Offset Lithographic printing is excellent for reproducing deep, rich colours which cannot be achieved with other forms of printing.

Offset lithographic Printing

Offset lithographic printing is based on the principle of oil and water working in repulsion.

The printer first has to create the printing plates from your supplied artwork file. Modern technique is to use a computer-to-plate (CTP) machine.

The first stage is to create a print ready PDF if artwork has not been supplied in this format. The file is then imposed for print (the pages placed in the correct position on a virtual plate). The imposed file is then ripped. This process splits the artwork file into plate artwork (1 plate for each colour). To see how the individual colours come together, read our 4 Colour Process guide.

The image is then burnt to the plate by lasers and the non image area is then removed when the plate is developed.

There are 2 types of plate with different methods to develop them. The first is an environmentally friendly method called processless plates (FM Print use processless plates). This method develops the plate using no chemicals. Instead the plates are cleaned and gummed using water based solution.

The second method uses a different plate which requires a chemical developer.

Printing plates are made up of two areas. Areas of the plate which are not to be printed (the "white" areas of the paper) absorb a water based fountain solution. When the plate is introduced to the printing ink, the oil based ink is naturally rejected by the areas filled with water. This ensures that non-printing areas remain ink free. The inked image is then transferred (or offset) from the plate to a rubber blanket, and then from the sheet to the printing surface.

While transferring the inked image to a rubber blanket first might seem an unnecessary additional step, it actually serves two important functions. Firstly, the printing plate is quite delicate, so the soft rubber protects it from possible damage during repeated use. And secondly, the flexible rubber conforms exactly to the surface to be printed. This results in a consistently sharp and clear print every time.
In fact, properly developed plates used with quality optimized inks and fountain solution can perform runs of more than a million or more impressions with little to no degradation in print quality.

Advantages:

• Consistently high image quality.
• Quick and easy production of the printing plates.
• Offset printing plates last longer than plates on direct litho presses, due to no contact with the print surface.
• Especially suited to print runs of high quantities.

Disadvantages:

• Most of the cost lies in press preparation, so the process is expensive for printing small quantities.