Flexo Printing

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Flexography

Flexography is the fastest growing conventional printing process, especially in packaging such as corrugated containers and flexible films.It has also made significant advances in publication printing, particularly newspapers.Because the quality flexo printing has improved so much, it is now used extensively for process color printing, as well as spot color, on a wide variety of substrates.

It is used extensively for printing tags and labels, many in full process color. Flexography was originally called �aniline� printing because of the aniline dye inks that were originally used in the process.

The aniline dyes were made for coal tar and were banned from food packaging by the FDA because of their toxicity.

Other coloring agents were developed which were safer, but the name aniline printing persisted. Because the name still carried bad connotations, Franklin Moss in 1951 started a campaign to change the name of the process.

Over 200 possible names were submitted by readers of Moss� publication �The Mosstyper�. A subcommittee of the Packaging Institute�s Printed Packaging Committee narrowed the choice down to three names,: permatone process, rotopake process and flexographic process.

Mail-in ballots from readers of The Mosstyper overwhelmingly chose flexographic process. Flexo plates are flexible and imaged in relief, a natural outgrowth of letterpress printing. The origin of these plates was in rubber stamps, which were formed in plaster molds that had been pressed with lead type.

Thus, the original plates for aniline printing were made of molded rubber. Moss�s Mosstype corporation was a pioneer in rubber platemaking for both aniline and letterpress printing. The first aniline press was built in 1890 by Bibby, Baron and Sons in Liverpool England. It used water based dye inks which were not chemically bleed proof. Because the colors smeared and ran, the device was called �Bibby�s Folly�.

In 1905 C. A. Holweg built an aniline press as a tail end unit on a bag machine. In 1908 he made it the first patented aniline press. The ink metering on these presses was crude until 1938 when the anilox roll was introduced. This roll, patented by Douglas Tuttle and Col. Joe Viner, employed a mechanically engraved copper coated roll with controlled cell sizes. The idea grew out of gravure printers laying down coating from a uniform cell-engraved roll. The anilox uses this process to coat the raised surfaces on the plate.

As with gravure cylinders, the anilox rolls were coated with chrome to prevent corrosion and wear. The original aniline inks gave way to ones based on polyamide resins. These stable, fast drying inks enabled web speeds to increase from 150-750 feet per minute. The 1980 clean air act lead to more extensive use of water-based inks in flexo. Water-based inks are now used extensively for printing on paper based substrates.

In the 1950s, cosolvent and polyamide inks were developed for flexo printing on transparent polymeric films such as cellophane, polyethylene and polypropylene. In the 1970s rubber plates began to make way for photopolymer plates. The use of photopolymer with their hard UV cured raised areas has enabled the significant improvement of the quality of flexographic printing. The quality is now approaching that of offset and is even impinging on gravure.

Flexographic Process

As stated earlier, flexography is like letterpress in that both print from a raised image surface. Flexo plates, whether molded from rubber or imaged from photopolymer, are generally made from flexible materials. Flexo inks generally are low viscosity, highly fluid and quick drying, although there is a trend toward higher viscosity inks. The inks are made from a dispersion of resins, solvents, color and additives, which are either organic or aqueous based.

In the simplest and most common form, the flexo process consists of four components;

1) fountain roll,

2) ink-metering (anilox) roll,

3) plate cylinder and impression cylinder.

These are illustrated in the figure below. The fountain roll is generally covered with rubber.


It rotates in a reservoir of ink and its purpose is to pick-up and deliver a relatively heavy flow of ink from the fountain.

The anilox roll is usually chrome or ceramic plated and is covered with tiny engraved cells. The number of cells varies from 80 to 1000 cells per linear inch. The roll of the anilox is to supply a fine film of ink to the printing plates. Thus, the fountain and anilox rolls are set to rotate against one another with the least amount of pressure required to form a puddle behind the nip. The anilox roll is often used with a (reverse angle) doctor blade to wipe excess ink for the roll.

The plate cylinder is usually steel and in installed between the anilox roll and the impression cylinder. Printing plates are attached to the plate cylinder with stickyback (a special double sided tape). The plates raised surface picks up ink from the anilox and transfers it to the substrate. The impression cylinder is smooth, highly polished and supports the substrate when it contacts the plate. Its speed must match that of the plate cylinder, the anilox roll and the substrate.

