Mysterious spotting

I’ve been dealing with this issue for a while now and have pretty much thrown in the towel. The picture may not help much. It’s as if parts of the photopolymer plate will randomly stop inking between pieces. Mostly on serifs but sometimes larger areas like this one. Whole letters will be crisp on one impression but spotty on the next.
I’ve tried all the usual adjustments and patches. It’s not ink contamination - I’ve at least isolated that variable. Could be some odd plate issue? A dead spot on the rollers? Printing on a platen press. I’m thinking of trying some ink additives like gamblin to see if that helps when it happens.

image: 20210301_200440.jpg


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Are you sure your rollers are making contact with the plate well enough? Try putting a sheet or two of paper behind the chase to bring the plate closer to the rollers. If that will give you too much impression pressure, take the corresponding amount of packing out from under the tympan.

Can you trace a path of the missing image? As an example can, you see the spot moving down that page? This would typically happen in a straight line going from the top to the bottom of the form and then starting again at the top.
If so it would indicate that the problem occurs in a cyclical way that is probably related to either an indented spot on a roller or a truck that is somewhat out of round. If you have adjustable Morgan trucks and one is larger than the others it would be moving at a different speed, therefor causing a slur or “squeegeeing” of the type as it is dragged across the plate.

Are smudges showing up anywhere else on the print? I sometimes notice “missing” ink first and then realize that what is happening is my sheet is curved or my pin is weird and the paper is lightly touching the plate in one spot, then fully touching upon impression. So little bits of ink get transferred to the paper elsewhere and are “missing” from the main print.

Basically, is your sheet totally flat and flush with your platen? :)

Sorry if that was a poor description! Haha.

Your reference to Gamblin makes me think you might have some experience with art materials. Are you using a traditional printing ink, or something else like waterbased inks or linoleum block inks? You’ll get the best results with standard printing inks.

Another problem could be as contaminated surface on the plate, perhaps something used in cleaning the plate after printing. If you are using something out of the ordinary for cleaning, make certain you at east give the plate a good clean with mineral spirits or other solvent prior to inking. Polymer plates do not have a very high surface energy, and can easily repel ink if conditions are not right.

John Henry
Cedar Creek Press

Another question would be, is your image taller than the diameter of your rollers? This can cause ink starvation where the roller has already inked one part of the forme and therefore lost some ink, and then has to ink another part of the forme with that lower amount. I’ve had this cause odd deletions and thin spots.

Michael Hurley
Titivilus Press
Memphis, TN

I have seen this exact problem with surface contamination (with copper engravings). I had a clean plate, good ink, and rollers, but my cleaning cloth had a tiny amount of press wash that was transferred to the plate when I wiped the plate down. Problem solved when I used a fresh cloth. It took a while to figure where the problem was coming from.