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Overprint & Trapping
Control your overprint, traps and chokes :-

To produce professional quality results you must understand and use these artwork techniques.

Mis-register Paper is basically a very thin layer of mushed up wood that we image by applying pressure in the presence of ink and water. This inevitably produces some degree of movement and stretch referred to as misregister.
When passed through a multicolour press, pressure is applied at high speed by successive sets of steel and rubber cylinders. Depending on the particular conditions, paper stock, ink coverage and tack factors the image on each unit may move slightly and may not fit exactly with the others. This misregister may only be very small but the human eye can often see it when the colour conditions and design of your job highlight the problem. The techniques of overprint and trapping are used to ensure that the problem is not highlighted.

All litho inks are transparent!
Litho inks are specifically designed to be transparent - even black. You cannot cover up one colour with another and expect the colour underneath not to show. It is like laying sheets of coloured glass over each other. If you lay yellow glass over blue glass and hold it up to the light you will see green.

What is Overprint?
Quite simply, as the name suggests, the technique of printing one page object or colour area over another.

What is Knockout?
The opposite of overprint. An image fits into a hole in the background colour, ie the background is knocked out.

Overprint Black vs Knockout Black - black is a special case
As a general rule you should set all small black objects to overprint as this illustration shows.
overprint black
Magnified 8pt black type and lines printed on a blue background.
As you can see the overprinted black image on the right is far superior to the knockout black image to the left. Although the mis-register is likely to be very small, much thinner than a human hair, your eye can sometimes see it. Avoid these sort of potential problems by setting your black on overprint.

Traps and Chokes
What if your image isn't black?. Remember that all litho inks (apart from metallics) are transparent. You cannot, therefore overprint an orange object on a blue background because it will change colour and probably become purple
In this case you must apply a trap or a choke. Trapping is where the top object is made slightly fatter than the hole in the background it fits into. Choking is where the hole in the background colour is made slightly smaller than the top object.

Automatic Trapping
Applying traps and chokes can be very complicated and most professional page make-up programs have some sort of automatic facility to handle this. You should read your help pages and user manuals in order to set it up correctly.
Lower end budget programs and programs not intended for this purpose (eg MSWord) do not understand trapping and are likely to set everything on knockout. Such programs are best avoided for this and other reasons.