Digital printing: technology, imaging and terminology

Put together, digital print-on-demand color printing has been steadily gaining an increasing volume share of the global label market (Figure 2.1), but growing much faster in terms of value.

Quite simply, digital POD color label printing is now regarded by most successful digital label printers as more profitable than analog label printing.

This same kind of volume and value growth is anticipated in the digital POD color package printing market over the next five years or so.

Digital color presses of this level of market sophistication are now widely installed (well over 2,000 to-date worldwide in the label industry alone) and there is a considerable amount of historical data to show the level of production performance and end-user acceptability that digital printing technologies now provide. So what digital printing technologies are available?

DIGITAL PRINTING TECHNOLOGIES

Digital printing of labels and packaging may be undertaken today by quite a wide range of different digital printing technologies, including:

• Electrophotographic (EP) liquid toner

• (HP Indigo)

• Electrophotographic dry toner (Xeikon, iSys)

• UV inkjet e.g. Domino, Durst, EFI Jetrion, Nilpeter Caslon, Stork Prints

• Water-based inkjet (Epson, Monotech and Shanghai Taiyo)

• Dye-based inkjet (Swiftcolor)

• Laser (Primera)

• Thermal Transfer (Matan)

• Nanographic offset (Landa)

At the time of writing this publication, around 85% of the installed base of digital presses for the printing of prime labels was EP – electrophotographic liquid or dry toner – but there has been a big push in the last few years in the direction of inkjet (now around 35 inkjet label press manufacturers in the label market alone), and specifically UV inkjet press manufacturers.

While inkjet is steadily gaining market share of the total digital installed base of label and package printing presses, and will continue to grow rapidly, it is still forecast to be overshadowed by the toner technologies in the short term (Figure 2.2) and may take another five or more years before the technology even begins to equal the annual installed base of electrophotographic presses.

SCALABILITY OF DIGITAL PRESSES

Looking at the digital printing technologies available, one of the basic arguments for inkjet is that it’s more scalable and that it’s potentially cheaper in terms of running costs in the long term. Not necessarily accepted by everybody yet, it is inherently, mechanically and physically a simpler technology. Also, UV chemistry in particular, offers potentially very high speeds that will allow, for example, more easy generation of hybrid products: a hybrid product being a printing system which can print in analog (flexo) as well as digital at the same time. This is the route adopted by Nilpeter Caslon for example.

Electrophotography can be single pass, full rotary (in the case of a Xeikon), or it can be effectively multipass (which is what Indigo uses even though it functions as a continuous system). Inkjet is a continuous system. Electrophotography can be liquid or dry toner chemistry, while inkjet may be UV, aqueous, solvent, dye or LED ink (mostly UV is offered for the prime label market). Until now, both electrophotographic and inkjet have mainly been operating at roughly a similar speed capability but, in the case of inkjet, it’s capable of higher speeds and some of the latest presses are now reflecting this.

One of the most interesting characteristics of inkjet, as already mentioned, is this issue of scalability. Scalability obviously meaning you can take the same technology and make it narrow or make it wide, or make it less fast. And it can be built in a modular fashion, particularly if converters want to put inkjet print stations inside or on conventional analog presses. So scalability is something which may hold promise for the future of inkjet.

Having said that, toner systems have the advantage of generally being able to go wider (width for toner comes relatively inexpensively), but width for inkjet is more expensive (more printheads). However, when thinking of short-run applications for labels or flexible packaging, does the printer/converter really need a much wider solution? How do you finish a wider solution for short run? These are important considerations in terms of technology fit. Finishing is certainly a key issue for any label (and folding carton) converter looking to invest in digital printing. There are also some possible issues with the need for temperature and humidity control for toner which may lead to color shift over time.

