Flexography is the fastest growing conventional printing process, especially in packaging such as corrugated containers and flexible films.

It has 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 on a variety of substrates.
It is used extensively for printing tags and labels, many in full process color.

HISTORY
Flexography was originally called “aniline” printing because of the aniline dye inks that were originally used in the process. Banned from the food packaging because of their toxicity.

Others coloring agents were developed which were safer, but the name aniline printing persisted.

Because the name still carried bad connotations, Franklin Moses 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 subcommitee of the Packaging Institute’s Printed Packaging Committee, narrowed the choice down to three names- rotopake process, permatone process and flexographic process.
Mail –in ballots from the readers of The Mosstyper overwhelmingly chose flexographic process.
Flexo plates are flexible and imaged in relief, a natural outgrowth of the 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, 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.
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 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 to 750 ft/min.

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 linerboard.
In the 1950’s, cosolvent and polyamide inks were developed for fllexo printing on transparent polymeric films such as cellophane, polyethylene, and polypropylene.

In 1970’s 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 print quality is rapidly improving.

Flexographic Markets and Press Characteristics

Wide Web  

Plastic Bags
Bakery Products
Snack food
Candy and Confectionary
Frozen Food
Dairy Products
Industrial Agricultural
Meat, Poultry and Seafood
Fresh Produce
Dry Mixes (e.g., cereal)
Beverages
Drugs, Surgical, Medical
Household and Sanitary
Toilet, Cosmetic, Cleaning
Apparel / Textile
Tobacco Products

Wide Web Press Characteristics

Press sizes                             32" - 65";                     (40-50 most common)
Number of color units             1 - 10;                          (3 - 6 most common)
Run lengths (lineal feet)         30,000 - 200,000;         (30,000 - 100,000 most common)
Press speeds (feet / min)         400 - 1,000                 (600-600 most common)

Narrow Web

Industrial
Primary Labels
Specialty Labels
Grocery
Pharmaceuticals and Cosmetics
Tags and Tapes
Decals

Narrow Web Press Characteristics
 

Press sizes                             1" - 18"     (7-10 most common)
Number of color units             1 - 10;         (1 - 3 most common)
Run lengths (lineal feet)         5,000 - 50,000; (5,000 - 20,000 most common)
Press speeds (feet / min)         10 - 400 (100 - 250 most common)

Folding Carton

Drugs, Surgical, Medical
Candy and Confectionery
Industrial / Hardware
Toilet / Cosmetic / Cleaning
Frozen Foods
Paper Products
Bakery
Automotive
Toys
Fast Foods
Dairy Products / Liquid Packaging
Apparel / Textile
Computer Supplies
Beverage Carriers

Wide Web Press Characteristics

Press sizes                                 28" - 44+";                (40"+ most common)
Number of color units                 1 - 10;                     (2 - 6 most common)
Run lengths (lineal feet)             10,000 - 1,000,000; (10,000 - 500,000 most common)
Press speeds (feet / min)             400 - 1000                 (400 - 800 most common)
 

FLEXOGRAPHIC PROCESS
Flexo prints from a raised image surface.
Flexo plates, whether molded from rubber or imaged from photopolymer, are generally made from flexible materials.

The simplest form of flexo process consists of four components:
Fountain roll
Anilox roll –ink metering roll
Plate cylinder
Impression cylinder

Fountain roll- (usually made of rubber) rotates in a reservoir of ink and its purpose is to deliver a relatively heavy flow of ink from the fountain.
Fountain roll and anilox do not rotate at the same speed- fountain roll is driven more slowly than the ink metering roll, causing it to wipe away excess of ink from the surface of the metering roll. The pressure and speed differential between fountain and anilox roller- considerable. Must be set to wipe off excess of ink from the surface of anilox, leave the ink only in engraved cells.

Anilox roll- Ink metering roll, made of either steel or ceramic materials. Surface is engraved with uniform cells that carry and deposit a thin uniform ink film onto the plate.
Number of cells- 80-170 lpi (80-70 cells per linear inch).
Anilox supplies a fine film of ink to the printing plate.
The anilox is often used with a reverse angle doctor blade to wipe excess of ink = two roll system.

