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HP Scitex UV Curable Inks
HP Scitex UV Curable Inks
HP Scitex UV Curable Inks
Since UVC inks are very different than inks
based on water and organic solvents, it is
important to understand what they are and
how they work.
There are two main types of UV curable ink used
UV Curable Inks in industrial inkjet printing: “Free Radical” and
“Cationic”. Today, about 95% of UVC inks are
Industrial inkjet printers can print hundreds of square
based on Free Radical chemistry. A key difference
meters per hour, but expanding markets for colorful,
between Free Radical and Cationic systems involves
high-quality, and durable point-of-purchase, vehicle
the solidification process under exposure to UV light,
and building graphics will push productivity needs to
and this will be discussed further under “Curing”. Free
even higher levels. Because the printhead and inks
Radical and Cationic UVC inks are offered in Solvent
are the key components of any inkjet printer, higher
Diluted and the so-called “100% Solids” formulation.2
productivity must be built on printhead1 and ink
technologies that offer reliable performance and high
image quality at high print speeds.
While a printhead can be designed to use different
types of industrial inkjet inks, that doesn’t mean that
the printhead and printer can produce acceptable and
dependable results with any ink. Only when printhead,
ink, and printer are designed and developed together
as a system can the user expect high levels of
performance and dependability.
HP developed HP Scitex UV Curable (UVC) inks, the
HP Scitex X2 printhead, and large-format printers to Solvent Diluted UVC inks use a volatile component,
meet the developing needs of industrial inkjet printing. either water or an organic solvent, to lower ink
viscosity for drop ejection and to get greater ink
spread (dot gain) on the print medium. The volatile
component is then evaporated in a dryer using hot
air or infrared (IR) lamps, and the ink components
remaining on the print medium are then cured by
exposure to ultraviolet (UV) light to produce a solid
film. The drying/evaporation process produces either
water vapor or volatile organic compounds (VOCs).
1 See the companion White Paper: HP Scitex X2 Printhead.
2 The term “100% Solids” should not be confused with a hot-melt
ink, also called a “solid ink”. Solid inks are a waxy solid at normal
temperatures. They are heated and melted for jetting, and then solidify
by freezing on the substrate.
Machine
Lenght
(m)
Machine
Footprint
(sq.m.)
Maximum
Power*
(kW)
HP Scitex
TJ8550
UVC inks
8 32 90
HP Scitex
TJ8350
Solvent inks
19 57 80
In solvent-based systems, the release of VOCs involves
legal requirements to meet local, state, and federal
environmental, heath, and safety regulations relating to
worker VOC exposure and to material handling, use,
storage, and disposal.
With 100% Solids UVC inks all the ink printed onto the
print medium converts into a solid without any material
loss to evaporation. Since no VOCs are released,
100% Solids UVC inks have significantly lower
environmental, health, and safety issues compared to
conventional solvent-based inks and UVC inks diluted
with organic solvents.3
Inks and the System
Prints made with aqueous or solvent inkjet inks “dry”
when volatile ink components evaporate leaving
behind colorants and other materials on the print
medium to form an image. Because industrial inkjet
printers can produce 100’s of square meters per hour,
a dryer is used to ensure that most of the volatiles
are evaporated or, at least, prints are sufficiently dry
for handling. Dryer power consumption, concerns
about solvent ink vapor ignition, and the response of
print media to high temperature (e.g., shrinkage, curl,
blistering, etc.) impose practical limits on the rate of
drying. Since the drying process must take several
seconds, printers with high linear feed rates may need
dryers that are several meters long.
A key difference between 100% Solids UVC inks and
other types of inks is that UVC inks don’t dry: they
solidify in about 0.1 second upon exposure to intense
UV light. This means that an in-line print dryer is not
needed, and a printer using 100% Solids UVC inks
requires less production floor space than a similar
printer using aqueous, solvent, or solvent-diluted
UVC inks.
The figures and chart at the right compare two similar
HP Scitex printers: the TJ8350, which uses solvent
inks and the new TJ8550, which uses UVC inks. The
TJ8550 offers the same level of productivity, but needs
only about half the floor space as the TJ8350.
Because they do not evaporate, 100% Solids UVC
inks can improve printhead operation and reliability.
