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Advances in UV-curable Ink Jet
by Mario Carluccio

Technology Feature

UV-curable jetting chemistries are rapidly emerging as the next key enabler for piezo drop-on-demand (DOD) ink jet technology. Many leading industry consultants predict it to be the primary growth opportunity for digital printing over the next decade. Why the excitement and what can be expected for this digital printing technology?

Ink Jet Technologies
Starting with the basics, there are two types of ink jet technologies: continuous and drop-on-demand (a.k.a. impulse). Continuous ink jet operates as a continuous stream of ink where ink droplets are deflected to the print area by a complex system delivering an electric charge, and where re-circulated ink is continuously replenished with solvents to maintain necessary jetting properties. These solvents are highly volatile and evaporate quickly, but are necessary in order to manage viscosity of the ink. Drop-on-demand propels ink droplets that can be very small from individual jets only on demand or when needed. There are primarily two types of drop-on-demand ink jet technologies: thermal and piezoelectric transducer (PZT). Thermal ink jet primarily uses thermal energy to create a vapor bubble in the ink that will burst to propel the ink out of the individual jet only as needed. The need to use heat-resistant or low vapor chemicals is why water-based inks primarily are used with thermal ink jet. Piezo ink jet technology also propels ink from individual jets only as needed, but it deflects the piezo material in the individual jet channel through an electric impulse. Piezo ink jet uses a wide range of ink chemistries, to include water-based, solvent-based, solvent-less UV-curable, and wax-based hot melt.

Advantages of UV-curable Ink Jet
Traditional screen and gravure/pad print technologies are labor-intensive, highly consumable processes with limited use in producing short-run, personalized or dynamic print jobs. There are multiple steps involved in making screens or producing gravure plates, with each process taking up to several hours to complete set-up. This includes time for adjusting ink viscosity, aligning print elements, breaking down, and cleaning up. Each process also consumes many different chemicals and printing elements that make the total cost per print expensive when producing low-to-medium print jobs (< 5,000 pieces). The advantage of UV-curable ink jet is that it is a single step, single consumable printing process that can be economical for print jobs of only one piece. It can be used for printing relatively flat or evenly shaped objects. It is typically viewed as a compliment more than just a potential replacement for traditional print technologies, which are best suited for long-run, static images and, particularly for pad printing, for odd-shaped objects.

UV-curable ink jet also offers a number of advantages over other types of ink jet technologies. Thermal DOD ink jet (TIJ), commonly used in home/office printers, is limited to water-based chemistries and, therefore, is used only on uncoated or specially coated papers. Continuous ink jet (CIJ) allows for a broader use of jetting chemistries, including aggressive solvents, but is limited in its use due to low print quality and high system maintenance issues. CIJ is typically used for producing simple text at high production speeds where stoppage is infrequent.

UV-curable Beats Solvent-based Chemistries
Piezo DOD ink jet also offers the capability of jetting solvent-based chemistries, which is where the technology made its initial inroad several decades ago. Issues related to use of non-volatile solvents that are only suited for porous substrates and frequent jet-outs due to solvent evaporation have hindered the acceptance of this digital print technology for many applications. UV-curable jetting chemistries are typically 100 percent solids containing no solvents. They require less maintenance, produce higher system uptimes, and are more environmentally friendly. UV-curable jetting chemistries also can be used for either porous or non-porous substrates, to include many types of engineering plastics commonly used in industrial applications.

Potential End-users
Primary industrial plastics end-users of UV-curable ink jet are companies currently using traditional screen or pad printing technologies to produce single- or multi-color images on relatively flat or evenly shaped objects (i.e. cylindrical or spherical). Markets are diverse and include printing simple text and logos on electronic, electrical, automotive, and medical components, or printing graphics to include on packaging containers, golf balls, and other general industry items. Primary commercial end-users of UV-curable ink jet are companies currently using thermal or solvent ink jet to produce short-run, variable, or personalized information on many rigid and flexible plastic materials. Markets include wide- to large-format printing of billboards, displays, and posters, as well as small-format printing of commercial signs, award plaques, and advertising specialty items.

