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Technology of Flexography
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.
1. Characteristics of Flexography
2. Advantages of Flexography
3. Limitations of Flexography
4. Applications of Flexography
Flexography Features:
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.
IMAGE CARRIER PREPARATION
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
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.
2. PHOTOPOLYMER FLEXOGRAPHIC PLATES
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.
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.
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.
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.
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.
3. LASER ENGRAVING ON RUBBER ROLLERS
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.
Computer to film technology- film setter:
Photographic masters, can create image not only of type, but also a wide range of graphics including line, tints and photographs, by reproducing in a predetermined dot or other shaped pattern.
Image setters are driven form application program which can output their information in a page description language called postscript.
Post script can support any level of graphic complexity, it is a page- dependent language.
The process of image setting essential consists of two parts:-
1. A raster image processor (RIP) and
2. A film setter.
IMAGE SETTER: Image setter is a high resolution generate and expose dots that can transfer electronic text and graphics directly to rolls or sheet of either film or bromide paper to a laser light source.
A image setter user a laser and a dedicated (RIP) and is usually post script resolution varies between 1270 or 2590 dpi with a maximum dpi of 4800. Image setter create the separated output by printing the image four times i.e. C, M, Y, and K.
Output on film: silver Helide coated plastic film.
On bromide paper: it use for proofing document before lithography plates are made.
Image setter are continuously improving include argon-ion, infra-red, (helium-ion) laser diode, NeNe, YAG and visible red laser.
Now holographic technology is higher speed and improve screen quality.
Light is high intensive, but cannot be easily switched on and off. To overcome this problem, the laser light passed through a crystal based prism which deflects the laser differentially, simulting switching on and off.
Image setter is depend on resolution with size.
The biggest presses print job according match image setter size like B3, B2, B1, or above.
B2 image setter can output a four page A4, imposed with 15 minutes.
B0 give 16-page A4 in under 15 minutes at 2540 dpi.
B3 image setter give small size printer and use polyester plate for commercial work.
Some devices expose film and polyester film in one matching and developed are being worked on to include CTP with metal plate.
Automatic film processors:
After exposed film transfer in this machine exposed films are transported through the developing, fixing and washing solution and delivered after dried by warm air.
Automatic processor reduce man power, improves quality, less space required in darkroom, time reduction, cost reduction and rollers are easy removal and cleaning.
Types of (AFP):
1. Litho-type processor speed 4 to 6 minutes.
2. Contact processor speed less than 2 minutes.
3. Rapid access processor speed 90 second but use in lower density film.
Feature
Rubber Plates
Photopolymer Plates
Material
Natural/synthetic rubber
UV-sensitive polymer resin
Durability
Lower (wears faster)
Higher (more impressions)
Resolution
Limited detail
Fine detail possible
Cost
Cheaper
Higher initial cost
Applications
Short runs, rough surfaces
Long runs, high-quality work
Unit – 2
THE FLEXOGRAPHIC PRINTING PRESS:
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.
2. Unwind section: most to the substrate come in form of roll or web. Firstly they are fed through in-feed draw rolls, which pull the web into press section. Now the speed of the web and press should be synchronized “to provide correct tension and register control. If the speed is more in unwind sections it is controlled by unwind breaking. An unwind section may also include a nest of internally heated steel rolls, or the rolls used for in-feed tension control may be heated for a secondary purpose is open the surface of heavily glazed or tight papers by preheating. Preheating in this manner is also beneficial with some plastic materials, as it ‘normalizes’ the web, making it flatter and reducing the tendency to wrinkles.
3. Printing section: a single color station with the four essential rolls are fountain roller, inking roller, printing plate cylinder and impression cylinder. The majority of printing presses are multi-color from two to eight color in printing section. In some presses there color units are arranged horizontally, inline. The unique of flexography is ‘stacks’ with a single stack of two to four color units, each color unit arranged vertically one above another.
'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.
1. Two-roll ink metering system: in this system, the anilox roller and fountain roller with tight contact and fountain roller turn slower than anilox, creating wiping action. This cause most of the surface ink to fall back into the fountain.
2. Modified two-roll with a doctor blade with inking metering system:- for wiping use doctor blade.
3. Reverse two-roll doctor blade inking metering system: where the anilox roll is suspended directly in the ink fountain (removing the need for a fountain roller) and the reverse angle doctor blade shears off the excess ink letting it fall back the ink fountain.
