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 user 2004-01-26 at 9:54:00 am Views: 61
  • #4822

    What is digital printing?

    the science of inkjet and electrographic printing processes, which look set to become the fourth major printing process in the next few years

    During the judging process of this year’s National Print Awards, there were a number of questions asked about the correct classification of some entries, and as chair of the judging panel of the Awards, I was given the task by the National Committee of examining the categories for this year’s competition.

    No category was more potentially confusing than that of digital printing, and this subject has been exercising the minds of some of the leading figures in the printing industry over recent months.

    We all think we know what digital printing is – we probably have examples of it on our desks and in our offices as we read this article. I certainly have two desktop printers on mine, and a laser network printer just on the landing outside, but the lines do get a bit blurry.

    There is absolutely no doubt that the advance of printed material that is generated from computer data (in digital form) is both important right now within the industry, and will continue to grow in importance over the coming years. Many have heard me quote Professor Frank Romano from the Rochester Institute of Technology who says that the big three printing processes litho, flexo, and gravure, will be joined in about ten years by a big fourth digital process.

    Here the good Professor hedged his bets, as he would not give a firm opinion on which process the big fourth would be, and neither will I.

    Every one of the current crop does have some inbuilt technical flaws, that cast some doubt on whether there will be a winner (as in the VHS versus Beta debate), or whether there will ever be a clear one fits all process.

    Ink jet printing is occupying a significant part of the research projects of the CRC for Functional Communication Surfaces, where most of the work is focused on the receptivity of the substrate and its influence on print quality. We all know that ink jet printers are effectively tiny spray guns that deliver droplets of ink to the surface to be printed, and have the huge advantage of being the only genuine non-impact form of printing.

    The design of spray heads has been the subject of much ingenuity, and the task of documenting these is beyond this author, but in the style to which you have become accustomed, I will have a go at describing how they work in hugely oversimplified terms.

    The most common ink jet units fall into two main categories. The first resembles an airless spray gun unit that pumps liquid around a loop and releases some when a valve is opened, these are referred to as CIJ for continuous ink jet. CIJ units generally produce large drops or large numbers of drops, and are mainly applied to the printing of simple designs like barcodes.

    The second class of unit supplied a drop of ink only when required by a signal (from a computer) and is called drop on demand (DOD) for this reason. The largest number of ink jet units worldwide are the thermal DOD type, as these form the delivery system of many desktop printers.

    A heating element on one wall of a chamber containing a small amount of ink is rapidly heated to around 350C, which instantly vapourises a small amount of the ink, (less than 1 per cent). The shock wave of the expanding vapour forces the remaining volume of ink in the chamber out through a nozzle. When the bubble collapses, more ink is drawn into the chamber.

    One limitation of the thermal type is that the inks are restricted to water based carriers. It doesn’t take too much imagination to realise that volatile combustible solvents could behave in a spectacularly unpredictable manner when treated to the thermal shock.

    The other DOD technology, and technically the system with the most potential, is the piezo-electric type. A piezo transducer is a slice of ceramic material that changes its shape when a voltage is applied across it. As the voltage varies, the piezo crystal shrinks and then expands.
    A number of configurations of the piezo material are in the market, including an action resembling a piston, to one more closely resembling a squeezed toothpaste tube, but all push out drops of ink on the expand stroke, and draw in another charge on the shrink cycle.

    Piezo units can accommodate many kinds of ink, from solvent based to UV and hot-melt types, and are capable of producing high quality images on photographic quality paper, but they can only handle relatively slow web or sheet speeds (up to around 150 metres per minute). When piezo heads are used in arrays to enable high print speeds to be achieved, the installation rapidly becomes expensive.

    The other principal form of digital printing is electrography, often called xerography, (but this name is based on a trademark, a bit like calling all ball point pens Biro).

    The electrographic process, seen in all photocopiers and laser printers, (and pretty much all other types of digital engines), comprises a cylinder coated with a photoconductive material, to which a uniform positive charge is applied. The cylinder is then rotated past either a laser or a light source reflected by a mirror, which causes the charge at the exposed points to decay.

    Ink, mostly a powdered combination of resin-like SBR, and carbon black, is electrostatically charged and released to the relatively stronger charge of the cylinder, the toner adhering to the less charged (exposed) areas. The paper is charged to accept the toner, the toner is then transferred to the substrate, and fused (melted) onto the substrate by heat and pressure.

    Important and versatile
    The electrographic process is certainly not new, the US army had a large format press printing maps back in the Sixties, and whether it is an Australian or American invention takes minor place to the fact that it is an important and versatile method of transferring an image.

    Electrography has the digital capability of producing a new image at every revolution of the printing cylinder, and whether the process then becomes one of conventional toner fusion, thermoplastic ink transfer like HP’s Indigo, or one of conventional ink transfer like the DI press, fast efficient image reproduction is possible. However, electrography is certainly not a non-impact process, and the substrate takes quite a beating during the toner deposition (and especially fusing) steps, so that in this regard ink jet could be said to have a distinct advantage.

    So where do we go with the category of digital printing? For the moment, it is status quo, as technologies are still evolving, but with absolute certainty it may be predicted that as digital printing becomes more of a mainstream process, the question of where printing starts and computer graphics stops will be the next question to address.