INVESTMENT IN DIGITAL TECHNOLOGIES

So where are the label and package printing industries today in terms of investment in digital print on demand full color label presses? Of the estimated 2.300 digital presses installed in the label industry by mid-2013, some 86% or so are estimated to be electrophotographic liquid or dry toner presses from HP Indigo or Xeikon. Inkjet has a much smaller market share, but the leading inkjet press manufacturers are undoubtedly ramping up sales. However, even projected forward to 2015, toner technology is still expected to be the dominant process over the next few years, but with inkjet beginning to gain substantial ground. Yet even these figures for the digital printing technologies will still be small numbers when compared to the total installed base of conventional analog presses.

Having said that, digital label presses at the end of 2012 already made up around 20% of the 1400 or so total label press installations worldwide.

This can be seen in Figure 2.3.

Installations of digital presses for package printing are not believed to be more that 1% or 2% of new press installations at the present time. This is expected to change in the coming years as the new generations of wider HP Indigo, Xeikon, Screen Europe, Landa and other digital package printing presses come to the market from late 2013 and through 2014.

What is not in doubt is that electrophotography, after many years of effort, has today gained a very respectable installed base, and still has a very ambitious target to aim towards. Inkjet has a much smaller installed base at the present time, but the label and packaging industries are undoubtedly going to hear a lot more about inkjet in the future. Indeed, a key and important discussion on the future merits of toner and inkjet needs to more fully reflect the equivalent cost calculations – some toner devices have a click charge. Inkjet in general does not. In the longer term economics of digital printing this may become increasingly important, along with speed of production, cost of inks and toners, and resolution.

While historically, the label and package printing industries have been looking at no more than just a handful of digital label press manufacturers/suppliers, investing in digital print-on-demand color presses for labels, cartons or flexibles in the future may mean examining anything up to a couple of dozen or more different suppliers and machines.

Just to give an indication, the number of digital label (and more recently, package printing) press makes and models in the market place has risen from just a handful ten years ago to more than 50 today, the majority of the new market entrants being inket – predominately UV inkjet. This can be seen in Figure 2.4.

With such a large variety of makes and models to choose from, the challenge for label converters or package printers is how to compare different press makes and models, processes, inputs, outputs, front ends, resolution, etc, of digital drop-on-demand label presses in a meaningful way that will help them to make the best decision. So let’s start with the digital imaging process.

THE DIGITAL IMAGING PROCESS

To aid label and package printers in understanding the digital imaging and printing process it is important to have some knowledge of the key factors that make comparisons more objective. Firstly in the way a digital label press operates and then in terms of some of the key areas of operation, starting with Figure 2.5. This diagram, and some of the explanation of technical details that follow, are based on an original White Paper document put together by Xeikon, with additional material specially prepared by Domino Printing Sciences.

As indicated in the illustration, the digital printing process is essentially in three stages: input, imaging and output. These are briefly explained as follows:

Input

This is a typeset page, label or package design created with, for example, Adobe Indesign and made up of images (contones), text and graphics (line work). This page or label/pack is typically device independent and is basically a file of instructions describing line work in the form of vector graphics, text, fonts, and images in a page description language which specifies all elements of the layout. 

Imaging process

Digital print engines typically print interfacing lines of dots. Contone images or line work can therefore not be printed without converting them to a series of dots. This is done by the RIP (raster image processor), a component of the digital front end (DFE) that interprets the instructions of the page description language and translates them into a bitmap, i.e. the actual pattern of dots, producing one bitmap for each process color. Typically every dot in the bitmap contains 8 bits per process color. The amount of information contained in such bitmaps is generally too large to be handled by the printing process. This is why, in order to print this bitmap it has to be ‘halftoned’. Halftoning or screening is a process that converts a bitmap containing dots of 8 bits per process color into a bitmap containing dots with fewer bits per color. These ‘re-sampled’ bitmaps are then printed by the print engine.

Output

This is the reproduction of the electronic original (input) printed by the print engine. The quality of the output is influenced by the combination of the DFE and the print engine. Although the design parameters or specifications of these components do not by themselves define the level of perceived (image) quality, they do have a direct influence on the ultimate quality of the output, and as such they can be manipulated to maximize image quality.

