Ink transfer and adhesion
Inks/toners for digital printing have different properties and requirements and they have to work with the different digital printing processes and in different environments. They make severe demands on print surfaces. For example, they may need to have electrical conductivity, with the images formed through the interaction of inks or powders with an electrostatic field, or to be formulated to pass through the very tiny nozzles of an inkjet head.
They may also need unique chemistries and physical properties, with the chemistries involved limiting the range of compatible surfaces for ink transfer/adhesion. These unique requirements and compositions for digital printing are very different from conventional label printing inks; getting the ink to stick or bond to the substrate or material is actually one of the last concerns/requirements of the digital ink/toner supplier.
So, the first challenge is to choose a substrate that is compatible with the technology that is being used, whether it is liquid toner, dry toner or inkjet.
Then the second requirement is the print quality and color that can be reproduced on the substrate.
Print quality and color
With digital printing now having largely become a mainstream printing technology for the label industry, and now starting to move into the package printing sector, the end-user market is not unnaturally becoming increasingly demanding on print quality and by quality we mean image quality and color consistency.
Providing a surface that inks will transfer and adhere to is not enough. The benefits of digital printing require that colors must be created and matched using process printing. The image must be printed using exceptionally fine dots of the different primary colors and the human eye will then synthesise these dot patterns as a color. This is the reason why it is said that quality is ‘color quality’.
Digital printing places extraordinary demands on substrates for dot quality. Broken dots, or even worse unprinted dots, will compromise image quality. Broken and missing dots will totally undermine quality control. So how do these various factors relate to the different digital printing technologies used today?
In order to fully understand all the unique requirements of digital printing that affect the substrate it is important to appreciate the landscape of the digital technologies used in the label and package printing market today.
The table below indicates the approximate market share of substrates used for each digital label printing technology, and then highlights the degree to which each technology is substrate sensitive – firstly to ink adhesion and then for dot quality.
To expand on this, the table highlights how the substrate sensitivity is different for each technology, and sometimes even within a technology, namely:
Liquid toner technology currently accounts for something like 72% of the digital substrates being converted in the market today. As can be seen from Figure 6.1, adhesion/transfer to the substrate is highly sensitive as is the dot quality.
Figure 6.1 - Substrate sensitivity of digital printing technologies. Courtesy Avery Dennison/ExxonMobil
Dry toner technology currently accounts for something like 19% of the digital substrates used, and is somewhat sensitive to the substrates used.
This is because many substrates work perfectly well, but some fade. For example, metallized paper does not work with dry toner printing. Dot quality here is also somewhat sensitive.
Inkjet technology has been growing steadily over the past 3/4 years but, as yet, there is no real accurate substrate market share information available.
Adhesion and transfer are really dependent on the specific inkjet technology being used, which may be solvent/water/acrylic/UV-based.
Dot quality is highly sensitive.
Each of these digital printing technologies will be discussed in more detail but at this stage is can be said that:
It is often necessary to modify substrate surfaces to achieve acceptable (or optimal) performance when used in digital printing
Surface modification is most frequently achieved through application of a coating/primer
The need for and benefits of applying coatings to achieve compatibility between substrates and digital technologies depends on the printing technologies used.
It is now possible to explore the factors that control the compatibility between the individual printing technologies and the materials that they will be printing on. It is possible to do this by focussing on each of the major technologies used in the market today, starting with the current market leading technology – liquid toner printing (ElectroInk) as practised by HP Indigo.
LIQUID TONER TECHNOLOGY
The first requirement of a material that is going to print successfully in a digital printing application is that the material must have a surface that will receive the ink and must provide adequate anchorage for the ink to be able to adhere to that surface.
Anyone who is familiar with liquid toner technology is aware that the vast majority of surfaces do not provide these characteristics in their native state, and these surfaces must frequently be modified, most frequently by being coated in order to make them compatible with liquid toner printing.
