Curing
On the UV LED wavelength
UV LED technology featured at the last drupa show in 2008, and since then, a significant
amount of effort has gone into research and development aimed at gaining greater
understanding and expanding usage in the printing sector. By David Atkinson.
lengths that range from UV through
visible light to IR. The quartz envelope of the arc lamp itself gets very
hot (in the order of several hundred
degrees centigrade) and this, along
with the incident IR energy, means
that a lot of heat is directed at the
substrate. This makes it necessary,
when printing thin heat-sensitive
substrates, to temperature control
the web itself by means of chill
rollers, as well as extracting heat
from the lamp head.
Advantages
Hönle’s PureUV Powerline
s a technology, LED (light
emitting diodes) has already
infiltrated our day to day lives
to such a degree that we are all familiar with it in one form or another, be
it in a torch, low energy lighting or
car lighting. The advantages that
LED technology offers in applications that we are familiar with, such
as long lamp life and low running
cost due to low power consumption,
are fundamental attributes of LEDs
that are available benefits for those in
the printing industry working with
energy cured inks. It is hardly surprising then that the industry is showing great interest in the technology.
To date, energy cured inks have
typically been cross-linked (dried) by
means of ultraviolet light emitted by
low pressure mercury arc lamps or
more exceptionally by means of EB
(electron beam) technology, but it is
also possible to cross-link energy
cured inks by means of LED.
A
The technology
LEDs are solid state devices (diodes)
that emit energy in the form of light
when an electrical current passes
through them in the forward biased
April 2012
direction. The wavelength of the
emitted light depends on the material
from which the diode is made, with
each material emitting light in a very
narrow waveband that is almost
monochromatic.
Solid state UV LED devices can
be constructed from diodes outputting wavelengths of 365 nm, 385
nm, 395 nm, 405 nm and 410 nm
(nm=nanometre), LEDs with outputs at 365 and 395 nm in the UVA
range being the most popular for
current applications.
UV LEDs convert about 10% to
25% of the electrical energy they
receive into light, with the rest being
converted into heat that is emitted
from the back of the array, hence the
need for cooling. They do not emit
IR energy. The higher the irradiance
of an LED, the greater the amount of
heat generated. As the output of these
devices increases, the more energy it
takes to drive them and the greater
the amount of heat that needs to be
effectively eliminated.
This is quite different to the situation with conventional medium pressure UV mercury arc lamps, which
emit a broad spectrum of wave-
UV LED has a number of advantages to offer compared with conventional UV lamps, and these can be
broadly grouped into environmental
advantages, cost advantages and
processing advantages, plus a few
miscellaneous advantages.
The main environmental advantages are that LEDs are mercury free,
do not produce ozone and consume
less energy than medium pressure
conventional UV lamps.
Economic advantages include lower
energy consumption in their running
state, and because LEDs switch on
and off instantly, no warming up time
is required. As a result, it is not necessary to keep LEDs on standby
power, as is the case with conventional UV lamps, and this further reduces
energy consumption. Lamp life is in
excess of 10,000 hours compared
with between 1000 to 2000 hours for
a conventional UV lamp, which
reduces the cost of consumables as
well as the amount of downtime as a
result of having to replace lamps.
Processing advantages include the
means to cure inks on heat sensitive
substrates without the need to install
expensive chill rollers, as the absence
of direct IR radiation from LEDs
means that it is a cool running technology. The relatively long wavelength of LEDs can penetrate inks
more deeply and this facilitates
through cure so thick ink films can be
more readily cured.
Other advantages include a light
and compact design, which enables
them to be easily integrated into
existing presses, and a simpler
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Curing
design because there is no need for
the shutters or head rotation that is
required for conventional UV lamps
when in standby mode. In addition,
there is the ability to manufacture
LED units at any length and width,
and they also offer greater safety in
the work place.
Challenges to be resolved
These advantages are offset to some
degree by a number of ongoing challenges that the manufacturers are
currently addressing.
Process speeds are largely dependent on the UV output of the lamps
and in particular the surface cure is
dependent on the peak irradiance
reaching the web. The output of UV
LED systems is increasing rapidly
and performance has almost doubled
year on year to date. This is, however, a bit of a balancing act because
higher output requires higher electrical input and this translates to
increased running cost, which
reduces the cost benefits of LED
technology. Also the higher the output the greater the amount of heat
generated, so the ability of the supplier to remove the heat in a cost-effective manner becomes a more crucial
aspect of the system.
