Magnetic stirrer from a 80 mm fan by WaveSupportApparatus

Last crawled date: 3 years ago

I needed to keep a solution of Chelex constantly stirring so that the beads wouldn't just sink to the bottom during pipetting. I could move equipment around the lab, or move my work to the equipment — OOOOOR I could make my own personal stirrer so that I never have to share equipment with anyone! I like that.
A demonstration video of this mixer is here.
This mixer is made using a standard 80 mm computer fan hooked up to a multi-voltage power adaptor, which is how you control the mixer's speed. I designed it so that the magnets would be as close to the platform's surface as possible, which keeps the stirring bar stable when you're mixing larger volumes at higher speeds, or mixing viscous liquids (like vape juice, apparently). Heck, put a stirring bar into your bubble tea and keep your bubbles well-distributed, whatever you like!
Bill of Materials
Tools
A long-stemmed screwdriver that can fit through the fan's mounting holes
Hot glue gun
Mallet
Magnetic stir bar (or a screw or nail or any other magnetic object depending on your needs)
Printed components
1 × stirrer enclosure.stl
1 × stirrer magnet mount.stl
Electronics
1 × 80 mm computer fan
4 × standard PC fan screw
1 × 3.5 mm female jack (AliExpress Example 1 or Example 2)
1 × multi-voltage power adapter (mine delivers 3–12 V with a maximum of 800 mA).
Vitamins
4 × self-adhesive rubber feet (AliExpress)
2 × neodymium magnets, 8 × 5 mm (OD × H) (AliExpress)
(Optional) Reusable double-sided mounting tape, e.g. Scotch restickable strips
Assembly
Something you should know: You can release a hot glue joint by soaking it with rubbing alcohol for a few seconds.
Hold both ends of your stir bar to a magnet to find the polarity of each end.
Surprisingly, my own stir bars had the same polarity at both ends, even though every other magnetic stirrer design on Thingiverse (and common sense) assumes opposite polarities.

Friction-fit the neodymium magnets into the holes of the magnet mount, remembering the polarity you found in Step 1.
Check this against your stir bar before you continue. Your stir bar should be tightly held down by both magnets, not just one end held down and the other end flopping free and being repelled by the other magnet.
If you get the polarity of these magnets wrong, your stir bar will be dragged in a big circle around the outside of the mixing vessel instead of spinning in place in the center.

Affix the magnets into the mounting holes by using super glue or a tiny dab of hot glue. Tap them down with a mallet to ensure they are seated properly.
Place a few dabs of hot glue on the flat side of the magnet mount, and then attach it to the central hub of the fan (the bit that rotates with the blades, not the reverse side which has a sticker that covers the wires).
There is always a dimple or tooling mark exactly at the center of the fan's hub. When you can see the tooling mark through the hole in the magnet mount, you are centered!
Don't use too much hot glue. Four dots distributed around the magnet mount, that's all you need.

Screw the fan (magnet side up) into the mounting holes on the underside of the mixing enclosure.
Wire the fan to the 3.5 mm female jack (see photo).
Ignore the fan's yellow wire.
Positive/negative doesn't really matter, because you can reverse the polarity of the power supply by reversing its removable male plug.

Attach a 3.5 mm male plug to the multi-voltage power supply and plug it in.
If the fan doesn't start turning, reverse the 3.5 mm male plug to swap the polarity and try again.

Hot glue all the dangling wires and things together, and attach the rubber feet to the flat pads of the enclosure.
Notes for using this stirrer
If you place a thin-bottomed vessel (like a small beaker) onto the mixing platform, you may find that the stirrer doesn't move until you turn the voltage way up. This is because I designed the magnets to be as close as possible to the platform's surface; the magnets are therefore so close to the stir bar that it's being pulled hard against the bottom of the vessel. This is a boon for thick-bottomed vessels like large Schott bottles, but not always ideal for everything else. The solution is to put the thin-bottomed vessel on a platform. In one of the example photos above, I use a thick petri dish as my platform. I have also used a pad of post-its and a protocol from a kit; you only need to raise the vessel a few millimetres until the magnetic force weakens enough.
I sometimes mix stuff in small 70 mL yellow-topped specimen bottles. These bottles are light, so they need to be held down somehow during mixing. The best solution I've found is to put reusable double-sided mounting strips onto the mixing platform. When they begin losing their stickiness, they can be washed in soapy water to bring it back.