4. Chambered doctor blade ink metering system: ink fountain is replaced with an assembly mounted against the anilox roll. On one side of the chamber there is a reverse-angle doctor blade that perform the meeting function. The other side of the chamber is sealed by a containment blade, which keeps the ink from escaping or leaking out of the chamber. The ends of the chamber are sealed with gasket like material. Ink is pumped into chamber and usually returned by gravity to the ink sump.
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:-
1. Cell count: the no. of rows of cells per linear centimeters in the meteric world-divide by 2.54 to convert.
2. Cell depth: which is the determiner of density in a given application.
3. Cell volume: the key of coverage and uniformity of solids. More volume results in more ink.
4. Cell angle: the cell are angled 45 degree from the axis of the roll. It is possible to fit more cells into an area when they are aligned at 60:60 degree are better uniformity with less ink and avoid moire pattern.
5. Cell shapes: inverted pyramid shape cells.
a. Quad rectangular
b. Tri-helical cell
c. Used to apply
d. Viscous
e. Coating
6. Anilox rolls based on roller surface
a. Laser: engraved ceramic anilox rolls: it is a steel roll and plasma sprayed chromium oxide surface built up to a thickness of 0.00 – 0.010 inch. The cells are burned into the ceramic with a CO2 laser that literally vaporizes the coating, leaving a precise cell. The ceramic surface is extremely hard.
b. Conventional engraved chrome anilox rolls: it is same electromechanical technology.
7. Roll cleaning:
a. Jet wash cleaning system: that utilize specific heated chemicals fixed at the roll under high pressure.
b. Powder blasting: it is dry system, this can be messy process.
c. Polymer blade blasting: it have entered the market over the last few years that deserve mention.
d. Dry ice system: it is very noisy operation.
e. Laser cleaning:
a. Ultra-sonics: it can be neutralized in certain circumstances and the volumes are low in comparison with jet wash system.
b. Alpha sound:
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.
1. Single layer (photopolymer plates): it consists of a relief layer (untreated photopolymer) that is covered with a protective film. A separation layer allows easy removal of the protective film. A polyester film on the reverse side of the plate serves to stabilize the untreated plate. The layer structure of a single-layer plate thickness from 0.75mm for (plastic bags, film, cardboard) to 6.35mm (corrugation board and heavy duty bags made from paper and film).
2. Multilayer sheet photopolymer plates: the base layer itself forms a compressible sub structure for the relief layer and consequently absorbs deformation during printing.
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.
a. Optical device: optical device on simple visual alignment of the position marks on the plate with grid or layout lines.
b. Pin registration: it has employed pins to align images for decades. The use of register pin had to stop with the final film in the flexographic process. Pin registration of images on film to the plate is achieved by drilling sheet photopolymer plate material with holes matches to holes in the final film. After back-exposing the plate material, the pins are placed through the holes and used to position the film during face exposure, just as is routinely done with litho plates. These same registry holes are then used for mounted onto the cylinder.
c. Video microscope: the position of the plate over the cylinder and attaching without any additional action. A pair of micro-target are imaged into all the plates. The mounting system has atleast two video microscopes.
Types of flexographic plate cylinder:
a. Integral plate cylinder: it most expensive. The one-piece cylinder is made for a single repeat length, circumference, and has a precision journals to hold gears and bearing.
b. Demountable cylinder system: it consist of a shaft or mandrel, art metal sleeves machine to specified repeat length. These steel or aluminum sleeves are installed for the specific job and corresponding gears and bearing are installed at the time of mounting. This system in narrow-web application.
c. Light weight sleeves: it are a solution for the larger sized of presses jobs can be set of up for mounting directly to sleeves. The principal of sleeve technology consists of a thin-walled metal sleeve the inside diameter of which is dimensioned so that the sleeve can be expanded under compressed air and pushed axially into the plate cylinder. Once the compressed air has been turned off, the sleeve sits firmly on the plate cylinder by fore fir. Before being pushes onto the plate cylinder the entire outer surface of this sleeve is covered with plate base material. The cylindrical plate is directly imaged using lasers in a round image-setter.
There are two modes of procedure for sleeve technology:-
a. Covering the sleeve with a laser exposable plate cut exactly to the size of its cylinder casing, in which case the sleeve has a seam.
b. The used of seamless sleeves which have been already fully prepared by the manufacturer with the relief layer-BASF dis-sleeve.
Impression cylinder: the impression cylinder is also made from steel. The substrate passes between the plate and impression cylinder, which generate light printing pressure.
