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Flexography and Screen Printing
FLEXOGRAPHY PRINTING
History of Flexography: Flexography also called “surface printing” often appreviated to ‘flexo’ is a method of printing most commonly used for packaging (labels, tape, bags, boxes, banners and so on). This process was invented by (BIBBY, BARBON & SONS,) England in 1890. In 1839, Charles Goodyear accidentally discovered a means of strengthening natural rubber, a process he called "vulcanization." In the mid- to late-1800s, various rubber products and patents began appearing.
In the late 1800s, letterpress (printing from raised type, typically bits of metal) was the dominant form of printing, with the alternate processes of lithography and gravure still in their formative years. It was found that letterpress type could be set into plaster and that unvulcanized liquid rubber could be poured into the mold and, after heating and cooling, could make a workable rubber stamp. Soon, it was found that the rubber stamp concept could be applied to the manufacture of printing plates, which could be useful for printing on surfaces that did not yield good results with conventional letterpress processes, in particular corrugated paperboard.
The invention in the 1930s of synthetic rubbers made the properties of the rubber stamps and plates much more reliable than they were with unreliable natural rubber. Advances in rubber platemaking were pioneered by the Mosstype Corporation, which developed effective processes for both aniline printing (as flexography was known until the 1950s) and for letterpress printing. In the 1940s, Mosstype developed effective off-press plate-mounting systems, which minimized downtime and made aniline printing more efficient. In 1938, two men at the International Printing Ink Corporation devised a way of accurately and effectively metering the film of ink transferred to the rubber plate. Their system was inspired by the etching of gravure cylinders, which transfers ink from cells to the substrate. They developed an ink roller, engraved with a controlled size and number of cells, and plated with copper and chrome that effectively metered the ink film transferred to the aniline printing plate. They called their roller an anilox roller, and it is still the basis of modern flexographic presses.
In the first decades of the twentieth century, as was mentioned, flexography was known as "aniline printing," taking its name from the type of dyestuff used in the inks. In the 1930s, the aniline dyes were declared toxic by the FDA. Although aniline printers were by then using different types of inks, the name remained. In the late '40s, it grew apparent to industry leaders that the name "aniline printing" had to go, as the name had bad connotations, since the process was widely used for printing food packaging. In 1951, the Mosstype Corporation, in its company newsletter, held a contest to rename the process. Alternate names were solicited, and a final choice would be voted on. Two hundred suggestions came in from printers around the country, and a special committee formed by the Packaging Institute pared the list down to three: permatone process, rotopake process, and flexographic process. On October 21, 1952, it was announced that the overwhelming choice was "flexographic process," or "flexography."
Working Principle of Flexography: Flexography is a process in which the printing image stands up in relief. A liquid is used which may be solvent-based, and dries mainly by solvent evaporation. Water-based inks are also widely used and UV-cured system are being introduced.
A low printing pressure is essential to the process because of the combination of very fluid inks and soft flexible printing plate that are used. The process has several distinctive features.
The principle on which a flexographic printing unit works is illustrated. The low-viscosity ink is transferred to the printing plate via an anilox roller that is evenly screened with cells, the so called screen roller/ anilox roller (screen width 200-600 lines cm) ceramic or hard chromed metal surface). The rubber or plastic plate is attached to the printing plate cylinder. Ink is transfer to the printing substrate by the pressure of the impression cylinder. The use of blade (together with the inks supply system) on the screen roller has stabilizing effect on the printing process resulting from even filling of the calls on the screen roller.
Flexography Market: The Flexographic Printing Market size is estimated at USD 196.24 billion in 2024, and is expected to reach USD 228.90 billion by 2029, growing at a CAGR of 3.13% during the forecast period (2024-2029).
The market for flexographic printing has gained significant attention in the packaging industry, where it is used for printing labels, cartons, and flexible plastics. The growth of industries such as food, beverage, and other packaging, is anticipated to aid the market for flexographic printing.
Flexography Products: 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.
Flexography Features:
Particulars
Flexographic Printers
Speed
High-speed printing, perfect for large volumes
Substrate Versatility
Capable of printing on a variety of substrates
Ink Usage
Can use water-based, solvent-based, and UV inks
Initial Cost
High upfront investment
How Flexo Printing Works:
Flexo typically utilizes an elastomer or polymer image carrier such as sleeves, cylinders, and plates. The image carrier is engraved or imaged to create the design for the final desired product. Ink is transferred from the ink pan via an anilox roll onto the image carrier, where it is then printed onto the substrate.