The reason for this is based on an understanding of the mechanism of ink adhesion and liquid toner printing. To explain that mechanism, one picture is worth a thousand words; see Figure 6.2. This shows a cross-section of ElectroInk – printed white BOPP top-coated film magnified 7000 times in a scanning electron microscope. The sample was prepared by taking an Avery Dennison top-coated white BOPP and printing it with black Electroink with a degree of adhesion that is 100%. So this is perfect bonding.
Figure 6.2 - Illustration shows liquid toner printing ink adhesion. Courtesy Avery Dennison
Next, the sample was prepared for the scanning electron microscope by metallizing the sample. This has an unfortunate side effect that all of the layers – the black ink, the clear top coat, the white film – now look mis-uniformed. But nevertheless it is possible to clearly see a sharp boundary between the two distinct layers.
The place where care is required, and the place where something can be learnt, is at this interface between the toner and the digital top coat.
When the samples are prepared the aim is produce an absolutely perfect vertical cut through the film and ink. However from time to time what is seen is the ink fracturing, and when it fractures it will leave areas at the surface of the ink where it will have extra electrons being reflected. These can be ignored; this is just an artefact of preparation. Where there is an absolutely clean cut however, there is a razor sharp boundary.
At 7000 X magnification it is possible to image as few as 50 molecular layers. Quite simply, this means there is hot toner coming off the blanket at about the temperature of boiling water, being applied to the substrate surface as a soft polymer with a softening point around that same temperature. So it would be logical to think that the mechanism of adhesion is that the toner is literally melting into the top coat, thereby producing an effective weld. Well at 7000 X if this was the case, the weld would show on the micrograph. There would be a fuzzy layer where the two resins have interpenetrated with one another.
However, that is not at all what is seen. Instead, what is seen is a razor sharp layer, which means the bonding between the ink and the coating is almost purely chemical. When it is understood that the principal method of adhesion is chemical bonding, it explains a lot. It explains, for example, why even on substrates like paper you can get adhesion by having the individual fibres encapsulated by the ElectroInk. Even so, it still is often very beneficial to add a digital coating because that coating provides more uniform and more predictable anchorage. And secondly it explains why, although the chemistry of the ElectroInk itself is understood, there are really so very few materials and so very few surfaces that actually provide the requisite sites to achieve this bonding.
In order to provide the best bonding, if the optimum surface isn’t already there, then it becomes necessary to add it. The easiest way to do this is through top coating. It also explains the fact that because it’s a chemical reaction, it reacts very favourably to an increase in temperature. All chemical reactions go faster at warmer temperatures and thus you would expect that when you compare the ink on a thick multi-layer label stock with a single layer of flexible packaging film, the thick substrate is likely to provide a bigger heat sink, cool down the ElectroInk faster and therefore make it harder to get good bonding.
Requirement for coatings when printing with liquid toner
The result of all of the factors above is that coatings are almost always universally required when printing on an HP Indigo press to achieve acceptable bonding with the ElectroInk. Figure 6.3 shows the current list of Avery Dennison qualified HP Indigo paper and film substrates that meet the requirements of acceptable ink transfer and bonding.
Figure 6.3 - Table shows Avery Dennison HP Indigo certified substrates. Courtesy of Avery Dennison
So, all of these qualified materials can all be said to perform well for adhesion, but how do they perform for control of color?
To talk about color control – the quality of color being printed – it is necessary to make two comments. Firstly, the need for color control with digital printing has largely become essential. Digital printing has become a mainstream technology and customers who are printing ultra-short runs of custom designs are usually forgiving about color. However, that’s not where the bulk of the substrates are now being used. The bulk of the material currently being used is in label printing applications where digital converters are quite commonly printing anything up to 10,000, 20,000, 30,000, 40,000 or even more commercial labels.
Frequently, these commercial labels are going to be side by side with conventionally printed labels, or will actually have been converted from conventionally printed labels that are now more efficiently and better produced in a digital environment. When that happens, the color accuracy requirements of those conventionally printed labels get carried over into the digital world, and the demand for high-quality, accurate reproduction of color, particularly Pantones and brown colors, just grows exponentially.