3. Drying section: the drying section require an after drier to remove the remaining solvent from all the colors before the web can be wound in to a roll, and may require between color drier between printing units on multicolor presses to permit the necessary printing of coloron color. The removal of solvent can be accomplished in several ways, hot air current being the most common.
4. Rewind section: the section is identical to the unwind section in most respects but with some significant differences. It need be nothing more than a shaft in plain bearing holding the winding role by means of core chucks. However, there is one important different. The unwind shaft is breaked to add necessary tension as the press pulls the web, off the roll. The rewind shaft must be driven.
Types of flexo inks:
1. Dye based ink: these have been developed from the original aniline inks. It is making solutions of the dye and lacking agent in the solvent either by cold or hot blending on high speed stirring equipment. Dye based ink are usually used for printing on paper, general purpose bags, wrappings, waxed bread wrappers.
Dying rate of the dye based ink is slow. You can increase it by adding acetone getting higher speed of production or you can slow down by adding glycol either but you will get good result with bad odours.
2. Water based inks: In this type of ink water is used as a solvent, where resins are mixed well with water. For example shellac is used as a main resin. This is mainly dry by absorption. It used to print on paper, paperboard, kraft, newsprint and corrugation but now is widely used in newspaper because they do not rub off on readers hand and has less slow through. The wax compound is added in ink for rub resistance and anti-foaming agent. But it is poor level of ‘gloss’ and slow drying.
3. Solvent based ink:-it is mainly dry by evaporation and it suited print on plastic films, aluminum foil and gloss ink. In this many types of resins and solvent used.
Following are the solvent used in ink are: alcohol, esters, aliphatic hydro carbons, glycol ethers.
TYPES OF FLEXO INKING SYSTEMS
Flexography can be distinguished from other printing processes by its inking systems. The metering roller is known as the anilox roll, and it is the primary determiner of ink film thickness. It determines uniformity and consistency. Developments in anilox rolls continue to be at the heart of process improvement. The laser-engraved anilox permits use of doctor blades, which together provide consistent image uniformity over long runs and over long periods of time. All of this is essential for repeatability and predictability. Today flexography is competitive with all processes, largely due to the modern anilox roll technology.
INK METERING
In every printing process there must be a method to meter the quantity of ink. One must control the film thickness of ink in order to use the least amount of ink required for proper density/darkness and solid uniformity or coverage. The surface of the anilox roll is covered with tiny cells, all equally spaced and of the same depth and shape. The cells are specified by the number of cells to the linear inch and the depth of the cell, or its volume.
Anilox cells are often described as fine or coarse, depending on the cell count. A roll having 200 cells per inch is rather coarse, one with 400-500 cells per inch is average, and one having over 700-800 cells per inch is considered fine. Figure 3.1 shows that cell count alone does not reveal all one needs to know about the ink delivery capacity of the anilox roll.
A 360-line anilox with a deep cell can carry more ink volume than a coarser 200-line roll with very shallow cells. Therefore, when specifying an anilox one must always define the cells per inch or cell count, and either cell depth or volume. Cell dimensions are specified in microns. A micron is a millionth of a meter. To appreciate this, consider that there are 25.4 microns in 0.001 inch. Volume is measured in bcms, “billion cubic microns” per square inch (an interesting measurement combining a mixture of metric and English units).
A volume of 1.0-2.0 bcm is a low volume, probably used for fine screens/halftones on smooth substrates. A volume of 4.0 bcm is a middle-of-the-road anilox roller while a 7.0 bcm roll is found where bold solids are being printed on a very rough and absorbent surface. To increase or decrease the amount of ink in flexography, one changes to another anilox roll that carries the desired amount.
The anilox roll is at the heart of the flexo process. There are several common formsof ink metering systems found on flexographic presses.
1. Two-roll ink metering system
The old standard system is called the “two-roll system” (Figure 3.2 top). The anilox receives a flood of ink from the fountain roll which is suspended in a pan of liquid ink. The fountain roll is run in tight contact against the anilox roll. The fountain roll turns slower than the anilox, creating a wiping action. This causes most of the surface ink to fall back into the fountain, leaving only the ink inside the cells on the anilox roll. The ink in the cells is then transferred to the plate as they come in contact. In the two roll system, the efficiency of the wiping action is affected by the durometer of the rubber fountain roll. A harder, higher durometer like 80 wipes the surface (lands) of the anilox more efficiently than a soft roll with a durometer of 50. (The lands are the tops of the walls between cells which support the rubber roll or doctor blade and define the cells.)