Flexo printing is most commonly associated with uses in flexible packaging and labels, utilizing a variety of substrates including film, paper, foil and non-woven. Producing these products generally takes 3 steps:
The first step in flexographic printing is designing your artwork. Creating a design that fits the specifications needed to produce the final desired print can be challenging. Failure to thoroughly proof your artwork can result in costly mistakes.
Image carriers cannot be changed once they are produced.
When mounted to a printing cylinder, image carriers created flat stretch and distort your image. You can learn how to calculate and compensate for the distortion -- or opt for In-The-Round, continuous print image carriers that are distortion free.
The rolling design of the flexographic printing press allows for continuous substrate materials (in roll-form) to be fed through the machine.
This feature of flexography is what allows for the continuous printing process that can achieve high speeds, maintain accuracy, and efficiently produce large print runs.
Litho printing is an offset process which uses a printing plate. The ink is first applied onto the printing plate and then transferred to a rubber blanket through multiple ink rolls, and finally applied to the substrate from the blanket.
This means the image is not printed directly onto the substrate from the plate.
Both flexo and litho printing can produce reliable prints in very high volumes. They can be used to produce quality product packaging, labels, and a wide variety of printed paper materials.
While lithographic printing might dominate some markets, flexo printing has increasingly gained popularity since the 1970s with the introduction of direct laser engraving.
Litho can accommodate foil stamping, spot gloss, embossing, and other embellishments, but these options also come at a higher cost.
Additionally, since the only way to print with litho is through an indirect pre-print, this guarantees an additional step in production and thusly raises costs.
Both processes can be very cost-effective when printing large runs with basic needs.
With proper maintenance and storage, as well as an investment in durable image carrier materials, flexo image carriers can be reused many times before they need to be replaced.
Litho is generally limited to printing on smooth, flat surfaces as the image must be pressed onto the substrate.
Printing on corrugated substrates requires an additional step in production where the images are first printed onto linerboard, which is then attached to the corrugated substrate.
Flexo can print on both porous and non-porous surfaces, making it ideal for a wider variety of substrates, including coated linerboard and paper.
Litho generally uses oil-based inks, and printing usually consists of the four process colors, each requiring a dedicated printing station.
Flexo also utilizes one image carrier per color and can use oil-based inks; it additionally accommodates printing with a wide variety of other inks -- including water-based, solvent-based, and UV inks. Curable inks, such as UV, afford faster drying times. Faster drying can lead to faster production runs.
Flexo is noted for producing superior print with fine line and text detail.
Making the final decision whether to print using the flexo or litho process depends on the substrate, budget considerations, and a number of other production requirements.
Digital printing presses function similarly to the printers we use at home. Unlike the other printing methods, digital printing does not require the creation of printing plates because a digital press uses electronic files to print images.
Digital printing is capable of rendering high resolution images. This method costs more per print than the other methods and moves more slowly, but without the need for printing plates, set up is less expensive.
This makes digital printing a great option for small volume printing, samples, or test marketing.
Flexo is a more cost effective option than digital for high-speed and high-volume printing needs. Being a continuous printing solution, there is little to no press downtime.
However, for short runs, digital printing is considered more cost effective due to the comparatively minimal initial investment in materials and quick set-up time.
INKS / SPEED:
Due to the fast-drying inks that are compatible with flexo printing image carriers and presses, flexo is a great option for applications where speed is of utmost importance.
QUALITY:
For some industries, digitally printed final products may be less durable than ones printed with flexography.
This is a particular consideration when printing products intended for outdoor use, or needed to withstand many years of display.
Gravure printing works by applying ink to a substrate with the use of a metal plate that is typically mounted onto a cylinder. This plate is often made of copper or chrome.
The image or text that is intended for printing is typically laser etched into said metal plate, a process that often delivers high quality and precise results with good repeatability.
Gravure image carriers are typically much more expensive than Flexo, making the number of prints required to break even much higher. Gravure, however, has a longer press run time as the cylinders do not require as frequent changing as Flexo.
Gravure lead time is usually 3-4 times that of Flexo in the time it takes to manufacture the image carrier.
One of the limitations of gravure printing is that it generally better suited for porous substrates. This is one of the reasons that gravure is best suited for high detail printing on applications such as magazine covers.
That isn’t the case with flexo printing, which is able to print on both porous and non-porous substrates, making the technology ideal for everything from film to paperboard.