Actually meeting that demand in digital is harder than in conventional printing. In conventional printing for conventional colors, for brand colors, for Pantones, the ink is mixed up in the ink room and the job of the press is simply to maintain a uniform thickness of the ink film. With digital technology, to maintain the ability to have quick changeovers, the color has to be prepared digitally. Digitally means the color’s process color. The color is created by transparent dots that are overprinting one another. The separation of those dots is the science of color management, and world class color management software like Esko works to make very accurate separations in as many as 7 colors in this technology.
The separations are then transferred to the HP Indigo as machine instructions – literally digital instructions that instruct the Indigo how to form the dots on the Photo Imaging Plate (PIP) and transfer them to the blanket. The very last step, the transfer of the dots from the blanket on the HP Indigo press to the substrate itself is chemistry. There is nothing that can be done at this stage by color management; there is nothing that can be done by the HP Indigo press. It is now a physical process and wholly dependent on the nature of the coatings and the nature of the surface that is going to actually touch the head.
To go back to the question – is a coating that provides good performing adhesion also a coating that provides uniform stable dots, because the consistency of the dots is the consistency of the color? The answer is in Figure 6.4 which used the same press, the same printing conditions, on the same day.
Figure 6.4 - Illustration shows liquid toner printing ink adhesion. Courtesy Avery Dennison
On the left is a white, smooth OPP with a top coat that was optimised for dot consistency. On the right is another HP certified material. This one has semi-gloss paper with a digital top coat optimised for ink adhesion. On both the left and on the right, dots form the color blue. Magenta dots and cyan dots over-print one another and the dark areas are simply the overlaps. These are identically the same separations; the separations were optimized for paper not for plastic. They were printed on the same machine, they were printed within a few hours of each other, yet when they are analyzed it can be seen that the dots printed on the plastic film are extremely uniform, both in size and in shape, and the dots printed on the paper are anything but.
A logical question to ask is why? Is this simply the difference between a smooth plastic film and a rough paper, or is it the difference between coating chemistry that has been optimised for dark quality and coating chemistry that is optimised for ink adhesion? Well, one way to answer that question is to only print one color of dots.
In Figure 6.5 mid-tone 30% cyan color dots have been printed, once again using identically the same separations; and on three different materials.
Figure 6.5 - Illustration of liquid toner printing with 30% cyan dots, courtesy Avery Dennison
On the left the dots are printed on a smooth white OPP film with a coating optimised for dark quality. The center sample is printed on an ExxonMobil synthetic paper. This is the same coating chemistry as the film on the left but with added particulates that provide a roughness to make the surface about the same as any typical paper. And finally on the right is the paper itself with the digital top coat optimised for adhesion.
Below the samples are the relevant statistics. For the first sample the statistic is simple – 35 dots, in an area of .0075. This shows that all the dots print. The smooth white OPP prints 35 dots out of 35; the synthetic paper with an optimised coating for dark quality also prints 35 dots out of 35; the paper sample however, misses a dot, now only printing 34 dots out of 35.
When looking at the average area of the dots under identical conditions, the dot areas for the plastics are roughly 40% larger than the dot area for the paper. This simply indicates that those coatings have been optimised for increased activity. However the real story is the variability and size of the dots, because at the end of the day the color management software is specifying a specific dot, and if those dots are printed per the color management software the color will be right. And if those dots vary from what the color management software is requesting, then there are color matching problems and waste in color matching.
So, in these three samples, there is a standard deviation as a percentage of the average area – for the white coated OPP, one standard deviation is 6.7%.
A good way to think about this is that a full range of variation is typically plus or minus 3 sigma. In this case the full range of variation is plus or minus 3 times 6.7 %, plus or minus 20 %.
When looking at the synthetic paper the impact of roughness of plus or minus 20% becomes plus or minus 25%. But with the semi-gloss paper with the adhesion optimised coating the full range of variation is now plus or minus 50%. A significant difference in mid tones is therefore part of the equation. But if it is necessary to hit Pantone colors with process printing, then it is necessary to print highlights – sometimes very severe highlights. Highlights like 3% dots.