Unlike aqueous and solvent-based inks, 100% Solids
not do not form a solid crust when exposed to air
and so require less maintenance of the printhead
nozzle plate. This characteristic reduces printer design
complexity compared to machines using aqueous and
solvent inks, and this is a significant consideration in
industrial inkjet printers that can use 100’s or even a
1000 or more printheads arranged in a large nozzle
array.
Industrial inkjet printing places high demands on
system productivity, reliability, and quality. To meet
these levels of performance, HP designs, develops,
and manufactures inks, printheads, and large-format
printers as a system with end-to-end engineering
and quality control. Inks are a critical element of the
system solution, and not just any UVC ink will meet all
industrial printing performance requirements.
To demonstrate the importance of each ink component
to the system, consider that every change in HP Scitex
UVC ink formulations undergoes exhaustive testing
both in the laboratory and in production environments.
That investment in quality control means that every liter
of HP Scitex UVC ink comes with the confidence that
the inkjet printing system performs to HP standards.
3 Refer to the Material Safety Data Sheet for instructions on handling, use,
and disposal of HP Scitex UV Curable Inks.
In fact, the physical, chemical, and colorimetric
properties of ink affect everything in the printing
system. The table below lists some primary ink-system
interactions. Failure to perform in any one of these
aspects means an industrial inkjet printing system
will not deliver the necessary level of productivity,
reliability, and quality.
Ink physical properties (e.g., surface tension, viscosity,
pigment particle size and distribution) affect
•
printhead maximum drop frequency (productivity)
•
ink supply pressure drops at high ink flow rates (printhead
ink starvation, productivity)
•
drop formation and break-off (satellites, drop placement
errors, quality)
•
wetability of the nozzle plate (reliability of drop ejection,
drop placement errors, quality)
•
wetability of the print medium (wet dot-dot interactions, dot
coalescence, quality)
•
dot gain (virtually everything related to image quality)
•
feathering (edge sharpness)
•
substrate penetration (optical density)
•
stability of pigment dispersion (pigment settling in the
printhead and ink delivery system)
•
consistent printhead operation over the industrial temperature
range (dependability)
•
Ink chemical properties affect
•
resistance of printhead and ink delivery system components
to damage from ink exposure
•
stability of pigment dispersion
•
robustness of the UV cure process (complete and rapid
conversion of monomer to solids)
•
dot gain
•
flexibility, hardness, adhesion, and durability of cured print
•
fade resistance
•
environmental, health, and safety considerations in handling,
use, storage, and disposal
•
shipping restrictions due to hazardous components and
world-wide product availability
Ink colorimetric properties affect
•
color consistency
•
color accuracy
•
color saturation
•
color gamut
UVC inks offer a number of important image quality
benefits. Once on the print medium and exposed to
UV light, drops of UVC ink undergo a rapid increase
in viscosity from a jettable liquid to a durable solid.
This minimizes dot spread, feathering, and ink
penetration into absorbent media. This characteristic
allows UVC inks to print with high quality on any
substrate with precise dot control for sharp lines
and edges and repeatable dot properties for color
halftoning algorithms. High color saturation is
achieved because UVC inks form a film of colorant on
the surface of print media.
After curing, UVC inks form a durable, mechanical
bond to the print surface: the solidified ink “keys” into
microscopic pores in the print medium. Prints come out
of UV cure ready-to-use and odor-free.4
4 Some post-cure polymerization can occur, meaning that maximum
image durability may be achieved a few hours after printing.
Curing
The UV cure station has a Lamp Housing assembly
that provides power, cooling, and beam control for a
lamp that emits intense UV radiation. This assembly
is shown schematically at right. UV light is typically
broken into three bands from the longest to shortest
wavelengths: UV-A, UV-B, and UV-C. UV-A and UV-B
are used in curing, UV-C is absorbed by air and
produces ozone, which can be effectively trapped
and eliminated by filtration. The Lamp Housing
contains optical elements, such as mirrors with dichroic
coatings, that reflect UV-A and UV-B but absorb IR and
some visible wavelengths. This minimizes heating of
the print medium during cure.
Mercury vapor lamps offer high efficiency at UV-A and
Dichroic Mirrors
Lamp Housing with cooling channels
air plenum
Lamp
Mirror/Shutter
media feed direction
UV-B wavelengths. But, as summarized in the figure at
left, only about 10-20% of the input electrical power
is delivered to the print medium as UV-A and UV-B
radiation. The rest is lost as IR (infrared) radiation and
heat. Heat is removed by forced-air cooling of the
lamp and by air or water cooling of mirrors and other
components in the Lamp Housing.
