The Biqu Magician is a great little pre-assembled delta 3d printer. But the part cooling design is flawed. Three fans are mounted at the top of the effector. One runs all the time to cool the hot end heatsink. The other two kick in for part cooling as required. But all three fans vent through radial slots downwards at the bottom of the shroud.
Good part cooling requires a transverse flow of air across the nozzle. For some models the downward airflow hits the model or the buildplate and this reflects enough air sideways to at least do some part cooling.
But for many models, particularly with demanding overhangs, there's a problem. That's why most 3d printers have a radial blower for part cooling that directs air across the nozzle.
While some people have tried to fix this problem by redesigning the effector shroud, the issue is that with the hotend fan venting through the shroud base, any redirection of the airflow will compromise first layer adhesion and cause significant issues with part warping as well. This is because you want independent control of part cooling, often leaving it off until the model has several millimetres of layers printed.
Luckily there's a cheap and easy fix. We replace the right-hand fan as you look at the front of the printer with a 30mm radial fan. The 3d printed duct then sends the airflow down and across the nozzle.
You can see the results in the before and after prints here, where the part cooling defects are significant on the model with the stock cooling.
30mm radial fan
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Step 1: Print the Fan Duct
I printed it with standard PLA at 0.1mm layer height, 100% infill and rotated 180 degrees with buildplate-only supports and a 5mm brim. This puts the outside of the duct (as it would face you when installed) nearest the buildplate during printing.
Once printed, tidy up the part in the normal way
Step 2: Replace the Right-hand Fan With the Radial Blower
Just unscrew the fan, cut the leads, and fit the new blower, using heatshrink tubing to make a neat join on the cables. You may need to ream out the mounting holes on the blower to fit the original screws as the holes are a bit smaller than on the fan you're replacing. Or you can fit M2 brass thread inserts into the original fan holes as I did and use M2 bolts to mount the blower.
Step 3: Attach the Fan Duct to the Blower
Use a drop or two of epoxy to fasten the fan duct to the radial blower
and another drop down at the base of the shroud so that vibration won't cause it to become loose.And that's it!.
Step 4: Add a Second Duct If Desired
After I wrote this instructable I figured it wouldn't hurt to replace the second part cooling fan in the same way. To do this print the STL again after mirroring it 180 degrees in Cura. Note that the clearances are a little tighter for the second duct so have a look at how the arms interfere with the duct before glueing it on and just file a 45 degree chamfer on the edge that nearly interferes with the arms.
With both cooling fans modified the part cooling is now exceptional. I have switched to a 0.2mm nozzle and can print incredibly fine detail.