Another key differentiator is the range of inks Flexo is able to print with. Gravure is more limited in the inks that it works with, often making Flexo the superior choice due to the ease of printing with a wider variety of inks.
Gravure requires much more ink per print, which can drive up the cost of printing.
Due to the fact that Gravure is traditionally associated with solvent based inks, although water based ink capabilities have been evolving, Flexo is commonly considered the “greener” options.
Flexo generally has more options for VOC considerations, and the press operations are associated with more environmentally friendly practices.
Gravure was once considered best for fine detail and tonal work, but Flexo technology is further enabling prints with higher resolution and detail as it evolves -- enabling it to move into print jobs that were previously associated exclusively with Gravure.
Similar to flexo, Offset also uses a plate, but it works in a different manner than flexo. Offset plates are usually made of metal, and the printing ink is transferred via the plate to a rubber piece of material (often this is called a "blanket") and then to the printing surface.
Offset is most commonly used to print on paper (think: newspapers and magazines), as a flat and smooth surface is required for printing.
Flexo is able to operate with many more types of inks.
Offset generally works with inks that are oil-based along with using some water-based and UV curable inks.
One thing to consider about offset is that the plates used in the process are susceptible to oxidation if they're not cared for properly. For this reason, extra maintenance is often necessary to ensure plate quality.
Flexo image carriers are generally a bit cheaper to create, and they also are typically more durable than the plates used for offset -- meaning they can be re-used a few times before replacement is necessary.
What's more is the wider range of inks that flexo works with (notably the faster drying times with low viscosity inks and UV inks) can accelerate print jobs and increase job profit.
As noted briefly above, offset printing can only be accomplished on a smooth, flat surface. Flexo, on the other hand, can print on a wide variety of substrates, making it a more convenient choice for many.
Combine this with Flexo’s ability to use many more types of ink and the ease of printing more large-scale tasks, it is often hailed as the superior choice.
So what process is right for you? It largely depends on the job, substrate and a variety of other factors of the job you're printing.
Screen printing has evolved from the two color machine presses that were utilized in the 1960s for various print applications. The old presses featured a long dryer between print stations, and wire screens that were rigid and contained photographic emulsion.
Since the 1960s, the screen printing process has evolved greatly with the improvement of technology and materials.
Label and Narrow Web magazine explains the modern process of screen printing:
“Screens, which come in a variety of meshes, are covered by chemical emulsions, which are the proprietary products of the manufacturers. To prepare a screen for use, a film containing a positive image is placed on the screen and exposed to light.
The screen is then washed with water, which removes unexposed emulsion and opens the mesh below the image areas. In exposed areas the emulsion fuses together, closing all mesh openings to prohibit ink flow.”
Though the aforementioned process of screen printing is standard, there are two types of screen printing, flat and rotary, that differ slightly in process.
During the process of flat screen printing, the exposed screen is arranged over the top of the chosen substrate. The ink is then deposited in front of a squeegee. The squeegee then moves over the screen, thusly forcing the ink through the mesh and creating a print.
The rotary screen process differs slightly. Rather than the squeegee moving atop the screen, it is fixed inside of the screen. The squeegee and screen are mounted ITR, and ink is introduced.
When the mounted cylinder rotates, the ink is forced through the mesh via the squeegee. The substrate moves outside of the screen and receives the ink, producing a print.
The substrates available for screen printing, specifically rotary screen printing, are limited when compared to flexo. The most common substrates used for screen printing include:
Though flexo is not a common choice for printing on textiles, it does support a wider variety of substrates and executes detailed prints.
In general, the majority of inks used for rotary screen printing are UV curable, especially for use in narrow web applications. There are some projects, however, that require solvent based inks, which can be used.
When it comes to color, white ink is the most popular in the screen printing market.
Flexo utilizes a wider variety of inks. A full guide to the inks used in flexo printing, and the considerations based on application, can be found here.
Due to the high number of jobs that utilize UV curable inks, print speed is largely dependent on the ink, and more specifically the amount of time the ink requires for curing.
Because of this, print run speeds are slower than that of flexo.
Flexographic printing figure: Today higher requirement, especially in the printing of packaging, photopolymer wash-off plates are used such as “NYLOFLEX” from BASF and “CYREL” from dufont. These allow screen resolution of up to about 60 lines/cm (150, lines/inch).
Main sections of flexography printing machine: all flexographic presses are made up of four basic section typically mounted in succession between studies.
