Overprint is a common mistake that happens a lot in print production. It is easy to avoid it but it is even easier to miss it. This is why you should carefully assign correct attributes to the objects and colors used in your design. Especially to the black colored objects and text.
Overprinting is a process of printing one color on top of another. When you set an object to overprint (Window – Output – Attributes) it will overprint any other object below. This means that the bottom object will be printed entirely. On the other hand top object that is not set to overprint will drill a hole in object below. This is also known as knock out.
In InDesign all swatches are set by default not to overprint. They knock out. But however, all text made of 100% black is set by default to overprint.
The reason why the text is set to overprint is because if you set it to knock out it can create problems in printing process.
Lets say you have picture captions that are set to 10pt, placed on top of an image and you switch off overprint. Problem known as a misregistration can occur and it will produce thin white lines between the image and the text. This is not good.
To compensate for missregistration black text should always be set to overprint. Since this is default option you should leave it this way.
Now you think, but then the image below will show through the text. No, it won’t. In fact it will but since the text is so small it won’t be visible to human eye.
But on the other hand if you have a large headline spreading across whole page and there is an object of another color below or an image then the objects below will be visible since text set in black is much bigger. Should you switch off overprint in this case? No!
Instead you should do another trick. Create another black swatch. It is usually known as “rich” black. “Rich” black is made out of all four process colors.
What are the perfect values for “rich” black? Well, it all depends. There is a debate about what is ideal “rick” black. Some say it should be 30%C, 30%M, 30%Y, 100%K (black). I set “rich” black in 50%C, 50%M, 60%Y, 100K values.
Different values produce different “rich” blacks. There is a warm black (35%C, 60%M, 60%Y, 100%K) and cool black (70%C, 30%M, 30%Y, 100%K).
Whatever values you use, the text to which you apply them will be made out of 4 colors thus blending with the object below which in the end won’t show through.
Should you use rich black on small-sized type and body copy? No, never! This is a big mistake because if missregistration happens it will result in something like 3D effect were missregistrated color will stick out.
So, to conclude. If you are working with type smaller than 48pt (this is font dependent), overprint should be applied. if your text is large and there is a chance bottom object will show through, use “rich” black color. Test few sizes of your desired font to see which values work best.
if you are using lines in your design, overprint black should be applied for lines below 2pt. In the case of the lines do not use rich black, instead use overprint option. Turn it on or off for desired line.
For other vector objects that are not set in black you can always leave overprint switched off, thus creating knock out.
If you have black object on top of another colored object, again use “rich” black or you can turn off overprint for this object only.
To avoid all mistakes it is best to check all PDF files before you send them to the printing-house. Best way to do it is to open each file in Acrobat then navigate to Tools – Print Production – Output Preview.
In Output Preview tick the Simulate Overprint check box which will simulate overprint issues on-screen. Now you will be able to spot and correct them.