Thursday, August 21, 2008

School's in, suckas!

Since getting paper to feed through the press reliably is the first hurdle in printing, you need to know when to use rubber suckers. The metal suckers on the sucker bar work surprisingly well for most jobs, but when they can’t pull the sheet up consistently it’s time to use (rubber) suckers. For most presses these will be in various conical shapes, but there are also pneumatically telescoping versions for wavy or rough paper, and other variations. The classic sucker for the Windmill however is a flat rubber disc. When snapped over the metal sucker and into the groove provided, it takes on a conical shape. These are available from numerous sources, perhaps the largest being Rudow, at They advertise “suckers by the zillions!” 19H is the model for the Windmill.

When I first began using suckers I tried the old red rubber ones that came in the tool drawer, but found that they left scuffs on the tail ends of stiff sheets as they whipped under the sucker bar on their way into the press. Happily, the standard 19H suckers come in a white rubber material, so this is no longer an issue.

The downside of rubber suckers is that they can work too well. They seal so well against the paper that the vacuum of the press can suck right through the sheet to the next two or three sheets, depending on the stock. To mitigate this you can open up one or more suckers that are not located over the sheet. This bleeds off enough vacuum that only one sheet is picked up. On a recent job that I ran at over 4000 impressions/hr, with a fairly hard and heavy stock, it still required three extra suckers open for the four that were over the sheet. Without bleeding off all that extra vacuum, it still picked up doubles and triples. I’ve found that 220 lb Lettra works very consistently with one extra sucker open, with separation blast up all the way and pile height at its highest, or one click down.

Since it’s good to rule out variables when troubleshooting, you want to make sure that you aren’t losing any vacuum or blast between the pump and the air openings. I added small hose clamps (Jubilee clamps to UK visitors) where the hoses meet their fittings, as the sag of the hoses had pulled their ends into a shape which didn’t seal perfectly anymore. It’s also good to clean out the air holes in the sucker bar, as they may well be half-closed with decades of anti-offset powder and oil mist. Find the largest drill bit that will fit, and spin it into the crud by hand. It augers out the stuff nicely, and then you will have all the vacuum possible. Our cylinder had plenty of this as it had previously been in a large offset shop, and additionally the suckers weren’t all aligned evenly or to their vacuum holes on the bar. Which leads to another lesson with these old machines: never assume that anything on the press is adjusted correctly.

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