Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress, evolved with high speed rotary functionality, which can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid colour).
In 1890 the first such patented press was built in Liverpool, England by Bibby, Baron and Sons. The water-based ink smeared easily, leading the device to be known as "Bibby's Folly". In the early 1900s, other European presses using rubber printing plates and aniline oil-based ink were developed. This led to the process being called "aniline printing". By the 1920s, most presses were made in Germany, where the process was called "gummidruck", or rubber printing. In modern-day Germany, the process continues to be called "gummidruck".
During the early part of the 20th century, the technique was used extensively in food packaging in the United States. However, in the 1940s, the Food and Drug Administration classified aniline dyes as unsuitable for food packaging and as a result, printing sales plummeted. Individual firms tried using new names for the process, such as "Lustro Printing" and "Transglo Printing", but met with limited success. Even after the Food and Drug Administration approved the aniline process in 1949 using new, safe inks, sales continued to decline as some food manufacturers still refused to consider aniline printing. Worried about the image of the industry, packaging representatives decided the process needed to be renamed.
In 1951 Franklin Moss, then the president of the Mosstype Corporation, conducted a poll among the readers of his journal The Mosstyper to submit new names for the printing process. Over 200 names were submitted, and a subcommittee of the Packaging Institute's Printed Packaging Committee narrowed the selection to three possibilities: "permatone process", "rotopake process", and "flexographic process". Postal ballots from readers of The Mosstyper overwhelmingly chose the last of these, and "flexographic process" was chosen.[1]
Evolution
Originally, flexographic printing was rudimentary in quality. Labels requiring high quality have generally been printed using the offset process until recently. Since 1990,[2] great advances have been made to the quality of flexographic printing presses, printing plates, ink systems and printing inks.
The greatest advances in flexographic printing have been in the area of photopolymer printing plates, including improvements to the plate material and the method of plate creation.
Digital direct to plate systems have seen a good improvement in the industry recently. Companies like DuPont, Kodak, XSYS, and Esko have pioneered the latest technologies, with advances in fast washout and the latest screening technology.
Laser-etched ceramic anilox rolls along with chambered ink systems, have also played a part in the improvement of print quality. Full-color picture printing is now possible, and some of the finer presses available today, in combination with a skilled operator, allow quality that rivals the lithographic process. One ongoing improvement has been the increasing ability to reproduce highlight tonal values, thereby providing a workaround for the very high dot gain associated with flexographic printing.
Process overview
1. Platemaking[3] The first method of plate development uses light-sensitive polymer. A film negative is placed over the plate, which is exposed to ultra-violet light. The polymer hardens where light passes through the film. The remaining polymer has the consistency of chewed gum. It is washed away in a tank of either water or solvent. Brushes scrub the plate to facilitate the "washout" process. The process can differ depending on whether solid sheets of photopolymer or liquid photopolymer are used, but the principle is still the same. The plate to be washed out is fixed in the orbital washout unit on a sticky base plate. The plate is washed out in a mixture of water and 1% dishwasher soap, at a temperature of approximately 40°C. The unit is equipped with a dual membrane filter. With this the environmental burdening is kept to an absolute minimum. The membrane unit separates photopolymer from the washout water. After addition of absorb gelatine for example, the photopolymer residue can be disposed of as standard solid waste together with household refuse. The recycled water is re-used without adding any detergent.[4]
Flexographic printing press
The second method uses a computer-guided laser to etch the image onto the printing plate. Such a direct laser engraving process is called digital platemaking. Companies such as AV Flexologic, Glunz & Jensen, XSYS, Esko, Kodak, Polymount, Screen and SPGPrints from The Netherlands are market leaders in manufacturing this type of equipment.
The third method is to go through a molding process. The first step is to create a metal plate out of the negative of our initial image through an exposition process (followed by an acid bath). In the early days the metal used was zinc, leading to the name 'zincos'. Later, magnesium was used. This metal plate in relief is then used in the second step to create the mold that could be in bakelite board or even glass or plastic, through a first molding process. Once cooled, this master mold will press the rubber or plastic compound (under both controlled temperature and pressure) through a second molding process to create the printing plate or cliche.
