Building a 3D Printed Magnetic Stirrer

One of the coolest things about 3D Printing is how easy it can be to use the printer to create new tools that let you build other cool projects. In my case, last Christmas I bought my lovely financé an airbrushing set and as she got to using it she quickly wished she had a better way to mix paints together. Always looking for an opportunity to use my hobbies to do something productive, I built her a 3D printed magnetic stirrer. The basic concept of a magnetic stirrer is really straightforward. Basically you start with a motor that spins two powerful magnets with opposite polarity from each other, and mount it under a sheet / platform to rest your beaker or whatever you are mixing on. Then, you put into the liquid you want to mix something that looks like a large plastic pill, except inside of it you have two more magnets oriented in opposing polarity. The magnetic attraction of the poles of both the pill and the motor-mounted magnets align and when you turn on the motor the spinning of the magnets below the platform spins the pill inside of your container and viola! You are using magnets to stir your liquid. Here’s what the final product looks like:

So how do we go about building one? It’s actually pretty easy with almost zero soldering required! Let’s start off with listing out the electrical components we will need:

As you can see, for our motor we are going to use a standard 80mm 12V DC computer fan. This is a much easier approach than trying to design something that will spin our magnets ourselves, by using the fan as a base we can just 3D print something to glue on the fan that will hold our magnets. Here’s my design:

The mount I printed and glued to the center of the PC Fan to mount the magnets

You’ll note that the parts inventory only calls for 2 10mm magnets, but this model has four magnet slots. This is because if you wanted to use a bigger stir rod you could use the outer slots, or alternatively use 4 magnets to support both cases. Either way, you need at least either two in the middle or two on the outside — and mind the polarity — each side should be opposite of the other. Next, we will need a base to house our magnet spinner. But before we do that we need to decide how fancy we want our magnetic stirrer to be. In my case, I wanted to create a stirrer that I could variably control the speed and see what that speed was, so my financé could hone in on exactly the speed she needed to mix her paints. Thankfully that’s easy enough to accomplish with an off-the-shelf PWM controller. The one I selected had everything I needed: An LCD display that indicates how fast the fan is spinning (in terms of a percentage), an on/off power button and a potentiometer knob to control the fan. If you wanted to make a simpler version of this mixer, for example one that always spins at 100% you wouldn’t need something quite so fancy as I built and you could get away with a simple on/off switch (for example). Anyway, so now it’s time to move on to the housing for the fan and its components. Here’s my design:

The housing for the magnetic stirrer with LCD speed display, power, and speed control

As always I try to design my 3D prints to be printable without supports and this one is no different! In my case the print did have a little drooping (using PLA) for the long bridge where the LCD screen went, but since that’s all hidden behind the lip of the LCD screen itself it wasn’t a problem. When building this device it is really important that the distance between the magnetics within the case, and the magnetic pill itself within the container be as close as possible. They are powerful magnets for sure, but for our purposes even a millimeter or two can drastically change how effective the device is at mixing. Remember, since the mixing pill itself will be inside of a container (which furthers the distance from the base magnets), as close as possible is best. With everything printed, it’s just a matter of putting it all together! This can be broken down into a few simple steps:

1. 1. Glue the magnet mount to the center of the 80mm fan using some superglue. Be careful to make sure you center the mount as exactly as you can on the center of the fan, otherwise it will wobble when spinning and that will be no good for you.
   2. Glue the fan, spinner-side facing the top of the housing, to the mixer housing. I designed the housing to actually allow you to screw the fan into the housing but I didn’t have the right size screws I needed to do that in my case so super glue on each riser works just fine. When securing the fan, make sure it’s again as centered as you can get it on each riser (especially if using super glue). You want the “visible” side of the housing’s built-in target to actually match where the magnets are! Also make sure you run the wires in such a way along the side of the fan as to not get caught or otherwise get in the way.
   3. Pop the cap off of the back of the LCD display to expose the plugs where the power and speed controls attach to the board. You’ll need to remove them so you can secure them to our case and then reattach them after everything is put together.
   4. When attaching the potentiometer, mine had a tab jutting up that was preventing it from mounting flatly against the back of the housing. This tab is designed to enforce an orientation of the potentiometer. Since my design didn’t include a little hole to enforce an orientation I just broke the tab off using a pair of pliers.
   5. Secure the potentiometer and power switch, which screw into place, and then insert the LCD screen.
   6. For power, I used a female DC power jack (listed in the parts) and a 12V 0.5A power supply (also listed), which I soldered leads to it in the correct polarity. When doing this step make sure you examine your power adapter to understand if the inner peg or outer shell is positive so you don’t miswire the PWM controller!
   7. Next install the LCD controller. It is a pretty loose fit, so I used again a little super glue under the lip of the screen to make sure it was nice and secure when mounted to the case. Install the LCD screen with the lead-blocks (green guys with the screws) facing down in the case otherwise your LCD will be upside down! Here’s a picture of how everything looked when it was completely put together:

The underside of the assembled magnetic mixer

With everything installed wiring it up is pretty straightforward. On the LCD casing it should identify which terminal blocks are used for what wires. If you used the same fan as mine you’ve probably noticed by now that the fan has three leads coming from it and hopefully they are colored red, black and yellow. For this project we only care about the power (red and black), so I clipped the yellow lead off entirely. In case you’re wondering what that yellow lead is for, it’s attached to something called a hall sensor in the fan itself which allows it to track how fast it’s actually spinning. In a modern computer this allows the computer to keep an eye on the RPMs of a fan, but we don’t care about it at all for this project. So wiring looks something like this:

12V +/- attached to power in of the LCD Power plug re-attached Potentimeter re-attached Motor power +/- attached to red/black leads of fan

Once everything is wired up, use a few zip-ties to secure the wires so they don’t get caught in the fan and maybe a dab of hot-glue to keep them from moving around and you’re done with the base. Now to finish the project, we need to make a stir bar / pill to put into our liquid. The stir bar / pill is super easy, it’s just two 3D printed halves of a bar / pill with space inside of them to stick some small yet powerful magnets:

One half of the stir rod /pill (print two and seal together)

However as boring as it sounds there is a really cool technique to putting it together that’s worth discussing a minute called Friction Welding. The idea basically is that instead of using superglue to secure these two pieces together we fuse them together by taking a (small) piece of PLA and sticking it in a rotary tool like a Dremel as if you were putting a drill bit in. Then with the Dremel running you press the PLA you inserted against the seam of the PLA thing you are trying to weld and the friction causes the PLA to heat up, melt, and fuse the parts together! This can be extremely useful technique when glue isn’t ideal and is actually quite strong. You can check out a video of how it works here. Gluing the stir rod together or using friction welding, it is important that you orient the magnets within the stir rod correctly before sealing it up! Since you installed the magnets in the base to be opposite polarities, make sure your magnets also are opposite polarity from each other in the stir bar. When properly assembled, the stir bar will firmly attach to the magnets in the base on both sides with zero repulsion! That’s it! I hope you’ve enjoyed the tutorial! You can find all of the models for this project on Thingiverse — including the source SketchUp model so you can modify it easily yourself! Be sure to follow us there and subscribe so you can learn the latest ways to make your house smarter! Don’t forget to leave a comment if you have any questions or feedback!