'Printing Unit'. The inked anilox roller is adjacent to the plate cylinder, a steel drum on which the rubber flexographic plate is mounted (usually by means of an adhesive backing, rather than the plate clamps used in offset lithography). The raised impression on the flexo plate picks up the ink and transfers it to the substrate passing between the plate cylinder and the smooth, steel impression cylinder. The plate cylinder can either be integral (the cylinder body, end-caps, and shafts are all one piece), demountable (the shafts are removeable), sleeve (the cylinder face is slid onto a bored cylinder using high-pressure air), and magnetic (the cylinder is magnetized, allowing metal-backed plates to be mounted magnetically, rather than by means of adhesive). (See Plate Cylinder: Flexography.)
In some applications (typically those in which ink strike-through is a problem, and is likely to cause ink buildup on the impression cylinder), the impression cylinder is replaced with an impression bar, a G:H-inch-diameter steel rod clamped into the proper position behind the web. The bar does not rotate, and as a result the moving web wipes off any ink likely to accumulate on it.
The printing unit have three basic parts:
Inking system: the function of the inking system is to meter out a fine and controlled film of liquid ink, and apply this to the surface of the printing plate. It typically consists of an ink trough, a rubber covered fountain roller and a screen (anilox) inking roller into which cell of uniform size and depth and engraved. The fountain roller lifts ink to the tip position, where it is squeezed into the cell in the screened inking roller and by a shearing action is removed from the roller surface. The ink in the cells in then transferred to the surface of the printing plate. To regulate ink firm thickness in printing, screened ink roller are available which have screens of from 40 to 200 cells/cm. these may be engraved or etched metal or ceramic. The engraved cells are generally square in shape (although many other shapes are available now) with stopping side walls.
'Ink Fountain'. Flexo ink, typically a thin, volatile liquid ink, is stored in an ink pan, where a rubber-covered fountain roller rotates. The fountain roller picks up a thick film of ink and transfers it to a metering roller, typically known in flexography as an anilox roller. The anilox roller is a chrome- or ceramic-covered roller whose surface contains small, engraved pits or cells (typically from 80:1,000 cells per inch).
The pressure between the fountain roller and the anilox roller is set so that the excess ink pools up at the top of the nip between them. The difference in revolution speed of the two rollers (the fountain roller typically turns at a slower rate than the anilox roller) causes a wiping effect on the anilox roller. The goal is to ensure that only the ink stored in the engraved cells on the anilox roller's covering is transferred to the plate. The difference in speed also eliminates a problem in flexography called mechanical pinholing (sometimes also called ghosting, and related to mechanical ghosting found in offset lithography), in which ink is not replenished uniformly to the surface of the anilox roller, causing the texture of the roller to be transferred to the substrate.
Some alternate configurations include a chambered or enclosed system, in which the anilox roller sits in the ink fountain itself (removing the need for a fountain roller), the ink metering performed by a doctor blade (a strong strip of steel, plastic, or other material) that is placed between the fountain and the nip between the anilox roller and the plate cylinder. The angle and pressure of the doctor blade ensure a controlled and uniform ink metering. Another fountain roller-less configuration pumps ink from an ink tank to the surface of the anilox roller (which sits above an ink pan, the latter acting as a catch basin). A doctor blade is also used in this configuration to meter the ink film. Another more elaborate system, called an enclosed inking system, features two doctor blades—one at the bottom of the anilox roller, the other at the top, the ink reservoir located between them. Ink is pumped onto the surface of the anilox roller, where the top doctor blade is responsible for metering. This system is typically used on high-speed presses, and is popular due to the fact that, since the inking system is not exposed to the air, ink viscosity can be tightly controlled.
Types of flexo inking system: flexography can be distinguished from other printing processes by its inking systems. The metering roller is known as the anillox roll and it is the primary determine of ink film thickness. It determines uniform and consistency.
Developments in anilox rolls continue to be at the heart of process improvement. The laser-engraved anilox permits use of doctor blades, with together provide consistent image uniformity over long runs and over long periods of time.
Inking metering: in every printing process there must be a method to meter the quantity of ink. One must control the film thickness of the ink in order to use the least amount of ink required for proper density/darkness and solid uniformity or coverage.
0.01inch = 25.4 micron.
Type of anilox cells and cleaning system:-
Anilox roll: the anilox roll is a uniformly imaged gravure cylinder. the illustrates cells of two specifications, showing the depth and the opening. It also shows the land area critical to print quality, including solid uniformity and clean printing screens or halftones.