2. Mounting For every colour to be printed, a plate is made and eventually put on a cylinder which is placed in the printing press. To make a complete picture, regardless of printing on flexible film or corrugated paper, the image transferred from each plate has to register exactly with the images transferred from the other colors. To ensure an accurate picture is made, mounting marks are made on the flexographic plates. These mounting marks can be microdots (down to 0.3mm) and/or crosses. Special machinery is made for mounting these plates on the printing cylinders to maintain registration. Earle L. Harley invented and patent the Opti-Chek Mounting and Proofing machine enabling the operator to check the registration before going to the press.
Precision mounting of flexo plates on tape with the aid of laser pointers on automatic mounting machine
Precision mounting is critical for producing good quality print which is in the register and has a direct impact on minimizing waste. The mounting process needs to deliver the same accurate result each time a job is mounted, consistent accuracy is the target. For simplicity we'll refer to print sleeves throughout this module but you can substitute cylinders if these are used in your operation.
Generally plates are mounted directly onto the print sleeve but for corrugated operations, plates are mounted onto a carrier sheet that is fixed to the print sleeve in the press when required, it is removed and kept in storage between print runs. You'll find out about carriers in the corrugated section of this module.
There are two key areas to achieve effective plate mounting, positioning the plate correctly and achieving a good bond.
Positioning is achieved by correctly lining up register marks that are common to each plate in the set. The skill is to plan carefully exactly where these marks should be. A variety of marks can be used, register crosses and micro dots. A good bond is achieved by means of special mounting tape. Accurate positioning is essential or the images of each colour will not be superimposed correctly, they will be out of register.
Types of registration marks
These are various types of mark used for aligning the plates correctly:
Registration crosses are commonly used but need to be positioned in waste areas as they are easily seen on the print, they can also be used as a guide for aligning the print to a bag or box structure if this is required.
Micro dots as the name suggests are tiny dots on the plate, usually about a quarter of a millimeter across in labels and flexible packaging but 1mm diameter in corrugated. Since they are so small they don't have to be in waste areas as they are not easily seen.
On most mounting machines the dots or crosses on the plates are lined up using magnifying cameras, the higher the magnification the greater the accuracy.
Register crosses are more common in label and corrugated printing where waste and hidden folds are normal, the microdot is common in flexible packaging where waste is kept to a minimum and no un-required marks are to be seen on the packaging (example meat, dairy and hygiene).
The location of registration marks, whether crosses or microdots, is essential to the success of plate mounting. If they are incorrect, mounting can be difficult, time-consuming and inaccurate, so they need to be planned carefully. The marks should be positioned symmetrically
Always have a pair positioned in the middle of the plate in line with the sleeve axis. It is also recommended to have two more pairs, one at either end so a simple rotation and check under the cameras confirms the plate has not twisted when it was laid/stuck down.
Register Mark Mounting
The most common form of mounting is register mark - also known as video mounting. When the print is run the register marks should print on top of each other indicating that the plates are correctly aligned. The register marks on the plate are lined up using magnifying cameras.
To align the plates accurately a mounting system using video cameras is required. Each print sleeve is transferred to the mounting system in turn.
Each sleeve is fixed in the mounter by a clamping system and mounting tape is then applied to the sleeve.
The high magnification video cameras (positioned on a precision machined camera beam) are moved into the required position for the plate to be mounted, accurate measurement of this setting is critical. The plate is then adhered to the sleeve using a mounting tape (see item 5) and the complete unit removed from the mounting machine.
The next sleeve is loaded and the plate mounted in position by positioning the register marks on the plate based on the previously fixed camera positions. This ensures that every plate is mounted in the same position and thus the print is in register.
The same principle is applied for multiple plates across the sleeve and so either two cameras per plate are used or two cameras are used which move into the correct position driven by servo motors and camera setting software. To step plates around the sleeve to minimise risk of bounce is done either mechanically using an index disc or by use of stepper motors to drive and lock it in position.
Each plate is mounted on a double sided adhesive tape - there are many types of mounting tape and it is important to use the correct thickness and hardness. Adhesive type also needs to be correct for your mounting process (see Tape Features)
The tape is applied to the sleeve being careful to lay it on without trapping air underneath it (must be flat to sleeve). A small strip of the tape liner is removed to leave the adhesive exposed initially.
The plate is carefully positioned over tape, usually by hand so that the register marks are seen directly below the cameras.
The cameras provide an enlarged visual display indicating whether or not the register marks are correctly positioned in line with crosshair targets. If necessary the plate position is adjusted.
Automatic Flexo Plate Mounter used for accuracy down to 5 microns without operator dependency
Once the plate is accurately aligned, it's pressed onto the strip of exposed mounting tape. The rest of the tape liner is then removed or a plate support table is moved so that the remainder of the plate can be laid onto the sleeve. This is done with each sleeve in turn so that all the plates register correctly.