THE DIGITAL FRONT-END

The digital front end is a combination of hardware and software, encompassing the entire suite of software modules necessary to process the files to be printed and to control the print engine. Depending on the press manufacturer, digital front end modules may be Esko, Xeikon-800, Fiery XF, Prinect, Screen Equios, etc. The elements of the modules or processes typically impacting output quality are calibration, color management and screening.

Calibration

The calibration of the imaging device in order to increase the accuracy of its output, i.e. to ensure its output matches certain criteria, is closely linked to all aspects of process stability. The discussion of calibration, although important, is not covered in this article. More will be discussed in the article Pre-press strategies for profitable digital label printing.

Color management

Each device in the digital workflow works with its own color space and therefore has its own color representation (e.g. RGB versus CMYK). Colors from the same input file may look different on different monitors and the colors on the printed output file will look different from the colors displayed on a computer screen.

Color management is therefore necessary to ensure color consistency and predictability among the different devices, i.e. users will know what a given yellow on a computer screen will look like once it is printed on paper. 

Screening

As explained above, most digital printing systems cannot print continuous tone images as they can only deposit dots with a limited number of bits per process color, which is why for printing purposes continuous tone images need to be converted to a pattern of dots containing the right number of bits per process color. The process used to do this is halftoning.

DIGITAL IMAGING CONSIDERATIONS

Imaging technology

One of the key considerations to take into account when deciding on a digital press is the imaging technology itself, which brings unique advantages and applications. The imaging technology might rule out particular applications from the start. So, for example, dry toner systems require heat to bond toner to the substrate, so are generally not suitable for some heat-sensitive materials. On the other hand dry toner systems do not generally require the substrate to have any special surface treatments and are extremely resilient without a protective varnish or over-laminate.

UV inkjet likewise does not require any special treatment of the substrate – although such treatment is often recommended for color management and consistency – but the process is able to deliver a heavier film weight, leaving a slightly ‘raised’ effect similar to UV screen. This effect may or may not be desired for a particular application (but does suggest the process can attack applications currently dominated by direct screen printing).

Drop on Demand inkjet

Inkjet printing offers the widest range of applications of all the digital printing processes, as well as the largest selection of inks, printing formats and substrates. In inkjet printing the printheads eject micro droplets of ink through nozzles so as to produce images on a wide variety of substrates (paper, plastics, glass, ceramics, textiles, etc) without any contact.

Inkjet printing may be continuous or drop on demand. In Continuous Inkjet (CI), a continuous flow of ink is deviated to create an image pattern; Drop on Demand (DOD) creates and ejects a droplet only when it is needed. For the majority of inkjet label and package printing presses the droplets are ejected when the ink reservoir wall is deformed by an electrical pulse applied to a piezo electric crystal. Depending on the electric current applied, an ink droplet of a uniform or variable size is ejected.

The piezo Drop on Demand process may be binary inkjet or grayscale inkjet. In the grayscale process the ink droplet size may vary, unlike binary inkjet in which the drop, when expelled, always has the same size.

This effect is particularly apparent on oblique edges, as can be seen in Figure 2.6.

Grayscale inkjet printing is able to achieve a much higher level of quality than that obtained from binary inkjet using the same resolution.

Imaging units

Although there are a large number of Inkjet press manufacturers, there are only a small handful of different printhead technologies that they use, such as Xaar, Kyocera, Konica Minolta, Memjet or Kodak Stream (Epson is an exception here with its own proprietary head technology). Presses which use the same printheads will generally share the same print resolution and speed, so how do you choose between them?

In fact there are a range of variables which impact the actual performance of the press, even if it uses a similar imaging unit. These include accuracy of web handling, software integration, the use of additional ink ‘pinning’ systems, and the interaction of ink with curing system. Clearly these characteristics require extensive print testing and cannot be ascertained just from a specification.