2. Modified two-roll with a doctor blade ink metering system
The second metering system is a modified two-roll with a doctor blade. The rubber fountain roll is backed away so it floods the anilox with ink. The doctor blade is set at a reverse angle to the direction of rotation of the anilox. This reverse-angle doctor blade is engaged with just enough pressure to wipe the surface areas clean of all ink. This produces a much cleaner wipe than the two-roll system. Figure 3.3 bottom illustrates the two-roll with doctor blade ink metering system.
3. Reverse angle doctor blade ink metering system
A third configuration of the metering system is the simple doctor blade design (see Figure 3.4) where the anilox roll is suspended directly in the ink fountain (removing the need for a fountain roller) and the reverse angle doctor blade shears off the excess ink letting it fall back into the ink fountain. Here the ink metering is 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.
4. Chambered doctor blade ink metering system
The last common print station design is the chambered doctor blade system. As shown in Figure 3.5 the ink fountain is replaced with an assembly mounted against the anilox roll. On one side of the chamber there is a reverse-angle doctor blade that performs the metering function. The other side of the chamber is sealed by a containment blade, which keeps the ink from escaping or leaking out of the chamber. The ends of the chamber are sealed with gasket-like materials. Ink is pumped into the chamber and usually returned by gravity to the ink sump. The chamber blade metering system keeps the ink enclosed at all times, reducing the loss of volatiles and maintaining the ink in a constant and clean condition.
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.
Conclusion
Flexography is a process where a precisely engraved anilox roll prints a thin film of ink onto the raised surface of the plate, which offsets the ink onto the substrate. It is imperative that the same amount of ink is delivered hour after hour and job after job if the process is to be predictable and profitable. Although there are many two-roll (sometimes called roll-to-roll) metering systems, the doctor blade is clearly the choice for repeatability. For some who prefer “art” over science, the two-roll system does allow the operator to vary the ink film. Of course, achieving the same variation on repeat orders poses a problem, complicated further when a different operator is at the controls.
The best of flexo printing requires precise doctor blade metering, and the chamber blade system is the system of choice, at least until something better is developed. The flexo press is easily retrofitted with the latest metering systems; many older machines still in sound mechanical condition are being retrofitted to bring them up to the print quality capacity of much newer presses.
3.2. TYPES OF ANILOX CELLS AND CLEANING SYSTEMS
THE ANILOX ROLL
The anilox roll is a uniformly imaged gravure cylinder. Figure 3.6 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 specifications of the cells in the anilox roll determine its capability for specific applications. For example, an anilox roll with 200 cells per inch, having a cell depth of 30-35 microns, will carry a volume of 7.5 bcm (billion cubic microns per square inch). This is a lot of ink. It would be like a six-inch paint brush, only good for very heavy applications of ink. You could paint a barn or rough siding with a six-inch brush and you could cover a very rough, absorbent kraft paper with a 200-lpi (lines per inch) 7.5-bcm anilox roll. However, if you wanted to do fine work, like fine lines and 133line halftones on a smooth and coated paper you might want a 600lpi 1.6-bcm anilox roll. Determining the best anilox roll for a given production scenario is a MUST; first an explanation is required of the specifications and how they relate to the substrates to be printed and the variety of graphics required to be reproduced.
3.2.1 ANILOX ROLL SPECIFICATIONS
CELL COUNT
Cell count refers to the number of rows of cells per linear inch (specified to linear centimeters in the metric world—divide by 2.54 to convert). A cell count of 180 would be very coarse, found only in coating or low-end imaging applications where substrates are poor and quality is not a priority. A cell count of 360, once considered fine, is now a middle-of-the-road roll used in good work on absorbent paper and paperboard substrates. Today cell counts of 700 and above are commonly used for very high-quality imaging on smooth, high-holdout (not absorbent) substrates. This explanation places importance on the substrate in choosing an anilox roll. Images, however, are also very important in determining the cell count.
CELL DEPTH
Cell depth is the next specification and is just as important as cell count. These two specifications determine cell volume, which is the determiner of density in a given application. Figure 3.7 shows that three aniloxes of the same cell count may have very different volumes depending on the cell depth. It is volume that interests the printer. When specifying an anilox roll determine the cell count and volume to do the job and leave the depth to the anilox supplier.
