MD. Mofakharul IslamInstructor (tech) & Head of the department Printing.

FlexographyThe modern letterpress

History• The first flexography

press was built in England and was called aniline printing

• The first rubber printing plates used aniline oil-based inks

• In the early days the technique was used mainly for the printing of food packaging

Evolution

Originally flexography could not compete with offset printing.

Since 1990 advances have been made in platemaking and flexography inks.

By today full color picture printing is available and print quality can rival the lithographic process.

Progress

Ammonia and Alcohol was used for the ink as drying agents, that raised concerns about air pollution

Photopolymer plates are replacing rubber based plates. They produce a sharper image.

Modern Flexography Inks

The oil-based inks used in letterpress can cause rubber to swell.

Traditional flexography ink contains ammonia and alcohol.

Pollution control equipment is required for older inks.

Water based ink and a dryer has been pioneered, that reduces costs.

Modern Flexography Inks

There are five types of inks that can be used in flexography: solvent-based inks. water-based inks. electron beam (EB) curing inks. ultraviolet (UV) curing inks and, two-part chemically-curing inks (usually based on polyurethane isocyanate reactions). Although these are uncommon at the moment.Water based flexo inks with particle sizes below 5 µm may cause problems when deinking recycled paper.

Flexography Ink Control

The ink is controlled in the flexographic printing process by the inking unit. The inking unit can be either of fountain roll system or doctor blade system. The fountain roll system is a simple old system yet if there is too much or too little ink this system would likely not control in a good way. The doctor blade inside the anilox/ceramic roller(Metering roller) uses cell geometry and distribution. These blades insure that the cells are filled with enough ink.

Modern Flexography Plates

Rubber plates are traditionally used, but the image quality is not prefect with them.

Photopolymer plates are replacing rubber plates.

Photopolymer plates have a sharper image.

A light process (UV) is used to harden the printing parts of the plate and the unexposed parts wash away.

Photopolymer plates are as flexible as rubber plates.

Platemaking

Method AThe most popular platemaking process uses the UV light sensitive polymer. A negative is placed over the plate that is exposed to UV. The polymer hardens at the printing area and the rest is washed away either by water or some solvent.

Method BThe second method uses laser engraving process to engrave the printing plate. This method is also called digital plate making.

Platemaking

Method CThe third method is to create a metal negative by an exposition and acid bath. In the second phase this metal plate can be used to make a mold from glass, plastic and once it’s cooled this master mold is used to press the rubber component through a second molding process.

Mounting

In flexography for every printed color a different plate has to be made, therefore mounting the flexographic plates need to be very accurate.

Plates in Flexography have mounting marks, but since 2007 an automatic mounting machine (FAMM) is also available.

Printing

The ink roll is immersed partly in the ink tank and transfers the ink onto the meter roll. The meter roll has a special texture that hold a certain amount of ink. Different meter rolls can be used for different print jobs. A doctor blade (meter scraper) removes excess ink from the meter roll.

Printing

The scraped meter roll ink the plate finally and then the substrate (paper) is “sandwiched” between the plate and the impression cylinder so the imprint transfers to the substrate. The printed substrate then moves into a dryer.

Basic parts of the press

Unwind and infeed section- The roll of stock must be held under control so the web can unwind as needed

Printing section- Single color station including the fountain, anilox, plate and impression rolls

Drying station- High velocity heated air, specially formulated inks and an after-dryer can be used

Outfeed and rewind section- Similar to the unwind segment, keeps web tension controlled.

Flexographic Presses

Stack pressColor stations stack up vertically, which makes it easy to access. This press is able to print on both sides of the substrate.Model NO. FKFS 20 30 40 STACK TYPE Application OPP. PP. PE. WOVEN BAG. PAPER etc. Printing Speed. 2~4 COLOR CYLINDERWIDTH *320~1000 6~COLOR CYLINDER WIDTH*320~800

Flexographic Presses

Central Impression pressAll color stations are located in a circle around the impression cylinder. This press can only print on one side. Advantage: excellent registry

Flexographic Presses

In-line pressColor stations are placed horizontally. This press prints on both sides, via a turnbar. Advantages: Can print on heavier substrates, such as corrugated boards.

Ink roll – Fountain roller

The ink roll is partially submerged into the ink tank, this roller transfers the ink onto the anilox or meter roller.

Meter roll – Anilox roller

This roller has engraved cells that carry a certain amount of ink, the excess ink is removed by a doctor blade.

Plate cylinder

Usually a two way tape is used on the print cylinder to hold the flexible printing plate. The ink is transferred to this cylinder from the meter roll.

Impression cylinder

The impression cylinder applies pressure to the plate cylinder so that the image is transferred to the substrate that is sandwiched in-between the two cylinders.

Applications of Flexography

Labels (mostly by flexography)

Food containers (mostly by flexography)

Flexible packaging (mostly by flexography)

Beverage carriers (mostly by flexography)

Corrugated packaging (mostly by flexography)

Paper sacks (mostly by flexography)

Applications of Flexography

Flexographic 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. Printing press speeds of up to 600 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.

ConclusionFlexography printing can give superb print quality and it is cost effective with a fast turn-around time. It allows for printing on the widest range of substrates using a wide range of ink types and custom colors.

ReferencesWikipedia [Flexography] – http://en.wikipedia.org/wiki/Flexography Accessed 21 January 2013

eHow [Flexography Printing Characteristics] - http://www.ehow.com/info_8475499_flexography-printing-characteristics.htmlAccessed 21 January 2013

ScienceDirect [A review of printing and coating techniques] - http://www.sciencedirect.com/science/article/pii/S0927024808003486Accessed 21 January 2013

TNT FlexPak [Technical Questions about Flexography and Flexible Packaging] - http://www.flexpak.co.th/about/faq_technicalAccessed 21 January 2013

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