The specification of the cells in the anilox roll determine its capacity for specific application. For ex, an anilox roll with 200cells per inch, having a cell depth 30-35 micron, will carry a volume of 7.5 bcm (billion cubic per square inch).
Absorbent kraft paper with a 200-lpi, 7.5 bcm anilox roll.
For fine work, 133 lines halftone on a smooth and coated paper you might want a 600 lpi, 1.6bcm.
Anilox roll specification:-
Plate cylinder: it is made of steel material structure of flexographic plate
The total thickness is determined by the space between the cylinder and the pitch line of the gear where the transfer of image to substrate is achieved. Thin plates are between 0.025 inch and 0.045 inch is news and narrow web label application.
Plates between 0.067 inch and 0.125 are most common and still common is 0.150 inch to 0.250 inch.
Flexographic plates:
Metal-backed plate: metal-backed plate which molds and vulcanizes the rubber to a metal backing. It is some of those used on offset presses, it have pre-punched holes for accurate mounting on plate cylinder registration pins
Several types of remounted plates are produced on a removable metal cylinder of sleeve that can be slid onto the plate cylinder. Some varieties also produce the plate on a mountable carrier sheet.
Magnetic plate have the rubber surface applied to a magnetic backing material, and it is easy mounting and removal.
Some are flexographic plates is a design roll which is a printing cylinder containing a layer of rubber. It engraved using laser and used for gift wrapping, linerboard, security paper.
Flexographic plate mounting: the plate mounted process is simply the precise positioning of the plate in the X and Y axes. If the plate are mounted perfectly square, the running adjustment on the press permit easy registration on the run. The flat plates are mounted in accurate registration onto the plate cylinder with double-sided adhesive film. Two side self-adhesive tapes are also called as sticky back. It is thin as 0.002 inch and most commonly 0.015 inch and 0.020 inch thickness.
Plate alignment concept: it is grouped into three categories: 1. Optical, 2. Register pin, 3. Video-scopes and micro-targets.
Types of flexographic plate cylinder:
There are two modes of procedure for sleeve technology:-
Impression cylinder: the impression cylinder is also made from steel. The substrate passes between the plate and impression cylinder, which generate light printing pressure.
Unit – 2
Flexographic Plate
The first plates developed for flexographic printing were made of natural or, more commonly, synthetic rubber, and were manufactured much like letterpress plates. Although photopolymer plates are now widely used in flexographic platemaking, rubber still has its adherents, primarly because of its economy, its simplicity, and its compatibility with ink solvents that cannot be used with photopolymer plates.
Structure of Flexographic Plate
Figure - Structure (physical parts) of the flexographic printing plate
The terminology used to describe the plate is detailed in the above figure. The face is the image that prints. It must be smooth and have sharp edges. The shoulders will be as straight as possible where they meet the face. Ideally they will angle out from the face to provide support to fine lines and small halftone dots. The floor is the nonimage area. The distance between floor and face is relief depth and is critical to the relief principle. Contrary to standard practice, large relief depths are unnecessary as proven by the newspaper printers and leaders in narrow-web printing, both of whom print with relief depths of as little as 0.015 inch.
The back or base of the plate, in the case of photopolymers, is a polyester sheet and provides dimensional stability. It may also be metal as with many newspaper plates and plates mounted to cylinders magnetically. Rubber plates, with limited exceptions, have no stable backing.
The total plate thickness is determined by the space between the cylinder and the pitch line of the gear where the transfer of image to substrate is achieved. Thin plates are between 0.025 inch and 0.045 inch, and are found most commonly in news and narrow-web label applications. Others are slowly moving in this direction.
Plates between 0.067 in. and 0.125 in. are very common in most industry segments, with the exception of corrugated. There it is still common to find plates between 0.150 in. and 0.250 in. Trends in almost all flexographic applications are to thinner plates, which are found to hold better resolution and print with less gain.
There are several kinds of image carrier used in flexography
1. The traditional rubber plate
2. Photopolymer plates
3. Laser-engraved rubber plates or rubber rollers.
Flexographic plate composition must match to some extent the type of ink to be used and to the substrate to be printed. Both rubber and photopolymer plates are used.
2.1.1. RUBBER FLEXOGRAPHIC PLATES PREPARATION (IN BRIEF)
Natural and synthetic rubber plates were the first type of flexo plates developed, and they are still used for some applications. The process of producing a rubber plate is not far different from the process used to produce photoengravings used in the hot type letterpress process (figure).
i. Preparation of Original Plate
A sheet of metal alloy coated with a light-sensitive emulsion is first placed in a specially designed vacuum frame. The emulsion is not only light-sensitive, it is also an acid resist.