CELL VOLUME
Cell volume is the key to coverage and uniformity of solids. More volume results in more ink and, thus, better coverage. However, too much volume of ink also results in dirty print. If there is too much ink to sit on top of the relief image of the plate, it will flow over the shoulders and result in dirty print.
High-resolution images require high-line, low-volume anilox rolls. There are rules of thumb for determining anilox cell count from halftone lines per inch. It is common to demand at least 3½ to 4½ times more cells on-the anilox than the lines per inch in the halftone. This is to prevent anilox moire, an objectionable pattern caused by the screen of the graphics interacting with the anilox screen pattern.
Figure 3.8 shows the importance of the cell count the ability to produce clean printing. It can be seen that a high-line count (“line count” and “cell count” are terms used interchangeably) roll has enough cell walls to support very fine screened images. A coarse cell will allow small percentage dots to fall inside cell, without being supported by a cell wall, and thus permit ink to flow around the image onto the shoulder of the dot. This causes “dirty print” or dots to join wherever a dot is unsupported by a land area. A high-line-count fine anilox roll will produce clean printing of fine screens and type.
The best anilox roll specifications yield just enough ink to deliver the required density and solid uniformity while not overinking the fine screens in the plate. This roll has enough cells to provide lands to support the finest image areas. Cell angle can also be controlled. While traditionally the cells are angled 45° from the axis of the roll, it is possible to fit more cells into an area when they are aligned at 60°. Since this provides more cell openings and less land area, or space between cells, 60° rolls achieve better uniformity with less ink. The 60° angle is also better in avoiding moire with traditional graphic screen angles since it no longer falls in line with the most desirable image angle of 45°. Today most new rolls are purchased with 60° cell angle. Sometimes a flexo printer concludes that the ideal roll is a very-high-line-count, even when printing on an absorbent substrate. The all that is needed is very deep cells to achieve the required volume. This introduces one last concept to be considered, depth-to-opening ratio. Figure 3.9 illustrates several depth-to-opening ratios. It shows that very high volumes might be engraved into an anilox of a high cell count. However, the bottom row illustrates that when the depth exceeds a certain point, no more ink is released to the plate. There is a range within which volume on the roll can be used to control in film on the printed substrate. Beyond that range, no additional ink can leave the roll and there will be no increase in density.
Until now the discussion might suggest that the anilox is the so determiner of ink film thickness. But this is far from the case. The ink itself is a major player. It has been assumed that the amount of liquid that is printed controls the dry ink on the product. Actually it is the amount of solids, particularly the colorant, or pigment. Figure 3.10 illustrates that one ink may require 40% more liquid be printed to result in the same density and solid uniformity. This, of course, would require a 40% higher volume anilox roll. Such an anilox would not print as clean. Therefore, when people talk of high-line-count low-volume aniloxes, you must realize they are also talking about inks with the maximum amount of pigment and the least amount of liquid necessary for transfer and adhesion.
3.2.2. TYPES OF ANILOX ROLL BASED ON CELL SHAPES:
Anilox Roll is engraved with tiny cells. They normally have an inverted pyramid shape. These cells or pockets when filled with ink from fountain roll carry up an exact quantity of ink to the printing plate. Choosing a proper anilox for the job is important for successful flexographic printing. If the cell count is more, the ink carrying is also less. The Anilox rolls come in various sizes with various shapes of cells. Three basic shapes of Anilox roll cells are
i. Inverted pyramid shape cells
ii. Quadrangular shape cells
iii. Trihelical shape cells
i. Inverted pyramid:
Anilox roll with inverted pyramid shaped cells are recommended for all types of flexo inks as well as varnishes and coating.
Quadrangular Cell:
Anilox roll with quadrangular shaped cells carry more volume of ink in comparison with inverted pyramid cells. These cells are oftenly used with reverse angle blade.
Trihelical Cell:
Used to apply heavy viscous coating. This type of Anilox roll can be used with or without reverse –angle doctor blade.
Anilox rollers are normally engraved. After engraving they are copper finished then hard chrome plating is applied to increase their life.
3.2.3. TYPES OF ANILOX ROLLS BASED ON ROLLER SURFACES
i. LASER-ENGRAVED CERAMIC ANILOX ROLLS
Laser-engraved ceramic anilox rolls are the dominant type of roll being used today. This is a steel roll that has been machined to very precise dimensions and tolerances. It has a plasma sprayed chromium oxide surface built up to a thickness of 0.00~0.010 in. The cells are burned into the ceramic with a CO2 laser that literally vaporizes the coating, leaving a precise cell. The cell count and depth are computer-controlled, meaning that theoretically any specification can be set.