Figure 6.6 shows what happens with a typical highlight, a 10% dot. All the setup is the same as before – same day, same press, same conditions.
Figure 6.6 - Examples of liquid toner printing on different substrates using a 10% dot, courtesy Avery Dennison
The samples show a smooth film with a coating optimised for dark quality, a synthetic paper (rough surface) optimised for dark quality, and a semi-gloss paper with a coating optimised for adhesion. But now, 35 out of 35 for the film, 35 out of 35 for the synthetic paper, but only 26 out of 35 for the paper. If the frame is moved around it can be seen that there are some frames with 20 dots and some frames with 30 dots and that’s a huge range of variation in the highlights.
It can also be noticed that the receptivity of the coating now almost doubles the ink carrier, while the full range of variation for the paper is plus or minus 70%. So, in summary it can be said that:
All of the materials tested work well from the viewpoint of ink adhesion
On the other hand, they are far from interchangeable in terms of color control:
Consistent dots are mandatory for consistent color.
Although surface roughness plays a role (smoother is better), coating chemistry and application technology play a bigger role.
Achieving high levels of dot quality is difficult for makers of pre-optimized materials.
Converter coating adds new variables and substantially increases risk.
Among all of the materials tested ... pre-optimized films are the most controllable
However, it has to be said that it is not just coating chemistry that is important. Go back to the first illustration. Everything that’s going to happen in the interaction between the dots and the chemistry is going to happen in perhaps just 6 molecular layers. That means really intimate contact.
Intimate contact that cannot be conceived in everyday life – it is a prerequisite for the chemistry to play its role. But how can that intimate contact be achieved? Well, through an extremely uniform and extremely consistent coating.
Consistent coatings for liquid toner printing
The best way to have a uniform consistent product in anything is to make it on a large scale industrial scale and that’s exactly the case with substrates for liquid toner printing. The degree of consistency required for high degrees of color accuracy is difficult to achieve even on a large scale industrial process. So achieving it in a short run, off scale environment is very, very risky. Of all the materials tested by Avery Dennison, pre-optimised films are consistently the best materials and the most controllable materials for color. For films that are optimised specifically for dot control, the coatings are so consistent that Avery Dennison are able to offer color guaranteed printing.
Converters who are printing with liquid toner using HP Indigo technology receive a design file from a designer and prepare it for press, judging color typically on a monitor, which is a highly inaccurate game. Ultimately after a minimum set of adjustments the digital file is sent to the press and there the press operator will be the first person in the production cycle to actually be able to visualise the color that’s in that digital file. All too often that color is wrong.
So, what does the press operator do? He uses the press adjustments to try and compensate for whatever’s wrong with the color. When those press adjustments are frequently not adequate he then shuts the press down and takes a copy of the print to the pre-press operator. The press operator walks in and says ‘My press is down, this is the color I printed, I need this to be a different color, fix this job please.’ And whatever else they are doing, pre-press has to drop it and fix the job. The job then goes back to the press where because it was fixed essentially blind, it may or may not be the right color. If it’s not the right color it circulates through this process iteratively until it is the right color The end result is that the press has become the proofer; the press is the only place where it is possible to see what the actual end color is going to be. All in all, a wasteful, disruptive process for hitting the right color.
Color guaranteed printing
Color guaranteed printing with liquid toner takes advantage of the fact that if the dots are absolutely stable then it is possible to build a highly reliable relationship between the dots of ink that are printed and the colors that are seen. That color profile relationship can be made up front in pre-press. Instead of visualising the colors on the press it now becomes possible to visualise them on an inkjet proofer. The inkjet proofer limits the gamut of the colors being printed to precisely the colors of the press. So, instead of printing the colors as the designer requested them, they print the colors exactly as those colors are going to be seen at the press.
