I work for a small printers on the pre press side of the business. We are seeing a problem in the paler tones of our jobs, especially skin tones, where the dots appear to make up a square pattern. However when we run progressive colours from the presses you can see a distinctive line pattern on the yellow plate even with no other colours overlayed, so it can't be Moire. It's somewhat similar to the material pattern of denim. This pattern cannot be seen on the individual Polyester plates before they are inked up and yet we get the same issue on both of our presses. We have a 4 colour Ryobi 524HX and a 5 colour Ryobi 525HXX. Both are showing this pattern on the yellow progressive only, but it is enough to affect the look of the whole job.

If we take the yellow plate off and run it on the magenta unit the pattern doesn't show, if we run the magenta plate on the yellow unit there is no pattern, and yet when you then combine the two it comes back again. If you only run the CMK plates as an image there is no pattern, it's a real mystery to me, and of course, being pre-press I'm being asked what the problem is... and I'm out of answers. We thought it was the blankets on the Presses, but they have now been changed, we thought it might be the units themselves (but not on both machines on the same unit surely?), then we thought it might be the ink affecting the plate, but surely not in a specific pattern, and the yellow ink doesn't affect the Magenta Plate.

My Screen angles are C:15 degrees M:45 degrees Y:0 degrees K:75 degrees, the frequency is 175lpi... Has anyone seen this problem before?