What is the best resolution for images to be printed in flexography?

Well, it depends first of all on the distance that the printed product will be looked at by the viewer: this will determine the suitable linescreen that the image will have on the printed product.

A general guideline for AM screening defines a minimum and an ideal ratio between image resolution and print linescreen at 100% size reproduction.

A minimum resolution is given by this formula:

ppi = lpi x 1,5

The ideal resolution can be calculated by this formula:

ppi = lpi x 2

Printing at 100 lpi (40 l/cm) would require images at minimum resolution 150 ppi.
Printing at 150 lpi (60 l/cm) would require images at ideal resolution 300 ppi.

Using images at resolution higher than ratio ppi:lpi=2:1 (example image at 600 ppi printed at 150 lpi) will only result in larger files, longer and slower processing times, without bringing any better quality in printing.