Offset Printing
Offset
printing is a method of single or multi-colour printing where
the required image is photographically etched onto thin metal
printing plates which are then wrapped around a roller. The
plate-covered roller is then coated with ink and the printed
sheet rolled past the plate to transfer the image to the sheet.
The majority of offset printing work is multicolour where up
to 6 colours are often applied wet-on-wet through a single machine.
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The
advantages of this method of printing for the point-of-sale
and display industry are: |
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High
print resolution. |
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Fast
throughput. |
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Cost
effectiveness of medium to long length print runs. |
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Disadvantages:
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Longer
machine set-up time. |
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Maximum
sheet size limited. |
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Lower
colour strength and UV stability of inks. |
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Only
a small range of suitable materials Ð limited mainly
to thin papers. |
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Unable
to print to rigid materials. |
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Step
1
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An
offset printed item starts life as a piece of artwork
which is then transferred to a clear plastic film as
a negative image of what is required to be printed.
It is necessary to produce a piece of film for each
colour that will be printed to form the final image.
The film is produced at the actual size of the finished
item. Smaller items may be produced with multiple copies
of the same image spread around a larger sheet to reduce
printing costs. Films fall into two broad categories.
Line Art: where solid blocks of colour, areas of colour
tones or lines are used to form the image. Process Art:
where the four process colours (Cyan [blue], Magenta
[red], Yellow and Black) are used in a dot pattern to
form the final image. The process colours can be used
to form photographic images or any desired solid colour.
In some cases a single image may include areas of process
colour and areas of line art.
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Step
2
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Each
piece of filmwork comprising the finished image is placed
over a light sensitive thin metal plate, clamped in
place by a vacuum, and then exposed to strong ultra-violet
light which, like with screenprinting, affects those
areas of the plate which are not protected by the filmwork.
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Step
3
The
exposed plate is then passed through a photographic processor
which develops the image on the plate making the areas of
the plate where ink is required to be placed become oil receptive
and water resistant, whereas the blank areas of the image
become oil resistant and water receptive.
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Step
4
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The
developed plates are fitted around the rollers in each
of the colour stations in the machine where a series
of rollers spread the ink (which is held in reservoirs
above each station) into a very thin film which is eventually
transferred to the printing plate only in the areas
required to be printed onto the sheet.
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Step
5
Blank
sheets are automatically fed into the press and pass through
the colour stations where each of the colours are applied
in sequence. There is no drying operation between each colour
as the ink is applied only in a very thin layer and is not
disturbed by passing through other colour stations.
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Step
6
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Printed
sheets are collected at the other end of the machine
and generally set aside in small piles to allow the
ink to dry. The small piles prevent excessive weight
being applied to the sheets at the bottom of each pile.
Trimming is usually by guillotine or by forme-cutting
as for screenprinted items.
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