Registration is a big problem with flexo printing. But there are quite a few things you can do as a designer to help mitigate the problem of your colours not lining up once on press.
Flexo presses typically have much greater trap lines to help with registration. Trap sizes will vary depending on the type of press, but I have seen everything from .5 pt (not too bad – similar to screen printing and some newsprint) to 1.5 pt or more! Heavier traps will have an impact on your line widths, the details you can have in your design and the type of colours you choose for your design. As an example, printing a blue object next to a yellow object will result in a very noticeable green trap line where the two colours meet up. This may obscure small type and details.
Another bad example occurs when blue and red are printing side-by-side. It isn’t too bad when the traps are small, but it gets pretty ugly once you get into larger traps. Having Complimentary Colours trapping to each other can be a real problem. Complimentary colours will always create ugly greyshish/brownish colours when they overlap. Good colour combinations to trap together would be yellows, reds and oranges, or greens and yellows. Blues and purples aren’t as good a choice, as I will get into next, but they work as well. Black lines can be your friend to hide unsightly trap lines, but your design may wind up looking like a kid’s colouring book if you are not too careful. Or try to keep the colours as separate as possible. No need to worry about ugly trap lines if your colours do not need to trap!
Getting out of Reverse
One big problem with bad registration is reverse type or graphics. Small type reversing out of process colour can sometimes be hard enough to reproduce when printing standard offset printing. Keeping your reverse type legible in flexo can be a nightmare. Best to avoid this altogether, but if you must, try to choose a nice open san-serif font. Serif type will fill in otherwise. And the fewer CMYK components your background colour is, the better. Reversing out of one or two colours is much easier than three or four. And check with your printer, you might be able to sub in a spot colour which will make reversing your type or other graphics better. You will also have to make certain your line widths are nice and thick. Quarter point rules reversed out of any background colour will fill in. Your printer should have information for you on just what line thicknesses you can use for reversing out of colour. Be prepared for minimum line thicknesses of 1.5 pts. or more though.
Colours that touch each other that are similar to each other in their process colour breaks can be a joy or a pain to reproduce. Take blues for example. Nice, pure blues are usually made up of combinations of cyan and magenta. If both cyan and magenta changes for each touching colour, there can be a “double image” printing effect. Seeing double may work fine for a night out at the bar, but not so much when reading the ingredients or other small type on your packaging. Only changing one of the components (like the magenta) and leaving the other one that same will greatly reduce this problem. See the illustration for a clearer example.
Printing process colours that have been surrounded with white can be a real problem. If a colour is made up of two or more components, you can get a double image or “fuzzy” type and graphics. The fewer the components, the better. Using all four process colours is a real bad idea. Three is better, but two is ideal. Red and greens are the most forgiving, blues and purples are not great. If you have complex colours that require a lot of different process colour components, try using a spot colour instead.
If you are using process colours in this way, you may find a hold line will help you out. A hold line is an outline of colour that only uses one process colour to act as a hedge against mis-registration. If you are printing a blue colour made up of cyan and magenta, try a cyan only outline around the object. The line thickness is usually half the trap amount. This way, if the colours mis-register (and they will) the cyan outline will prevent the magenta from peeking outside the cyan, creating nice ugly, unwanted, pink shapes. See the illustrations for some examples.
Next post, I’ll wrap things up and give my final thoughts on flexo design and some links to check out as well.