Straight out of the booth, the powder is held on only electrostatically. Brush it and it falls off — there's no film yet, just charged particles sitting on the surface.
Two things happen in sequence in the oven. First the powder melts and flows into a continuous, even layer. Then a chemical cross-linking reaction begins and locks that layer into a solid film.
It's this reaction that gives the coating its hardness, adhesion and chemical resistance. It can't be achieved by air-drying or by a brief blast of higher heat.
Every powder has a defined cure schedule — a metal temperature and a hold time. What matters is the part's own temperature, not the oven air, which is why thicker sections need longer.
The error shows itself. An under-cured film stays soft, scratches easily and loses adhesion; an over-cured film can yellow or drift from the specified RAL shade.