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Screenprinting
Screenprinting,
or silk-screen printing is a method of single or multi-colour
printing where a finely meshed screen is made into a stencil
of the printed image required. The mesh is left open where
ink is required to pass through the screen, and sealed where
ink is not required. Each colour is applied in a separate
pass through the printing machine or through a multicolour
machine where each colour is applied and dried before the
sheet passes to the next colour stage.
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The
advantages of this method of printing for the point-of-sale
and display industry are: |
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Short machine set-up time. |
| Cost
effectiveness of short to medium length print runs. |
| Suitability
to a large range of sheet sizes from small through
to very large. |
| High
colour strength and UV stability of inks. |
| Large
range of suitable materials from thin papers to
thicker boards, metal, timber and plastic sheets. |
| Ability
to print to both flexible and rigid materials. |
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Disadvantages:
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Lower print resolution than
offset printing. |
| Slower
throughput than offset printing. |
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Step
1
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Step
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A
screenprinted item starts life as a piece of artwork
which is then transferred to a clear plastic film as
a positive 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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The
screen which will carry the required image is coated
with a photographic emulsion that is applied in a thin
layer to each side of the mesh by a coating machine.
This coating is initially dried in a heated drying cupboard
to prepare for the making of a printing stencil.
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Step
3
The
film positive of the required image is placed in front of
the coated screen, held in place by a vacuum and then exposed
to intense Ultra-Violet light which cures and hardens the
emulsion in the areas not protected by the image on the film
positive.
After UV light exposure the film positive is removed from
the screen which is then washed with water to remove the uncured
emulsion - leaving a stencil of the image to be printed. After
the job has been finished, the emulsion is dissolved from
the screen with a stencil remover and the screen can be re-used.
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Step
4
The
finished stencil is placed in the screenprinting machine with
the desired colour ink spread over the surface of the screen.
A pair of blades move backwards and forwards over the screen
- one blade (scraper) scrapes the surface to leave an even
layer of ink over the image area, the other (squeegee) squeezes
the ink through the stencil to form the image on the material
below the screen.
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Step
5
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Most
machines are automatically or semi-automatically fed
with paper or board which leaves the machine after printing
via a conveyor belt and passes through a drying machine.
Most modern drying machines use high intensity Ultra-Violet
light to dry special UV sensitive inks instantly, without
heat.
Qualified
printers inspect each print for defects as they leave
the back of the dryer for re-stacking. Each colour is
added by a separate pass through the machine.
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Step
6
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Finished
prints are either trimmed to final size by guillotine
(for square-cut items) or die cut (for irregularly shaped
items).
Other
finishing processes also include folding, glueing, partial
assembly or just simply wrapping and delivering.
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Step
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