The plate cylinder- is usually steel and 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 metal cylinder. Impression cylinder backs up and supports the substrate as it contacts the printing plate.
The contact point, or nip, gives “kiss impression”- the lightest possible impression that transfers the ink to the substrate.

DESIGN
The design of a printing job for flexo is similar to that of all other processes.
The desired printed image is designed and laid out on the computer.
Negatives are produced photomechanically or digitally. Direct to plate systems for flexo are appearing on market.
Compensation and shape distortion- issue, which is not of importance for other printing processes.
Printing plate elongation takes place when the plate is curved around the plate 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.
 

FLEXOGRAPHIC PLATES
Rubber plates –solid rubber pieces made from a mold. Mold can be used over and over to make new plates.
Photopolymer plates have taken an increasing share of the flexo plate market.
Photopolymers - light sensitive polymers that crosslink when exposed to UV light.
Steps of preparing photopolymer plates are:
1. Back exposure of base to UV light to harden (cure) the floor
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 shoulders
6. Finish plates with chemicals or UV light to remove residual tackiness

Photopolymer plates- 150 lpi- produce accurate multicolor registration and hold fine halftone dots.
Acrylates commonly used as photopolymer material. Require photoinitiator to achieve curing in a reasonable time.
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.
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.
The photoinitiator accelerates the reaction by UV light. The photoinitiator absorbs UV light, it splits apart into free radicals.
The unpaired electrons on the free radicals attack the -C=C- (carbon carbon double bonds) of the elastomer or the acrylate, which open up and react with other double bonds.
Typical photoinitiator- e.g. benzophenone.
The photoinitiators generally become part of the polymer chains and therefore initiate very long polymer chains.
The washout solvents are frequently chlorinated solvents- such as perchloroethylene. Because of environmental hazards of chlorinated solvents, substituents are being sought.
Introducing carboxyl groups into the elastomer improves the solubility of the non-cured elastomers in washout solvents.

Most photopolymers work well with alcohol and water based inks.
Oil based inks tend to swell natural rubber plates. Butadiene-acrylonitrile elastomers are used for oil based inks.
Overall plate thickness and relief height vary with the substrate.
Plates for printing corrugated greater relief than for printing film or paper.

FLEXOGRAPHIC PRESS TYPES

The three types of flexo presses are most common: 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 press frame.
There may be one to eight stations with six being the most common.
There are three advantages of the stack press:
The web may be reversed to allow perfecting.
The stations are very accessible
Stack press 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. Its greatest advantage- holding registration.

The in-line press has separate color stations (new Comco press) in separate complete units driven by a common lineshaft. They are arranged in a horizontal row, each standing on the floor. They can handle wide webs. They are used for folding carton, corrugated, and multi-wall bag presses. Used also for pressure sensitive and standard labels. In line operations can be included right after printing section.
Flexo presses print webs as narrow as 4 in., (10.1 cm) and less for labels, and wide as 100 in., (25.4 cm) for shower curtains, toilet tissue. Speeds up to 2,000 ft/min (609.6m/min).

ANILOX ROLL
The anilox roll (metering roll, engraved roll, ink aplicator, ink transfer roll) is engraved 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 inverted pyramid is given by:
                                 _____
V= h/3 [ At + Ab + VAt Ab ]

Where h is the depth of the pyramid
At is the area of the opening
Ab is the area of the cell bottom.

Anilox cell volumes are typically measured in units BCM (Billion Cubic Microns per square inch). Unit obtained by multiplying the volume of a single cell by the screen count in line per inch squared.
A more sensible unit is micron ³/micron ².
As a unit of length it gives a measure of the linear size of the cells.

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.

Anilox screen count  should be at least three to four times the value of the plate screen. E.g. printing plate has screen of 133 lines per inch, it would require an anilox roll with a minimum screen count of 400 to 530 lpi.
Typically, a converter would choose a 550 lpi or 600 lpi anilox when using  a 133 line plate, depending on a ink volume required.