Flexo plate mounting equipment includes many options to increase efficiency. These include tables to lay the plate on to make maneuvering the plate into position simpler, lay down (pressure) rollers to eliminate air bubble inclusions, tape application options, plate and tape cutting options and moving cameras, replacing multiple camera mounting systems.[5]
In recent years higher quality demands from customers, shorter and more frequent job runs result in an increasing relative cost of the prepress department.[5]
To counter this, automatic mounting provides up to 10x faster plate mounting than traditional plate mounting, no operator dependency and the highest possible accuracy and consistency down to 5 micron (0.0002 inch) per plate.[5]
3. Printing A flexographic print is made by creating a positive mirrored master of the required image as a 3Drelief in a rubber or polymer material. Flexographic plates can be created with analog and digital platemaking processes. The image areas are raised above the non image areas on the rubber or polymer plate. The ink is transferred from the ink roll which is partially immersed in the ink tank. Then it transfers to the anilox or ceramic roll (or meter roll) whose texture holds a specific amount of ink since it is covered with thousands of small wells or cups that enable it to meter ink to the printing plate in a uniform thickness evenly and quickly (the number of cells per linear inch can vary according to the type of print job and the quality required).[6] To avoid getting a final product with a smudgy or lumpy look, it must be ensured that the amount of ink on the printing plate is not excessive. This is achieved by using a scraper, called a doctor blade. The doctor blade removes excess ink from the anilox roller before inking the printing plate. The substrate is finally sandwiched between the plate and the impression cylinder to transfer the image.[7] The sheet is then fed through a dryer, which allows the inks to dry before the surface is touched again. If a UV-curing ink is used, the sheet does not have to be dried, but the ink is cured by UV rays instead.
Basic parts of the press
Unwind and infeed section – The roll of stock must be held under control so the web can unwind as needed.
Printing section – Single color station including the fountain or ink chamber, anilox, plate and impression rolls.
Drying station – High velocity heated air, specially formulated inks and an after-dryer can be used.
Outfeed and rewind section – Similar to the unwind segment, keeps web tension controlled.
Operation
Operational overview
Diagram of a typical flexographic printing apparatus, illustrating the general operating principle.
1. Fountain roller The fountain roller transfers ink located in an ink pan to a second roller, an anilox roller. In modern flexographic printing, the anilox roll is referred to as a type of meter or metering roller.
2. Anilox roller The anilox roll is a unique characteristic of flexography. The anilox roller transfers a uniform thickness of ink to a flexible printing plate. The anilox roll has finely engraved cells with a particular ink capacity, viewable with a microscope. These rollers are responsible for transferring inks to the flexible printing plates mounted on the plate cylinders.
3. Doctor blade (optional) An optional doctor blade scrapes the anilox roll to ensure that the ink to be delivered to the flexible printing plate is only what is contained within the engraved cells. Doctor blades had predominantly been made of steel, but advanced doctor blades are now made of polymer materials with several different types of beveled edges.
4. Plate cylinder The plate cylinder holds the printing plate, which is made from a soft flexible rubber-like material. Tape, magnets, tension straps and/or ratchets hold the printing plate against the plate cylinder.
5. Impression cylinder The impression cylinder applies pressure to the plate cylinder where the image is transferred to the image-receiving substrate. This impression cylinder or "print anvil" is required to apply pressure to the plate cylinder.
The nature and demands of the printing process and the application of the printed product determine the fundamental properties required of flexographic inks. Measuring the physical properties of inks and understanding how these are affected by the choice of ingredients is a large part of ink technology. Formulation of inks requires a detailed knowledge of the physical and chemical properties of the raw materials composing the inks, and how these ingredients affect or react with each other as well as with the environment. Flexographic printing inks are primarily formulated to remain compatible with the wide variety of substrates used in the process. Each formulation component individually fulfills a special function and the proportion and composition will vary according to the substrate.
There are five types of inks that can be used in flexography:[8]
solvent-based inks
water-based inks
electron beam (EB) curing inks
ultraviolet (UV) curing inks
two-part chemically-curing inks (usually based on polyurethaneisocyanate reactions) - although these are uncommon at the moment
Water based flexo inks with particle sizes below 5µm may cause problems when deinking recycled paper.
