Uneven press inking - moving position

Hi! I have a 10x15 c&p that has started printing unevenly. I’m stumped because sometimes it happens and sometimes it doesn’t. And when it does, the area progresses down the plate and then returns to the top again. I have photos to demonstrate. One is perfectly even, one is light on the left, heavy right, and the third is heavy left, light on the right. This all happens in the same print run and the heavy/light slowly moves across the print. (This is part of a 5x8 plate that is horizontal in the machine)
Some info:
We have a boxcar base, use kf152 plates. I tape the rails the same way I have for 10+ years. We recently replaced our trucks when I saw this happening but there was no improvement. I use both rubber and soy inks and it happens with both.
Any wisdom? Thanks!

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You have to think like the press. It wants to do a good job, but it sometimes needs you to help it. Get back to basics. Troubleshoot one thing at a time. Only by eliminating a potential problem will you arrive at the solution.
Start with your chase and your lockup. Is the form and the chase flat-flat on the stone. A warped piece of furniture or a poor lockup can cause a problem. When locked up, test the chase by pressing down on diagonal corners. Is it true or wonky?
Next you check for inking. Without proper inking you can go no further and cursing and messing with the packing will do no good. Ink up the form and remove the chase from the press. Inspect the form with a magnifying glass. Is the ink good and uniform and kiss inked?
It looks like you are printing with some impression and dent into the paper. Thus, that is not a problem.
If you find something from the tests listed above, you have information. All you have to do is use it.

Get some ink on your shirt

Hi! Yes I’ve tried all of those things but thanks for the tips!

You must still think like the press and get back to basics. Unless some bolts and nuts have become quite loose, the bed or platen have not come loose and cannot move. If they did, it would not be a random movement. Your well locked chase is going to stay flat on the bed. I assume you are printing with a poly plate. Absent something come loose, it has to be in the inking.
Try turning the form 90 degrees in the chase and printing. What have you discovered and how does it compare with the previous way?
Are you printing with a three roller press and using all rollers? Try turning one end for end and printing again. Perhaps each of the others one at a time.
Lastly, print with one roller in the single saddles. Cycle the press to give triple inking and then make one impression.
With these tests you should make some discoveries. Then figure out how to help the press.

Get some ink on your shirt.

You might run a search on Briar Press for:

“Why do I get these dead spots?”

This discussion a while back tried to solve a similar problem.

John Henry

Ok! Yeah I read other posts and that’s why I bought the trucks. That was recommended on another post. I’m not getting full on dead spots, rather heavy and light ink that travels as the press is run.

actually jhenry i thought better and reread the post you referred to and you’re right, i missed something very simple! i took my rails down just the slightest and was able to run a job with no problem. crossing my fingers that’s all it was! thanks!

There is always some variation in roller concentricity. If set in the middle of the variation, the inking will be , literally, hit and miss. By lowering the rollers so that there is consistent contact, you will get the best inking.

Each time another concentric is added to the mix (the trucks, for instance) the variation can be multiplied. There is always a + or - in any machining process. If the low side on the roller happens to coincide with the high side of the truck, you get the worst case.

Photopolymer plates are very unforgiving, as they have a low surface energy as compared to copper or other metal plate surfaces. They do a wonderful job, but are a bit more “persnickety” than some other plate materials or typemetal.

John Henry