Offset Printing: How it is done?
Offset printing is a process of printing from a plate that has been coated with an ink. The plate is offset from the paper, so that the ink will not be transferred to the opposite side. The image on the plate is transferred to the rubber blanket, then onto paper through wet inking and pressing.
In this article we'll take a closer look at how offset printing works and see how it builds on newspaper production methods of paste up and stereotyping – which are still used today in certain specialist applications such as large scale phototypesetters, production of film negatives for color separation, letterpress poster making, typesetting etc.
The offset printing process is a similar to the rotary and flexographic printing methods. The offset printer prints on one side of paper, then views the image through a mask that has been overlaid onto the paper; then impression is taken of this image object onto a rubber blanket, which forms the reverse side of the final printed piece.
Paper used with an offset press is generally known as "bulldog" or "coated" or "hot" or "ink jet". This paper is made from cotton fibres with a water-based adhesive applied to it. The inking fluid used in offset printing is supplied by 3M (St. Paul, Minnesota, USA) and has been in use since 1933.
Offset printing is an environmentally friendly printing process. It only uses clean water to wash the ink and does not require harmful chemicals to clean the machine. It also separates the ink from water for reprocessing into other products.
Offset printing has the highest productivity rates. A print run of 500,000 pieces can be done in just a few hours and much faster than flexographic printing or letterpress printing which are one of the slowest in comparison. The environment in which it operates also helps speed up production time, as it uses warm running temperature and low humidity, this keeps its high speed operation without getting poor quality prints as experienced by newspaper printers during bad weather conditions.
Offset printing has several different processes that can vary depending on the company.
Roll to Sheet: Used for commercial printing and for publishing magazines or catalogues. The process is cheaper than sheet fed, but not as high in quality. It runs on full mesh offset press.
Sheet fed offset: Used for most commercial printing, it average print runs varies from 1,000 to 5,000 per sheet. An operator would lay out the page in pages of four and ink each with a light coat of varnish (drying agent) which keeps the ink well adhered to paper. It is then felt fed through the press by a slitter and die cutter unit (which cuts the image into pieces). The printed sheets are then filtered and dried.
Spot Color: Printing is done on sheets of paper using a plate that has one color on it. Each color is individually inked and printed, not using the 4 color process. The plate and press are also used for letterpress printing. There are 2 types of spot colors:
Process spot colors: This is a CMYK registration mark (small rectangular shape) or lined rectangle shape or circle shape that indicates that there is a separation to be made. Where each part of the rectangle has a different set of values for the CMYK primary colors.
Discharge spot colors: This is a plate with a spot color on it. The plate has an opaque black ink or varnish applied to the area that is to be printed. A discharge ink is then applied over this, which reacts with the varnish and transfers the color to the paper.
Ink jet technology: Some offset presses are now being replaced, by newer technologies like ink jet technology. The press ejects thousands of microscopic droplets of water based inks onto special photo paper using piezo-electric print heads and applying thousands of tiny dots within each printing cell (pixel). This gives a high quality image at a much lower cost than traditional screen printing methods. (Ink jet is also the most popular way to print a computer printer).
Offset printing has become an important part of the US economy. It employs millions of people and provides for millions more with jobs being created in the paper and ink industry.
It is also used for posters, advertisements, calendars, magazines and greeting cards.
Source: http://en.wikipedia.org/wiki/Offset_printing#Types_of_offset_printing
Source: http://www.webopedia.com/TERM/O/Offset_printing.html
Source: http://www.softschools.com/printing/offset_printing_articles/1
Source: http://www.intute.ac.uk/printing/printingtheory/offset.html
Future Scope: Offset Printing
Source: http://www.publishingindia.com/article-17-future-scope-of-offset-printing.html
Different Types of Printing (Part 2): Offset Printing
Source: http://bearpawdesigns.com/v1_content/newsletter_030810_edition2.pdf
Source: http://www.ehowzine.com/facts/general/offset-printing/.
Source: http://www.gutterseminar.com/how-is-offset-printing-carried-out/
Source: http://www.gutterseminar.com/why-is-offset-printing-used/.
Image Source: http://www.gutterseminar.com/4c_color_process/.
Source: http://www.dmnews.com/Features/20071006174/FeatureStory21883412881.htm
Source: <a href="http://www.arcondesignsltd.co.uk/" target="_blank">http://www.
Conclusion
Today, offset printing is the most commonly used method of printing large format and small format pages, posters, brochures and leaflets. The main reason for this is that offset printing has many benefits over other processes. It has a speed advantage over letterpress and flexographic because there are no inks to clean up afterwards and it can be printed on less than perfect paper. It also has a higher quality using four color reproduction than letterpress or lithographic methods because each color is applied separately instead of being screen printed. Blueprinting images is also easier with an offset press. A letterpress printer cannot print in reverse, whereas an offset printer can print double-sided prints without flipping the page over on its own axis.