Nd:YAG provides direct laser gravure printing

Mark Greenwood and Rene Hartmann

Information technology has transformed printing to a greater extent than possibly any other industry. Because design and layout are now normally conducted electronically, the manufacturers of printing equipment are developing new systems that are fully compatible with the speed, precision, and sustained accuracy of computers. The general aim is to shorten processing times without deviating from the rigorous quality standards demanded by the end users.

To shorten processing times without deviating from the quality standards, laser rotogravure system is based on a dual 400-W source in which the beams are combined to give a 70-kHz pulse train to create an 11-mJ pulse lasting a few hundred nanoseconds—essential to ensure the proper removal of material from the roller—every 14 µs with a stability level of 5%.

Of the three different technologies that continue to be improved by the industry as a whole—offset lithography, flexography, and rotogravure—rotogravure is most suitable for high-quality, high-volume print runs and is therefore typically used in the production of packaging and color magazines. Ink is transferred to the medium (paper or plastic, for example) via metal printing cylinders that are normally several meters long and about 50 cm in circumference.

Prior to use, each roller is machined clean, replated with copper, and engraved with tens of millions of minute inkwells, corresponding to the required distribution of one color in what may be a four- or six-color process. The standard means of performing the final operation has been to pit the drums with electromechanically driven diamond styli, prior to chromium plating the cylinders and mounting them into the printing machine. This is a time-consuming task, however, as a single-head mechanical engraver takes at least ten hours to complete a drum. There was and is a clear market demand for a quicker alternative.

Energy-beam engraving

The idea of replacing the diamond styli with an energy beam is not new. The English company Crosfield tried direct laser engraving in the 1980s, generating cells in rollers coated with epoxy. This material was selected due to its low ablation energy (that is, the ease with which it vaporized), though in other respects it proved unsuitable, and the project was terminated. In the same decade, the German company Hell experimented with electron-beam engraving, but it was discovered that this would require an excessive investment in data processing and vacuum technology.

In the early 1990s, more progress was made in the field of indirect laser gravure. Here, the copper roller receives an even coating of a black substance that is removed by a fairly low-power beam. A 60-W multimode continuous-wave Nd:YAG laser equipped with an external acousto-optic modulator capable of handling the high data rate necessary is appropriate for this purpose. The actual inkwells are then created by chemically etching the roller before it is chromium plated. (Spectron collaborated with Schepers-Ohio GmbH—now a subsidiary of MDC—to develop a laser for this application.)

Though indirect laser gravure produces cells that are hemispherical and thus optimal in shape, it is not ideal because the etching stage cannot be fully controlled at a reasonable cost. In particular, the occurrence of underetching remains a limiting factor. Consequently, in 1992, MDC began work on the Laserstar, with the goal of manufacturing a product that could perform direct laser gravure on the surface of metal cylinders (see figure). Spectron subsequently became involved with this project.

Demanding requirements

The design and manufacture of a laser suitable for incorporation into the product required meeting an exceptionally challenging specification. The underlying need was for long, extremely stable pulses, at a high repetition rate, with a beam quality high enough to allow the beams to be coupled into the 400-µm optical fiber transferring them to the drum being treated. In addition, however, total reliability was vital, as there could be no dropped pulses.

Following the construction and testing of a series of prototypes, Spectron found a solution in the form of a dual 400-W system in which the beams are combined to give a 70-kHz pulse train. (Each laser features a 2.2-m resonator incorporating twin Nd:YAG rods and delivers a Q-switch frequency of 35 kHz). This creates an 11-mJ pulse lasting a few hundred nanoseconds—essential to ensure the proper removal of material from the roller—every 14 µs with a stability level of 5%.

Modifications to nearly all the company's standard components were implemented during the refinement of this system. This included a customized power supply and coolers designed to operate at 12 kW and an enhanced mechanical-optical rail. The system is also configured to run on an interbus controller for ease of integration with the other equipment comprising the Laserstar system.

The finished product

Up to 140,000 inkwells per second are generated by the Laserstar, with the walls between the cells being just a few microns. It takes less than 15 minutes to complete a square meter of drum surface engraving.

The advanced software incorporated into the product allows an experienced operator to convert in a matter of minutes a document formed by virtually any desktop-publishing or cylinder-engraving process program into a cylinder set, from which the individual engraving files are quickly produced. The software features queuing, allowing the operator to begin another task while the first is still being processed.

Focusing optics follow a spiral path along the roller, which is ablated in accordance with the data stored within an engraving file. The Q-switch synchronizes the laser pulses to the rotation of the cylinder, while the modulator relates the energy delivered to the image information. The beam profile defines the type of inkwells generated, which may be depth-variable or have a constant surface-to-volume ratio.

Tight control is provided over the size of individual cells. These are hemispherical—the best shape for efficient ink release—and so need to be only two-thirds the depth of mechanically engraved pits to produce the same print density. If extra-thick ink layers are required, deep cells can be created using the full power of the laser.

First unveiled at the Drupa exhibition in Düsseldorf in 1995, the Laserstar has been in industrial use for several years. The potential market for this product is estimated to be 40-50 major magazine printers. All of these companies, it is anticipated, will eventually want to switch to direct laser gravure.

MARK GREENWOOD is managing director, Spectron Laser Systems, 8 Consul Road, Rugby, Warwickshire, CV21 1PB England; e-mail: [email protected]. RENE HARTMANN is managing director, MDC MAX Dätwyler AG, Flugplatz, 3368 Bleienbach, Switzerland; e-mail: [email protected].