Pattern of engraving:
The cell structures may be of one of five types:
Trihelical, Pyramid, Quatrangular, Hexagonal, Hexagonal Chanelled screen.
 

A. A trihelical screen is an unbroken line inscribed at 45deg angle to the roll axis. These are 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 truncated inverted pyramid. These are very versatile when used with wipe roll and doctor blade systems.
D. A Hexagonal cell is a 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 the cell.This releases heavy body inks. Gives the most even laydown of the ink.
 

The anilox 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.
Anilox wear- profound effect on the volume of ink transferred.
20 % reduction in depth of a pyramid cell can lead to 40-50% reduction in volume depending on the screen count and cutting angle.

It has been estimated that the approximately bottom third of the cell does not release ink.
So when the cell is worn down, the actual ink deposited may be only 40% of the designed amount.
The wear of anilox surface is caused by the use of doctor blades (DB).
The doctor blade removes the 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.

There are several varietes of anilox rolls to choose from.
Smooth Ceramic- Long wearing, the least expensive anilox roll. Do not allow controlled metering of ink because of smooth finish.
Usually employs flame sprayed Aluminum or Chrome 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.
Available in screen counts 500 lpi. Cells are burned into the ceramic using a CO2 laser.
A wide variety of cell patterns is available, cells deliver repeatable metered volume of ink.

Stainless Steel or Nickel/Copper- Flame sprayed to a smooth finish similar to ceramic.

Engraved and chromed- most common engraved anilox roll for high quality print result.
Chrome plated for surface protection. Can be reengraved when worn.
Electronic engraving has come to flexo from the gravure industry.

The reverse angle DB used with anilox is very efficient. The theoretical ideal angle is 30 deg.
Newer chambered blade systems employ double doctor blades, one on the up side and one on the down side of doctor blade holder.

FLEXOGRAPHIC INKS

Water- based
Solvent – based
UV- curable

Water Based vs. Solvent Based:

Low viscosity: Shipped in concentrated form- let down to running viscosity at press – using solvent specified by manufacturer. Close viscosity control- essential for print quality – color control.
Flexo inks- evaporate on the press- change viscosity- change color value.
Continuos viscosity control- adding solvent.
Variation in viscosity - major cause of color variation in flexo printing
Low viscosity needed to transfer properly. Too low viscosity - ink will not remain on the surface of the plate or have adequate density. Runs down the sides of image, builds halo around the printed image, fills in halftones.

Ink formulated with the correct balance of solvents- prints at lower viscosity than improperly formulated ink.
Solvent evaporates from ink fountain and rollers- change of solvents balance- more volatile solvent evaporates more readily- proper solvent mixture to replace evaporating solvent

Aqueous inks- ammonia, morpholine base- volatile. They can evaporate on the press. Insufficient base is present- resin will precipitate- pH decreases, resin separates from the solution before ink is printed. WB- flexo- pH should be monitored while printing, additional base can be added

Drying of ink can be regulated by adding faster, or slower solvent to change drying rate of ink.
Multicolor printing- several colors are printed in sequence. Building ink layers, influencing drying rates.
Drying rates- must be graduated- selecting right solvents- to ensure proper trap.
First down-fastest solvent, last down- the slowest solvent.

Formulating Principles
3 main components: colorant, binder, solvent, and additives.

Water-based Inks
Styrene-maleic, rosin-maleic resins (base: ammonia, morpholine)- low gloss, poor rub
Acrylic resins in solution and emulsion form. Good gloss, and rub. Too viscous. Emulsions too unstable. WB inks used for non-absorbent substrates (films). Dry by evaporation.
Problem: Organic pigments are not compatible with the resins for WB - coagulate
Solution 1: Pigment chips- use of acrylic carrier to produce acrylic color concentrate
New pigment application must be utilized to produce pigments that have water stability
Stabilize pigment not to gel, flocculate, or separate over time.
60% organic pigment 40% water soluble acrylic resin (compatible with other w-b resins
Pigments: Azo , Naphtol Red, Diaryllide Yellow, Phtalocyanine Blue and Green, Methyl and Carbazole Violet. No additives, dispersants, defoamers
Pigment and acrylic resin combined via pre-mixing. Dry mix is dispersed on high horse power heavy duty two-roll mill with power knives. Two roll mill sheets are cooled and chipped via a mechanical chipper to uniform non-dusting particle size.
Dry mix: no settling, bodying, dusting.
Color chip: pigment locked into a vehicle system. Dry color concentrate cut at customer site. pH of cutting solution must be above 8.5 (morpholine, ammonium derivatives)
Solution 2: Addition of surfactant to disperse the pigment