Design Considerations

The design of a printing job for flexo is similar to that of other processes.

The desired printed image is designed and laid out on the computer using the usual drawing, image editing and page layout tools.

Negative, right-reading negatives are produced either photomechanically or digitally with an imagesetter.

Direct to plate systems for flexo are just now appearing on the market.

One issue flexo, which is not of importance for other printing processes, is compensation and shape distortion.

Printing plate elongation takes place when the plate is curved around the printing cylinder. This causes the print repeat length to increase and all design elements to grow in the circumferential direction.

To compensate for this the image must be �preshrunk� along this direction using page layout or imposition software.

The scale factor can be determined from the relief height and the cylinder radius.

Flexographic Plates

Rubber plates are solid rubber pieces made from a mold. The mold can be used over and over to make new plates. Photopolymer plates have taken an increasing share of the flexo plate market. As the name suggests, photopolymers are light sensitive polymers that crosslink when exposed to UV light. Photopolymer flexo plates are flexible and resilient like rubber plates.

There are many candidate materials for photopolymer plates. They may be viscous liquids or solid sheets. The steps for preparing photopolymer plates are

1. Back exposure of base to UV light to harden (cure) the floor and establish relief depth.

2. Face exposure of surface to UV light through a negative to cure the image area.

3. Washout of unexposed polymer with appropriate solvent.

4. Drying to remove solvent and restore gauge thickness.

5. Post expose to final cure of floor and character shoulders.

6. Finish plates with chemicals or UV light to remove residual tackiness.

Negatives for flexo platemaking may be produced either digitally or photomechanically, although most are made digitally today. Photopolymer plates have made it possible to print process color jobs with halftone screens up to 150 lpi. They produce accurate multicolor registration and hold fine halftone dots.

Solid photopolymer plates are generally produced by crosslinking a rubber, or elastomer, with a photoreactive material Because of their low activation energy, acrylates are commonly used. Even these require the use of a photoinitiator to achieve curing in a reasonable time. The photopolymer layer in an unexposed flexo plate contains a thermoplastic elastomer, a polyfunctional acrylate monomer, a photoinitiator and additives. UV light activates the photoinitiator, which starts the crosslinking.

Many different elastomers may be used: even natural rubber can be crosslinked with acrylates. A popular photopolymer plate is produced by crosslinking poly(2-chloro-1,3-butadiene, commonly known as chloroprene) with trimethylolpropane triacrylate. Upon exposure the trimethylolpropane triacrylate crosslinks the polymer rendering it insoluble. Other flexo plates employ styrene-isoprene rubber with a polyacrylate. The photoinitiator accelerates the reaction by UV light.

When the photoinitiator absorbs UV light, it splits apart into free radicals. The unpaired electrons on the free radicals attack the carbon-carbon double bonds of the elastomer or the acrylate, which open up and and react with other double bonds. This process crosslinks the elastomer. Typical photoinitiators are benzophenone, benzoin and benzil. The photoinitiators generally become part of the polymer chains and can therefore initiate very long polymer chains.

The washout solvents are frequently chlorinated solvents, such as perchloroethylene. Because of environmental hazards of chlorinated solvents, substitutes are being sought. Introducing carboxyl groups into the elastomer improves the solubility of the non-cured elastomers in washout solvents. Copolymerizing nitrile rubbers with acrylic acid allows them to be washed out with glycol ethers. Liquid photopolymer plates are made from partially polymerized acrylates or other unsaturated products.

Most photopolymer plates work well with alcohol and water based inks. Oil based inks tend to swell natural rubber. Sheet photopolymer for oil based inks generally consist of butadiene-acrylonitrile elastomers. Overall plate thickness and relief height vary with the substrate. Plates for corrugated printing tend to thicker with greater relief than for printing film or paper. For corrugated a typical thickness is .28" with a plate thickness of .25" and a carrier of .3".

Press Types

The three types of flexographic presses are stack, central impression and in-line.

