5.3.5 Digital printing

The result of the computer publishing system operation is a numerical image of the page or multi-page print sheet. This information is sufficient to properly reproduce illustrations, text, elements of graphic design, their placement in the page, etc.² With the subsequent stages being strictly normalized, the possibility of full automation of the whole printing process in the concept computer –to - print or the so-called "digital printing" is created. The input of such systems receives a flow of publication data while the output almost simultaneously and without the use of any intermediate manual operations provides prints, and in some cases, folded and bind print products.

"Digital" presses of DI (direct imaging) type differ in that they include the entire above-mentioned CtP system of, bypassing the mechanicals, the plates manufacture, as well as the means of their automatic installation and removal [5.16].

In the majority of digital presses there is no plate in its usual meaning, despite existence in some cases of an intermediate image carrying surface. The latter objectively fixes the received data individually for an each print and then comes into contact with a paper, ink, toner, offset blanket…. Such a light-sensitive surface of optical photoconductor has, for example, the cylinder in the most common digital printing systems with electro photographic (with dry or liquid toner) principle of image registration [5.17].

In devices with thermal transfer of the dye from the tape, its thermal sublimation or non-impact printing of the ink jet type, intermediate images are excluded at all, and the numerical data received at the input is imaged only on the print substrate [5.18].

In general, as defined by [5.19], a digital press is a device that:

- is able to accept digital data from computer (mainly in the PostScript format);

- is equipped with a raster processor (RIP or driver) that converts a PostScript file to a bitmap;

- registers the bitmap on a substrate with the use of a particular physical process.

However, such a set of features also meet the usual, office printer and digital color proof device. Therefore, it is also important to see how this machine is an alternative to traditional printing in terms of performance, efficiency and other parameters.

Most appropriate in technical and economic terms, the runs of various digital presses were initially in the range of 50 - 5000 prints and the format had rarely exceeded A3. Simpler versions of such machines combined several conventional laser printers.

The current digital presses are mostly designed to take into account the specifics of the "computer-to-print" concept and the optimal use of its advantages [5.20]. Ongoing is the research and development of novel methods and, for example, a continuous tone printing elcography –the deposition of pigment on a substrate from the electrolyte under the image signal control [5.21]. A systematic analysis of the digital printing structure and trends was given in the end -1990s in [5.22].

The specific advantages of digital printing are characterized by such concepts as personalization, "just-in-time" and "print-on-demand". This implies the following features that are not available in traditional technologies:

- getting each copy of the publication and any of its pages with the desired individual differences from others;

- making corrections and additions at the last moment and, in particular, remotely over the network, the peripheral device of which is essentially a digital press;

- print to the exact appointed (up to minutes) date with reduction of problems associated with warehousing and forwarding of print matter;

- print the runs in parts, including single copies, at different times and places, which significant reduce the mailing expenditures.

The last of these features marks the trend of transition in the new media from the usual practice of “print - distribute" to the principle of “distribute - print".

The feasibility of using digital printing in each case is determined by the ratio of these advantages and disadvantages. In comparison with the traditional offset, for digital printing machines of various types, such disadvantages were initially expressed in the following:

- 3 to 10 times less performance, which among other things was limited by the speed of raster processors;

- limited format (A3, A4);

- limited choice of papers;

- the high cost of monopoly supply of consumables (color toners, for example, were many times more expensive than offset inks);

- limited outside the triad, the choice of colors;

- relatively low print quality;

- high investment.

In addition to the awareness in illustration printing specific, the system operator was required to be able to handle PostScript files, at least for their display on the screen due to lack of proofs.

Being decisive for some, especially small-run, types of printed products, the advantages of digital technology seemed to be in little demand in the future in other fields, such as the production of labels and packaging. However, over time, and here the emphasis has shifted significantly in favor of digital printing. With the improvements its scope is steadily expanding not only for print packaging, but also in the manufacturing of a wide variety of industrial items, such as electronics.