This document summarizes UV color inkjet printing technology for plastics. It discusses how UV inkjet printing works, the components involved like printheads and UV curable inks, and key advantages over other printing methods like reduced VOCs and digital non-contact printing. The document also outlines considerations for ink and substrate compatibility and the need for integrated systems to bring together the various hardware and software components required for robust UV inkjet printing manufacturing.

1. TECHNOLOGY FEATURE
UV Color Inkjet Printing for Plastics:
Technology Developments for Digital Applications
By Scott R. Sabreen, The Sabreen Group, Inc.
This is the first of two articles featuring UV color inkjet
printing for plastics. This first issue will present basic process
principles, UV inks and surface preparation, and technol-
ogy advancements. Part 2 will discuss
application solutions including equipment
selection, ink chemistry/curing, production
systems integration, and process control
techniques for robust manufacturing.
UV color inkjet printing on plastics is a highly effective
process for applications which require variable information
and low volume production. UV inkjet printing has a long
history originating in the early 1970s for industrial coding,
labeling, direct mail, and paper stock packaging, on mostly
flat surfaces. Inkjet is a computer-to-print process and does
not require consumable printing plates, screen fabric mesh,
or foil/plastic transfer carriers. Major system components Figure 1: Diagram of Inkjet Printing. CIJ is used for
generally consist of the following: lower-end print quality, very high speed coding. Impulse
or DOD is used for applications requiring high quality
a) printhead assembly images and versatility. Courtesy: XAAR.
b) printhead drive electronic controllers
c) UV inks
d) curing irradiator and
e) motion-controlled parts handling. As reference, Advantages of UV Inkjet Processes
toner-based printing and laser marking are also As with all printing processes, whether ink or inkless,
digital processes. digital or non-digital, UV inkjet is a niche technology and
the results are highly dependent upon the product applica-
One of the earliest inkjet technologies is termed “Continuous tion and precise engineering development and integration.
Inkjet” (CIJ) printing. With CIJ, printheads incorporate a Strengths of UV inkjet offer numerous advantages over other
single jet and can run at very fast speeds using air drying conventional printing and decorating methods:
solvent-based inks, although alternative heat cure inks are
also used. As the terminology implies, CIJ operates in a 1) UV inks offer superior vibrancy and opacity
“continuous” stream mode. (compared to solvent-based inks) and totally cure
within 24 hours.
A second type of inkjet technology is termed “Drop-on- 2) They greatly reduce the use of VOC’s (volatile
Demand” (DOD), or piezoelectric DOD inkjet. It is DOD organic chemicals).
printing which provides new excitement for decorating, 3) The process is digital, non contact, and can produce
marking, and coding of flat and cylindrical plastic products. full color printing CMYK (cyan, magenta, yellow,
DOD inkjet is a non-contact printing technology in which black).
droplets of ink are jetted from small diameter vessels (rang- 4) UV inks can deposit a thicker layer of print material
ing from a minimum of 128 to 256, 768 and more depending than most other digital print methods which can
on the print area and speed) directly to a specified position provide advantages.
on the substrate to create an image. Thus, the big difference 5) UV inkjet can increase productivity and reliability
between these two inkjet print technologies is with multi while reducing costs associated with solvents and
nozzle drop-on-demand heads there are large numbers of other chemicals. The plastic identification card
nozzles which are in intermittent use. This high-end quality shown in Figure 2 demonstrates the capabilities of
inkjet process can be either “binary” or “grayscale”. Imprint DOD inkjet printing.
resolution can be 600x600 dpi and higher.
20

