Air duct for 40mm fan

Chinese 40mm fans often has poor design (too big gap between blades and fan duct). So, using them on a thin air ducts are not efficient, because air tends to come back in the gap between blades and fan duct.

If you have this issue, then this small cover effectively reduces opposite air flow.

It is very easy to try. Just print it and then feel with your hand air flow behind the fan with and without this cap.