Heidelberg Windmill & Double Thick Stock (Curl)

Setup: Heidelberg Windmill 10x15, running on guides, 9”x12” boxcar base, 6”x8” photopolymer, for a 5”x7” finished size. Stock is Lettra 220/600 gsm paper cut to 6.5”x8.5”. Job is 3 color art print.

First time running a 3 color job with Lettra 220/600 gsm paper so hadn’t experienced this particular issue before. We were hitting register fine on the first two passes but by the 3rd pass the paper starting coming out completely wonky. We realized that the paper had a slight curl which caused it to completely miss the left side lay gauge. We ended up having to hand curl the paper in the opposite direction in order to finish the job.

We hand curled out of desperation, however, I’m sure there are more elegant ways to solve this issue and I’m wondering what other people do to either eliminate curl from the start or manage the curl throughout the job. Did we use too much impression, did the paper curl due to environmental changes throughout the day, something else?

I’ve read about using frisket fingers and lay gauge pins but haven’t used them yet and I’m a little unsure of the proper ways to utilize them without hitting the base. Any advice there? Would they even be appropriate? How do you get around them hitting against the base and/or the poly plate? Do you just cut your paper larger in order to miss the plate?

Oh, another thing we ran into was the the rubber suckers started leaving black marks on the paper. Anyone have this issue before? We cleaned them multiple times on and off the press but they kept leaving rings and streaks on the paper.

We only used the rubber suckers when we had to since the stock became difficult to feed throughout the run and would often miss the gripper bar on pickup when not using them. This scenario actually saw the press continue to run and not realize the misfeed occurred resulting in impression on the tympan sheets. The suckers solved this issue perfectly but then started leaving marks which was maddening.

Sorry for the rambling, happy for any advice that is given. Thanks!

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Not too sure about running a close register job anymore. Been years. But I don’t think your hand curling is out of the ordinary. Sometime with multiple impressions that are too deep may change the character of the paper. If your paper was exposed to a severe change, temperature and moisture throughout the job, it may contribute. You have consistent heat? The suckers are picking up ink or dirt from some where. Streaks— Possibly the ink previously printed, not dry yet as the suckers slide across paper as it feeds thru. Maybe ever so slight you can’t see them touch. Sometimes once you get ink on them you can’t clean them enough. The rings— Look like it may have oily? Does it resemble any color you have printed or are running? You can remove sucker bar and wash it out with Press Wash, especially the sucker area. Air compressor it clean. Dry good. IF the circle marks look like part oil. The paper dust can leach the oil all thru your hoses over time. Keep them clean to.
I would also check your screen in your hose near your vacuum trip, if that gets full it can cause trouble at slower speeds. Your sheet separators can cause trouble if they are out into the stack too far. Try using the side fingers on heavier stock. Lower pile, higher blast to blow up pile. Angle of sucker bar on heavier stock has to be almost or dead-on as straight as possible. Some times placing a small piece of furniture under the stack. As so… reaching across the width under your paper. Just behind the first rail of your paper standard guide. This will elevate the rear and lower the front. Requiring more air blast to raise the sheet level, thus giving a straighter sheet, allowing for curl. Most of the time it will change the character of the paper as it floats for an instant, making it “listen” to the sucker bar and getting it to the gripper bar. If you hear a slight slurp sound when feeding check the angle of your gripper bar. -OR for one that isn’t touching the paper but open.

If the paper is curling the problem is going to be humidity as the cellulose fibre starts to absorb moisture. After coming out of the paper wrapper you have to get the stock through all the processes asap otherwise you will have registration problems. Our digital friends never have this problem as theirs is one pass multi colour.

Having to bend the stock to deal with paper curl was part of the apprentice training, the stock on the platen can have a small curl and the frisket will then flatten it before impression.

As already mentioned putting wedges in/under the stock on the feed table to get the edge of the stock roughly flat under the suckers is part of the practice of working a letterpress machine with auto feed.

If the suckers are putting rubber marks on the paper then they a perished, get some new ones. Also it may be that the sucker bar is pressing heavily onto the stock,, run with the table at a lower level and adjust the blow so that the paper is blown up to the suckers.