I read a good book on DTP.

The author stated, that one should do the trapping at the design stage when working with metallic inks, instead of automatic trapping in the RIP. Unfortunately the author doesn't explain this further.

I would like to ask, why do the trapping at design stage for metallic inks? What is the problem?

1 Answer 1


Trapping is normally done by the print house and in most cases the designer doesn't need to worry too much about this. But of course there is no rule without exceptions.

It's hard to reduce graphic design to a set of rules. It's better to learn the background for the rules so you can make better decisions yourself, so I have to explain a bit.

Please note that my illustrations are exaggerated to prove the point. They are zooming in on tiny objects.

Trapping is added to avoid that misalignment between the inks leads to unwanted thin white stripes where two colors meet.

Let's assume your artwork consists of two overlapping rectangles in two spot colors. The topmost rectangle knocks out the rectangle below.

If no trapping is applied and there is a tiny misalignment the result could look like this:

Instead trapping is applied. The RIP software automatically decides which colors should "bleed" into which colors and the prepress worker supervises this. Where two colors meet the contour of the darkest color is preserved and the lightest color is expanded a little bit to create an overlap (maybe about 0.2-0.5 pt.).

Now if there is a misalignment, the trapping can hide the problem.

The downside is that there will be a dark contour where the two colors overlap, but it's better than the alternative.

This example is simple and you could easily make this kind of trapping yourself. In complex vector drawings with lots of colors it gets so complicated that trapping software is needed.

The misalignment and trapping width sets a natural limit to how tiny details you can print in multiple colors that knockout. Be careful with thin lines like this:

With trapping they can end up looking very different from what you expected.

Trapping is only applied on overlapping objects with knockout. If the topmost object overprints (often done with multiply blend mode) trapping is disabled, because it wouldn't make any sense. The risk of white lines is non-existing.

And now we can finally get to the point.

As I mentioned in an answer to one of your other questions, it's recommended to always overprint metallic spot colors. They are opaque and will cover what lies beneath. There is normally no need to knockout and introduce all the troubles that comes with that.

It's even recommended to print black under a metallic ink to give the color more depth.

Let's assume you have a design using a metallic gold spot color and black. You want it to look like this:

But you want the gold to be as deep as possible, so you add black beneath it. You can either use a mixed ink swatch with 100% gold and 100% black or simply create a black duplicate and multiply the gold on top (or the other way around, it doesn't matter since the printer decides the order of the inks). Annoyingly the preview now looks nothing like the result. That's a tricky thing with opaque colors.

But when we overprint inks trapping is disabled. So with misalignment we might get black shadows on the gold objects (and a slightly lighter shade of gold on the opposite side).

To prevent this we could manually make sure that the black shapes beneath the gold is a tiny bit smaller.

When printed we avoid black shadows, but have to live with a slightly lighter shade of gold along the edges.

Avoid using black beneath small text or thin lines in metallic inks. The risk of visible misalignment is big and if you apply manual trapping the black disappears anyway.

An alternative to printing black beneath metallic inks is to print the metallic ink twice. In this case you need to treat the duplicate metallic ink as the black in the example above to avoid that everything gets bolder than intended.

If you ever need to make artwork with metallic colors I strongly recommend that you get in contact with the print house and ask for their advice. They can tell you how much trapping they normally use and they might tell you that their misalignment is so small that manual trapping isn't needed. Perhaps they even have an automatized way of taking care of these matters.

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