UV intensity can also be increased
by focusing LED output, and
when this is done, it is possible to
achieve better surface cure and
UV LED curing process
higher processing speed. Curing in
the absence of air by using a nitrogen
blanket is yet another way of improving curing speeds, and early tests
indicate that a significant improvement in curing speed can be
achieved with this approach.
The cost of a UV LED unit,
increases in direct proportion to its
22
size, because the larger the
unit the greater the number
of LEDs required to build it.
With conventional UV systems, the
price difference between lamps of
different widths is not so great. As a
result, where wide presses are concerned, the capital cost of a UV LED
unit is higher than that of an equivalent width conventional UV lamp. As
demand for UV LED systems
increases, this should lower their
manufacturing cost and the price differential between the two systems.
Because LEDs have a low output
compared to conventional UV
lamps, it is necessary to place them
within a few millimetres of the substrate for efficient curing. Having to
place the lamps so close to the substrate can cause difficulties with
some applications and it may be necessary to redesign certain aspects of
various presses.
As with all energy curing technologies, the absorption spectrum of the
inks has to be closely matched to the
peaks of the lamp emission spectrum
and this requires suitable materials
for formulation. Lack of commercially available ink systems is probably
the main factor restricting uptake of
the technology at the moment. That
said, a number of ink companies,
including Sun Chemical, Flint
Group, Zeller+Gmelin, Siegwerk
and Marabu (which primarily produces inks for
screen printing and
inkjet applications)
have already developed various inks
for use with LED
and the list of suitable base materials
is growing.
Because the output of LEDs is near
the visible end of
the spectrum, there
is a greater risk of
inks curing with
exposure to daylight than is the case
with conventional
UV inks. There is
the possibility of shorter wavelength
LEDs being developed in the future,
but experience to date indicates that in
general the shorter the wavelength of
the device, the higher the production
cost of the LED.
In the longer term, ink development will have to be led by demand
from printers. As demand increases
Phoseon Technology’s
watercooled FirePower 300
so will availability of inks. At the
moment it is a bit of a ‘chicken and
egg’ situation, with ink companies
waiting for demand from printers and
printers searching for suitable inks.
For specialised applications, where
demands are much more focused,
development on the ink front has
been greater.
System suppliers
It is early days still, but a number of
manufacturers are already offering
UV LED lamps of various descriptions, which may be suitable for
flexo applications.
Phoseon Technology has a number
of products that are said to be well
suited to flexo applications, including
its FireLine and FireFlex, which can
be sized to cover any web width. It
also offers the watercooled FirePower
range. With UV LED peak irradiance
outputs of 12 W/cm2 and 16 W/cm2,
this unit should offer the highest
potential press speeds.
Hönle AG, supplier of conventional UV curing systems, has developed
the PureUV Powerline for pinning
and full curing in wide format and
web printing applications. The unit
is watercooled and available in a
range of wavelengths. It can be
extended in 40 mm steps and the
output can be varied in 1% steps
between 10% and 100%. Because it
is of a modular construction, it is
possible to selectively switch modules
on and off so as to match the output
to the width of the print.
Lumen Dynamics Group Inc
recently launched its Excelerate
FC-200 series of aircooled full cure
UV LED products. The units have
been designed for single pass narrow
format digital applications. They
deliver an irradiance of 9 W/cm2 at 2
mm working distance and the company said that this is the highest irradiance available from any aircooled
UV LED system for the
digital print industry.
IST has drawn on its experience in
the production of conventional UV
curing systems for the development
of LUV, its new modular watercooled
UV LED system, which is available
in five different wavelengths (typicalApril 2012
Curing
ly 365–395 nm). Each module is 50 x
70 mm, and by combining modules,
the lamp length can be extended in
70 mm increments until the required
length is achieved. A maximum
intensity of 10.5 W/cm2 can be
achieved at 395 nm. If one of the
LEDs should fail, the output of the
remaining LEDs in the module
remains constant and the service life
is not affected. If failure does occur,
it is detected by means of a current
control operation. Output can be varied in 5% steps from 0% to 100%.