2. Figure 2: Drop-on-demand UV-curable color inkjet printing Figure 3: DOD Spectra® Nova Figure 4: DOD Spectra®
on plastic identification cards, including product security 256 Channel Jetting Assembly. Nova 256 Channel Inkjet
features. Courtesy: Impika Aubagne, France. Courtesy: FujiFilm Dimatix. Printhead Assembly. Cour-
tesy: FujiFilm Dimatix.
Weaknesses of UV Inkjet Processes UV Curable Inks
No printing process is without weaknesses and UV inkjet is Ultraviolet (UV) curable inks provide superior print image
no different. Weaknesses include the following: quality and physical properties. An important operational
process factor to remember is that during manufacturing
1) UV inks use more costly low molecular weight downtime, inkjet nozzles should be covered to prevent clog-
monomers and oligomers to achieve the low ging. Similar to all UV ink process decorating applications
viscosities necessary for printing. (including pad print, screen print, spray-coating, etc.), major
2) UV inks will solidify in the print head nozzles when ink components include the following:
UV curing lamps are positioned too closely.
3) UV inks can offer superior gloss appearance if continued on next page
allowed to flow out before exposure to UV radiation,
but this will produce dot gain which needs to
be corrected (optimized) through graphic file New VOTAN W™
manipulation. Laser Welding Systems from Jenoptik
4) Turnkey inkjet systems are extremely expensive
($USD 100,000 - $200,000 or more is not uncommon)
Jenoptik www.lasertech-
and require very low vibration material handling nologies-jenoptik.com
automation systems.
Piezoelectric DOD Printhead Design
In the piezoelectric drop-on-demand ink-jet method (Figures
3 and 4), deformation of the piezoceramic material causes
the ink volume change in the pressure chamber to generate a
pressure wave that propagates toward the nozzle. This acoustic
pressure wave overcomes the viscous pressure loss in a small
nozzle and the surface tension force from ink meniscus so
that an ink drop can begin to form at the nozzle. When the
drop is formed, the pressure must be sufficient to expel the
droplet toward a recording media. In general, the deformation • Diode laser welding method prevents scratching
of a piezoelectric driver is on the submicron scale. To have • Excellent choice for welding plastic consumer
large enough ink volume displacement for drop formation, the products, medical components, automotive parts,
physical size of a piezoelectric driver is often much larger than and more
the ink orifice1. Each pixel on the substrate is either covered • We can test your application on our machine
with ink or not – a binary choice. Grayscale inkjet works in a before you buy!
similar way to binary inkjet but has the ability to fire a range
of drop sizes, normally 8-16 different sizes. The result is sig-
nificantly higher apparent resolution using the same native
Call us today to learn more at (248) 446-9540
resolution as binary. End users need to carefully examine the
or visit www.lasertechnologies-jenoptik.com
potential advantages versus disadvantages.
e-mail us at jlt.sales@earthlink.net
21

3. TECHNOLOGY FEATURE continued from page 21
a) photoinitiators with resin molecules to form resin free radicals, then the
b) monomers small amount of heat from the infrared (IR) component
c) oligomers in UV lamps accelerates the polymerization crosslinking
d) colorants and additives reactions of the resin molecule free radicals. This IR heat is
minimal due to the brief dwell time of parts in the UV cure
The application of ultraviolet (UV) coatings is a photo- zone, but it is enough to give a fully-cured coating. Some
polymerization process – formation of molecular chains radicals often remain for a brief time (1-2 minutes) after UV
by fusion. This category of coatings contains various ac- exposure which give a minor degree of added post-curing
celerators or catalysts that are dormant until acted upon by to the film. Abrasion, mar, and scratch resistance of UV
ultraviolet light. The UV light or electron bombardment coatings are therefore excellent.
triggers a free-radical reaction among chemical groups
that results in cross-linking (curing) of the paint resins. Ink – Plastic Substrate Compatibility
UV coatings consist of liquid oligomers (polyester resin), All ink printing processes require the liquid ink chemistry
monomers (generally acrylates as dilution agents), photo- (UV, solids, thermal, etc.) be compatible with the plastic
initiators, and various additives and pigments as required. substrate so that proper “surface wetting” is achieved. UV
Applications typically cure with electromagnetic radiation inks are typically lower in viscosity (approximately 25
wavelengths in the range of 300-450 nanometers (near UV- dynes/cm) than pad or screen printing inks.
A light spectrum).
A major contributing factor to ink-substrate compatibility is
The chemical photoinitiators are sensitive to UV light, which that many plastics are chemically inert, nonporous surfaces
changes the chemical bond structure of the photoinitiators, with low surface energy. Surface pretreatments on today’s
forming free-radical groups that trigger resin cross-linking. high performance engineering resins will solve most ink
Curing happens in a 2-step sequence; first a photoinitiator adhesion. As a general rule, acceptable ink adhesion is
absorbs UV rays and forms free radicals. These interact achieved when the surface energy of a substrate (measured
in dynes/cm) is approximately 10 dynes/cm greater than the
surface tension of the liquid. In this situation, the liquid is
In-Mold.
Chicago Decal Company
www.chicagodecal.com
said to “wet out” or adhere to the surface. Surface tension,
which is a measurement of surface energy, is the property
In-destructible. (due to molecular forces) by which all liquids through con-
traction of the surface tend to bring the contained volume
onto a shape having the least surface area. Therefore, the
In-mold decorating (IMD)
inserts your graphics
directly into the mold,
higher the surface energy of the solid substrate relative to
the surface tension of a liquid, the better will be its “wet-
tability”, and the smaller the contact angle.
making them a permanent
part of your product. They The degree or quality of treatment is affected by the clean-
are virtually impossible to
liness of the plastic surface. The surface must be clean to
remove, resistant to scratch-
ing, scuffing, chemicals and achieve optimal pretreatment and subsequent ink adhesion.
liquids. Surface contamination such as silicone mold release, dirt, dust,
grease, oils, and fingerprints inhibit treatment. Material purity
If your product needs the is also an important factor. The shelf life of treated plastics
highest level of durability, depends on the type of resin, formulation, and the ambient
you need IMD graphics. environment of the storage area. Shelf life of treated products
is limited by the presence of low molecular weight oxidized
For the highest quality, most responsive service and
materials (LMWOM) such as antioxidants, plasticizers, slip
strongest technical support, contact Chicago Decal.
and antistatic agents, colorants and pigments, and stabilizers,
We’ll guide you through the in-mold process.
etc. Exposure of treated surfaces to elevated temperatures
increases molecular chain mobility - the higher the chain
mobility the faster the aging of the treatment. Polymer chain
mobility in treated materials causes the bonding sites cre-
101 Tower Drive, Burr Ridge, IL 60527 ated by the treatment to move away from the surface. These
(630) 850-2122 (888) 332-2577 components may eventually migrate to the polymer surface.
Therefore, it is recommended to bond, coat, paint, or decorate
www.chicagodecal.com the product as soon as possible after pretreatment.
continued on page 24
22