In the stack press the different color units of the press are stacked over one another on one or both sides of a press frame.

There may be one to eight stations with six being the most common.

There are three advantages of the stack press:

1. The web may be reversed to allow perfecting.

2. The stations are very accessible.

Stack presses can print on a wide variety of substrates.

The central impression press, sometimes called drum, common impression or CI, supports all of its color stations around a single steel impression cylinder.

The impression cylinder supports the web and helps to maintain color registration between print stations.

The Mark Andy three color label press in the small press lab is a CI press. The greatest advantage of a CI press is in holding registration.

The In-Line press has separate color stations in separate complete units driven by a common lineshaft.

They can handle wide webs since a single frame need not support all colors. They are used in folding box, corrugated and multi-wall bag presses.

They are common for printing on pressure sensitive and standard labels. They can be made to perfect by turning the web over a turn bar.

Dryers

Flexo presses have integrated drying systems.

Generally there are individual dryers after each print station. These dryers are known as between color or interstation dryers.

The dryer after the last print station is called the main tunnel or overhead dryer.

The interstation dryers remove a sufficient amount of volatile liquids from the ink so that the next station can apply another color without altering the previous one.

The main tunnel dryer removes the final traces of volatiles from the substrate.

These systems generally consist of gas fired burners along with both supply and exhaust fans. The burners raise the temperature, which evaporates the solvents from the inks.

The fans maintain the air flow, which helps remove the evaporated solvents. The length of the dryer provides enough residence time for the drying process to occur.

UV inks don't dry like conventional liquid based flexo inks. Instead they cure by an acrylic polymerization similar to that employed in photopolymer plates.

The reaction is similar to that of the discussed earlier for flexo plates.

To cure these inks UV units are attached to the press between printing units.

We'll return to UV inks later when we discuss inks.

Plate Cylinders

The plate cylinder must be very accurate.

Its TIR (Total Indicated Runout) should be in the range .0002-.0005" for good process color printing.

TIR is a combined measure of both roundness and concentricity of a cylinder.

It is the difference between the maximum and minimum radius of the rotating cylinder.

Plate cylinders may be integral construction (part of the press) or demountable.

There are several ways to attach a demountable cylinder to the shafts.

One type mounts with its end wall heated so that it will expand and slide over the shaft. After it cools it shrinks into position and fits tightly.

Another type has a threaded opening on the end wall.

The shaft is threaded and they lock together when tightened with a spanner wrench.

Another type locks into position with a pressure system using a grease gun fitting.

Still another type is fastened with screws which activate a hydraulic system.

Anilox Roll

The anilox roll is known by several other names. These include form roll, meter roll, knurled roll, engraved roll, ink applicator roll and ink-transfer roll. The name anilox is a throwback to the times when the process was called aniline printing. Its construction is similar to a gravure cylinder in that it has a series of engraved cells. Like gravure cylinders they may be mechanically or laser engraved.

The anilox roll may be either chrome plated or ceramic-coated. Laser engraved rolls are generally ceramic-coated prior to engraving. The cells in a mechanically coated anilox are in the form of an inverted pyramid. The volume of the inverted pyramid is given by V = h/3[At + Ab + (AtAb)1/2] where h is the depth of the pyramid, At is the area of the opening and Ab is the area of the cell bottom.

Anilox cell volumes are typically measured in units of BCM (Billion Cubic Microns per square inch, a truly bastardized unit obtained by multiplying the volume of a single cell by the screen count in line per inch squared). A more sensible unit is microns (�3/�2). As a unit of length it gives a measure of the the linear size of the cells. The conversion is Cell volume/Area(�) = 1.55BCM Since the opening is the base of the inverted pyramid, it is the part most sensitive to wear.

Since the region around the opening contains the greatest volume per change in depth, a little wear can cause a significant change in the amount of ink transferred. The cell structures may one of five types;

A) Trihelical, B) Pyramid, C) Quadrangular, D) Hexagonal or E) Hexagonal Channel Screen.


A) A Trihelical screen is an unbroken line inscribed at a 45� angle to the roll axis. These are primarily used in coating applications of viscous fluids.

