OFFSET PRINTING (UNIT 4)

Offset printing is a commonly used printing technique in which the inked image is transferred ( or offset ) from a plate to a rubber blanket, then to the printing surface. Today over 40% of all print jobs in world are carried out using offset printing.

Based on the repulsion of oil and water, the offset technique employs a flat image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film, keeping the non-printing areas ink-free. Development of the offset press came in two versions: in 1875 by Robert Barclay of England for printing on tin, and in 1903 by Ira Washington Rubel of the United States for printing on paper.Compared to other printing methods, offset printing is best suited for cost-effectively producing large volumes of high quality prints in an economically sound manner that requires little maintenance.


Three Cylinders
Offset printing works in a simple manner. It uses three cylinders to transfer the image onto the substrate. The first cylinder is mounted with the printing plate. The image on the printing plate is ‘right’ reading or written with the right side up. The first cylinder is inked and the image transferred or offset onto the second cylinder, which is mounted with a rubber blanket. The image on the second cylinder is thus reversed or becomes ‘wrong’ reading. Finally the image is transferred from the blanket cylinder onto the third cylinder or the substrate. The substrate is mounted on the third cylinder also known as the impression cylinder. The image once again is reversed and becomes ‘right’ reading or right side up in the final printed version

Ink-water balance
Ink and water balance is an extremely important part of offset printing. If ink and water are not properly balanced, the press operator may end up with many different problems affecting the quality of the finished product, such as emulsification. This means the water overpowering and mixing with the ink.It leads to scumming, catchup, trapping problems, ink density issues and in extreme cases the ink not properly drying on the paper, resulting in the job being unfit for delivery to the client. With the proper balance, the job will have the correct ink density and should need little further adjustment except minor ones. Such as when the press heats up during normal operation, thus evaporating water at a faster rate. In this case the machinist will gradually increase the water as the press heats up to compensate for the increased evaporation of water. Printing machinists generally try to use as little water as possible to avoid these problems.


Advantages of offset printing

Advantages of offset printing compared to other printing methods include:
1. high image quality - Consistent high image quality. Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
2. Quick and easy production of printing plates.
3. Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
4. Cost saving -  Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.


Photo Offset
The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates.
In current use, original materials may be an actual photographic print and typeset text. However, it is more common — with the prevalence of computers and digital images — that the source material exists only as data in a digital publishing system.

Web offset
Offset lithographic printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production.
Ink is transferred from the ink duct to the paper in several steps :-
The ink duct roller delivers ink from the ink duct to the ink pyramid. Also called the  Ink Train.

The ductor roller, sometimes called a vibrator roller due to its rapid back and forth motion, transfers ink from the duct roller to the first distribution roller. It is never in contact with both rollers at the same time.
The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers. The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
The printing plate transfers the ink to the offset cylinder (typically called blanket cylinder) usually covered with a rubber 'blanket'. The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.

Sheet-fed Litho
"Sheet-fed" refers to individual sheets of paper or paperboard being fed into a press. A lithographic ("litho" for short) press uses principles of lithography to apply ink to a printing plate, as explained previously. Sheet-fed litho is commonly used for printing of short-run magazines, brochures, letter headings, and general commercial ( job work ) printing.

Heat set offset

Web-fed refers to the use of rolls (or "webs") of paper supplied to the printing press. Offset web printing is generally used for runs in excess of 5 or 10 thousand impressions. Typical examples of web printing include newspapers, newspaper inserts/ads, magazines, catalogs, and books. Web-fed presses are divided into two general classes: "Cold" or "Non-Heatset," and "Heatset" offset web presses, the difference being how the inks that are used dry. Cold web offset printing dries through absorption into the paper, while heatset utilizes drying lamps or heaters to cure or "set" the inks. Heatset presses can print on both coated (slick) and uncoated papers, while coldset presses are restricted to uncoated paper stock, such as newsprint. Some coldset web presses can be fitted with heat dryers, or ultraviolet lamps (for use with uv-curing inks). It is also possible to add a drier to a cold-set press. This can enable a newspaper press to print color pages heatset and black & white pages coldset.