Resolution

Imaging resolution has tended to become a source of confusion for some potential digital press buyers in recent years, particularly when looking to understand the difference between ‘binary’ and grayscale’ press models.

Over the last few years a number of companies have released new digital label presses, particularly inkjet presses. Fundamentally, the basis of the print quality and productivity achieved by these new generations of digital presses is largely determined by the type of print heads integrated into them; two of the dominant inkjet heads in label printing today for example, being the Xaar 1001 and the Kyocera KJ4.

With these new (ink jet) presses has come a new language which has been adopted to help define the print resolution and quality of the output from different digital print heads, with dots-per-inch (dpi), effective dpi and grayscale commonly being quoted.

Print resolution, although not the only measure, is one of the key measures for print quality. For those more used to flexo printing this is commonly quoted in the form of lines per inch (lpi) or lines per centimeter (lpc). Ink jet, including the desk top versions that readers may have at home, as well as the numerous UV-curable ink jet-based digital presses that are now a rapidly growing market presence, will all tend to quote dpi or dots per inch.

Dots-per-inch defines the density of dots that can be printed in an inch. The more dots per inch available, the higher the print resolution of the image; consequently, the more smaller dots that can be utilised, the better the image definition is going to be.

Consequently, the print resolution depends on the dot size and therefore, in turn, on the hardware of the print engine – and on its imaging device in particular.

For example, in Figure 2.7, a 1200 dpi print engine can place 1200 discrete dots per inch, i.e. dots with a size of 1/1200 inch = 21 µm dot size. A 600 dpi print engine could place 600 dots per inch, with each dot being 1/600 inch = 42 µm. Using LED imaging technology this would correspond to a LED array with 1200 and 600 LEDs respectively. The number of LEDs on a diode array being fixed, resolution, as it is defined here, is a hardware specification.

In some inkjet systems resolution can also be affected by print speed – higher resolutions are achievable but at lower speeds and vice versa. So buyers should always check what speeds a quoted resolution refers to. Similarly, the highest quoted print speed may only be achievable with a reduced resolution.

But once again, there are a number of variables which affect actual print quality. These include the surface tension and absorbency characteristics of the substrate, as well as the screening algorithms used in the digital front end.

Grayscale capability

Another related characteristic that impacts print quality is the range of ink drop sizes selectable for printing, often referred to as grayscale capability.

A grayscale converts an original image (or photograph) to a pattern of dots that simulates the continuous tones of the original. Lighter shades of grey in the original consist of smaller ink jet dots spaced far apart; darker shades of grey typically contain larger dots with closer spacing. The latest ink jet print heads, all support grayscales, however lower resolution (lower dpi) print heads will be printing more grayscales with, on average, larger drops being used in order to create the required ink coverage.

‘Effective’ or ‘Apparent’ resolution

With digital presses it is possible for the print resolution to be different in the vertical and horizontal axis of the image. This can be explained as follows:

On an ink jet press, the dpi perpendicular to the material travel direction is fixed and defined by the number of nozzles per inch in the print heads printing the image; not the case if they interlace vertically.

In the material travel direction the dpi is determined by the jetting frequency of the print head, the number of grayscales being used as well as the operational speed. Where there are fewer grayscales being printed, print density and consequent color range available will be impacted, but a higher running speed is possible. However, it should be noted that higher dpi presses like Domino have less grayscales but still provide a higher density.

Also, for example, if you halve the print resolution in the material travel direction it can be possible to double the print speed, recognising the print head has a defined jetting frequency. For this reason in some cases dpi is defined in two axis, for example 600x300dpi would mean 600dpi in one axis and 300dpi in the other.

There are currently two dominant ink jet print head technologies being used in digital label presses today: The Xaar 1001, used by companies such as Durst, EFI Jetrion and Stork, and the Kyocera KJ4A used by Domino, Linoprint and Monotech.

As can be seen in Figure 2.8, there is a difference in the grayscale or drop sizes produced by the Xaar and Kyocera print heads and the image resolution and ‘Effective’ image resolution when the two head technologies are compared side by side.