Ink controls
The ink is controlled in the flexographic printing process by the ink system. The ink system contains an ink pump, anilox roll and either a fountain roll system or doctor blade system. The fountain roll or two-roll system has one roll spinning in an ink pan pressed against the anilox roll to transfer a layer of ink to then be applied to the printing plate. This system is best used for low quality print such as flood coats and block lettering due to its inability to produce a clean wipe of the anilox roll. The doctor blade system can either be an open single blade system or an enclosed dual blade system. The single blade system uses an open ink pan with a roller that is then sheared with one doctor blade to create a uniform layer of ink to be distributed. The remaining ink sheared from the anilox roll will collect in the ink pan to then be pumped back into the system. The cylinder plate, anilox, and doctor blade are independently controlled by hydraulic, pressure and/or pneumatic systems. This system is best used for low to mid quality print work - usually found in corrugated box printing. The dual blade system is an enclosed system that has one doctor blade for doctoring the ink and one containment blade that contains the ink in the chamber and allows ink from the anilox roll back in. Dual blade systems require 2 end seals and adequate chamber pressure in order to maintain the tight seal between the ink chamber and the anilox roll. This system is best used for high quality, intricate print designs, like those found in the label industry.
Applications
Flexo has an advantage over lithography in that it can use a wider range of inks, water based rather than oil based inks, and is good at printing on a variety of different materials like plastic, foil, acetate film, brown paper, and other materials used in packaging. Typical products printed using flexography include brown corrugated boxes, flexible packaging including retail and shopping bags, food and hygiene bags and sacks, milk and beverage cartons, flexible plastics, self-adhesive labels, disposable cups and containers, envelopes and wallpaper. In recent years there has also been a move towards laminates, where two or more materials are bonded together to produce new material with different properties than either of the originals. A number of newspapers now eschew the more common offset lithography process in favour of flexo. Flexographic inks, like those used in gravure and unlike those used in lithography, generally have a low viscosity. This enables faster drying and, as a result, faster production, which results in lower costs.
Printing press speeds of up to 750 meters per minute (2000 feet per minute) are achievable now with modern technology high-end printers. Flexo printing is widely used in the converting industry for printing plastic materials for packaging and other end uses. For maximum efficiency, the flexo presses produce large rolls of material that are then slit down to their finished size on slitting machines.
Related Research Articles
Lithography is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone or a metal plate with a smooth surface. It was invented in 1796 by the German author and actor Alois Senefelder and was initially used mostly for musical scores and maps. Lithography can be used to print text or images onto paper or other suitable material. A lithograph is something printed by lithography, but this term is only used for fine art prints and some other, mostly older, types of printed matter, not for those made by modern commercial lithography.
Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The earliest known form of printing evolved from ink rubbings made on paper or cloth from texts on stone tablets, used during the sixth century. Printing by pressing an inked image onto paper appeared later that century. Later developments in printing technology include the movable type invented by Bi Sheng around 1040 AD and the printing press invented by Johannes Gutenberg in the 15th century. The technology of printing played a key role in the development of the Renaissance and the Scientific Revolution and laid the material basis for the modern knowledge-based economy and the spread of learning to the masses.
Corrugated fiberboard or corrugated cardboard is a type of packaging material consisting of a fluted corrugated sheet and one or two flat linerboards. It is made on "flute lamination machines" or "corrugators" and is used for making corrugated boxes. The corrugated medium sheet and the linerboard(s) are made of kraft containerboard, a paperboard material usually over 0.25 millimetres (0.01 in) thick.
In printing, anilox is a method used to provide a measured amount of ink to a flexographic (flexo) printing plate. An anilox roll is a hard cylinder, usually constructed of a steel or aluminum core which is coated by an industrial ceramic, typically chromium(III) oxide powder, whose surface is engraved with millions of very fine dimples, known as anilox cells. In the printing process, the anilox roll is coated in a precise layer of ink that is then transferred to the raised portions of the printing plate. The number, size, and geometry of the anilox cells vary and will determine the amount of ink that the anilox roll delivers to the plate.
A rotary printing press is a printing press in which the images to be printed are curved around a cylinder. Printing can be done on various substrates, including paper, cardboard, and plastic. Substrates can be sheet feed or unwound on a continuous roll through the press to be printed and further modified if required. Printing presses that use continuous rolls are sometimes referred to as "web presses".
Offset printing is a common printing technique in which the inked image is transferred from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier. Ink rollers transfer ink to the image areas of the image carrier, while a water roller applies a water-based film to the non-image areas.
