I know I’m a little late to the party, but I believe all the suggestions posted here are dealing with the symptoms and not the root cause. If you are getting contamination in the yellow, odds are you are also contaminating m and c as well, though not as noticeable. Here’s what’s happening: your plates are running too dry. Look at your blankets. If you see “picture farming “ outdide the dimensions of the sheet, that is confirmation that you are running too dry. Also, scumming the first few startup sheets is a yellow flag that your water settings are too low. Wet ink from previous units naturally transfers a little to subsequent units, and because there is not enough moisture on the plate to repel the ink, that ink that has traveled from previous units migrates to the plate, ink train, and ultimately fountain. The heavier the coverage, the more noticeable the contamination will be. Someone suggested that no pressman should run ink zones outside the image area on 0. Actually, when properly adjusted, a zero setting still feeds a very light ink film. My suggestions are to make sure your water rollers are running fast enough and metering rollers are not too tight. Check your dampening solution temperature and conductivity. Temps above 70 will usually cause problems. Many modern one-step solutions will also accept additives to help make the water “wetter.” Also, if you are conditioning your incoming water you may need to add hardener, or conditioner. Finally, make sure fans ir air conditioning is not blowing into the print units. Of course, all of this assumes that the rollers and roller settings are in good form.
