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FLEXO Magazine : October 2009
54 FLEXO OCTOBER 2009 www.flexography.org The real disadvantage of laser cutting technology---and the reason that most companies that use them do so in conjunc- tion with one or another tool-based system---is that it is less cost-effective for many relatively straightforward long run applications which are not beyond the reach of mechani- cal cutting. If part geometries are easy for a physical tool to achieve, if the substrate is not too thin, too sticky, too abrasive or in some other way troublesome for a physical die, and especially if it involves a relatively long run length where the cost of the die becomes a negligible factor, tool-based cutters (platen presses, rotary die cutters, electro-optically controlled gap press technology) often prove the better finishing tool. QUALITY AND THE SOFT MARKING STANDARD Laser cutting systems that were engineered just a few years ago were often not up to the challenges of cutting complex designs, especially when there were many sharp angles in the artwork geometry. One can still find inferior laser cutting systems being sold today that similarly are plagued by the quality problems usually evidenced by pinholes at the start and stop of cutting sequences or burn-throughs. For example, Figure 4 shows the difficulties that less sophis- ticated machines have whenever turns are required in sharp edges. Here you can see the telltale black burnthrough marks at turning points that show where the lasers lingered too long. One might think of the analogy of a car making a turn, and the usual need to decelerate in order to make the turn. Figure 5 shows a laser cutting machine that has just the opposite problem. In attempting to avoid the burn-throughs shown in Figure 4, the lasers were accelerated. However, the control of this acceleration was inadequate. Instead of the sharp corners that the artwork requires, the edges are rounded. Here, the laser beams are moving too fast to make the sharp corner details. Improvements in the software engineering of today 's better laser cutting machines obviate these historic quality prob- lems. Soft marking, where the laser movements are better synchronized with artwork geometry and tightly controlled during the entire cutting sequence eliminate the burnthrough problems yet make the sharp angles required, as shown in close-up in Figure 6 and in the finished product Figure 7. Older systems often left pinholes at the start of a cut because of the time it took to move the scan head (mirrors directing the laser beam) off from that initial start point. In contrast, the better quality laser cutting systems of today create better edges, don't leave pinholes at the start of cuts, don't leave burnthroughs at sharp corner turns. This is not because better lasers are used but rather because better algorithms improve control of the move- ment of the mirrors that point the laser beam. Soft marking is no small feat for the control software to achieve, and it is only the manufacturers of laser cutting technology that have made significant R&D investments in better software engineering that can deliver the defect-free soft marking that most applications require. To examine how cutting speed potentially affects quality, consider Figures 8, 9, 10 and 11 showing the laser cutting of a small folded box. In Figure 8, the frequency of the laser output is so slow, 10 kHz, that the single pulses of the laser give the cut more the appearance of a dot- ted line as opposed to the con- tinuous line cut that is desired. Figure 9 shows a laser cutter without algorithms for optimizing the laser movement to geometry and cutting speed when it is operating at a fast cutting speed. Here the cutting speed is too fast for the scan head mirrors to follow the contours of the artwork in a synchronized way. What results is not exact. Contours that should be sharp are rounded. What you are looking at is the output of a less sophisticated laser cutter where the mass of the scan head mirrors and what it takes to move this mass are not adequately handled by its software. These problems are even more pronounced when the cutting speed is doubled as shown in Figure 10. In contrast, laser cutting systems that can match the cutting speed to the part geometry and optimize the powering on and off of lasers accordingly is shown in the greatly improved quality output of Figure 11. Here, the algorithms the laser cutting software is using can match the speed of cutting to the design in an optimized fashion. Improved quality in today 's better quality laser cutting systems is seen not only in better edge quality but in the far more consistent cut-to-print ac- curacy afforded by the new level of systems integration in the best-in-class laser cutting machines. For example, earlier systems had no way to compensate for the rotation in the working field that can occur as the web moves through the laser cutting machines. Today 's best-in- class systems not only use high resolution cameras but also integrate the camera information with the laser software that is controlling cutting. This means that as the camera systems determine any X/Y offset values, they communicate these to FIGURE 4. Burn-throughs. FIGURE 5. Round corners. FIGURE 6. Soft marking. FIGURE 7. The final product. TECHNOLOGIES & TECHNIQUES
Sustainable Fall 2009