Overprinting

If you have a dark object which is placed over an object of a lighter color, you may want to overprint it to eliminate the variable of registration and to increase the density of your dark ink in the area where the two objects overlap. The question of whether to overprint is mainly one of contrast. If the overprinting color is dark enough that having the other color underneath it won't alter your printed piece in an undesirable way, then it should usually be overprinted. Black overprinting another color is the most common overprint situation- it can print on top of most inks without any adverse effects, and in fact, usually appears more solid and dark, generally a favorable thing, when it does so. However, there are some other color combinations which will work with overprinting as well.

You must use caution if you are printing the dark color over a background with drastic contrasts, such as areas printed with different inks, or some areas which have ink and others which don't. You must make sure that each object is on a consistent background. If you run the darker ink over backgrounds of different contrasts, you will see these variations in contrast through the darker color because some parts will be darker than others. See the illustration below.

This can be fixed by making a consistent background for the darker color to print over. The simplest, most common approach is to eliminate the overprint and allow the darker color to simply knockout (trapping will be done automatically during the RIPping process) over the other objects and print mainly without any ink underneath it. However, using this approach will eliminate the advantages of more solid color and worry-free registration.

To preserve these advantages, you must place a second object, identical to the original one, underneath the original object, coloring the second object with a 30-80% tint of the (lighter) underprinting color. Then allow the original object to print on top of the second object by setting your trapping settings/attributes for the original object to overprint.

The first example in the illustration below shows how objects printed with dark ink will appear if simply printed on top of those objects in the other inks. The second and third examples simulate results from the two corrective approaches we've discussed here. Note that the text is darkest and most vibrant in the underlaid example. Also note that the underlay on the third example was made from Ink 2. It could have also been made from Ink 3, or from both.