Sciencemadness Discussion Board

Modifying an existing stir plate with a stronger magnet

SuperOxide - 31-5-2022 at 09:11

I recently just purchased a series of the fabric heating mantles (1L - 3L sizes), and they seem pretty awesome. I planned on using the stirring capability of one of my hotplates (this one to be specific), but, and I expected this to happen, since the fabric mantle is around 1 inch thick, it decouples easily at anything over 350 rpm. The oval stir bar is definitely not the problem, it's a decent quality one and still has its magnetic properties, I just think it's too far away from the magnet in the stir plate.

I know I could probably make a DIY stir plate using something like a PC fan and a potentiometer, but I was wondering if anyone has just "upgraded" a stir plate before (something like this for example). I imagine it would be pretty easy to pop it open and glue a neodymium magnet to the existing magnet.

Alternatively - Does anyone have an [affordable] stir plate make/model they would recommend that has a stronger magnet than is usually found in hotplates? Would a stronger magnet mess with anything in the stir plate (ie: the motor that's in there)?

Thanks.

P.S. For reference, the hotplate I was using was this JOANLAB hotplate which can go from 200 to 2000 RPM's, and can easily stir large volumes if the flask is sitting directly on the surface. So I'm certain it's just the 1 inch gap that causes the magnet to decouple.

[Edited on 31-5-2022 by SuperOxide]

Rainwater - 31-5-2022 at 11:35

Increasing the strength of your magnet can be done in two ways.
1) bigger, stronger magnets, preferably U ( or horseshew)
2) electromagnet.

With the right core material and number of windings, an electromagnet will outperform a permanent magnet for a given volume of space.

I have tried to upgrade my cheap, hotplate stirrer with little success.
The easiest to do was to add a neodymium bar magnet to the existing spinner. Made a large improvement after I increased the distance between the hot plate and the case with some standoffs.
Next, I'm trying electrically isolated bearings, to feed power to the core. Still haven't finished it. Too many projects. But my idea is to connect it to a current source with a U-shaped electromagnet.
The U shape has an increased flux density at the tips, which kinda focuses the field where I want it.

Mateo_swe - 1-6-2022 at 06:09

Ordinary neodymium magnets does not work in high temperatures, it will demagnetize if temp is too high.
Somewhere around 80°C for ordinary neodymium magnets.
There are high temp variants but they are more expensive.
There are also other types of magnets that can handle high temps better but they are also more expensive.

Maybe just add some magnets from a harddrive and see how long they last, maybe you dont do much high temp stuff.

From the net:
N42, N40SH ... what does it mean?

Neodymium magnets are all graded by the material they are made of. As a very general rule, the higher the grade (the number following the “N”), the stronger the magnet. Any letter following the grade refers to the temperature rating of the magnet. If they are not letters following the grade, then the magnet is standard temperature neodymium.

For example: A magnet of grade N40SH has a maximum energy product of 40 MgOe and can operate to about 150°C.

Standard NdFeB magnets retain full magnetic strength up to 80°C. Customers are recommended to consult us on application involving temperature near to 220°C.
Neodymium magnets will lose magnetic properties when environmental temperature exceeds those they could withstand.

Fyndium - 1-6-2022 at 10:34

I tried to replace my heating mantle magnet with stronger one, with little success. I just gave up and moved to overhead stirring. It does work with vacuum as well with paraffin oil lubed PTFE bearing ground joint mount with O-ring. Even the cheapest stirrer can easily stir 10 liters reaction volumes with high viscosity, so you will never outrun it unless you go production scale.

Afaik high temp magnets do exist and they work much better, but a little one can cost as much as an entire overhead stirring setup.

Mateo_swe - 2-6-2022 at 02:03

I have one of those cheap PTFE stirrer bearings made for a 7mm paddle rod.
But i havent tried using it yet and it would be nice to know if they work good and if they can be used with vacuum.
The model i have is nothing more than something similar to a temperature adapter, it uses an o-ring for the sealing.
I think there are some different variants of these PTFE stirrer bearings, maybe some work better than others.
Mine look like this:


Fyndium - 2-6-2022 at 03:01

I have multiple of those and they are what I use. The tightness can be adjusted, be careful not to screw it too tight, the stirrer shaft will eat a groove in it and eventually also ruin the O-ring. I bought a pack of teflon rubber rings for replacement. I have used generally paraffin oil for lubrication for all my glass joints and those rings, and had no issues with vacuum distillations.

