This is a parametric design for a centrifugal fan. I wanted one that could take a knock or two to blow air into my forge with, so I designed one with parametric wall thickness, fan clearance, diameter, height, output nozzle and inlet hole diameter etc. It can be printed entirely without support material and the two halves are held together with 3xNo.8 screws.

The sample fan is 70mm in diameter and 45mm high. It uses a small 12V motor I salvaged, but can easily be modified (physically or programmatically) to suit a range of motors.

The forge now has an RP'd 110mm diameter blower, and that blows plenty of air into the tuyere. The motor I chose does not have a very high RPM. It can only just reach white heat with one fan, but does not melt even in PLA due to the high airflow. A much faster motor on a smaller printed blower produces a higher flow, so I'm sure I can get more out of it later.

If you want one with a handle on, see http://www.thingiverse.com/thing:8096

Print case, impeller and lid. Attach motor to lid with 2 x M3 screws or similar. Warm motor shaft and insert into hole in base of impeller. Do not trim printing goo strands off the interior of the impeller.

Ensure that:

a) Impeller clears motor mounting screws, and b) Impeller is fitted centrally on the motor shaft. c) Impeller will rotate in the right direction to "brush" air out of the outlet.

Fix the lid to the case with 3 x small countersunk woodscrews. Hook motor up to relevant volts.

A nozzle may be made of any convenient tube or sheet metal and fixed to the outlet with duct tape. Hey, it's a ducted fan so this is a legitimate use.