Airflow Backflow Preventer Stop Valve

I designed this to prevent damp external air from back-flowing into my printer enclosure when my exhaust fan is idle. The valve opens automatically when the fan starts, and the valve closes when the fan stops. *The valve is very sensitive: it opens wide with my fan set to it's slowest speed; when the airflow stops the valve closes fully against the edge stops.*

I integrated this valve into my HEPA Air Filter Scrubber Tower https://www.thingiverse.com/thing:2751455, and this valve is backward compatible (as an add on) with the Air Filter design from https://www.thingiverse.com/thing:2394452

The design primarily consists of a Collar, and a Flapper. The outer diameter of the 42mm Collar mates with the interior of the Grommet provided at https://www.thingiverse.com/thing:2394452