keep the material from becoming wedged between the blades, however it compromises the squareness of the edge. As far as equipment is concerned, the machine consists of a shear table, work-holding device, upper and lower blades, and a gauging device. The shear table is the part of the machinery that the workpiece rests on while being sheared. The work-holding device is used to hold the workpiece in place and keep it from moving or buckling while under stress. The upper and lower blades are the piece of machinery that actually do the cutting, while the gauging device is used to ensure that the workpiece is being cut where it is supposed to be.

The design of press tools is an engineering compromise. A sharp edge, strength and durability are ideal, however a sharp edge is not very strong or durable so blades for metal work tend to be square-edged rather than knife-edged. Typical workpiece materials include aluminum, brass, bronze, and mild steel because of their outstanding shearability ratings, however, stainless steel is not used as much due to its tendencies to work-harden.

Other types of geometrical possibilities include the squaring shear, angle shear, bow-tie shear and bar shear. All of these have many different uses and are all used regularly in certain manufacturing fields.

Power shears

A power shear is electrically or pneumatically powered hand tool designed to blank large pieces of sheet metal. They are designed to cut straight lines and relatively large radius curves. They are advantageous over a bandsaw because there is not a size limit. Large versions can cut sheet metal up to 12 gauge.

An alternative to the hand tools are hydraulically powered tools attached to heavy machinery. They are usually used to cut materials in situ that are too bulky to be transported to a cutting facility, too big or dangerous for the hand tools and are stored at remote locations (e.g. mines, forests).

Throatless shear

Closeup of shear jaws

Throatless Shear

A throatless shear is a cutting tool used to make complex straight and curved cuts in sheet metal. The throatless shear takes its name from the fact that the metal can be freely moved around the cutting blade (it does not have a throat down which metal must be fed), allowing great flexibility in shapes that can be cut.

Shearing (manufacturing)

Shearing, also known as die cutting, is a process which cuts stock without the formation of chips or the use of burning or melting. Strictly speaking, if the cutting blades are straight the process is called shearing; if the cutting blades are curved then they are shearing-type operations.The most commonly sheared materials are in the form of sheet metal or plates, however rods can also be sheared. Shearing-type operations include: blanking, piercing, roll slitting, and trimming. It is used in metalworking and also with paper and plastics.

Principle 

A punch (or moving blade) is used to push the workpiece against the die (or fixed blade), which is fixed. Usually the clearance between the two is 5 to 10% of the thickness of the material, but dependent on the material. Clearance is defined as the separation between the blades, measured at the point where the cutting action takes place and perpendicular to the direction of blade movement. It affects the finish of the cut (burr) and the machine's power consumption. This causes the material to experience highly localized shear stresses between the punch and die. The material will then fail when the punch has moved 15 to 60% the thickness of the material, because the shear stresses are greater than the shear strength of the material and the remainder of the material is torn. Two distinct sections can be seen on a sheared workpiece, the first part being plastic deformation and the second being fractured. Because of normal inhomogeneities in materials and inconsistencies in clearance between the punch and die, the shearing action does not occur in a uniform manner. The fracture will begin at the weakest point and progress to the next weakest point until the entire workpiece has been sheared; this is what causes the rough edge. The rough edge can be reduced if the workpiece is clamped from the top with a die cushion. Above a certain pressure the fracture zone can be completely eliminated. However, the sheared edge of the workpiece will usually experience workhardening and cracking. If the workpiece has too much clearance, then it may experience roll-over or heavy burring.

Straight shearing 

The processes of straight shearing is done on sheet metal, coils, and plates. It uses a guillotine shear.

Tool materials 

Low alloy steel is used in low production of materials that range up to 1/4 in. thick

High-carbon, high chromium steel is used in high production of materials that also range up to 1/4 in. in thickness

Shock-resistant steel is used in materials that are equal to 1/4 in. thick or more

Tolerances and surface finish 

When shearing a sheet, the typical tolerance is +0.1 or -0.1, but it is feasible to get the tolerance to within +0.005 or -0.005. While shearing a bar and angle, the typical tolerance is +0.06 or -0.06, but it is possible to get the tolerance to +0.03 or -0.03. Surface finishes typically occur within the 250 to 1000 microinches range, but can range from 125 to 2000 microinches. A secondary operation is required if one wants better surfaces than this.

Stamping (Metalworking)

Stamping (also known as pressing) includes a variety of sheet-metal forming manufacturing processes, such as punching using a machine press or stamping press, blanking, embossing, bending, flanging, and coining.This could be a single stage operation where every stroke of the press produces the desired form on the sheet metal part, or could occur through a series of stages. The process is usually carried out on sheet metal, but can also be used on other materials, such as polystyrene.

Operations

Bending

Blanking

Coining

Drawing

Deep drawing

Repoussé and chasing (embossing)

Forming

Piercing

Progressive stamping

Simulation

Stamping simulation is a technology that calculates the process of sheet metal stamping, predicting common defects such as splits, wrinkles, springback and material thinning. Also known as forming simulation, the technology is a specific application of non-linear finite element analysis. The technology has many benefits in the manufacturing industry, especially the automotive industry, where lead time to market, cost and lean manufacturing are critical to the success of a company.

Recent research by the Aberdeen research company (October 2006) found that the most effective manufacturers spend more time simulating upfront and reap the rewards towards the end of their projects.

Stamping simulation is used when a sheet metal part designer or toolmaker desires to assess the likelihood of successfully manufacturing a sheet metal part, without the expense of making a physical tool. Stamping simulation allows any sheet metal part forming process to be simulated in the virtual environment of a PC for a fraction of the expense of a physical tryout.

Results from a stamping simulation allow sheet metal part designers to assess alternative designs very quickly to optimize their part for low cost manufacture.