The FEP film is the multi-layer transparent foil installed on the resin vat, allowing UV light to run through it and curing liquid resin during the printing process.
If your printer starts to produce low-quality objects, an increase in printing failure rates, or notice severe dents, scratches, deformation on the FEP film. The film may require a replacement.
The following chart is the size of the FEP and PFA (nFEP) films:
FEP and PFA (nFEP) Film Size
Check this post for the guide on replacing the FEP film.