B) A Pyramid cell is a full inverted pyramid. These are primarily used in wipe roll metering systems.

C) A Quadrangular or Quad cell is a truncated inverted pyramid. These are very versatile when used with wipe roll and doctor blade systems.

D) A Hexagonal cell is an truncated inverted hexagonal pyramid. These can be packed very efficiently in two dimensions and have very good release characteristics.

E) The Hexagonal channel screen is a deep hexagonal cell with a shallow vertical channel that links each cell. This provides a means for even pickup and release of heavy body inks.

This screen gives the most even laydown of ink. The anilox roll is changing more rapidly than any other single component of a flexo press. They are made from a special grade of seamless steel tubing treated for engraving. As stated earlier, anilox wear can have a profound effect on the volume of ink transferred. For example, a 20% reduction in depth of a pyramid cell can lead to a 40-50% reduction in volume depending on the screen count and cutting angle. Sometimes pigment particles collect at the bottom of the cells.

Its been estimated that the approximately the bottom third of the cell does not release its ink. So when a cell is worn down, the actual ink deposited may be only 40% of the designed amount. The wear of the anilox surface is compounded by the use of doctor blades. The doctor blade removes excess ink from the surface of the anilox and provides better control of ink transfer to the plate cylinder. This is particularly important in printing process colors and halftones.

The �reverse angle� doctor blades used with anilox rolls is very efficient, literally shaving the excess ink from the roll. The theoretical ideal angle is 30�. The TIR of the anilox must not exceed .0005" in order to maintain proper blade pressure. Newer chamber blade systems employ double doctor blades, one on the up side and one on the down side. These have provided a tremendous boost to flexo print quality.

There are several varieties of anilox rolls to choose from. Smooth Ceramic - The longest wearing and least expensive anilox rolls. Do not allow controlled metering of ink because of smooth finish. Usually employs flame sprayed Aluminum or Chromium oxide.

Textured Ceramic - Long wearing like smooth ceramic, but uses a coarser ceramic powder. The surface roughness provides the mechanism of ink transfer.

Ceramic-Laser Engraved - Longest wearing engraved metering rolls. Process is becoming less expensive. Available in screen counts up to 500 lpi. Identical to smooth ceramic except that cells are burned into the ceramic using a CO2 laser. A wide variety of cell patterns is available and the cells deliver a repeatable metered volume of ink.

Stainless Steel or Nickel/Copper - Flame sprayed to a smooth finish similar to ceramic. Chrome plated for longer wear.

Engraved and Chrome - Most common engraved anilox roll for high-quality print results. Fabricated from high grade seamless steel. Can be reengraved when worn. Chrome plated to protect the surface.

Engraved and Ceramic - An engraved roll flame sprayed with fine ceramic powders of aluminum or chromium oxide. Coating is very fine, about .0005" in thickness. Usually a coarser screen than for chroming.

Forerunner of laser-engraved ceramic rolls.

Electronic engraving has come to flexo from the gravure industry. The advantages are

Elimination of striations because of computer controlled diamond stylus.
Harder copper surface than for mill engraving and therefor longer-wearing.
Tighter TIR tolerance is possible because engraving is done without pressure.
Screens up to 600 lpi are available, but the angles of the cell walls and cell configuration cannot be varied much because of diamond surface shape.

Fountain Rollers

Fountain rolls consist of a shaft or core of metal or other hard material covered with a rubber or elastomeric coating. The coating thickness may vary from a few thousandths of an inch to an inch or more. Many different rubbers are used in fountain roll coverings.

Neoprene - polymerized chloroprene (2-chloro-1,3-butadiene) crosslinked with sulfur.
A tough elastomer with good oil, chemical and water resistance and high resilience.
Nitrile or Buna N - Copolymer of butadiene and acrylonitrile.
Main attribute is oil resistance.
Good chemical and water resistance.
Ethylene Propylene Diene Monomer (EPDM) - Ethylene and propylene copolymer with small amount of a diene copolymerized for crosslinking purposes.
Because the backbone is chemically saturated, it has excellent chemical, heat and ozone resistance.
Resistant to polar solvents such as ketones but not to oils.
Polyurethane - Backbone of a diol that has been reacted with diisocyanate to form urethane linkages.
Diol is usually a polyether or polyester resin.
Urethanes soften in high humidity and high temperature.
Natural Rubber - Polyisoprene crosslinked with sulfur.
Poor oil, ozone and heat resistance.
When an elastomer is deformed, it doesn't completely return to its original configuration.