A negative is placed over the emulsion and light is passed through the negative. The acid resist hardens where light strikes the emulsion (image areas).
During processing, the unhardened resist in the non-image areas is washed away, leaving hardened resist only on the image areas. The metal alloy is then etched, which lowers the non-image areas and leaves the image areas raised. The remaining resist is washed off.
ii. Preparation of Mold or Matrix
The completed engraving is then moved to a molding press where a matrix (mold) of the engraving is made by pressing matrix material against the engraving with controlled heat and pressure. The matrix material sinks into the metal engraving to form the mold.
iii. Preparation of Rubber Plate
The rubber plate is made from the matrix by pressing a rubber sheet into the matrix, again under controlled heat and pressure.
Preformed sheets for rubber plates are available in a variety of thick nesses. The thickness depends on the job to be printed and the press to be used. The major disadvantage of rubber plates is that they are more costly to make than photopolymer plates. Also, because they are made from an engraving, any plate problems identified during proofing must be corrected by remaking the engraving, which further increases the expense of the process.
Photopolymer plates are made from light-sensitive polymers (plastics). When they are exposed to ultra violet light, they undergo polymerization, or the chemical conversion of many small molecules into long-chain molecules. The result is that they will be harder and more insoluble in exposed areas and softer in unexposed areas. Photopolymer plates eliminate many of the disadvantages of rubber plates. There are two basic types of photopolymer plates used in flexographic platemaking - Sheet photopolymer plates & Liquid photopolymer plates.
2.1.2.a. SHEET PHOTOPOLYMER FLEXOGRAPHIC PLATES PREPARATOPN (IN BRIEF)
Sheet photopolymer plates are supplied in a variety of thicknesses for specific applications. These plates are cut to the required size and placed in an ultraviolet light exposure unit (figure). One side of the plate is completely exposed to ultraviolet light to harden or cure the base of the plate.
The plate is then turned over, a negative of the job is mounted over the uncured side, and the plate is again exposed to ultraviolet light. This hardens the plate in the image areas.
The plate is then processed to remove the unhardened photopolymer from the nonimage areas, which lowers the plate surface in these nonimage areas. After processing, the plate is dried and given a postexposure dose of ultraviolet light to cure the whole plate.
2.1.2.b. LIQUID PHOTOPOLYMER FLEXOGRAPHIC PLATES (IN BRIEF)
Liquid photopolymer plates are made in a special ultraviolet light exposure unit. In this process, a clear plastic protective cover film is mounted over a negative transparency which is placed emulsion side up on the exposure unit (figure a). A layer of liquid photopolymer is then deposited by a motorized carriage over the transparency and cover film. The carriage deposits the liquid evenly over the cover film and controls the thickness of the deposit. While the carriage deposits the liquid, it also places a substrate sheet over the liquid (figure b).
The substrate sheet is specially coated on one side to bond with the liquid photopolymer and to serve as the back of the plate after exposure. Exposure is made first on the substrate side of plate. This exposure hardens a thin base layer of the liquid photopolymer and causes it to adhere to the plate substrate. A second exposure through the negative forms the image on the plate (figure c). As with sheet materials, the image areas are hardened by this exposure. The non-image areas, however, remain liquid.
Processing removes unwanted liquid in the non-image areas to leave raised image areas. A post-exposure is then made to cure the whole plate (figure d).
2.1.3. LASER ENGRAVING
Rubber suitable for flexographic printing can be engraved by laser techniques. The equipment will handle black and white positive copy for line work, and screened negatives or positives for halftone work. Screen rulings of 47 lines/cm (120 lines/in) are possible, and is expected to improve to 60 lines/cm. Engraving by this method can be done on either separate pieces of rubber, or rubber rollers. The ability to engrave rollers is unique, and an advantage in the printing of continuous designs. Because flexographic printing is done from an image in relief it is essential that the shank of the image has a steep angle and is smooth. A suitable depth in the non-image area is also essential.
2.1.1. RUBBER PLATES PREPARATION (in detail)
Rubber plates are made by a series of steps starting with a negative, specially sized and distorted for the specific rubber being used. Since the rubber molding process includes two steps where heat is involved, the changes in size caused by heating and cooling materials must be compensated.
i. Preparation of Original pattern plate
The negative is exposed onto the light-sensitive coating of the metal or photopolymer pattern plate. A variety of materials including magnesium, lead type, copper, and hard photopolymer are imaged to make the original pattern plate. Magnesium is the most commonly used pattern plate material. Hard photopolymer is gaining in use because of its preferred interaction with the environment and the workplace.
