Understanding Trapping Concepts for Flexo and Offset Printing

[Schiming]

Hi everyone,


I'm a graphic designer with strong knowledge of Adobe Illustrator. I started working at a label factory four months ago, and now I’m transitioning into prepress.


I know how to apply trapping and overprint functions in software, and I’m currently learning Esko ArtPro+. However, I want to understand the key concepts behind trapping. Most tutorial videos only cover basic color trapping (e.g., yellow on cyan), but I need to go deeper.


What factors should I consider when setting up trapping for flexo and offset printing? How do different materials, inks, and press conditions affect trapping decisions? Are there specific scenarios where I should handle trapping differently?


Any insights or recommended resources would be greatly appreciated!


Thanks in advance!

 

[Magnus59]

I can't speak for flexo, that's a completely different world, where trapping, from what I have read can be quite a can of worms.
As far as offset goes, providing your machines are well maintained and print in register, most of the time trapping light colours to dark colours with your chosen trap values is fairly straightforward, although there's always going to be the occasional job which needs extra attention.
One situation which needs special attention is black or other dark colours over metallics. If you print the metallics first, then dry trap the black, it's not an issue, but if you print wet on wet, you need to ensure the black is knocked out (Indesign, Illustrator and your Rip will want to overprint black type & linework below a certain size) The metallic is usually printed last with the black or dark colour trapped towards the metallic. Most Rips will see the Metallic as lighter than the black and will trap the metallic towards the black by default.

 

[Schiming]

I can't speak for flexo, that's a completely different world, where trapping, from what I have read can be quite a can of worms.
As far as offset goes, providing your machines are well maintained and print in register, most of the time trapping light colours to dark colours with your chosen trap values is fairly straightforward, although there's always going to be the occasional job which needs extra attention.
One situation which needs special attention is black or other dark colours over metallics. If you print the metallics first, then dry trap the black, it's not an issue, but if you print wet on wet, you need to ensure the black is knocked out (Indesign, Illustrator and your Rip will want to overprint black type & linework below a certain size) The metallic is usually printed last with the black or dark colour trapped towards the metallic. Most Rips will see the Metallic as lighter than the black and will trap the metallic towards the black by default.

I didn’t know this about metallic inks. That’s really useful! Thanks for the info—I'll keep that in mind for future jobs.

 

[SteveSuffRIT]

Trapping is a prepress technique to compensate for misregistration that occurs in printing from the materials or the process.
Less dimensionally stable materials/media (thin+ vs thick-, paper- vs film+) will need more trap compensation.
Narrow web flexo is usually in-line or unit/station/tower/deck after another. Wide web flexo is usually on a CI or Common Impression cylinder. The less distance between ink transfer points, the better the registration fit.
All process color inks are transparent. Most spot or special PMS colors can be formulated as either transparent or opaque. All metallic inks (gold, silver #877, bronze) are opaque.
The old darkroom (analog) method for trapping was called spreads (outward) & chokes (inward). Based on the lightness value (L*) of the adjacent colors, digital trapping will determine the direction of the trap.
Most sheetfed litho printing is wet trap (wet/wet). Most flexo is dry trap (wet/dry).