1. How can offset printing be described in two sentences?
Offset printing is an indirect printing process. The printing plate transfers the print image to the print carrier (the paper) via a blanket – rather than making direct contact with the carrier.
2. What is special about offset printing?
Offset is the most widespread printing technology used in book, advertising and packaging printing. The ‘indirect’ process means the printing plate is ‘spared’ from excess impact making it possible to print very high print runs on a wide variety of substrates e.g. very smooth or rough paper and board or cardboard. The speed of Offset printing makes it extremely efficient – producing significantly more impressions per minute than the various digital print technologies.
3. Where does offset print come from? Historical? Geographically?
Offset printing developed through the evolution of lithography. German-based Alois Senefelder was looking for a cost-effective duplication procedure in 1796. Initially he produced high-pressure molds made of limestone, covering the printing area with a greasy cloth and etching the drawing-free areas with a slightly acid solution then wetting the freshly etched image-free spots with water; these areas did not accept the ink with only the greased areas accepting the ink. With his discovery Senefelder laid the foundation for the flat printing principle on which today's conventional offset printing is based. Subsequently Senefelder designed the cylinder press which allowed the printing of paper sheets by means of a stone plate and a counter-pressure cylinder. With the introduction of the zinc plate, at the end of the 19th century, the stone could be replaced by a rotating cylinder with a clamped metal plate.
4. How does offset printing work?
a. General (the technique)
In offset printing the principle of grease repelling water is paramount. The printing plate is first moistened by dampening rollers with each revolution of the cylinder before ink is applied by the ink rollers. The areas that have previously absorbed water remain color-free. The other areas accept the ink and are coloured. On the printing substrate these areas represent all the colour-bearing elements such as text, lines or grid points. The pressure plate is usually composed of aluminum. The ink-bearing sites are formed of a photopolymer. The image-free areas are moisture-wicking. As soon as sufficient dampening solution is offered this thin layer of water blocks the access to the image-free areas. Without a dampening solution the entire plate surface would assume color.
In principle, a distinction is made between sheet and roll offset. As the name implies, in the sheetfed offset the print carrier is a sheet already cut. In a roll offset press, the print carrier runs directly from the roll through the machine.
b. What is possible / what is not?
Possible: Two-sided printing, special varnishes such as UV varnish, soft touch lacquer or also effect pigment varnish.
Not possible: Too thick inflexible material, individualization of the printed product, embossing inline
c. Are there restrictions?
i. To dye
As a rule four-color printing (CMYK) is applied. Special colors can also be used. The number of special colors depends on the number of available printing units on the press; alternatively printed material can be passed through the press for a second and third print-run.
The largest sheetfed offset presses can print sheets up to a size of 149 cm x 205 cm. Small sheets down to the format DIN A3 (297 x 420mm) can be accommodated on compact offset printing machines which continue to be used for smaller print jobs
iii. Papers (thicknesses, etc.)
The paper should be resistant to scratching, should have a good color retention and high abrasion resistance.
Grammages from approx. 60 gsm to max. 700 gsm are possible – depending upon the printing press at your print house.
iv. Number of copies
Sheet-fed offset: Small to medium runs up to 100,000 sheets
Roll offset: very high print runs up to several million
5. In each sentence: The advantages and disadvantages?
The advantages are very high print speeds can be achieved with a very good output quality. With UV offset inks dry very quickly, which is particularly advantageous for uncoated papers.
In contrast, the energy-intensive drying is a disadvantage in web offset printing. Compared to digital printing, offset printed pieces cannot be individualized or personalized.
6. For which applications / projects is offset printing particularly suitable? What are two typical offset projects?
Sheetfed offset: flyers, posters, brochures, books, reports, booklets, calendars, etc..
Roll offset: catalogs, magazines