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 generally consist of the following:

• printhead assembly
• printhead drive electronic controllers
• UV inks
• curing irradiator and
• motion-controlled parts handling. As reference, toner-based printing and laser marking are also digital processes.

One of the earliest inkjet technologies is termed “Continuous Inkjet” (CIJ) printing. With CIJ, printheads incorporate a single jet and can run at very fast speeds using air drying solvent-based inks, although alternative heat cure inks are also used. As the terminology implies, CIJ operates in a “continuous” stream mode.

A second type of inkjet technology is termed “Drop-on-Demand” (DOD), or piezoelectric DOD inkjet. It is DOD printing which provides new excitement for decorating, marking, and coding of flat and cylindrical plastic products. DOD inkjet is a non-contact printing technology in which droplets of ink are jetted from small diameter vessels (ranging from a minimum of 128 to 256, 768 and more depending on the print area and speed) directly to a specified position on the substrate to create an image. Thus, the big difference between these two inkjet print technologies is with multi nozzle drop-on-demand heads there are large numbers of nozzles which are in intermittent use. This high-end quality inkjet process can be either “binary” or “grayscale”. Imprint resolution can be 600x600 dpi and higher.

Advantages of UV Inkjet Processes
As with all printing processes, whether ink or inkless, digital or non-digital, UV inkjet is a niche technology and the results are highly dependent upon the product application and precise engineering development and integration. Strengths of UV inkjet offer numerous advantages over other conventional printing and decorating methods:

• UV inks offer superior vibrancy and opacity (compared to solvent-based inks) and totally cure within 24 hours.
• They greatly reduce the use of VOC’s (volatile organic chemicals).
• The process is digital, non contact, and can produce full color printing CMYK (cyan, magenta, yellow, black).
• UV inks can deposit a thicker layer of print material than most other digital print methods which can provide advantages.
• UV inkjet can increase productivity and reliability while reducing costs associated with solvents and other chemicals. The plastic identification card shown in Figure 2 demonstrates the capabilities of DOD inkjet printing.

Figure 2: Drop-on-demand UV-curable color
inkjet printing on plastic identification cards,
including product security features.
Courtesy: Impika Aubagne, France.
 

Weaknesses of UV Inkjet Processes
No printing process is without weaknesses and UV inkjet is no different. Weaknesses include the following:

• UV inks use more costly low molecular weight monomers and oligomers to achieve the low viscosities necessary for printing.
• UV inks will solidify in the print head nozzles when UV curing lamps are positioned too closely.
• UV inks can offer superior gloss appearance if allowed to flow out before exposure to UV radiation, but this will produce dot gain which needs to be corrected (optimized) through graphic file manipulation.
• Turnkey inkjet systems are extremely expensive ($USD 100,000 - $200,000 or more is not uncommon) and require very low vibration material handling 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 of a piezoelectric driver is on the submicron scale. To have large enough ink volume displacement for drop formation, the physical size of a piezoelectric driver is often much larger than the ink orifice1. Each pixel on the substrate is either covered with ink or not – a binary choice. Grayscale inkjet works in a similar way to binary inkjet but has the ability to fire a range of drop sizes, normally 8-16 different sizes. The result is significantly higher apparent resolution using the same native resolution as binary. End users need to carefully examine the potential advantages versus disadvantages.

Figure 3: DOD Spectra® Nova 256 Channel Jetting Assembly. Courtesy: FujiFilm Dimatix.	Figure 4: DOD Spectra® Nova 256 Channel Inkjet Printhead Assembly. Courtesy: FujiFilm Dimatix.

UV Curable Inks
Ultraviolet (UV) curable inks provide superior print image quality and physical properties. An important operational process factor to remember is that during manufacturing downtime, inkjet nozzles should be covered to prevent clogging. Similar to all UV ink process decorating applications (including pad print, screen print, spray-coating, etc.), major ink components include the following:

• photoinitiators
• monomers
• ligomers
• colorants and additives

The application of ultraviolet (UV) coatings is a photopolymerization process – formation of molecular chains by fusion. This category of coatings contains various accelerators or catalysts that are dormant until acted upon by ultraviolet light. The UV light or electron bombardment triggers a free-radical reaction among chemical groups that results in cross-linking (curing) of the paint resins. UV coatings consist of liquid oligomers (polyester resin), monomers (generally acrylates as dilution agents), photoinitiators, and various additives and pigments as required. Applications typically cure with electromagnetic radiation wavelengths in the range of 300-450 nanometers (near UV-A light spectrum).