The result is, if the colors aren’t right, pre-press makes the adjustment upfront as part of its normal preparation. Once pre-press is happy with the proof, the digital file that created the proof, or at least the separations that were behind that emulation, are sent directly to the press. The press with stable materials, stable printing conditions, is now able to match the color of that proof with little or no adjustment. The result: less waste, great increase in productivity, and a competitive edge for the supplier.
The foundation for this result is press repeatability, and half of press repeatability is dot consistency. The other half is control of the press itself and the environment that the press lives in. This can be seen in Figure 6.8.
Figure 6.7 - The table shows the current list of HP Indigo qualified substrates by end-use printing sector, as well the number of substrate suppliers worldwide
Xeikon qualified substrates
Figure 6.8 - The illustration shows the foundation of press repeatability with liquid toner printing
The HP Indigo is a precision machine. It does its job very well, but it does its job very well only when it’s surrounded by a highly controlled environment in terms of setup procedures, calibration procedures, and diagnostic tools that enable a numerical visualization of how well the Indigo is repeating itself, and if it’s not repeating itself provide the necessary guidance to troubleshoot and fix it.
In summary, only pre-optimised coatings for use on HP Indigo presses are color guaranteed. The analysis that is done to get to the basics of the chemistry and the actual interaction of molecules is expensive. The reward for that expense comes in how to optimise coatings. The coatings in turn are only as good as their application. A well controlled industrialised coating process in long runs on large scale industrial equipment with statistical quality control processes builds quality into the product.
DRY TONER TECHNOLOGY
The biggest advantage perhaps of dry toner technology is that it is compatible with a wide variety of substrates when compared to the liquid toner technology. This is the reason it is said to be only ‘somewhat sensitive’ in terms of adhesion and transfer. This technology represents about 19 % of the substrate used for digital label printing today. The dot quality is also ‘somewhat sensitive’.
So, for dry toner technology it is true that most of the time the substrate does not need to have a top coat. Yet there are some substrates that are simply not printable. For example, metallized paper. This is really linked to the features of the paper. There are also some materials that are too temperature sensitive to print with this technology – they will simply melt during the fusing process.
In terms of dot quality in dry toner printing it is worthwhile looking at the illustration below. This example shows an example of two different substrates which have been printed with micro text with the dry toner printing technology. One has been printed on white PP which has been top coated; the other on semi-gloss paper with a standard coating. Neither substrate has been optimized for dry toner printing.
On the PP substrate the printing is much more dense. On the paper substrate there is a reduced density in the printing, so it is possible to say that the density and darkness are really influenced by the substrate. Historically there were no substrates that had been optimised or qualified for dry toner and dot quality. Today, there is an increasing range of materials that have been qualified by the various materials suppliers for dry toner printing. This can be seen in the list on the previous page of Avery Dennison qualified Xeikon substrates.
Maybe there is some room for improvement here by all the suppliers in the future. But the forgiving nature of dry toner is generally such that it tends to enable substrate suppliers to sell their conventional products into this market segment.
Until fairly recently inkjet was still very much an emerging technology. The reason for this was because the term ‘inkjet printing’ was an umbrella technology for different inkjet printing solutions: for solvent based printing, for water-based printing, and for UV inkjet printing. So the situation today is that inkjet is being widely used outside the label industry but has taken some time for inkjet suppliers to achieve the combination of speed, quality and application – all the specific features required in the label market – that have now come to the market from a wide variety of different manufacturers/suppliers, with different inkjet head technologies and different ink formulations.
Figure 6.9 shows and demonstrates a few points relating to color accuracy with inkjet printing.
Different substrates differences
Figure 6.9 - Illustration courtesy of Avery Dennison/ExxonMobil
Proofs were printed on an inkjet proofer under identical conditions
Switching to HP Premium Photogloss paper doubled overall color accuracy
The illustration shows a third party coated paper which has not been optimised for inkjet printing compared with another paper which is an HP premium instant dry gloss paper used for inkjet printing.
