Colour Trapping
Colour Trapping
If your spot colour print job has colour that butt up or overlap then it will probably need some colour trapping to make your printer's life easier and ensure you get a good looking job.
Even the best four colour presses aren't perfect, due to paper stretch and mechanical tolerances, so on your average instant printer's single colour press some misregistration is inevitable. The difference is just the amount of trapping required.
If you use too much trapping, the areas where the colours overlap may become obvious and detract from the job. If you use too little then you may still have white showing through between the colours. More trapping may be required when printing on a large sheet, as paper stretch will have more effect on a bigger sheet of paper.
Your printer is the best person to advise the amount of trapping required, but as a rule of thumb use 0.1 to 0.3 points for jobs being printed on metal plated and better presses and around 0.5 points on paper plate jobs being run on small offset machines.
It is always a good idea to discuss trapping requirements with your printer to check on his preferences.
Depending on the program you use you may have to go to different levels of trouble to get trapping. By default most programs will produce no trapping at wall when printing colour seps, which means you get an exact cutout of the foreground object appearing in the background object. Your aim when trapping a job is to either eliminate the cutouts by overprinting the objects, or change the size of the foreground object or the cutouts slightly to provide the trapping required.
Some programs such as CorelDraw version 4 & 5 have an automatic colour trapping feature, but you still need to understand how it works before relying on it, as it will not trap all types of objects in all situations. That said, it can save a lot of work.
You need to look at each object individually to decide what sort of trap is required.
If you have a dark object on a light background, just overprint it. Most packages that support colour seps will have a setting - just select it.
The other methods involve applying an outline (stroke) to the foreground object that is set to overprint but with a fill that cuts out the background.
The colour of the outline depends on the relative darkness of the colours in use, as a rule choose the lighter of the two colours to avoid visual "thickening" of the object.
When the background colour is darker than the foreground, use the foreground colour. This is called a spread.
The variation on this is called a choke, where the outline is set to match the background colour. This is best when the background colour is lighter than the foreground.
When deciding the thickness of the outline keep in mind that in most programs the outline will be centred along the edge of the object, so the trap achieved is actually half of the line width that you have chosen.
Always try to design jobs so that small type will overprint, because it will be difficult to trap it without distorting the letters. It's better to put up with a different colour, than unreadable text!
Beware of adding an outline to one object that butts up to another of the same size (say two boxes) because one may appear to change size. Instead, draw the objects so that one actually overlaps the other by a tiny amount. Then have no outline, but an overprinting fill on the foreground object.
To provide overprinting of an unclosed path can be a bit tricky, you may need to make a copy of the object, adjust the line thickness and overprinting to provide the trapping required and also change the end caps of the line to provide trapping at the ends of the curve.
How you achieve the overprinting effect varies from program to program, you need to refer to the manual for the exact method used by each package, but usually it's an option somewhere in the spot colour definition menu, often supplemented by an over-ride setting which can be used on an object by object basis.
Remember while doing all this that the overprinting is not visible on the screen while you are working, so you will need to print separated proofs of your trapped job to a laser printer to check that all is set up correctly. Sometimes on a laser it is hard to see if a trap is working as the laser is not accurate enough to show you. If the program you're using will let you print selected objects only, and enlarge them to fit the page size it's a handy way to "zoom in" on a section of the job you're unsure about. Then overlay the pages on a light box or window.
A known hassle is with graphics imported into page layout packages from a draw program. Even when correctly trapped, the page layout program may not honour the trapping. There may be a way around it, but you'll need to find a solution that works with your software. You're welcome to ask too, the best way is to drop a question on the fax.
It may be that the only way to achieve trapping is at finals time.
We use Publisher's Prism, which works with Postscript files from just about any package, but you will need to talk to us when designing your job to make sure you set up your colours in a way that can be trapped correctly.