4. TECHNOLOGY FEATURE continued from page 22
Surface pretreatments are used to increase surface energy include carbonyl (C=O), carboxyl (HOOC), hydroperoxide
and improve the wetting and adhesive properties of polymer (HOO-), and hydroxyl (HO-) groups. Even small amounts of
materials. A variety of gas-phase surface oxidation pre- reactive functional groups incorporated into polymers can
treatment processes are used in the industry including low be highly beneficial to improving surface characteristics
pressure cold gas plasma (Microwave/RF), electrical (corona and wettability.
discharge), flame plasma, and low temperature voltage-free
atmospheric plasma. Each method is application-specific and Systems Integration
possesses unique advantages and potential limitations. Each The inkjet printing process departs from conventional ink
of these processes is characterized by its ability to generate printing techniques in that engineering is required in many
“gas plasma” – an extremely reactive gas consisting of free distinct disciplines for turnkey systems integration. In con-
electrons, positive ions, and other chemical species. In the trast, manufacturers of pad and screen printing equipment
science of physics, the mechanisms in which these plasmas almost always can provide turnkey systems including inks,
are generated are different but their effects on surface wet- printing consumables, curing equipment, automation and
tability are similar. chemical clean-up equipment. Inkjet system components
consist of the printhead, drive electronics, inks, parts-han-
Free electrons, ions, metastables, radicals, and UV gener- dling and motion control hardware, and curing irradiator.
ated in plasma regions can impact a surface with energies Further, digital information needs to be communicated to the
sufficient to break the molecular bonds on the surface of printhead through hardware/software file protocol including
most substrates. This creates very reactive free radicals on the main controller drive software. No single manufacturer
the polymer surface which in turn can form, cross-link, provides all of the mentioned components for every custom
or in the presence of oxygen, react rapidly to form various application. A high degree of engineering knowledge of all
chemical functional groups on the substrate surface. Polar the inkjet components and piece-part compatibility are criti-
functional groups that can form and enhance bondability cal to achieving robust manufacturing operations.
Innovative Marking Systems
www.padprinters.com
24

5. Conclusion
UV DOD inkjet is a relatively new advancement in the world
of digital printing and it has an amazingly broad range of
2-dimensional and 3-dimensional plastics applications. This ad-
vancing digital technology is poised toward changing traditional
printing processes. UV DOD inkjet technology research and
development will continue to advance and bring industrial users
another niche digital printing capability for cost-effective short
print production runs using environmental-friendly inks. n
References
1. Society for Imaging Science and Technology, Volume 42,
Number 1, January/February 1998, Hue P. Le
2. Plastics Decorating magazine “Breaking Down UV
Curable Coatings”, Scott Sabreen & Norman Roobel, The
Sabreen Group, Inc., May 2004
Scott R. Sabreen is founder and president of The Sabreen UEI Group www.ueigroup.
Group, Inc. (TSG). TSG is a global engineering company com
specializing in secondary plastics manufacturing processes
– surface pretreatments, bonding, decorating and finishing,
laser marking, and product security. For more information
call toll-free (888) SABREEN or visit www.sabreen.com and
www.plasticslasermarking.com.
Enercon Industries Corp.
www.enerconind.com
Enercon’s blown-arc and blown-ion
atmospheric surface treaters improve:
� mechanical and ionic bonding
� surface activation
� cleaning and functionalization
Contact us for a free demonstration.
Enercon Industries Corporation
www.enerconind.com
262-255-6070/info@enerconind.com