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Print Quality Matters in UV Ink Jet Printing



UV jet inks are being developed that allow for appropriate jetting control, printhead compatibility, performance on a wide variety of media and can respond to real printing conditions such as varied print to cure time



By Dr. Alexander Grant, SunJet



Published October 29, 2009
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Ink jet printing is attractive for many well documented reasons, such as fast, customized output without lengthy pre-press processes, speed to market and reduction in warehouse inventory.1 While the presence of ink jet in the small office home office (SOHO) markets has been known and widely accepted for some time, the emergence of ink jet in more conventional, traditional printing segments such as packaging and newspaper printing is creating much interest. The main challenges therein are providing jet inks that not only print reliably and offer full compatibility with a range of ink jet printheads but are able to offer resistance properties, odor, color and image quality that is consistent with existing printing technologies.

Despite these challenges, there are numerous examples of patented jet ink formulations, such as water-based textile and UV curable jet inks, that are capable of offering excellent application properties on a wide variety of substrates, often comparable to conventional printing inks.2,3 However, there is little in the way of published material in matters concerning print quality in one of the most rapidly growing ink jet sectors, 100 percent UV curable jet inks. These inks are typically composed of monomer, oligomer, photo-initiator, colorant and additives such as surfactant or stabilizer, combined to produce jet inks with low viscosity (typically 8 cP – 20 cP at jetting temperature) and with fine particle size (<1μm).

The ability to tailor the components of such complex formulae to that of the media is key to satisfying print quality specifications and will help differentiate ink and media suppliers as the market grows and end user requirements become more demanding. To illustrate some of the challenges in this task, Figure 1 shows an example of how two very similar UV jet inks that offer equivalent resistance properties can wet completely differently despite being printed on the same substrate. As a consequence of this, resultant print quality would be dramatically affected. Therefore, an emerging fundamental focus for the ink formulator is to provide solutions to customers’ print quality requirements through an understanding of ink-media parameters, without compromising application properties.

To help with this strategy, SunJet is a key partner in the IMAGE-IN project.4 This is a multi-partner collaboration, funded by the EU, bringing together partners from a diversity of backgrounds to focus on the challenge of improving ink-media understanding with a key output being how print quality can improve in an ink jet process.

From SunJet’s side, UV jet inks are being developed that allow for appropriate jetting control, printhead compatibility, performance on a wide variety of media and can respond to real printing conditions such as varied print to cure time. The focus of this paper is to look at some examples where simple ink, media and printing parameters can affect print quality, using printed dot size and line width values as key indicators of this.

Print quality is often in the eye of the beholder; one print can simply “look better” than another despite being printed on the same substrate, with the same printer and cured under the same conditions. However, usage of ISO 13660 is a recognized technique for ink jet print quality measurements that can help move away from more subjective analyses.5 In this study, a QEA personal analysis tool-kit is used that embraces ISO13660, to investigate several key areas of print, focusing on dot size and line width with a range of 100 percent UV jet inks.

Specifically, examples are given of ink-media interactions in the following areas:
1. Media surface energy and ink wetting.
2. Media roughness and ink wetting.
3. Effect of print to cure time on ink wetting.
4. Off-line testing to predict how an ink might wet.
5. Media chemistry and ink wetting.

Media Surface Energy



Media surface energy varies widely and is an important consideration when designing jet inks and achieving desirable wetting characteristics. Substrates such as certain grades of glass (170 dyn/cm) and gold (57.4 dyn/cm) have inherently high surface energies while some polyethylenes (31 dyn/cm) and polyurethanes (22.1 dyn/cm) are considerably lower. All of these could find application in ink jet printing. In a perfect situation, the ink formulator would like full wetting control irrespective of media energy. This means ideally that dense areas of print, such as blocks and large lettering, would be filled completely and show no mottle, while lines and dots would offer sharp definition, measured in terms of circularity and raggedness, respectively.

In order to achieve satisfactory wetting, a common practice among ink formulators is to ensure that ink surface tension is lower than the media surface energy. This is a sensible approach for dense areas of print such as blocks, where a high degree of ink spread is required, but for dots or lines, excessive wetting could cause poor definition and hazy images, ultimately reflecting poor print quality. Hence, in general, it is usually a balance of wetting characteristics that is needed.

The above ink-media energy and wetting relationship is often overly simplified. The general assumption that a fluid will wet if surface energy is greater than ink surface tension is true in many cases but should not always be considered as a “rule of thumb.” For example, it is possible to tailor ink formulations such that a fluid with lower surface tension than the media’s surface energy does not spread, if so desired. This is exemplified in Figure 2, where two UV jet inks have essentially identical static surface tensions but when applied to a higher surface energy substrate, do not wet in the same way.

Even within a class of substrate, surface energy can vary widely, depending on surface treatments or coatings applied by the media supplier. Hence, the ability to rapidly adapt an ink to match the media or vice-versa is critical. Chart 1 shows an example where a PET substrate is treated to give a range of surface energies, which in turn gives a wide range of wetting characteristics for two UV jet inks.

The plot shows that in several cases, we are able to offer inks that give narrow or wide line widths according to customer needs, irrespective of substrate surface energy.

Media Roughness



Variations in media roughness are common in the printing industry. Often this is an inherent feature, or coatings and treatments are specifically applied to modify them in a controlled way. This could be undertaken to improve gloss, adhesion or slip characteristics.