Advances in UV-Curable Ink Jet
UV-curable inks for piezo DOD ink jet have been available since the mid-1990s. Many of the materials used to formulate these inks were limited during initial introductions, but are readily available today. The most common UV-curable jetting inks are acrylate-based (a.k.a. free radical) liquid types.

Liquid UVC inks. Liquid UV-curable jetting inks are relatively low in viscosity (< 50 cPs) at room temperature. They are jetted at slightly elevated temperatures (~50° C) as a means for controlling the jetting viscosity. When these liquid inks are jetted onto porous substrates, the ink can wick into the substrate resulting in poor print quality, bleed-through, and color dilution because the substrate can show through the ink. To overcome these printing issues, the substrate is surface-treated with processes that make the ink more receptive. However, these treatments are tailored to a specific ink(s) and other inks may not perform well. Furthermore, when printing on non-porous substrates, some use pinning UV cure stations to rapidly surface-cure the jetted ink drop, thus minimizing drop spread. This method only surface-cures the ink and does not eliminate the issue with ink bleed-through. It also complicates system reliability since it requires the UV source to be in close proximity to the ink jet printer, which can cause ink to cure in the jetting array.

With the requirements necessary for obtaining good print quality and performance, many users have found UV-curable ink jet technology too cumbersome and expensive. Also, liquid UV-curable jetting inks often do not provide acceptable bi-directional print quality due to gloss banding issues, further hindering production output.

Hybrid UVC inks. Hybrid UV-curable jetting inks, which were first introduced for select industrial print applications in 2002, address many of the limitations of the liquid UV-curable jetting inks. Hybrid is a term used to describe the functional properties of these inks. These inks, also referred to as semi-solid or paste inks, are highly viscous (>200 cPs) at room temperature. They are printed at elevated temperatures (~70 ° C) to lower ink viscosity to the required levels for jetting with piezo DOD ink jet technologies. The ink viscosity will rapidly increase as it cools after contact with the substrate, offering excellent drop control for extended periods without requiring special surface treatment or UV pinning of printed ink before curing. The printing process for these inks is independent from the UV-curing process, which can occur off-line and hours afterwards, without degradation of print performance.

Superior print quality is achieved with hybrid UV-curable jetting inks for high resolution printing with no bleed-through or color dilution on a wide variety of substrates. Maximum processing flexibility is gained with no immediate setting, pinning, or curing requirements. Maximum output is attained with bi-directional printing capability without gloss banding print quality issues. The broadest substrate reach is achieved by not being constrained to using low viscosity chemistries in the formulation of these inks. Hybrid UV-curable jetting inks are much easier and cleaner from a handling standpoint. These inks also contain materials that minimize health and safety issues associated with other UV-curable jetting chemistries.

Cationic and epoxy UVC chemistries. Cationic or epoxy-based jetting chemistries also are gaining momentum in the industry, driven by the need for higher performance inks in industrial applications. These inks are typically targeted at more difficult substrates such as untreated glass, metals, and ceramics. They are less susceptible to the UV-curing environment (oxygen inhibits cure for acrylate-based chemistries). Their final performance properties also can be increased when using a post-thermal cure process. Cationic chemistries can be offered in both liquid and hybrid UV-curable versions. There are only a few offerings in today’s market, but this list is starting to grow rapidly as more materials for ink formulations become available.

Aellora Digital is a relatively new business unit within MARKEM Corporation, focusing on providing piezo DOD ink jet printing systems and chemistries for commercial, graphics and industrial printing, imaging, and dispensing applications. Current product offerings from Aellora Digital include liquid and hybrid UV-curable, acrylate, and cationic chemistries; white, spot, and process colors; and high-resolution single- and multi-color ink jet printers. For more information, contact Mario Carluccio, Business Development Manager, at (603) 357-1715 or visit