Letterpress printing is a technique of relief printing for producing many copies by repeated direct impression of an inked, raised surface against individual sheets of paper or a continuous roll of paper. A worker composes and locks movable type into the "bed" or "chase" of a press, inks it, and presses paper against it to transfer the ink from the type, which creates an impression on the paper.
Rotogravure is a type of intaglio printing process, which involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, it uses a rotary printing press.
Photogravure is a process for printing photographs, also sometimes used for reproductive intaglio printmaking. It is a photo-mechanical process whereby a copper plate is grained and then coated with a light-sensitive gelatin tissue which had been exposed to a film positive, and then etched, resulting in a high quality intaglio plate that can reproduce detailed continuous tones of a photograph.
Laser engraving is the practice of using lasers to engrave an object. Laser marking, on the other hand, is a broader category of methods to leave marks on an object, which in some cases, also includes color change due to chemical/molecular alteration, charring, foaming, melting, ablation, and more. The technique does not involve the use of inks, nor does it involve tool bits which contact the engraving surface and wear out, giving it an advantage over alternative engraving or marking technologies where inks or bit heads have to be replaced regularly.
Textile printing is the process of applying color to fabric in definite patterns or designs. In properly printed fabrics the colour is bonded with the fibre, so as to resist washing and friction. Textile printing is related to dyeing but in dyeing properly the whole fabric is uniformly covered with one colour, whereas in printing one or more colours are applied to it in certain parts only, and in sharply defined patterns.
Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise difficult to print on products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as appliances, sports equipment and toys. It can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants.
Waterless printing is an offset lithographic printing process that eliminates the use of water or the dampening system used in conventional printing. Unlike traditional printing presses, waterless offset presses do not use a dampening solution to clear the press of ink. Proponents of the technology claim efficiency and ecological benefits.
In color printing, print registration is the layering of printed patterns to form a multicolor pattern. Registration error is the "position misalignment in the overlapped patterns." Machine components such as the print cylinder, doctor blade assembly, printing plates, stress/friction and more, affect the registration of the machine. Inconsistencies among these components can cause the printing press to fall out of registration; that is when press operators will begin to see defects in their print. There are many different ways to achieve proper registration, many of which employ the alignment of registration marks. Many press manufacturers have installed automatic register systems to assist the operator in getting the print back into proper alignment.
Roller printing, also called cylinder printing or machine printing, on fabrics is a textile printing process patented by Thomas Bell of Scotland in 1783 in an attempt to reduce the cost of the earlier copperplate printing. This method was used in Lancashire fabric mills to produce cotton dress fabrics from the 1790s, most often reproducing small monochrome patterns characterized by striped motifs and tiny dotted patterns called "machine grounds".
Flexographic inks are inks transferred by the process of flexography, primarily used in the printing of packing materials. The inks and method of printing continues to grow in popularity due to its low cost and environmentally friendly nature. The most important part of the printing process is the application of the ink.
In printing the doctor blade removes the excess ink from the smooth non-engraved portions of the anilox roll and the land areas of the cell walls. Doctor blades are also used in other printing and coating processes, such as flexo and pad printing for the same function. It is believed that the name derives from the blades used in flatbed letterpress equipment for blades used to wipe ductor rolls, and "ductor" became doctor.
Chimigraf is a Spanish multinational company engaged in the production of inks for flexography, rotogravure, digital systems, and screen printing. It has its own technology in the development of ink-jet inks. It has a presence in over 40 countries around the world.
Cold foil printing, also known as cold foil stamping, is a modern method of printing metallic foil on a substrate in order to enhance the aesthetic of the final product. Cold foil printing can be done two ways: the older dry lamination process common in the offset printing industry, or the newer, more versatile wet lamination process, which is dominant in the flexo label industry.
Chemence is a speciality chemical and medical device manufacturer which products include cyanoacrylate adhesives, anaerobic adhesives, impregnation sealants, adhesive activators, epoxy resins, UV adhesives, photopolymer resins, custom printer ink cartridges, Gas pipe sealants, and soak off nail polishes. Chemence is a supplier of photopolymer and commercial printers to the flexographic industry in the US and Europe, as well as the primary sealant supplier to British Gas, and a primary supplier of private-label adhesives to companies including Tesco, 3M, Bostik, and Bondo. The company's catalogue of patents includes processes, packaging devices, and chemical combinations.
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