The chemical photoinitiators are sensitive to UV light, which changes the chemical bond structure of the photoinitiators, forming free-radical groups that trigger resin cross-linking. Curing happens in a 2-step sequence; first a photoinitiator absorbs UV rays and forms free radicals. These interact with resin molecules to form resin free radicals, then the small amount of heat from the infrared (IR) component in UV lamps accelerates the polymerization crosslinking reactions of the resin molecule free radicals. This IR heat is minimal due to the brief dwell time of parts in the UV cure zone, but it is enough to give a fully-cured coating. Some radicals often remain for a brief time (1-2 minutes) after UV exposure which give a minor degree of added post-curing to the film. Abrasion, mar, and scratch resistance of UV coatings are therefore excellent.

Ink – Plastic Substrate Compatibility
All ink printing processes require the liquid ink chemistry (UV, solids, thermal, etc.) be compatible with the plastic substrate so that proper “surface wetting” is achieved. UV inks are typically lower in viscosity (approximately 25 dynes/cm) than pad or screen printing inks.

A major contributing factor to ink-substrate compatibility is that many plastics are chemically inert, nonporous surfaces with low surface energy. Surface pretreatments on today’s high performance engineering resins will solve most ink adhesion. As a general rule, acceptable ink adhesion is 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 said to “wet out” or adhere to the surface. Surface tension, which is a measurement of surface energy, is the property (due to molecular forces) by which all liquids through contraction of the surface tend to bring the contained volume onto a shape having the least surface area. Therefore, the higher the surface energy of the solid substrate relative to the surface tension of a liquid, the better will be its “wettability”, and the smaller the contact angle.

The degree or quality of treatment is affected by the cleanliness of the plastic surface. The surface must be clean to achieve optimal pretreatment and subsequent ink adhesion. Surface contamination such as silicone mold release, dirt, dust, grease, oils, and fingerprints inhibit treatment. Material purity is also an important factor. The shelf life of treated plastics depends on the type of resin, formulation, and the ambient environment of the storage area. Shelf life of treated products is limited by the presence of low molecular weight oxidized materials (LMWOM) such as antioxidants, plasticizers, slip and antistatic agents, colorants and pigments, and stabilizers, 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 created by the treatment to move away from the surface. These components may eventually migrate to the polymer surface. Therefore, it is recommended to bond, coat, paint, or decorate the product as soon as possible after pretreatment.

Surface pretreatments are used to increase surface energy and improve the wetting and adhesive properties of polymer materials. A variety of gas-phase surface oxidation pretreatment processes are used in the industry including low pressure cold gas plasma (Microwave/RF), electrical (corona discharge), flame plasma, and low temperature voltage-free atmospheric plasma. Each method is application-specific and possesses unique advantages and potential limitations. Each of these processes is characterized by its ability to generate “gas plasma” – an extremely reactive gas consisting of free electrons, positive ions, and other chemical species. In the science of physics, the mechanisms in which these plasmas are generated are different but their effects on surface wettability are similar.

Free electrons, ions, metastables, radicals, and UV generated in plasma regions can impact a surface with energies sufficient to break the molecular bonds on the surface of most substrates. This creates very reactive free radicals on the polymer surface which in turn can form, cross-link, or in the presence of oxygen, react rapidly to form various chemical functional groups on the substrate surface. Polar functional groups that can form and enhance bondability include carbonyl (C=O), carboxyl (HOOC), hydroperoxide (HOO-), and hydroxyl (HO-) groups. Even small amounts of reactive functional groups incorporated into polymers can be highly beneficial to improving surface characteristics and wettability.

Systems Integration
The inkjet printing process departs from conventional ink printing techniques in that engineering is required in many distinct disciplines for turnkey systems integration. In contrast, manufacturers of pad and screen printing equipment almost always can provide turnkey systems including inks, printing consumables, curing equipment, automation and chemical clean-up equipment. Inkjet system components consist of the printhead, drive electronics, inks, parts-handling and motion control hardware, and curing irradiator. Further, digital information needs to be communicated to the printhead through hardware/software file protocol including the main controller drive software. No single manufacturer provides all of the mentioned components for every custom application. A high degree of engineering knowledge of all the inkjet components and piece-part compatibility are critical to achieving robust manufacturing operations.

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 advancing 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 Group, Inc. (TSG). TSG is a global engineering company 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.