Our recipe for success when printing our flannel is below. Please do your own testing prior to production.
The key is to use a Grey Poly Blocker to limit dye Migration and it also helps flatten the fuzzy fabric surface so the print is smoother.
The reasons these inks are recommended on Polyester is because at 300 Degrees Polyester Dyes can gas off and migrate into the ink altering the color. This is normal for all polyester garments. The print inks used are important, as well as the temperatures reached. The poly blocker protects against dye migration by absorbing gasses if the garment is over heated. It is also a good opaque base to cover the flannel pattern.
Washing the printed sample garment(s) after 24+ hours (so hardener has set) is recommended to make sure you have proper curing and ink adhesion.