Web-fed vs. Sheet-fed (photo offset)
Sheet-fed presses offer several advantages. Because individual sheets are fed though, a large number of sheet sizes and format sizes can be run through the same press. In addition, waste sheets can be used for make-ready which allows for lower cost makereadies, so that good paper is not wasted while setting up the press, for plates & inks (waste sheets do bring some disadvantages as often there are dust, offset powder particles that transfer on to the blankets and plate cylinders, thereby creating imperfections on the printed sheet in the form of "hickies").
Web-fed presses, on the other hand, are much faster than sheet-fed presses, with speeds in excess of 20,000 cut-offs per hour. Their speed makes them ideal for large runs such as newspapers or magazines. However, web-fed presses have a fixed cut-off, unlike rotogravure or flexographic presses.

Types of commercial offset processes

Perfecting press
A perfecting press, also known as a duplex press, is one that can print on both sides of the paper at the same time. Web and sheet-fed offset presses are similar in that many of them can also print on both sides of the paper in one pass, making it easier and faster to print duplex.

Offset duplicators
Small offset lithographic presses that are used for fast, good quality reproduction of 1- and 2-color copies in sizes up to 12”X18”. (Romano, & Riordan 139–141) Popular models were made by A.B. Dick, Multilith, and the Chief and Davidson lines made by A.T.F./Davidson.
Offset duplicators are made for fast and quick printing jobs; therefore have faster make-readies and turn-around time, printing up to 12,000 impressions per hour.
They are able to print business forms, letterheads, labels, bulletins, postcards, envelopes, folders, reports, and sales literature.

Sheet-fed offset
In sheet-fed offset, “the printing is carried out on single sheets of paper as they are fed to the press one at a time.” Sheet-fed presses use mechanical registration to relate each sheet to one another to ensure that they are reproduced with the same imagery in the same position on every sheet running through the press.

Process
The actual process of printing is quite involved. One of the most important functions in the process is Pre-press Production. This stage makes sure that all files are correctly processed in preparation for printing. This includes converting to the proper CMYK, finalizing the files, and creating plates for each color of the job to be run on the press. The sheet fed press consists of different systems that complete the actual process; feeder system, printing system, inking/dampening system, and the delivery system.

Feeder system
The feeder system is responsible for making sure paper runs through the press correctly. This is where you load the substrate and then correctly set up the system to the certain specifications of the substrate to the press.

Printing /inking system
The Printing Unit consists of many different systems. The dampening system is used to apply dampening solution to the plates with water rollers. The inking system uses rollers to deliver ink to the plate and blanket cylinders to be transferred to the substrate. The plate cylinder is where the plates containing all of the imaging are mounted. Finally the blanket and impression cylinders are used to transfer the image to the substrate running through the press.

Producer of offset press
Manroland  (Deutschland, Germany ) is the largest producer of newspaper presses in the world. Every third newspaper world-wide, gets printed on a Manrolannd web press. In 1911 the first Roland sheetfed offset press was built.
In India Hindustan Times uses MANROLAND COLORMAN Web offset. Colorman presses to 90,000 copies per hour.  Colorman E-Line web press with automatic wireless control which can scan the unit for its wear and tear, ink control console equipped with iPad for single-touch operations and chemistry.
Times Of India (TOI) owner, Bennett & Coleman group also uses MANROLAND COLORMAN. TOI has a long term relationship with Manroland with multiple press lines of Colorman, Geoman, Regioman and Cromoman installed at numerous production sites across India. http://www.manroland.com/ 


Mitsubishi is another big producer of web offset press. Mitsubishi  L1100 is a eight color web offset press.


SOME OFFSET PRESS Manufacturing co. in INDIA

Orient web offset press, ( The Printers House )  http://www.tph.co.in/ - a leading exporter and Manufacturer of Single width , Single circumference Web offset Printing Presses from  India. TPH’s manufacturing facilities at Ballabagarh (Faridabad) near New Delhi , India   ( User – Amar ujala, Dainik Bhaskar, Dainik jagran etc.)

Manugraph - Manugraph is India's largest manufacturer of web offset presses. http://www.manugraph.com/ ( user – Tribune, Dainik Bhaskar, Dainik jagran etc) ( Speed – upto 70,000 Copies per hour )
Ronald Web Offset, Ballabhgarh, (HR)- http://www.ronaldindia.com/
Newsweb Engineers Private Limited, Ballabhgarh  (HR)
Rotta Print India Private Limited, Faridabad. ( HR)

For more see - http://www.printweek.in/