News Inks
Flexo newspapers- acrylic polymers amonium salts, amonia evaporates - free acid groups- precipitate. Good rub resistance.
According to Research Institute  of the American Newspaper Publishers Association (ANPA/RI) –evaluated most suitable processes to print newsprint: Flexo was the most suitable. Flexography meets the requirements of: Low cost press, high quality reproduction, minimum printed waste, a low cost plate, minimum personnel needed for operation and maintenance, adaptability to different types of inks.
Newpapers printed flexo: Pittsburgh press, the Easton Express, the Providence Journal Bulletin, the Greater Buffalo Press (four color comics), the Washington Post.

Flexographic  Inks for Various Substrates
Solvent inks- coated papers, lightweight stock.
Polyethylene – coated paper and boards – need corona treatment to increase surface tension – to print with  alcohol-based polyamide inks.
Polyolefin films, nitrocellulose, polymer coated nitrocellulose- printed with polyamide inks (excellent adhesion, gloss, flexibility). Nitrocellulose increases grease and heat resistance.
Polyamide co-solvent (alcohol and aliphatic hydrocarbon) gives good resistance to water, acid and alkali.
Polyvinylchloride films contain plasticizers- may bleed back to ink, soften it, create blocking problem.
Alcohol reducible nitrocellulose and polyamide inks- useful for printing aluminum foil.
 
Surface printing, Reverse printing, and Laminating

Surface printing- to any substrate.
Reverse printing- to underside (corrugated, laminated films).
Combination of film or foil on paper or other foil- protection from oxygen, moisture
Ink reverse-printed before other material is laminated to it. Ink must adhere well to both materials used. Ink must have low solvent retention- solvent must not be trapped in between the layers.

UV Inks for Flexography
Recent years-development of UV inks.
First- modified letterpress UV inks with lowered viscosity, now different formulations.
Difficult- high pigmentation and good flow characteristics- heating up to 40oC (ink must be stable at this T).
Label Flexo UV Inks
Main flexo market- flexo labels : Self-adhesive label printing , flexo is cheaper and simpler than letterpress.
Formulations- almost like at letterpress, but more reactive diluent, less prepolymer and filler.
 
 

Example of UV ink for printing labels:
Constituent                                                     Parts by weight
Epoxy acrylate                                                             15
GPTA (glycerol propylated triacrylate                        30
TPGDA (tripropyleneglycol diacrylate)                      31
Triethanolamine                                                               3
Benzophenone                                                                  6
Polyethylene wax                                                             1
Pigment                                                                         14
TOTAL                                                                          100

Carton UV Packaging Inks

Cost of UV flexo = ½ of offset litho
Good cure and rub resistance, low odor for food applications.

Example of Low Odor Flexo UV Ink for Carton Packaging
Constituent                                                     Parts by weight
Polyester acrylate                                                                10
Epoxy acrylate                                                                       5
GPTA ((glycerol propylated triacrylate)                             30
TPGDA (tripropyleneglycol diacrylate)                              31
IXT                                                                                        2
Amine acrylate                                                                      3
Stabiliser                                                                               1
Polyethylene wax                                                                  1
Pigment                                                                                14
TOTAL                                                                              100

NEEDS:
Flexible packaging inks- need for solvents emission reduction- especially in  food packaging- low odor, need for good adhesion (frozen food),  combined with low viscosity, good flow and substrate wetting characteristics.