Irrespective of these modifications, UV jet inks are required to perform on these substrates and offer appropriate resistance and image quality performance. The main challenge in wetting adequately on a primarily rough surface is to minimize penetration of the ink into the peaks and valleys of the media and give a smooth, even ink lay. This is especially challenging for ink jet as ink drop volumes are small, typically in the range of 20-100 pL. If this cannot be achieved, then a highly roughened substrate can reduce ink spread and give sharp, small dots. This of course could be advantageous for some applications such as circuit board notation inks, but where a good degree of ink spread is required, such as filled areas of print on a foil or paper, formulation modifications are necessary.

The ability to match the ink with the media and achieve target dot sizes on uncoated and coated PET is shown in Figure 3. The figure shows AFM pictures of how roughness of the PET substrate changes when coated, giving a dramatically different profile to that of the uncoated case. Pictures from optical microscopy of dots printed with two UV jet inks A and B are shown alongside the AFM images. Ink A gives large dot size on uncoated PET and sharp rounded dots on the coated version. However, by some ink modification, it is possible to achieve opposite effects on each of the media, as is shown with ink B. Hence, irrespective of media topography, ink wetting features can be altered depending on output requirements.

Effect of Print to Cure Time



As printing speeds vary from machine to machine with a general trend toward much faster printing, it is important to appreciate how the ink will wet at various print to cure times. This is essentially the ultimate step before the end-user receives their final product. In turn, this can help considerations regarding lamp positioning and the subsequent impact on print quality.

To illustrate the dramatic effect that print to cure time can have on printed line width, Figure 4a shows how widths vary between 0.2s and 40s print to cure time, with a typical UV jet ink (A) applied to white coated aluminum media. At short time scales there is very little spread, gradually increasing at 2s and finally giving a very large degree of wetting at 40s. In terms of numbers, the width varies from 194 μm to 690 μm over a 40s period. Control of cure time is therefore critical in achieving the desired wetting properties.

In order to control this, the printer could engineer the equipment accordingly for each media type. However, realistically this is not usually an attractive option as lamps tend to be in a fixed position. A way around this is again through careful ink formulation modification, as Figure 4b shows. In essence, a re-design of the ink printed in Figure 4a applied to the same substrate can give much less wetting even at times up to 40s.

Off-line Testing



In order to speed up the ink to end user supply chain, it is useful to have some tests that can help predict how an ink will wet on the end-users media. Conventional techniques such as drawdowns of various film weights on media are useful to quickly screen a large number of candidates but often does not relate to the much smaller drop volume of ink jet printing.

A tool that can do this 100 percent accurately will never exist but an example of one key technique, dynamic surface tension (DST) using a bubble tensiometer, can give useful information. This method can show reasonable correlation between an ink’s surface tension under a range of surface ages and final printed output.

Figure 5 shows how dynamic surface tension varies with surface age for four different UV jet inks. The surface age time scale is such that low values relate to high bubble frequency and large values to low. From a jetting perspective, undertaken at typically 4kHz, then jetting data is related to low surface age while time to cure at ink-media impact, approximates more toward high surface age times. The inks vary from a standard to three modifications. As the DST trace shows, each of these perform differently as surface age increases.

Looking at printed dot sizes of these four inks on white aluminum foil shows the smallest dots occur with ink Mod A. This ink has the highest DST values of the four, while the largest dot size is evident with ink Mod D, which gives the lowest DST profile. As Table 1 shows, it should be noted that static surface tension of these inks are similar in most cases, indicating this is not always a good indicator of how an ink should wet. Furthermore, reducing ink viscosity, does not give any effect on ink wetting, as noted with Mod C.

Overall, this tool, which really examines the ink-air interface, can serve as a good indicator of how an ink might wet on media. By using this, it should be possible to further tailor ink modifications to follow a pre-determined DST profile to help achieve desired print quality.

Media Chemistry



Perhaps the most important aspect of all in terms of controlling how an ink will wet on media is a consideration of the media chemistry. For example, acid-base interfacial matching is a well described technique that can allow controlled dot sizes on a variety of materials such as lithographic plates.6 By understanding the nature of the plate chemistry, the ink can be modified in such a way as to create large or small dots. Among the benefits in so doing are production of controlled resolution prints and simplification of manufacturing techniques.

Often substrates have a highly hydrophobic or hydrophilic nature. This too can have a very marked effect upon dot size and shape, as exemplified by some work undertaken by the University of Oxford.7

Conclusions



Several key areas have been identified that need careful consideration when developing jet inks to perform on a variety of media. This includes chemical and physical matters such as media surface energy and roughness, but importantly also includes hardware aspects such as print-to-cure times. Some useful off-line techniques such as DST have proven to be useful tools for predicting how an ink will wet and some good correlation between expected and actual print quality data has been made.

With the printer, end user and media supplier working closely together, it should be possible to offer full solutions to print quality issues and provide output that can not only match conventional printing resistance requirements but can also meet increasingly demanding print quality specifications.



References



1. D. Bucknall, Proc. NIP 19, pp 552-554, 2003.
2. PCT/GB02/004360.
3. WO Patent 9,704,964.
4. Funded by the European Community through a Framework 5 grant (contract number: GRD1-CT-2002-00663). Partners: SunJet (ink), AGFA-Gevaert N.V. (media), Dotrix N.V. (manufacturer of printing technology), Teich AG (industrial printer), University of Oxford, University Joseph Fourier, Ardeje SARL (visualization specialists).
5. International standard ISO/IEC 13660 was prepared by Joint Technical Committee ISO/IEC JJ1, Information Technology, Sub-committee SC 28, office equipment.
6. US Patent 6,451,413.
7. D. Bucknall et al Proc. NIP 19, pp 257-260, 2003.


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