UV curable inkjet printing : where is it headed?
Although UV curable ink is not a new technology (developed in the 1970s), its application in drop-on-demand inkjet prints has come of age in the last five years. David Newman, technical marketing representative, Avery Dennison - Graphics Division Aus & NZ, discusses the features and limitations of printing on flexible and pressure-sensitive adhesive films.
Today, the digital printing
market seems to be
undergoing a UV curable
revolution, essentially because of
the flexibility and productivity
they provide. But is it all peaches
and cream for a technology, which
is shaped to replace more
established inkjet methods like
solvent? Let's take a look at a few
of the features and limitations of
printing on flexible and pressuresensitive
adhesive films, and what
hurdles this evolving technology,
needs to overcome.
Breathe that fresh air
One of the biggest selling points
of UV curable flatbed, and ever
increasing roll-to-roll UV curable
printers is the lack of solvents and
the volatile organic compounds
(VOCs) they emit. This reduces
the requirement for special
ventilation and air purification,
making it easier to comply with
environmental regulations and
less expensive to set up the
printer environment. But it’s
important to note that some UV
lamp systems generate ozone and
this must be vented or
neutralized. UV curable inkjet
printers generate a very small
amount of mist that must also be
removed from the printer
enclosure.
Since UV inks don’t dry in or on
the printhead, they can eliminate
some of the annoying problems
associated with cleaning and
purging the head, to remove
insoluble material. This allows to
achieve greater print production
rates with UV ink because you
don’t have to wait on drying
solvents, and will also reduce the
need to perform as much printer
maintenance. By instantly drying
the inks with UV light, you all get a
lot more control over dot gain and
printability. Solvent inkjet printing
quality and production rate is often
limited by dot gain, which is
directly related to the time it takes
for the solvents to flash off.
Added versatility
There is no question that UV
curable inks can print to a much
broader range of substrates from
paper, through flexible vinyl and
on to rigid media such as acrylic,
polycarbonate and polyolefin (PE/
PP) materials. UV ink technology
copes very well with this range of
substrates, which are very difficult
if not impossible with solvent inks,
but in some cases you will face
issues of ink key to the substrate.
Just because UV cured digital
inkjet can image a wide range of
different substrates, doesn’t mean
the end for pressure-sensitive
adhesive material. Films in many
cases will provide higher levels of
ink key. It is, therefore, very
important to test each substrate
before production. The challenge
for ink makers is to continue
to develop inks that will meet
the application viscosity
requirements, adhesion and
flexibility and cure speed.
Many of these substrates are also
more expensive than pressure
sensitive adhesive vinyl, which can
make proofing, colour matching
and printing errors extremely
costly. You will find in many cases
it is far more economical to print
onto self-adhesive films, as they
allow a greater margin for error.
When it comes time for reimaging,
you will be able to remove
the films and re-use the substrate.
Stretch it
Have you ever tried to stretch and
conform a UV curable inkjet print
on cast vinyl into irregular shapes
like rivets, corrugations, and
recesses on vehicles? If you have,
you probably didn’t have much
success. Not only does the ink
crack but also the vinyl would have
felt much more rigid and brittle
than before it was printed. When
UV curable inks are cured by UV
light, the chemical reaction and
instant drying will have a negative
effect on the vinyl film. This will
cause the film to become a lot less
conformable and will cause the film
to crack and tear quite easily. So if
you want to conform a print to
outdoor durability, solvent inkjet is
currently the only solution.
Protect it
Ink film tends to give a matte or
satin finish. To obtain a high gloss
finish, they need to be
overlaminated with a film or liquid
laminate, but often these surfaces
are hard to adhere to. Some
manufacturers will offer thinner
cast overlaminates now, which will
give much better surface contact
to these textured finishes to
provide much better ink adhesion.
As UV curable ink technology
advances, new generation inks are
sure to overcome this issue, it’s
just a question of when.