Figure: Steps in producing a rubber plate
The pattern plate is processed into a hard, letterpress-type relief plate. This becomes the “original” relief plate that will be duplicated in rubber for use in flexographic printing. Metal pattern plates are developed after exposure to remove the acid-resistant coating. The plate is etched with acid to the desired depth. This determines the relief depth of the final rubber plate. Then the plate is inspected and flaws are removed to prepare it for making the matrix, a mold.
ii. Preparation of Matrix / Mold
The rest of the rubber platemaking process takes place using a precision vulcanizer, or molding press. Figure below shows a vulcanizer and a diagram of its key parts.
Figure: A Volcanizer (top) and a diagram of its key parts (bottom)
Matrix board, sometimes called bakelite, is cut to size, brushed to be sure it is free of foreign particles, and inserted face up into the molding press. The pattern plate is placed on top, image side down, and pressed under heat and pressure into the matrix board.
Thickness control bearers are placed along both sides of the molding surface, called the serving tray, to control the thickness of the matrix. The matrix is a thermal plastic resin and cellulose material. The resin provides a smooth hard surface for molding the rubber plate.
The matrix is molded to a specified floor thickness, the thickness between the face of the image and the back of the matrix board. Figure below shows the assembly of pattern plate, matrix, cover sheet, and the thickness control bearers.
Figure: The assembly of pattern plate, matrix, and cover sheet and the thickness of control bearers
iii. Preparation of rubber plate
After checking the floor thickness and uniformity of the matrix it is placed back into the molding press, image side up, for molding the duplicate rubber plate. It is a duplicate because it is a copy of the pattern plate. In fact it is a third-generation plate, the first and second generations being the pattern plate and the matrix. The gum, which becomes rubber when vulcanized, is placed over the matrix. A cover sheet is placed on top of the gum to protect the upper platen of the molding press from any buildup of material. The exact total thickness of bearers is positioned at the left and right of the serving tray and the entire assembly is inserted into the heated plate molder. The bearers are calculated exactly to determine the thickness of the plate. The heat and pressure from the molding press soften the gum while hydraulic pressure pushes it into every part of the matrix. The assembly of matrix and gum is held for a specific time at 307°F until it is completely vulcanized, changed to rubber.
Quality checks often reveal slight irregularities in total plate thickness and uniformity. Small amounts of unevenness in rubber plates are often corrected by a grinding procedure
2.1.2.a. SHEET PHOTOPOLYMER PLATES PREPARATION (in detail)
As the name implies, photopolymer plates are light-sensitive, and the platemaking procedures employ multiple exposures to light to determine their relief depth and shoulder angles. The workflow figure shown above describes the sheet photopolymer production flow.
The raw materials are either in a liquid or a precast sheet form. Figure below describes the sheet type of plate, available in a wide variety of sizes from small (12 x 15 inches) up to 50 x 80 inches and possibly larger today; change is constant.
Figure: The sheet type of plate, which is available in a wide variety of sizes.
There are many sizes and types of exposure devices. The diagram in shown below is just one typical exposure system. The procedure for exposure and processing is simple.
i. Back / Base (Plain) Exposure
The plate material has a base and a face side. The base side is determined by the firmly attached polyester sheet. This provides the plate with dimensional stability. The base resists size changes and cannot be stretched during handling, particularly mounting. The first exposure is made through the base. Its duration determines floor thickness. Since total plate thickness is a specification of the sheet plate as it is supplied, floor thickness is the determiner of relief depth. Relief depth is a major factor in determining print quality. The longer the back exposure, the thicker the floor. Back exposure also affects the length of the face exposure.
ii. Main (Face) Exposure with negatives
The face side of the plate also has a polyester sheet, but it is easily peeled off prior to imaging. Face exposure is the imaging exposure made through the negative held in contact by a vacuum and a flexible drawdown sheet. The length of the face exposure determines the shoulder angle, which controls support of the image. Fine lines will be wavy if there is insufficient face exposure. Very small highlight dots will fail to image or be weak and move during impression without enough face exposure. Stochastic images require more face exposure to image the highlight “spots” since they are farther apart, somewhat independent of adjacent spots. Too much face exposure causes excess dot gain, particularly in highlights and quartertones.
iii. Washing out the non image areas
Once the plate is exposed the material has been rendered stable or insoluble. The unexposed material is still a soluble monomer. It is processed by simply dissolving in an appropriate solvent or detergent. The plate is also scrubbed with brushes during washout to speed the process by removing the unexposed material as it is dissolved. Solvent-washed plates require a blotting step to assure all solvent and plate material are removed from the printing surface. This is a simple but critical part of the platemaking process. Any foreign material left on the face of the plate causes noticeable defects in the printed image.
