I have a four-color separation that appeared ok on the screen and in the proof, but a moir� appeared in the printed image. To our knowledge our screen angles and RIP settings are correct. Why is this happening?

I have seen this happen when the TVI (tonal value increase or dot gain) on the proof is low and/or very high on the litho press. In one example, the TVI was too high because the press operator had to make the magenta ink film very heavy (the magenta or yellow separations are too weak) to match the proof. There is always a slight moire pattern in 4/C printing, but the other colors hide the moire. If the TVI is too great in one or more colors, the increased gain can make the moire pattern pronounced and visible. Check the solid ink densities of the color bar on the press sheet to see if they are within a normal range; if so, the TVI should also be normal. Check solid process ink densities and the TVI of your proof. Slurring can also significantly increase TVI and make a moire more apparent. A random, faint moire pattern could be related to slurring.