The ceramic surface is extremely hard, which is very important to print quality. Since high-quality flexo printing is achieved with doctor-bladed ink metering systems, the rolls must not wear or repeatability would be impossible. While ceramic rolls do wear, it occurs over an extended period of production.
ii. CONVENTIONAL (OR) MECHANICALLY ENGRAVED CHROME ANILOX ROLLS
While today the vast majority of new rolls being purchased are laser engraved ceramic, there are still many rolls in the industry of the engraved chrome technology. These rolls, also called mechanically engraved, or simply “chrome,” are manufactured by a displacement process, the same as knurling. A hard, precise tool called a mill contains a male pattern of the cells (Figure 5-9). The mill is forced under tremendous pressure into the steel- or copper-covered steel roll. During several passes over the roll the cells are made deeper and deeper until the roll has reached full engraved depth. Just as ice dropped into a glass of water raises the level of water in the glass, this process displaces the metal up into the mill while the mill is pressing deeper into the surface. Since every cell is produced from the same “master” the conventional engraved chrome roll is a very uniform “gravure cylinder.” The roll is electroplated with a hard chrome to provide protection from wear, hence the name engraved chrome. Figure 3.11 illustrates the two most common cell shapes used in flexo printing, quad and pyramid.
Engraved chrome has limitations that helped to move the market to laser-engraved ceramic, the greatest being its lack of resistance to the wear caused by doctor blades. Since new cell specifications require a lengthy process, demanding very high craft skills, to make the engraving tool, it was not possible to perform quality improvement experiments in a timely and economical fashion. These two factors were major contributors to the early acceptance of laser engraving as an alternative approach to anilox roll production. In little over a decade the dominant roll of choice changed from engraved chrome to laser-engraved ceramic.
There are other types of anilox rolls. Conventionally engraved rolls can be plasma sprayed with ceramic instead of chrome and yield better life. These rolls are called engraved ceramic. This approach, however, has never been widely adopted. Another approach to anilox cell production is electromechanical engraving. This method uses the same machines employed in the production of gravure cylinders. The Ohio Engraver and the Helioklischograph are the two most common tools to employ a diamond stylus in cutting precise cells into a copper surface. The copper is then electroplated for wear resistance.
One last technique known as random ceramic has been employed. This is a roll which is simply plasma-sprayed with chromium oxide particles. The coarser the particle the more ink carrying capacity. Like sandpaper, the rougher the surface, the more ink, and the finer the particles, the less ink. This is a simple system, not as uniform in its ink delivery, and is used relatively little compared to other types.
DEVELOPMENTS
Lightweight cylinders are now being used to replace the standard steel construction. Modern materials such as carbon fiber can be used to build the base roll without the weight of steel. These are much easier to handle, and shipping issues are reduced. The same is true of the use of sleeves, similar to those being used for plate cylinders. It is important to note that many new ideas continue to develop in this and other aspects of flexo printing, which is a sign of the atmosphere change and development that characterizes flexo technology.
3.2.4. TYPES OF ANILOX ROLL CLEANING SYSTEMS
i. Roll Cleaning System:
The level of cleanliness of anilox roll and indeed the ease of achieving, is one of the most important problems facing the flexo printer today. This is due to the rise in screen counts and the ever increasing requirement for quality improvements.
ii. Jet wash type system:
These are very simple mechanical device that utilize specific heated chemicals fired at the roll under high pressure. They are not generally not screen counts sensitive and will work over a range of screen counts. On the downside, their success is heavily reliant on the type and condition of the chemicals employed, which can be expensive, being applied at by volume with water. Performance can drop dramatically as chemical becomes contaminated.
iii. Powder blasting system:
Generally single roll system, they use the impinging force of a particle to knock out the contamination and are supplied as either wet or dry systems. Work well up to moderate screen counts when the operator is fully lined and interested. Gaps between roll and nozzle, air pressure, feed rate and the feed itself all need to be monitored and controlled. Also these system can have an issue with ink. Powder is a total loss and can be used for on press cleaning, however this can be a messy process.
iv. Polymer bead blasting system:
The same rules apply as with powder blasting. The units work well with the correct beads up to moderate screen counts when the operator is fully trained but the units suffer from the same limitation as powder blasting. These cannot generally be used as on press system is that there is no waste to consider. There are couples of fringe systems that have entered the market over the last few years that deserve mention.
