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