To go along with my previous post about plate movement, I noticed the leading and trailing edges of my prints have a noticeably deeper impression than the middle. I ran off a blind print of a small leaf plate so you can see it here. My packing is 4 sheets of tympan and 1 sheet of press board (total .042″). The print sheet is Lettra 110. What’s causing this? What can I do to eliminate it?
Thanks for your help as always!
This looks like a packing issue to me. You mention press board. Not sure I would use that on a cylinder press as it could keep the packing from laying tight to the cylinder. Below is my strategy for packing. It does not cause problems.
The main thing is that all the packing lies as flat as possible on the cylinder. Make sure the grain direction is correct to facilitate this. Note that this is an old post, I no longer distribute packing material.
Have you checked that the eccentrics on your press are set correctly?
Last and final issue to consider with roller bearers: Are your rollers perfectly flat across/without swelling at the ends/wear in the middle?
It’s true that rollers should be re-ground or re-cast if they’re excessively worn, but there is some potential to continue using a perfectly serviceable roller if the ends appear ‘swollen’ or the middle is slightly lower (as this is where we see most of the use- the centered form).
Anyhow, all I mean to point out is that there is potential for a situation where the portion of the rollers that ride the ‘bearers’ is of a different height than the portion that should be inking the form. It’s not fool-proof.
To me, and for my money, correct setting of the roller height has always ‘done the job’.
I have a lollipop
i found the discussion about ink roller bearers interesting – if they work, its a “sure way” of not have the rollers set too low, but would not prevent from setting too high.
And, of course, if i printed on wide paper, the bearers would put a stripe down each side.
Lad, if you don’t have a lollipop gauge, I’d get one from Fritz (NA Graphics). It’s invaluable in setting roller height. If you’re inking your base, your rollers are much too low. It’s really not difficult to learn how to use the gauge correctly.
Having roller bearers will help, but you’re putting unnecessary stress on the rollers by having them hit something low and then have to pop up to type high. You also need to make sure they’re not in line with your paper or you’re going to get long lines down the sides of your sheet.
Maysorum – i read the link about ink roller bearers. Are you/Gerald suggesting that bearers be added to a SP15 so that the ink rollers (at ends) will run over the top of the bearers. If i understand what the bearers are doing, they would eliminate the worry about adjusting rollers with a type-high lollypop in that the bearers will not let the ink rollers drop any lower than the height of the bearers. Gerald suggested type-high (.918). I would think the bearers need to be a bit lower than .918 so that the rollers can ink the type/form.
Personally, i tend to get ink on my boxcar base because i am not so good setting my roller height. Bearers would seem to solve my problem. Of course, i need to find bearers or figure a way to make some.
Yes, very clear Ray. (And I’m definitely NOT fast-rolling.) And thanks, Eric and maysorum. I figured it was probably something I could alleviate with makeready, but wanted to be sure it wasn’t something funky like carriage balancing (which I had done last year). I think I’ll remove one layer of tympan and build up from there to get my impression even.
A platen does not exert the same pressure equally over a 4″ square. The four edges will be heavier than the center even in a perfectly centered form, and if uncentered things get more complicated. This is why makeready–in the sense of registered overlay packing–is part of any careful letterpress job whether platen or cylinder. You need to adjust pressure to the specific paper and form and ink conditions.
Ray’s description of adjusting packing above is good but realize that the higher it is placed, the more effect, and lower there is less effect. A torn spot packing can be placed high in the hanger sheets, a cut spot pacing may need to be placed low, to get a similar print result.
The Ink Roller Supports (Bearers) article by Gerald Lange may help.
It depends somewhat on what you are printing. Yours offers a perfect example of the problem. The cylinder is essentially locked up-and-down to keep it from lifting up when it rolls over your printing surface. When the cylinder contacts the first are to print it hits that HARD. Then as it rolls up onto a broader surface the pressure is spread out over a lot more printing area. I’m not sure if the cylinder somehow raises a very small amount (say 1/1000″) but things at least change.
You might think about the difference between a C&P platen press and the Vandercook, both printing a 4″ square. The C&P exerts some amount of pressure spread evenly over 16 square inches. The Vandercook prints only about 1/2″ as it rolls over the same 4″ square. If everything was equal (and I would think that the Vandercook can exert seriously more downward pressure as it is) that pressure is focused on only 2 square inches (4 times .5). So, the Vandercook would be exerting 8 times more pressure per square inch.
Even as the Vandercook is rolling across your leaf there are so few square inches in play as it crosses the top and bottom of the leaf it will exert more force per square inch. It does this with type a lot. Almost always where you hit first will print heavier (may also be a bit of inertia in the cylinder if you are rolling the cylinder fast).
The way to correct it is to pack the center of the leaf. Let’s say the leaf is exactly 4″ tall. You might look at the point where you seem to be losing your impression both at the top and bottom. Tear (and I would suggest this instead of cutting as it will give you a more uneven edge otherwise it might show in your print) a strip of tissue paper (ours is 2/1000″ thick) that is maybe 2.5″ x 6″ and put it at the BOTTOM of your packing (to try and hide the edges of the tissue) and completely across the leaf.
Then in the end you might even add a bit extra to the stem to essentially impress the end of it a bit more which will add to the dimensional quality of the leaf.
Try printing again.
You may notice that you need to do this one more time nearer the middle of the leaf. After a few tries you’ll have a much more even impression across the whole piece.
I think that sounded clear. I hope that helps.