v. Dry ice system:
These as the name suggest utilize the dry ice to blast out the contamination. The equipment is also used as a general press cleaning system and although when properly used and controlled will clean the roll well but not enough is known about the longer term effects on the ceramic and the units are very noisy in operation. Again, there is no waste to consider with these systems.
vi. Laser Cleaning System:
These systems utilize generally the same laser used to cut the cells as to clean them. This vaporization of the contamination will render the roll clean but the systems are very expensive in comparison with other devices available and as it is generally the lower skilled operative that are left to clean the rolls, the question of skill level should be raised.
vii. Ultrasonics:
This system work by the flexing of the base of a filled tank at very fast rate. So fast, that on the downward stroke a vacuum is created under the water microns. On the upward stroke the vacuum is closed and pushed up into the fluid in the form of microscopic vacuum bubbles that collapse on contact with the roll surface, sucking out the contamination. When correctly controlled and combined with a suitable cleaning chemical (usually at 10%with water), this method will give excellent results. The system generally do not require a skill to operate and when used regularly are very quick and effective, however waste is a consideration, although this can be neutralized in certain circumstances and the volumes are low in comparison with jet wash system.
viii. Alpha sound:
This equipment utilizes ultrasound but, there are differences compared with ultrasonic cleaning systems generally available today for this purpose. This technology embraces and manipulates various frequencies and power levels to specifically target screen count ranges. Tight control over the base technology is the key and other various system features stop operator error and protect the roll. There is enough room in the marketplace for all the above roll cleaning methods but what is the best? That is the operator to decide.
3.3. SELECTION OF SUITABLE ANILOX ROLLER
CHOOSING THE ANILOX ROLL
There are always several, if not many, considerations to be made in the choice of anilox rolls.
1. Substrate. If only one substrate is to be printed then the choice is easy. Many times one anilox roll must be used for a range of substrates. This calls for the anilox which delivers the least ink required to achieve density and solid uniformity on the most absorbent of these substrates.
2. Anilox cost. If one roll costs $15,000-20,000 for a five-color press, one may have to settle on just one or two sets of rolls for all one’s needs. This is especially true in the corrugated industry. Therefore, even to print a variety of substrates and types of graphics, a compromise must be found or economic reasons.
3. Time. Modern flexo presses generally provide for quick changes of anilox rolls; however, most presses in use today are not so equipped. This means that to optimize the anilox to the job at hand, the changeover times may be prohibitive, again, a compromise is necessary.
4. Graphics. It is common for customers or designers to specify graphics with fine screens on substrates of less than ideal surface. Since most jobs mix screens with solids this scenario presents problems. In this case the anilox will probably be chosen to achieve adequate solid density and uniformity while delivering more ink than necessary to the screens; another common compromise.
5. Productivity. While there are many influences on productivity one common example is the availability or lack of dryers. The classic case is in the envelope world where very high speeds are expected without availability of dryers. Drying relies more on absorbance into the paper. The substrate is also generally rougher than ideal so that viscosities (fluidity of ink) must b lower, more liquid, and thus anilox volumes higher in order to deposit sufficient pigment for density and uniformity. Bottom line: fine screens are likely to suffer.
These realities point to the value of planning all jobs with input from the entire production team. It is unrealistic to expect a customer and designer to understand so much. Since everyone seeks total success in any project, working together from the beginning will result in the best achievable results given the specific realities hand.
Types of flexographic printing presses: flexographic presses are usually rotary web presses equipped for the printing of one or more colors. To meet high speeds with consistent printing quality.
It presses are used for packaging printing, they print only one side of the web but they can also be made to print both sides of the web. Web width vary form 4” to 99”. It is first printed and may therefore be sheet-fed, slitted, die-cut etc.
In addition to the variety of accessories available, presses vary in their die-cut etc.
1. In line type: in this type of presses, the printing unit consist of an impression cylinder and a plate cylinder is separate and remaining printing units also have similar structure. The web travels from one unit to another unit in a straight line. It requires elaborate register control system for multicolor printing.
2. Stack type press: the stack type press has two or more printing and inking units vertically arranged on one or both ends of the main press structure. The inking units and impression units are mounted on each side of the central side frames, so that a four color machine the first and second color units are on one side of the frames and the third and fourth units are on the other side of the frames. The closeness of the printing units in this press design provides satisfactory register and will produce excellent result on highly absorbent surface which old not require special drying.
It are only used for simple print jobs because of their poor register accuracy around +/- 0.2mm in longitudinal register.
