Inkjet Printing: Drop by Drop and without a Printing Forme

(NPES International E-commerce Website, August 28, 2008) Inkjet printing is a digital printing process and furthermore it is the only printing process that does not require a forme for transfer of ink to the paper. Ink transfer occurs via nozzles that are moved over the paper and which eject fine drops of liquid ink at high speed under the control of a computer.

In terms of function, inkjet offers the broadest spectrum of applications of all the printing processes, the largest selection of inks, substrates and print formats. In this Technology Guide we will present the operating principle as well as the state of technical development in fields such as large format printing, wide format printing, flatbed printing as well as transaction printing. Each of these application sectors has different requirements and properties and is a world unto itself.

Inkjet printing got under way in the printing industry in the 70s as an addressing system with which the details of the recipient could be printed in black ink for postal dispatch. A second application involved the growth of CAD plotters and their extensive uptake by industrial design and architectural practices in the wake of the PC revolution. By now, for example HP has sold several hundred thousand of these inkjet plotters. In prepress, inkjet printing was introduced by Iris Graphics and its high degree of colour accuracy and image resolution made it the dominant digital printing process in the market in the 90s. In Israel a number of firms developed the inkjet process for large and wide format printing and these came together in the activities of Scitex Vision, which now, as HP Scitex Vision and with the takeover of Mac Dermid Colorspan and NUR Macroprinters Ltd., offers a complete range of 25 different large format printers.

The nozzles
Currently, there are three different nozzle technologies: continuous inkjet, thermal inkjet and piezo inkjet.
Continuous inkier: In the continuous inkjet process the drops of ink are emitted from the nozzle in a continuous stream during printing. The ink drops are either directed at the substrate or deflected by a deflection lattice and returned to the ink reservoir. The advantage of the technology is its ability to deliver a large quantity of ink to the paper within a short space of time, and it is therefore used in particular for wide format posters with widths of between three and five meters and for high speed transaction printing.　
In other applications, the technology suffers from the disadvantages that it is only possible to control the size of the drops and their positioning poorly and that its ink consumption is high.

Thermal inkjet (Bubblejet): In the bubblejet process the ink in the print head is heated so rapidly and strongly that gas bubbles form and shoot the ink out of the nozzle. When the heating element is switched off, the pressure falls and fresh ink is sucked from the reservoir. This occurs at a frequency of up to 48,000 drops per second.
Such a nozzle has the advantage of having no moving parts and so it is highly reliable and has a very long lifespan. It can print water or solvent-based inks and even pigment inks without becoming caked or blocked. It has the disadvantage that the drops that are produced are all the same size. Modern thermal inkjet printing systems have 1.200 nozzles per inch, 10.560 nozzles in a print head and can have any number of print heads mounted alongside each other. As a result, the thermal inkjet process can also be used for wide format printing.

Piezo inkjet: In the piezo process, the application of a voltage across a piezo crystal in the nozzle causes it to stretch and force ink out onto the paper. The quantity of ink that is elected in this way can be controlled very precisely, making it possible to achieve a very high resolution of up to 2,880 dpi (dots per inch), a photographic image quality and sharply defined type.

The piezo process allows the droplet size to be controlled and with three data bits it is already possible to generate eight different droplet sizes at will. The result is the production of precise tonal values and a photographic quality and the process is therefore particularly suitable for use in the proofing and the poster sector with dye-based inks.

Two further printing processes-thermal transfer and airbrush-have almost completely disappeared from the market today.

The inks
The inks can essentially be divided into inks for indoor and outdoor use, and these need to fulfil widely differing requirements in terms of resolution, light and weather fastness of the prints, and, in consequence, there are the following different types of ink.
Dye-based inks: In these inks, tiny color particles are contained in an aqueous solution. The inks penetrate into the substrate so that the drops break down and form a smooth continuous tone image. They produce photographic quality, brilliant colors on gloss paper and very detailed reproduction of type. Their disadvantage, however, is that they are not particularly light fast and in direct sunlight they fade rapidly. They are therefore only recommended for indoor use.

Pigment inks: Pigment inks contain relatively large. Colour pigments and their colors also withstand bleaching by UV rays for longer. They are therefore used for outdoor advertising. However, since light fastness is required indoors, pigmented inks are increasingly used for indoor advertizing.

There are three different types of such inks. Water-based inks: Standard printers that cannot print more than five or ten square meters per hour and that currently print with dye-based inks, are also available for water-based pigment inks. The relatively slow drying of water-based inks is perfectly adequate for such applications. In order to make such printers more productive, the directed application of warm air in the thermally isolated printer can considerably accelerate the drying.