The amount of residual deformation is known as �set�. This is usually expressed as a percentage. Resilience is the ratio of returned to impressed energy.
The term �rebound� is often used in this context. Hardness is measured by plastometer, densimeter or durometer.
The Plastometer is the most accurate because there is no hand actuation. The weight of one kilogram depresses a steel ball, 1/8" in diameter, into the rubber.
The scale records the depth of depression in units of 10 microns (10-3 cm). The Adams Densimeter is a hand actuated instrument which uses a rounded impresser pin projecting through the bottom of the meter and connected to the scale pointer.
Higher densimeter readings indicate softer rubber and are roughly half the plastometer readings. The Shore durometer is similar except that higher numbers indicate greater hardness.
Since hardness is usually measured by an indentation, the thickness of the rubber can effect the hardness reading.

Folding Carton Presses

Folding cartons have been printed flexo for many years.
In the past 20 years the industry has upgraded to newer CI presses.
The ability to print wide webs and vary the repeat have made the process attractive.
Improved quality of flexo, water-based inks and the lower cost of equipment have have led to adoption of these presses by converters.
Folding carton equipment differs because of substrate.
The heavy board stock used requires higher tension levels, larger idler rolls and more sophisticated splicing and control units.
Unwinds must handle 72" diameter rolls and automatically splice them.
Idlers must be of large diameter to avoid breaking or creasing the board.
The cutter-creaser involves intermittent motion.
Web control is critical since we must print a quality job in register and then relax the web and die-cut in resister.
Electric eyes register the cutter to the web

Narrow Web Presses

Narrow web presses are generally less than 24" in width.
The original narrow web presses were used for printing self-adhesive tape with simple markings or company names and logos.
There was no need for large equipment for this process.
With the printing of pressure sensitive and self-adhesive labels, some for of die cutting had to be included.
The primary distinction of narrow web presses is the ability to perform multiple converting operations along with printing on the same pass through the press.
This provides a complete converting system for pressure sensitive labels on sheets, in roll form or fanfold stacks.
Most converting on flexo presses is rotary rather than reciprocating.
Narrow web presses are used for short run, quick changeover work.
Products printed on narrow web flexo presses include Small folding cartons, Business forms, Closures (lids for juice and yogurt), Tickets, Tags, Flexible packaging, Multiple layer coupons and Pressure sensitive labels.
The most critical variables for best performance of narrow web presses are tension control, anilox roll selection, precision impression setting, diecutting pressure and the quality of prepress.
Tension control is essential for good register.
Impression setting establishes print quality.
Without these dot gain cannot be controlled, colors look muddy and quality suffers.

Substrates

All sorts of materials are printed flexo from toothpick wrappers to huge bags for mattresses. Films, foils, tissues, enormous corrugated sheets, limp plastic films, glass and textiles are printed with flexo.

These materials weren't chosen for their printing characteristics, but for their functionality. Thanks to the versatility of flexo, there is almost no material that can't be printed.
Flexo is growing faster than other conventional processes because of this versatility.
New polymer blends and plastic alloys will expand the number of plastic substrates that can be printed flexo.
Coextrusion will increase as plastic materials are combined in multiple layers.
Oriented polypropylene film can be obtained with a saran adhesive surface on one side and a heat seal on the other.
Coating and laminating of films will continue. We briefly review some of the wide range of substrates printed with flexo.
Paper and Paperboard

Includes a wide variety of applications. Paperback books , envelopes, and gift wrap (uncoated free sheet, coated, ground wood and unbleached Kraft) may be printed flexo.
Liner board and folding cartons (bleached and unbleached Kraft, coated and uncoated), are printed with flexo.
Paper cups, tubs and milk cartons (bleached) are printed flexo. Flexible packaging and multiwall bags (unbleached Kraft) are printed flexo.

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