The process of Lithography

Lithography

This process uses a flat aluminium printing plate and works on the principle that oil and water do not mix. The image is transferred onto the plate photographically through ultra-violet light shining through a negative. The plate is then washed with a chemical that makes the image area attractive to the oily ink. The non-image area is later dampened with water to repel the ink. To lengthen the life of the printing plate and prevent the paper becoming damp through contact with the water, a rubber blanket transfers the ink from the plate to the paper. This is called offset lithography and is used for most commercial printing such as books, magazines, brochures etc. If rolls of paper are used rather than flat sheets the process can be speeded up and higher volumes printed. This is called web offset lithography.

4 colours(process colours) - black, cyan, magenta and yellow - are used to build up all other colours and tones.
Ready mixed (spot colours) are used for one, two or three colour designs.
Additional spot colours are sometimes used for special colours such as gold.
Economical for between 1,000 and 1,000,000 copies.
Good quality can be achieved.

Gravure

This uses an engraved plate where the printing surface is lower than the main area of the plate. The image is photo-etched onto copper-faced printing cylinders which are expensive but will stand up to very long print runs. The engraved recesses are then filled with ink, the surface is scraped clean and then the ink is transferred to an impression cylinder in much the same way as lithography. The process can be sheet fed or roll fed, as used for higher quality printing and very high volumes such as magazines and books. Limited edition fine art prints and postage ...

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Gravure

4 colours (process colours) - black, cyan, magenta and yellow - are used to build up all other colours and tones.
Excellent for half tones.
Economical for between 500,000 and 5,000,000 copies.
High quality can be achieved.

Letterpress

This type of printing is much less common these days but is still economical for small print runs. It uses a raised surface for printing off, usually made of aluminium sheet. The ink is rolled onto the plate and this is transferred directly to the paper. The quality is very good but the range of available type is very limited. It is used for items such as invitation cards, and short runs of books.

Colours generally limited to one or two ready mixed colours.
Economical for between 500 and 5,000 copies.
High quality in terms of sharpness.
Limited typefaces available.
Used mainly for text only - not suitable for half tone illustrations.

Flexography

This is similar to letterpress in that it uses raised images but they are made from a rubber or plastic sheet. The flexibility of the plate means that it is particularly suitable for printing on to plastic film, corrugated card etc. and is extensively used for both plastic and paper carrier bags. Wallpaper is sometimes printed using this method. Very small print runs are possible and can be used to print onto ready made carrier bags.

Each colour is ready mixed and can be applied in large solid areas.
Limited to combinations of single colours (typically limited to three).
Economical for between 250 and 5,000 copies using hand fed machinery.
Economical for very high volumes (250,000 to 10,000,000) using fully automated machinery.
Limited quality can be achieved.

Screen printing

This is completely different to other printing processes as it uses a stencil through which ink is forced. The stencil is attached to a fine mesh, which controls the amount of ink passing through the stencil. Stencils can be hand-cut, painted out with a waxy resist or photographically produced. Machine or hand methods are used according to the product but the process is relatively slow. Screen printing is usually done on to manufactured products such as T-shirts, bottles, cartons, folders etc. Also used for printing fabrics and wallpapers, a process known as continuous flat bed screen printing when a series of screens are laid end to end. The fabric or paper is precisely moved under each screen to ensure accurate alignment.

Each colour is ready mixed and can be applied in large solid areas.
Limited to combinations of single colours (typically limited to three or four).
Large areas can be printed.
Economical for under 100 copies using hand fed machinery.
Economical for higher volumes using continuous flat bed screen printing.
Very fine detail is not possible.
Good quality can be achieved as long as images are relatively simple.

Photocopying

In addition to traditional printing methods, photocopying is used for a considerable number of single colour printed products such as low cost leaflets. Often uses coloured paper for greater effect. Black on white masters are copied and printed. Reductions and enlargements are possible and the process is especially suitable for very small runs. Full colour photocopying is less common due to expense but reasonable quality can be achieved.