Solvent-based inks: In these inks, the colour pigments are dissolved in a solvent. As a result, the drying is substantially faster and this allows higher printing speeds. It does, however, require good extraction of the exhaust air to remove the solvent. Printers using such inks achieve the highest speeds and the HP Scitex TJ 8300, for example, prints 400 m2 per hour, and the manufacturer offers a three to five year light fastness guarantee.
However, on environment grounds there is a move away from solved-based inks and towards eco- or mild-solvent inks, which do not deliver absolutely the same colouring power as solvent inks but which still offer a high degree of light and weather fastness. They also contain considerably less solvent and can therefore be used in areas without elaborate extraction systems.

UV-cured inks: These are inks without solvents and are cured by means of UV light after printing. They therefore dry immediately and emit no environmentally harmful volatile compounds. In recent years, they have achieved very rapid acceptance, especially in conjunction with flatbed printers for rigid materials, because they bind well to non-absorbent materials such as metal or glass. Now they are also often being used in wide format rollfed printing systems across widths of up to five meters.

Quality criteria
Essentially, three factors define image quality in large format printing: resolution, colour printing and printing speed.

Higher print resolution
Print resolution means the number of nozzles per inch arranged alongside each other in a print head. Since each nozzle corresponds to a printed colour dot, the value is given in terms of dots per inch (dpi). For a wide format poster printer producing large areas intended to be viewed from some distance, the choice would fail on continuous inkjet heads with resolutions of 360 dpi or even 180 dpi in order to achieve a high printing speed. Should type that is easy to read need to be printed as well, then 720 dpi might be chosen.
For photographic quality to be viewed indoors piezo print heads offer resolutions of 1440 x 2,880 dpi, whilst thermal heads offer 2,400 x 1,200 dpi. This geometrical resolution is influenced by the drop size, which is constant with continuous inkjet and thermal inkjet nozzles but which can drop to two picolitres with piezo inkjet heads.

In all inkjet roll printers the print head traverses horizontally over the substrate and the material is then forwarded by the width of the print head.

Improved color printing
As in offset, so in inkjet almost all printed matter can be printed with the four CMYK (Cyan, Magenta, Yellow, Black=K) process colors. However, inkjet colors are normally more intense and with fine tonal gradations this can result in breaks. This is prevented by adding a light magenta and cyan to the process colors as the fifth and sixth colors, which clearly improves the reproduction of detail in the highlights and at the same time allows bright tonal gradations such as skin tones to be reproduced without breaks.

Besides process colors, there is also often a need to print with special colors, in so far as such inks are available. Here too, it is necessary to have a printer with more than four colors. Some suppliers offer a primer as a fifth color, which ensures that the substrate has a uniform surface to which the ink can bond or create a better color effect. In the case of flatbed printers in particular, white is often laid down as a base layer on colored substrates or on metal or glass, and the color image is then printed on this.

Some printer manufacturers offer printing with Pantone hexachrome inks, in which the CMYK gamut has been expanded with a red and a green. These secondary colors can also be luminous. Alternatively, to increase output, with eight color printers it is possible to have a second CMYK set.

Higher printing speed
In order to increase the productivity of an inkjet printer one can increase the number of ink heads, coarsen the resolution or construct larger print heads.

To achieve this, staggered lines of nozzles can be mounted in a wide print head, and this print head can be duplicated side by side until the desired width is achieved. In transactional printing systems this is the 21.5 cm width of the page for one head, and three of these mounted alongside each other can print a 45 cm wide web. Such transaction printers can today produce more than 2,000 A4 pages per minute and every page can contain completely different information.

Flatbed printers for rigid materials can print one or two inch wide strips (2.54 or 508 cm) in a single pass or can make several passes to achieve higher inking before the material is forwarded.

Three figures are usually quoted to define the productivity of a roll or flatbed printing system: the maximum speed (and therefore the lowest quality), the production speed and the lowest speed with the correspondingly highest quality.

Key Drupa trends
Virtually all manufacturers of inkjet printing systems offer a whole range of models for various inks, printing widths or printing formats. The real development is in the high speed printing of invoices, statements and mass mailings, which is otherwise known as transactional printing. The same printers can also produce promotional letters and flyers for direct mail and general promotional purposes.

As a result, the term transpromo has recently been coined. With a 45 cm wide web, all systems can naturally print the A3 format, three upright A5 pages alongside each other for book printing, or even short run newspapers.

Kodak Versamark is being pressed by companies such as Oce with the Jetstream 1100/2200, Screen with the Truepress Jet 520 duplex and HP with its recently an-nounced Inkjet Web Press. Kodak has responded by offering slower, cheaper models; whilst at the same time announcing its new Kodak Stream technoto~,v. Scheduled to be available in three years time, this should be more economical than offset for print runs of up to l0000 A4 pages.

In evaluating these printers, one should bear in mind that the new Xerox 490/980 reel-fed printer is capable of color laser printing at up to 980 A4 pages per minute.