The newest printers use pigment ink instead of dye ink because it resists fading for up to 99 years. This improves family snapshots, but doesn’t help the making of UV opaque positives. The nano porous structure of the coating controls the ink spread for image sharpness. Pigment ink particles are coated with a resin so they can be controlled with piezo print heads.
Modern inkjet inks used for positives are water based and still need time to evaporate and dry. Ink manufacturers need to balance nozzle maintenance and the desire to have an ink that dries fast after printing, so a positive can be used to make a screen. Glycol or glycerol humectant additives in the water based carrier that are used to to keep the pigments in suspension, retard drying, and prevent nozzle blockage need to evaporate the same way stencil coatings need to dry.
Imagine a nano porous coating as a jar of marbles. Pigment or Dye ink is absorbed into the coating by capillary action and stains the microscopic particles in the coating. It appears to be dry to the touch, but the liquid carrier still has to evaporate. A benefit of the nano porous coating is that the positive can be stacked and handled very quickly. If it is a humid day, the ink might not dry and the vacuum of the exposure unit can pull the wet ink out of the film coating and stain the stencil during exposure, because the positive has lost some ink during the process, it shouldn’t be used again.
Pigment Inkjet films are coated on 5 mil polyester. The coating loves water which is the carrier for the ink. The polyester side is very shiny and doesn’t absorb water. You can test the film for the coated side by wetting your fingers and pinching the film on both sides. The coated side will get sticky just like capillary film. Rolls are coiled with the coated side out, ready to go into the printer.