The curing process begins by exposing printed dots of
ink to UV light. This causes an ink component called a
photoinitiator to break down into reactive species that
cause liquid monomers to link together to form chains,
called polymers, and polymers to crosslink forming a
solid ink film.
For Free Radical chemistries, the photoinitiator
produces negatively-charged species (free radicals)
that are consumed as monomers link and polymers
crosslink. Continuous exposure to high-intensity UV
light is required to produce free radicals as long as
monomer is available for conversion.
In addition to reflecting different wavelengths of visible
light to produce a colored image, ink pigments absorb
different wavelengths of UV. This is shown in the plot of
%Transmittance versus wavelength at the right.
Notice that these four pigments (CMYK) pass UV-A
and UV-B wavelengths to varying degrees, but
cyan, yellow, and black inks are nearly opaque to
short wavelengths of UV-C. UV light is said to be
“blocked” by the pigments at wavelengths with lower
%Transmittance.
For a complete cure and adhesion to the substrate,
Free Radical chemistries require UV light to penetrate
the entire ink layer from the outer surface to the ink-
substrate interface. To minimize the effect of pigment
blocking, the photoinitiator is made of several different
materials, each having sensitivity to different UV
wavelengths produced by the lamp.
Most pigments transmit UV-A efficiently, and this allows
UV-A energy to penetrate the ink layer down to the ink-
substrate interface.
For Cationic chemistries, the photoinitiator breaks
down into positively-charged cations. Once initiated by
UV light, Cationic chemistries undergo a self-sustaining
chain-reaction that can continue in the dark until all
the monomer is converted. This “dark cure” needs less
UV energy for curing and avoids the effect of pigment
blocking, which could prevent the conversion of all
monomer into crosslinked polymers with Free Radical
chemistry. Cationic chemistries may require heating of
the substrate to ensure a reliable cure.
On the other hand, Cationic chemistry requires
formulations with high thermal stability and resistance
to triggering under low levels of UV light (e.g.,
sunlight, fluorescent lights, etc.). While a manageable
risk, the exposure of any part of a Cationic UV ink
system to initiation-levels of UV light could potentially
solidify the ink throughout the ink delivery system from
the printhead to the ink bottle.
HP Scitex UV Curable
Inks
HP formulated and developed UV Curable inks
to work with the HP Scitex X2 printhead and with
OEM printheads in large-format industrial inkjet
printers. These inks use a 100% Solids, Free Radical
formulation and are offered in 4-, 6-, 6 plus white
and 8- ink configurations for different HP Scitex large-
format industrial inkjet printers.
The inks are produced by HP in its own state-of-the-art
factories to strict quality-control standards. Each batch
is tested and certified to meet an array of rigorous
physical, chemical, and colorimetric specifications.
HP Scitex UV Curable inks offer 24 months outdoor
fade resistance5, high durability, resist abrasion,
and can be cleaned with soaps and other common
solvents. Images are waterfast, and have excellent
adhesion to a wide variety of rigid and flexible
substrates.
Color gamut is a key measure of ink performance, and
HP Scitex UVC inks deliver brilliant, saturated colors
comparable to HP Scitex solvent inks. The figure at left
shows the color gamuts in the a-b plane of the CIE Lab
color system for 6-ink systems6 of HP Scitex UVC and
solvent inks.
Summary
HP Scitex UV curable inks are formulated and
manufactured by HP to work together with HP Scitex
X2 printheads and large-format industrial inkjet
printers. These dependable systems produce dry,
ready-to-use, durable images with a large color gamut
on a wide variety of rigid and flexible substrates.
Designed for high-volume printing, HP Scitex UV
Curable inks and large-format printers do not release
volatile organic compounds and offer reduced
production floor space requirements compared
to aqueous- and solvent-based industrial printing
solutions.
5 Based on tests conducted by HP according to ASTM 2565--99
protocols.
6 These 6-ink systems use cyan, light cyan, magenta, light magenta,
yellow, and black inks.
http://www.hp.com/go/graphic-arts
© Copyright 2006 Hewlett-Packard Development Company, L.P. The information contained herein is subject to
change without notice. The only warranties for HP products and services are set forth in the express warranty
statements accompanying such products and services. Nothing herein should be construed as constituting an
additional warranty. HP shall not be liable for technical or editorial errors or omissions contained herein.
4AA1-9916ENW, October 2009