Halftone Print Issue
Hi,
I’ve been printing some halftone images on a golding jobber #7 using a boxcar base and polymer plates from boxcar.
the problem is that regardless of my pressure or the amount of ink initially on the press, I’m getting a small line of increased density on the top edge of the print, no matter its location on the platen—the line is approximately 1/8th of an inch, and it’s on any halftone I print.
the kicker is that if I ink the plate twice, it goes away, while still giving me a clean print, that is, no clogged areas, and good line screen differentiation.
My plates are at 65lpi.
I’ve tried more ink on the platen (clogs the plate), more packing, less packing, less ink, I’ve been searching and haven’t found any tips on this.
any thoughts?
thanks!!!
I’ll post a photo tomorrow—I left the cards in the studio.
First off- some terminology correction is in order. You don’t normally put ink on the platen- rather, on the ink disc. Just FYI. Onward.
It’s possible your problem is a ‘lap mark’ from one/all of the rollers?
How big are the images VS the length of the roll out from your rollers, for example- if you have a 1.5” diameter roller, it should be roughly 4.7” or so. This is the lap length; having rollers that have a shorter lap-length than your form can pose issues if your inking system doesn’t have distributors or rider rollers (as is the case for many platen presses).
Another thing to look at are your rails- are they taped to modify the inking/form roller pressure? If so, are they taped evenly? Could you have missed a spot with the tape, causing the roller to dip slightly lower? Or does the problem persist in the same spot on the plate, no matter where you put it on the base?
Your issue could be caused by the inking system, or it could be caused by pressure.
If your platen is off balance, you could be putting a bit more pressure down on the top of the form when the press closes. This could be because the bolts aren’t leveled out properly, in which case it would require some adjustment. If for some reason this is your problem it’s a fairly routine adjustment to make and there is information elsewhere about it on Briar- do a search.
This is less likely to me than being an inking problem, but from your description- the fact that it goes away when double inking- I’m not ruling it out as possible. If the form carried more ink and the problem was less visible as a result, that would be a logical cause/way for me to diagnose the problem. Take a close look and see if it’s still visible at all after 2 passes; ALSO, try and print once to paper without ink and see if this creates an impression that is visibly uneven.
Next also try cutting a piece of tissue that leads almost up to the problem area but doesn’t go behind the heavier stripe, insert this behind a sheet of paper, print it one time, see if that has an effect.
Pictures and more of a description of the press setup and other possible variables would be good.