Integration Technology Limited
(ITL), which works in collaboration
with IST combining the range of
curing systems, has recently
launched the miniature high-output
UV LEDZero Module. Each module
contains 25 closely packed individual
LEDs and has a built-in heat sink.
The output of the module is
enhanced by a lens that directs laterally transmitted light towards the
substrate. The modules are available
in 365 nm, 385 nm, 395 nm and 405
nm wavelengths so that the output
can be matched to the application.
With this design, it is possible for
end users to configure their own
designs for various applications.
ITL and Lumen Dynamics Group
have just announced that they are to
cross-license their patents. This will
enable ITL products entry to the
North American market and will give
Lumen Dynamics enhanced access to
its European counterpart. The
patents allow for greater control of
the curing process, while increasing
its energy saving potential.
Potential for flexo
UV LED curing is able to offer a
number of cost, environmental and
processing advantages over traditional medium pressure UV lamp systems and these potential benefits are
generating a lot of interest in the
technology from the printing industry
in general. The technology has
already made inroads into digital
inkjet printing, where it is reported to
have captured about one-third of the
energy curable market and is now
moving into screen printing. Growing
confidence outside the flexo markets
has encouraged an increasing number
of printers and suppliers to investigate and develop its potential for
flexo. With the establishment of the
International LED-UV Association in
2011, through the cooperation of a
select group of technology, ink and
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end users, the speed of adoption of
this technology for flexo and offset
litho is likely to increase.
As LED technology has already
started making inroads into screen
printing, it may initially be of greatest
interest to narrow web flexo printers
with multi-process platform presses,
and in particular, for companies that
print on transparent films where a
solid white ink is required to be laid
down before being overprinted with
coloured elements of a design.
With conventional UV technology,
white inks can be more difficult to
cure than other colours, because
white tends to reflect a lot of the incident UV energy. With LED, it is possible to cure very thick white ink films
at effective cure speeds. This was
shown three or four years ago by IST
when it demonstrated the printing of
a polypropylene film with a 10
micron thick screen ink at 50 m/min
with UV LED lab system. With higher output LED systems now available, there is little doubt that greater
speeds would be possible.
There is also the possibility of
hybrid systems consisting of medium pressure UV lamps with LED
units, where the LED is used to
partly cure (otherwise known as fixing and pinning) the ink, so that it
can be readily overprinted.
‘Previously LED systems did not
supply enough curing power for
flexo manufacturers to take advantage of this technology. Now that
LED systems provide 12 W/cm2 and
higher, flexo machine builders are
quickly evaluating LED systems and
pushing their supply chain to provide complementary capabilities,
such as inks, substrates and specialty
finishes. Just as the digital inkjet
market has quickly adopted LED
technology, the flexographic market
will rapidly transition from outdated
arc lamps to new, solid state LED
technology and bring along all
the benefits associated with it: economic, advanced capabilities and
environmental improvements.’
The future
As with all new technologies, there are
various challenges to be overcome,
but the technology is constantly
improving and over a relatively short
time frame the radiation efficiency of
UV LEDs has increased significantly.
Various companies are already working in partnership to advance the
technology specifically for flexo. At
Labelexpo Europe 2009, UV LED
curing technology was shown on a
Gallus press fitted with Phoseon’s UV
LED curing technology, which was
used to cure Siegwerk UV LED inks.
The view of Chad Taggard, vice
president of marketing and business
development at Phoseon, is very positive with regard to flexo. He commented, ‘The flexographic market can now
benefit from UV LED curing technology with the introduction of higher
power UV LED curing lamps. These
lamps will enable smaller footprint
flexo systems with simplified media
paths and more varied substrate choices. End users will benefit with higher
productivity, safer work environments,
and reduced operating costs.
Excelerate FC-200 from
Lumen Dynamics
It is unlikely that UV LED will
have much impact on well established conventional UV flexo in
the short term, especially where
production speed is the overriding
factor, but the usage of UV LED in
flexo is sure to increase with time.
As more printers adopt the technology, the production cost of the
lamps is likely to reduce and greater
adoption will put pressure on the
ink companies to increase the number and range of available inks, thus
making the technology more accessible to an increasing number of
printers. The big question is, how
long it will take? I
April 2012