Solvent-washed plate material absorbs some of the solvent, and time is required while drying for this material to escape from the plate. Detergent-washed plate materials don’t absorb liquid and thus require less time for drying. Dryers provide hot air and exhaust for rapid removal of moisture and vapors.
iv. Post Exposure
After the plate is processed and dried, it requires post exposure to cure all remaining unexposed material and finishing to eliminate a tackiness on its surface. While there are alternative methods, finishing is usually done by a UV light finishing process.
2.1.2.b. LIQUID PHOTOPOLYMER PLATES PREPARATION (IN DETAIL)
The figure below illustrates the process of making a liquid photopolymer plate. Liquid polymer plates are made following exactly the same exposure and processing steps of sheet photopolymer plates. The difference is that the parts of the plate come as separate items to the liquid platemaking department. The base, or substrate, of the plate is a sheet of polyester. One side has a matte surface to assure its firm attachment to the polymer resin. The polymer is in liquid resin form comparable to honey in appearance and consistency. There is a thin plastic cover sheet used to keep the resin off the negatives during exposure.
i. Preparation of liquid photopolymer layer
While there are many features of liquid platemaking systems, the basic process is the same. The operator positions the negatives, emulsion up, on the lower glass which is cleaned before every plate is made. A very thin cover sheet is pulled over the negatives and drawn down with vacuum.
The resin supply carriage moves across the negatives, pouring a metered quantity of resin and simultaneously laying down the polyester base of the plate. As soon as the carriage is clear of the plate area the top of the machine is closed and vacuum is applied between the top and bottom glasses. This is done to assure the plate completely fills the space between the two glasses. This space is the critical plate thickness and determines plate uniformity required for quality flexographic printing.
ii. Exposure
The exposures are made. First the back exposure lamp is switched on. As with a sheet system, this is timed to establish the floor thickness (and relief depth) while also increasing the sensitivity of the resin to the face exposure. While the back exposure is being made the face exposure is started from the bottom lamps. This exposure is made through the negatives and determines the imaging and the shoulder support of the plate.
iii. Washing out
When the exposures are complete the unit is opened and the plate is removed. The cover sheet is discarded and unexposed resin reclaimed. The plate is placed into the processor washout unit; the processor washes out all the unexposed resin using a heated detergent and water solution.
Once washed out, the plate is rinsed and moved to the finishing unit where it is post exposed and finished simultaneously, in a special solution to remove the tackiness and to leave the plate ready to be used once it has been dried. Drying is done only to remove water from the surface since there is no absorption into the plate.
v. Finishing
Liquid platemaking departments almost always reclaim a significant amount of the unexposed resin before the plate is washed out. This is done by placing it on a vertical surface where, after removal of the cover sheet, a high-velocity air knife is passed down over the plate causing the unexposed resin to roll off into a catch basin. This resin is used again in the platemaking process, saving both material cost and pollution of the washout and subsequent wastewater.
DIGITAL FLEXOGRAPHIC PLATES
Currently there are several varieties of direct-to-plate, or digitally imaged flexographic plates. As with all printing processes, the motivation is to eliminate film imaging costs and improve throughput. Of course, improvements in quality are also expected.
The first direct-to-plate process was laser engraving rubber. In the process gum is vulcanized and precisely ground to final plate thickness. It is then mounted to a drum and rotated in front of a CO2 laser. The nonimage area is burned away leaving the image in relief and the plate ready for mounting (see Figure 8-11). Laser-engraved rubber plates have precisely controlled shoulder angle, and resolution as high as 120-line halftone screens can be produced. One of the most appealing applications of this technology is the production of continuous-pattern images. Conventional plates always leave a gap of line where the two ends of the plate come together on the cylinder. Continuous patterns are laser-engraved onto rubber-covered rollers. Rubber is vulcanized to roller bases and ground to the exact repeat length. This roller is then laser-imaged. Gift wrap and wall covering often require uninterrupted patterns, and laser imaging is a popular solution. This process also eliminates any plate mounting and the cost of potential register flaws that go with mounting.