What can be seen is a difference in the quality of the dot. For example, on the third party paper there is an inconsistent print; the size of one dot is slightly different to the other one. On the optimised paper can be seen consistent print. Again, the samples were printed with the same ink, on the same proofer, but with the different paper. It can be seen that when printing was switched to HP premium paper the color accuracy was doubled.
There has certainly been quite a lot of work undertaken in inkjet printing technology today, whether it is by the ink vendor or, typically the print vendor, the print OEM and the print head supplier, with all three parties trying to best optimise their inkjet solutions. The substrate suppliers have also been working hard to qualify substrates suitable for inkjet printing, and even for specific inkjet presses. This can be seen in the two tables on the next page supplied by Avery Dennison for their materials that have been qualified for EFI Jetrion and Durst inkjet presses.
Today, inkjet printed results can be said to be very positive. However, in summary, it should be pointed out that:
UV inkjet includes a wide variety of ink/solvent chemistries with differing requirements to achieve ink adhesion and dot quality - general statements about all UV Inkjet technologies are simply not meaningful at this stage.
UV inkjet to some degree is still a fast emerging and changing technology, although now growing rapidly. However, the user base is still relatively small when compared to the toner technologies and material consumption is split over 30 or more different press makes, models and ink drying/curing processes.
Until the market settles on fewer of the more well established press manufacturers and ink/solvent chemistries, it is probably not possible to make specific statements concerning the need for and value of top coatings.
INPUT FROM INKJET LABEL PRESS SUPPLIERS
Having made the above statements from the substrate supplier’s perspective, some of the leading UV inkjet equipment suppliers state that UV inkjet is one of the most robust digital printing technologies around in terms of substrate interaction. In general, whether it is a BOPP, a vinyl, a PP, any sort of synthetic substrate, most matt papers and a lot of semi-gloss papers, they generally claim that they can actually be printed by the inkjet process with absolutely no coating. In fact, EFI Jetrion have stated that most of their 3000 series and 4000 series UV ink customers use exactly the same substrates in their flexo press as they do on their inkjet devices.
The only area where they see UV Inkjet might require an undercoat would be on some of the highly glossy papers where there is a need to control ink spread because of the smooth surface. So in terms of UV inkjet as a general statement, it is today a robust technology in which the areas where substrates for most of us come into play are mostly to do with controlling dot spread and image quality… it is not a question of ink adhesion.
As a general rule when an inkjet press supplier gets a customer’s print sample into the lab they measure the surface energy of the material. If it’s above 44 they know they will probably not have any trouble; any kind of regular UV ink is going to work well – give a nice film and nice adhesion.
Below 44 then it may be necessary to do some pre-treatment. This may be Corona, it might be Flame, it might be Heat. As an indication, they look for inks all the time that will go lower and lower on surface energy.
To achieve good application of Inkjet onto a substrate the surface energy is very important. Overcome that by any sort of pre-treatment, and ink adhesion can very good. And the stability of the ink is also excellent. Things like rub resistance and so on never appear to be a problem with inkjet and UV inks. That’s not just on substrates used for labels but things like high density polyethylene plastic caps for instance.
With UV inkjet it may also be necessary to look at cure time on different surface energy materials. Certainly have a powerful enough lamp to run a full ink load at the highest speed. Some substrates will allow the ink to spread a lot slower than others. So having a movable lamp actually makes a lot of sense because then people can change the time to lamp and allow for a bit more spread.
SPECIAL EFFECTS WITH INKJET
One of the interesting things about inkjet is that it is the world’s only non-contact printing technology, enabling printing on to uneven, embossed or difficult substrates. For example, there are now some label converters that have taken a pre-embossed label, say with a shape, and then inkjet printed a full color image in the embossed shape on top of that because it’s not contact – and that has created a very unusual packaging effect.
Indeed, when the label is wrapped around a bottle it actually is truly 3D.
There are also converters printing inkjet on to the top of embossed objects. Yet others have produced print that’s been textured with inkjet to give a 3D structure and feel. UV Inkjet is also being used to spot varnish on conventional pre-printed labels, while companies such as Atlantic Zeiser have a Braillejet press. This prints little white Braille spots, single pass in-line. It is just treated like any other single pass inkjet unit that they sell.
