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FLEXO Magazine : April 2009
TECHNOLOGIES & TECHNIQUES Sharp details result from a new screening technology and dots imaged at 4,000dpi. All art courtesy EskoArtwork. Flexo’s New Resolution High-res Optics and Special Screening Deliver Full Tonal Ranges By Ian Hole (computer-to-plate) system that approaches the quality of “just like off set.” While that has been the goal, fl exo has certainly ap- proached off set quality in the past 30 years, as plates migrated from rubber to polyester in the early 1980s, and digital imaging was introduced with laser ablatable mask system (LAMS) CTP plates in 1995. The time and prepress skill for fl exo has a signifi cant impact W on print quality. Why? The minimum dot in fl exo limits the tonal range. Small dots need to be printed reliably, so the range of highlight tones is limited. Jobs have to be adjusted manually to work around the limitation, which is typically expert work by skilled people. All of this contributes to increased prepress costs. NEW, COMPLEMENTARY SYSTEM Recently, technology was developed that allows digital LAMS plates to image at 4,000dpi —much greater than the 2,540dpi typically used to image plates. While a 4,000dpi pixel is not going to be noticed by any increase in imaging time, the exceptional screening that is possible from 4,000dpi will. The system that off ers high-resolution optics is accompanied by unique screening technologies that take advantage of the sharper and more accurate imaging. The combination of high screen rulings up to 250lpi and high-resolution optics on the im- ager brings off set quality to a fl exo press. With the higher resolution optics also comes more clearly defi ned screen dots, because the dots consist of more pixels. The rounder dot shape not only helps the screening, but also remains stable throughout the printrun—borne out by all initial testing (see Figure 1). With longer print runs, the press is stopped less often for wash-ups, resulting in greater productivity. 26 FLEXO APRIL 2009 hile a fl exo press is easier and less expensive to use than an off set press, the challenge has always been the promise of developing a completely digital CTP The high-resolution optics also deliver sharper imaging of text and line art. With high-resolution optics, line art and text are im- aged more cleanly, especially for small text, as shown in Figure 2. EXTENDED TONAL RANGE The second part of the new technology is its screening, allow- ing highlights to be printed better, extending the tonal range. For many years, a round dot shape has typically been used in fl exo printing. This works very well across most of the tonal range, particularly in the midtones. However, as any fl exo printer will tell you, in the highlights, dots become too small to print reliably. About 10 years ago, a hybrid screening technology was cre- ated to help resolve this. The thought was that rather than mak- ing smaller dots, why not keep them the same size, but remove some? There was signifi cant improvement, but the technology still had some limitations: gaps remained in the pattern (where the dots were removed), which could sometimes lead to a grainy appearance. Also, because there were fewer dots, there was too much space between these dots in the extreme highlight, so a stable shape was not formed on plate. Dots tended to “fall over” while on the press, leading to a dirty print. Responding to this, a novel solution was developed: a new “dif- ferentially modulated” screening technology. Dots remain on a regular grid as in standard halftones, but in the highlights they change size at diff erent rates. There is a mixture of mostly smaller dots with a few larger dots. FIGURE 1. New high-resolution optics facilitate rounder dots. www. f le xography. org