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FLEXO Magazine : November 2008
TECHNOLOGIES & TECHNIQUES dency of being the deciding factor of when a job will be going up on press. Hand sharpening is a very expensive and a very labor-intensive process which requires very skilled staff. This also contributes to the overall cost of the tool. The greatest and most effective ad- 5. vancement in flexible die technology was the machine sharpening process (Figure 3). The process of machine sharpening as a repeatable exercise eliminates any possible flaws that are common during the hand finishing process. This process made the die more cost effective to manufacture, more consistent, much faster to manufac- ture, cutting the lead-time in half when compared to a rotary die and in general offered a better quality cutting tool. Because machine sharpening is a fully automated process, the need for hand sharpening was eliminated thus drastically reducing the cost of the tool to printers and converters. Human beings are wonderful creatures but are flawed in nature. The process of ma- chine sharpening as a repeatable exer- cise eliminates any possible flaws that are common during the hand finishing process (Figure 4). The next phase in the flexible die 6. FIGURE 1. Early flexible dies were weak and did not hold up too long after contact. FIGURE 2. Hand-sharpened dies offered better quality, but were coun- terproductive for those seeking shorter leadtimes. evolutionary process was back grinding (Figure 5). In this process, the flexible die is placed with the cutting blades facedown on a precision-ground sur- face magnetic table, transferring any blade height imperfections to the back of the die. Once the die is set in place, a high-precession grinding process removes around 1 micron off of the back of the die with each pass. This of- fers extreme tolerance concerning the blade height and the kind of die strike that will result during the converting process. As synthetic liners continue to get thinner, this specific variable becomes increasingly more critical to control. For years, a common hurdle with flexible dies was die life. 8. FIGURE 3. Machine sharpening has been the most effective advance- ment in flexible die technology. There are presently two common methods for hardening rotary dies—vacuum hardening and induction hardening. Each has their advantages and disadvantages, but one fact is constant between the two: neither process will work on flexible dies. The nature of a rotary die being one solid piece of steel with a specific diameter gives that tool the ability to distort through the harden- www. f l e x o g r a p h y. o r g NOVEMB E R 20 0 8 F LEXO 39