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Adhesion
Problems with UV Inks?
The Significance of Constant Efficiency
Control of UV Dryers
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Nowadays UV inks have wide use in
graphics as well as in technical screen printing applications. With new
raw materials and innovations in formulations, the range of possible uses
grows almost daily.
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Basically UV inks require UV radiation
in order to cure (or polymerise). Under
UV radiation the initiator molecules (so-called photo initiators) are
stimulated (the UV radiation is only necessary for this part of the
process), then degrade and initiate the radical polymerisation. UV
radiators used are mainly Hg-high pressure lamps. These are mercury vapour
lamps with a high watt efficiency, which emit UV radiation within a high
spectral range. According to manufacturers, the typical life-time of such
UV sources is about 1000-2000 hours of operation, depending on the quality
of manufacture, use, (switching on and off frequently has an effect) as
well as maintenance. Often, an immediate decline in efficiency occurs
after that period which results in inferior drying properties, lack of
adhesion and insufficient chemical resistance. It is therefore essential
for printers using UV inks to keep measuring the efficiency of their
lamps, to guarantee constant quality of print. Naturally, one needs to
keep a good record of such measurements.
Measurement of intensity should be carried out at
the lowest belt speed. In addition each lamp should be measured
individually. Furthermore efficiency of UV lamps should really be checked
at the chosen belt speed prior to each production run.
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Currently there are two ways of
carrying out these measurements:
1) Intensity Measuring Device (UV Integrator)
This is a flat disc, measuring the intensity of the radiator within a
certain spectral range by means of a photo cell. The measurement can be
repeated as often as required. A UV-integrator is suitable for belt dryers
with an inlet slot of at least 3 cm. Efficiency of the UV source is
indicated as a number value in mJ/cm˛. It needs to be remembered that
measured values from UV-integrators are only
comparable if taken with devices of similar make as various manufacturers
use different measuring geometries.
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2) UV Control Labels.
These self-adhesive labels, known as 'Green Detex' have a UV-sensitive
layer, and are attached to the substrate to be printed. After being
exposed to UV radiation the sensitive layer discolours and after a colour
comparison one can identify a UV-dosage in mJ/cm˛. UV control labels are
especially suitable for container printing, roll label printing and all
dryer types not allowing
UV-integrators for constructional reasons. UV
control strips are available in sensitivity ranges of 10-200 mJ/cm˛ and
200-600 mJ/cm˛, and are certainly recommended for anyone not already
using them. They are readily available, as Green Detex labels, from label
printers, Sessions of York (tel; +44 (0) 1904 659224).
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