The Xaar 1001 print head has a native print resolution of 360dpi, typically operating at 25m/m with 360x360dpi and eight grayscales. Xaar have combined the grayscale capability with print resolution and quote ‘Effective’ or as EFI Jetrion quote, ‘Apparent’ print resolution. This is broadly calculated as the square route of the number of grayscales multiplied by the native dpi. Hence Xaar claim an ‘Effective’ resolution of over 1000dpi; albeit there are not 1,000 drops per inch. The print head has eight (no drop is counted as one of the drop sizes) grayscales with drop sizes between 6pl (pico litre) and 42pl.

The Kyocera KJ4A print head as used by Domino on the other hand, has a native resolution of 600dpi, operating at 50m/m with 600x600dpi and four grayscales (five including the ‘no drop’). This would imply an ‘Effective’ resolution of 1,340dpi. Within a square inch there will be up to 360,000 drops compared to 129,600 typically larger drops for the Xaar 1001 print head. The print head has four grayscales with drop sizes between 6pl and 14pl. With more drops available for printing, the average drop size is much smaller and it is therefore claimed that fewer grayscales are really required given the higher native resolution available.

Bit depth

The bit-depth determines the number of output levels that can be printed. While a typical laser exposure unit is either ‘on’ or ‘off’, the light intensity of LEDs can be modulated to create different output levels. A laser for example has, by definition, a bit depth of 1, which means it can produce 2 output levels. The laser is either ‘on’ or ‘off’, i.e. the device dot is black (laser ‘on’ and therefore toner is deposited) or white (laser ‘off’ and therefore no toner is deposited). In an LED imaging system the light intensity can be modulated. If the imaging system has for example a 3 bit dot density, then the number of available output levels equals 23 = 8.

Digital ink systems

The characteristics of different ink/toner systems has a major impact on the choice of digital press. In inkjet, for example, there are water-based, solvent-based, dye-based as well as UV or LED ink chemistries, each with their own qualities of adhesion, migration, light fastness and rub resistance – dictating, for example, whether a varnish or over-lamination is required for a particular application.

In addition, the availability of extended ink sets may be an important consideration. For most standard label applications simple CMYK will be sufficient. But if clear films or metallic substrates are going to be used, a sufficiently opaque white will certainly be required. When matching a wide range of Pantone shades is key, 6- or 7-color ink systems should be considered, adding combinations of Orange/Green/Violet. Look for presses with at least five color stations. In 5-color digital presses the fifth station will usually contain the white ink, but could also contain a special color.

This type of work could also be ideal for a hybrid digital/conventional press such as the Nilpeter Caslon, where UV flexo can be used to lay down the white and special colors in-line with a CMYK digital module.

Variable print capability

Digital press manufacturers contacted prior to writing this publication all said they supplied variable print software, either bundled with the press or as an option. Generally this will handle variable codes, serial coding and variable text and picture elements from frame to frame. If this is a key application, the capabilities of the software should be examined in more depth. There are also various excellent third party software programs available.

OTHER PRESS QUALITY CONSIDERATIONS

There are of course many other factors that will determine the ultimate print quality of a digital press output, such as the substrate being used, environment, color management, registration control, print density, ink flow, etc; but the print resolution of the underlying head technology will always be a key factor determining the level of detail in the images. The higher the native resolution the greater the detail that can be reflected in the image. Lower native resolution print heads will partly compensate for this through offering more grayscales. This difference is especially noticeable for smaller point size text.

So what is the best way for a label converter or package printer to determine what digital press, head technology and print quality they should invest in? Probably the best solution is to print a range of different images from any inkjet (or toner) press, on a range of different stocks, before drawing any conclusion on what is the best print quality for their particular company and range of applications. It will depend on the type of work produced, their customer quality requirements, desired running speeds, and compatibility of print quality with other printing technologies used, e.g, UV flexo.