Offset is traditionally used for printing the same design in huge numbers . This requires an artwork of the design which can be converted to a Positive . This in turn is exposed on to a printing plate , one plate for each of the four basic colours of Cyan, Magenta,Yellow and Black and the extras for special colours like silver, gold etc if any.
On the Printing Press, there are separate units or sections for different colours and one unit is meant for a specific colour only. As a result the design gets printed on paper one colour at a time,in a sequence. The number of colours a machine can handle in one run depends on the number of units it has ie. a two colour press prints in two colours in one run. The consistency of colours is largely dependent on the positives exposed onto the plates. These encode the percentages in which the different colours are transferred on to the paper in the different units. However, the printer physically checks the flow of inks to ensure consistency in colour on all the printed sheets.
On these Printing Press' only one side of a sheet is printed at a time. It must be then allowed to dry before it can be printed on the reverse.if that's not required ,it still needs to dry before it is cut to size. Offset printing gives a remarkable replica of the design , it has great clarity, sharpness and brightness..it is the best method for printing on paper, and becomes the most economical as the volumes increase .
Screen Printing :
Screen Printing is conventionally used for printing the same design in small numbers . Screen Printing requires an artwork that is photographed to produce a Positive . This positive is exposed onto a fine silken screen using a simple tubelight or at times just the sun's rays. This process allows the silk to be porous in different places based again on the number of colours and the design.
The paper is placed in a marked out area on a table and inks are placed one at a time on the screen. The inks are "squeezed" out by running a flat wooden piece over the screen . Here the paper needs to dry even between colours and not just between the different sides of the paper. As is clear this method is much slower than Offset printing!
Another drawback is the lack of clarity and sharpness of the images in this method..therefore it is better suited for texts rather than images.
The consistency in colour largely depends not only on the exposed screen but also the judgement of the printer.
Advantages of Screen Printing :
- This method is ideally suited for printing on Fabrics, Ceramics, Plastics, Glass etc.
- It is mostly suited for texts rather than images.
- Screen printing has been widely used for low volume printing; as quantity increases it becomes less economical in time as well as in money.