How strong is the Invisi Flap door? Shelly August 08, 2019 03:31 Updated The clear Invisi Flap Door is made of polycarbonate which allows the product to be relatively thin but strong. Related articles Can I install my Invisi Flap Door into a wooden panel or door? Can I install an Invisi Flap Door myself? Can I install an Invisi Flap Door in my sliding door? How does the locking mechanism work? Patio Pet Door Installation Video Comments 0 comments Please sign in to leave a comment.