For Digital Finishing, Static=Havoc!
Static has been a perennial villain (and sometime friend) in both the print and finishing process. Static is actually a key technology used in gravure printing, and we used static charging bars all the time in both saddle and perfect-bound offset book production (it assists in keeping book stacks together during production).
But most of the time, static is the enemy, causing all sorts of problems within finishing machinery, preventing sheets and book blocks from being properly jogged, among other issues. With the advent of continuous web digital printing and in-line saddle-stitching and perfect binders directly connected to the press, things have gotten even more technical.
The first issue arises with the relatively low moisture content of the paper emerging from a digital press. In toner presses, the final process of heat-and-pressure fusing of the toner to the substrate surface drives almost all moisture out of the paper. Even inkjet presses, which spray a lot of water during printing, then blast the paper with high heat to eliminate the water.
The web that emerges is then potentially charged with high-voltage. Traveling at high velocities through the cutting equipment, the stitcher or binder creates a potential to pick up even more of a charge along the way. This static results in the "havoc." Sheets jam due to misalignment. They will skew in the various infeeds, develop "waves" in the paper and momentarily stick to pieces of the equipment. The end result? A jam and stoppage of the entire print-and-finishing line or a poor quality end product.
Managing static requires a small arsenal of weapons, which are typically deployed in combination with each other. The first line of defense is humidity. Humidity levels must be adequate in the print and finishing area—typically no less than 40 percent. The second solution is to re-moisturize the web. WEKO has long sold web in-line units that will re-apply moisture to the moving web.
This can be very effective in knocking down static in the paper, but it's a careful balance. Apply too much water and the integrity of the paper is compromised. Proper grounding of components is crucial. Air ionization static bars can be fitted, anti-static sprays can be used on specific parts of the machinery that are causing problems, and teflon tape can be used to cover particularly troublesome pieces. The tape both eases friction between the metal and the paper, and acts an insulator.
Controlling static is vital in the digital finishing process, but fortunately, the tools are there.