In terms of difficult to print surfaces, such as certain grades of Tyvek which are often used for labels, they can printed perfectly with inkjet because the heads are not touching the substrate surface. That is often not achievable with conventional print. You either have to put the rollers down too far and you get a lot of smudge of the ink, or you leave it high and only touch the high spots. So there are definitely some specific areas where inkjet is attractive for a very specific reason.
CO-ORDINATION OF INKJET RESEARCH
As already explained, inkjet today – although producing excellent printed results – is still something of an on-going emerging technology, but a technology which has a massive potential for the future. At the present time most of the substrates used for inkjet printing can be said to work, although there are some issues on all the substrates which still need to be investigated.
A lot of inkjet developers continue to do their own trials on their own sets of substrates, using treatment or without pre-treatment. What is still perhaps needed today is some kind of co-ordination of research. If there could be some sort of forum for co-operation and co-ordination of the inkjet companies, or some kind of testing regime that could be set up it would be of benefit to the industry.
IN-HOUSE TOP COATING
Currently, some label converters do their own top coating of, say, film substrates and it is often asked what are the benefits, whether there is any quality difference between in-house coated films and the substrate supplier optimized films, and what are the risks?
Benefits of top coating
When looking at top coating versus pre-optimised supplier materials there are a number of positives:
Top coatings can be applied on any material when it is needed
Existing factory personnel and machines can be utilized as required, so minimising additional costs
Some press manufacturers, like HP Indigo, will even support in house top coating although they will not qualify the materials.
Certainly converters that coat their own materials will generally get acceptable ink adhesion. The in-house coating process however offers the possibility to add some inconsistency. This is simply because of run length changeover, the quality and type of coating equipment used, and the quality of the process control which is being used to apply the coating.
In addition to that, the coatings that are available on the market for converters to treat normal (but digitally unprintable) substrates, have been optimized on a single dimension. That dimension is ink adhesion. However, ink adhesion isn’t everything. Proprietary coated substrates have been optimised to achieve a balance between adhesion, dot quality and consistency in the coating application process. So what may be seen with in-house coating is:
a difference in dot quality
a difference in consistency.
The difficulty that can arise with in-house coated substrates is in trying to track back where a print problem has arisen. The end users can get very upset with any inconsistency of quality.
GOING BEYOND LABELS
The ability to expand digital printing beyond the pressure-sensitive label market has been the focus of considerable development and research in recent years, as it has been for all narrow-web printing and converting technologies.
It has already been seen that both HP Indigo and Xeikon have found increasing applications in narrow-web folding carton production, while HP Indigo is also used for shrink sleeves, some sachets and pouches.
New wider web and/or sheet-fed presses from HP Indigo, Xeikon, Screen Europe and Landa are now specifically looking to target the folding carton sector, with wider web and sheet-fed HP Indigo and Landa machines targeting sheet cartons and flexible packaging. This focus will undoubtedly result in more substrate variety becoming available to both the label and package printer.
However, initially the challenge of obtaining a range of optimized carton board or flexible substrates for HP 20000 or 30000 printing is being met with in-house priming/top coating using quick-change gravure coating rollers, so allowing any off-the-shelf substrates to be processed.
How the inkjet technologies will develop to meet these changing sector and market requirements will be interesting to see.
The launch of Nanographic sheet- and web-fed presses specifically targeted at printed packaging will further see challenges for substrate suppliers, press manufacturers and printers.
IS THERE A CONCLUSION?
To bring all of the preceeding information, illustrations and tables together it can be said that to maximize the opportunity and potential of digital printing, it is important for label and package printers to know:
Which paper, board, film or other substrates can be used with each of the digital printing technologies?
Which paper, board, film or other substrates need to be top coated to achieve optimal adhesion, print quality and color consistency?
Get these two aspects correct and there is no reason why the label and package printer should not be able to achieve optimum results and print performance every time.