3. Satellite type presses: the common impression or central impression press has one central impression cylinder with the plate and inking units groups around it. The central impression cylinder may be larger as five or six feet diameter. It give better registration than the stack type press. But it is more difficult of ink between the units, where over printing is necessary.
CIC diameter greater than 2 meter and web width of 300 to 300nm.
Flexographic printing units are interchangeable and can also be combined with offset and gravure printing units for the construction of hydrid system.
Flexographic printing presses machine:
1. 8 color CIC with state of the art control technology equipping of the press with plate cylinder by means of a robot system from a loading trolley, a change of plate cylinder sleeve through the side frame, it takes around 12 minutes to change 8 cycles, cylinder diameter is 2275mm web width up to 1770mm, printing speed 6m/s 34DS/8-CNC, fischer & krecke.
a. Astraflex, W& H ----------------------------- an 8 color press with CIC.
b. Lemanic 82F, bobst----------------------- flexographic press with in-line unit stand with intergrated, die-cutting unit for folding box production, speed -3.5 m/s.
c. Arsome EM 510, Heidelberg (gallus------- flexographic label printing press with U.V drying and rotary die-cutting, 2.5m/s.
d. Flexicourier, KBA ------------------ newspaper press for multicolor printing with 144 printing units.
e. Flexogold, Aurelia --------------------- sheet fed flexographic printing press, two color for 70 cm X 100 cm (28 inch X 40 inch) format, 12000 iph.
The commercial printing industry has recently seen the popularity of hybrid presses grow. Consumer products are requiring more intricate prints and durable final products, which in turn has required printers to adjust their capabilities to meet demands.
This has resulted in many printers opting for hybrid presses which delivers more of the “pros” of each of the above printing methods, without sacrificing quality or efficiency.
One of the main reasons that printers are opting for digital flexo hybrid preses is it allows the flexographic printing process to better accomplish shorter runs, without sacrificing the quality of a final flexo print.
Hybrid presses enable full color digital inkjet technology to be utilized with an existing flexo press. Brand new digital presses that feature flexo printing stations are also available.
'Hybrid Flexographic Presses'. There are several varieties of "hybrid" printing processes that combine aspects of flexography with other methods.
Flexo Gravure. Flexo Gravure is a form of offset gravure. Offset gravure printing essentially replaces the flat offset plate with a longer-lasting gravure cylinder, transferring the image to a rubber blanket which, in turn, transfers the image to the substrate. In flexo gravure, offset gravure is performed on a flexographic press, with the gravure cylinder replacing the anilox roller. A rubber blanket (such as that used in offset lithography) is mounted on the flexo plate cylinder. The ink is transferred to the engraved cells of the gravure cylinder (which, unlike conventional gravure, need to be engraved so that the image is right-reading); the image is then offset onto the rubber blanket (where the image becomes wrong-reading), and is finally transferred onto the substrate. Flexo and offset gravure are utilized when the desire for the high-quality gravure image carrier and long life of the gravure cylinder are needed for substrates that are not easily printed by traditional gravure. The flexible rubber blanket ensures high-fidelity image transfer on a wide variety of surfaces.
Offset Flexo. Offset flexo is a hybrid of flexography and offset lithography in which the anilox roller transfers the ink to a flexo plate (which needs to have its image in positive-reading form) which then offsets the image to an offset blanket cylinder mounted between the plate and the impression cylinder. Cylindrical plastic containers need to be printed in this manner. In some presses, all the color stations are positioned around a single blanket cylinder, and a multi-colored image is registered on the blanket, a single multi-color image being transferred to the substrate in essentially one pass.
The main advantage of flexographic printing, as was mentioned earlier, is its ability to print on many different types of substrates. There are far too many flexo substrates used to provide a comprehensive list here; flexo presses print everything from breath-mint wrappers to plastic packages that hold king-size mattresses. In the past, different types of polymers (i.e., plastics) mixed together tended to yield poor substrates with low print characteristics, but new advances in chemistry and manufacturing are producing new blends of plastics—known as "plastic alloys"—which can impart different qualities to the final product, such as increased strength, chemical resistance, resistance to the penetration of oxygen or other gases, etc. As the substrates change, so must the ink; cooperative efforts between ink manufacturers and the manufacturers of substrates ensure that for each new substrate that can be printed a compatible ink will enable printers to utilize it effectively, efficiently, and economically.
UNIT-3
Flexo inks are selected based on final product requirements:
Unit – 4