Adhesion promoter.

An adhesion promoter is a thin primer laid on a substrate before printing so the cured ink bonds to a surface it would otherwise only sit on top of. Low surface energy plastics such as polypropylene and polyethylene are non-polar and barely let ink wet out, so the cured film is held by very little; the promoter bridges that gap, keying into the plastic on one side and into the ink on the other. It works differently from corona or flame treatment, which raises the energy of the plastic's own surface rather than adding a layer, and no preparation lasts indefinitely: corona-treated stock loses its effect in storage, and promoters carry a drying and over-print window that varies by product. Weak adhesion passes every check at the press and turns up later as a tape pull that lifts the graphic whole, or as a run of parts coming back after handling, with the substrate, the machine time and the reprint all on your side of the ledger.