With overheads all vacuum bumping issues are gone. Also atmospheric distillations are much more efficient, you can heat the flask much more strongly and gain much faster condensate rates when stripping solvents. No other method offered nearly as good bumping prevention than overhead stirring. It also allows you to use any heating method desired, instead of being restricted to flask specific mantles with underpowered magstirs, worse yet hotplates. My go-to bath is CaCl2, which is good to slightly over 150C, is very efficient at heat transfer, dirt cheap and does not burn, con is it's a bit messy and CaCl2 is very corrosive to everything, including stainless.

Those PTFE collars will jam themselves VERY tight to ground glass joints upon heating alone due to teflon's larger thermal expansion, add to that vacuum. I have had to use pipe wrench to very carefully torque it off, even when I applied oil to it. I have been worried glass cracking due to expanding force, and I actually bought reducer adapters for 14/28 collars with theory that they apply much less pressure to area under vacuum, and also offer a layer of protection, if the reducer cracks, it's $10 shot, if the flask cracks, it can be two orders of magnitude higher. As the stirrer shaft is attached to the teflon collar, in case of implosion I presume it may shoot down the shaft, but be stopped by the stirrer paddle unit at least enough to prevent it blowing the flask bottom.

I have not heard of them cracking glassware by anyone at least in here, but better safe than sorry. For the record, I have had zero incidents with overhead stirrers, but be careful when detaching the shaft from the stirrer, it's long, heavy steel shaft which may easily crack the flask if it drops down there, at least don't count on the teflon collar o-ring holding it, either lay it manually to the bottom of the flask or remove it entirely. I have always held it with other hand when installing the allen screw and secured it tight.

monolithic - 2-6-2022 at 03:26

I had the same problem with stir bearings sticking inside flasks. If you're not doing vacuum work, putting a single layer of PTFE plumbing tape around the outside of the stir bearing helps a lot.

Mateo_swe - 2-6-2022 at 08:25

Would silicon oil be a good lubricant for the O-ring?

I just bought a 5 Liter pail of it for heated oil-baths.
Nice to hear the PTFE bearings are OK, i will try mine soon.
PTFE plumbers tape seem very useful for a hobby chemist, i read good things about it often.

SuperOxide - 2-6-2022 at 18:42

Seems like everyone pretty much agrees that upgrading the magnet may not be a wise idea.
I realize that heat would be a problem, but the fabric heating mantle outside doesn't get too hot, and the stir plate wouldn't be doing anything but stirring.

In regards to overhead stirring, I actually do have everything needed to do that - except for the actual overhead stirrer, lol. I got various types of stir bearings and was surprised to see that some of them are basically just a fancy way to cinch down an o-ring onto a PTFE or glass rod. I bet a regular thermometer adapter would be sufficient.

I was looking at this overhead stirrer, but I haven't actually gotten it because I feel like with the stands, the lab jacks, the heating mantle/hotplate, etc, it already gets pretty crowded on the table. But I suppose it won't be used frequently.
I did get a fume hood, so I would absolutely like to get a distillation lattuce which would make this easier, but that won't be for a while :-(

Thanks guys.

SuperOxide - 2-6-2022 at 18:43

Quote: Originally posted by Mateo_swe  
Would silicon oil be a good lubricant for the O-ring?

I just bought a 5 Liter pail of it for heated oil-baths.
Nice to hear the PTFE bearings are OK, i will try mine soon.
PTFE plumbers tape seem very useful for a hobby chemist, i read good things about it often.


Take this with a grain of salt since I haven't actually used mine yet, but really - it's just a fancy thermometer adapter with an o-ring. I would bet you could do the same thing with a regular thermometer adapter.
I could be wrong though.

Mateo_swe - 3-6-2022 at 00:29

Maybee a regular thermometer adapter will work.
If you are good with DIYíng things together you could make a overhead stirrer.
You already have the stirrer bearing and the rod with the paddle so its just the motor, motor controller and some mecanics.

Mush - 3-6-2022 at 01:35

PTFE stirrer bearing.
Mine stirrer shaft (ptfe coated steel) had been chewed up by the o -ring, marking the shaft in multiple place only after about 10 uses.
Using under vacuum don't seem to be a good idea after this.

Fyndium - 3-6-2022 at 01:46

You only need to tighten it so it bears against the shaft and rotates with little resistance. The oil, and replenishment of it is part of keeping a good seal under vacuum. For me, vacuum distillations have been just for lowering temp, not protecting from oxygen, so small leak that is unavoidable is allowed for me. Stuff that boils at 240C will come down to 140C with this setup. If you need vacuum with inert gas, you could add a straight adapter with hose joint and add another bearing and let some inert gas flow into this compartment to leak in. For high vacuum work, the stirrer may not be suitable.

And you might not need actual stirrer motor, I used electric hand mixer for the first half year before I found out actual stirrers were sold cheap. It has got enough torque to crush your fingers and destroy everything around it due to worm gear drive, downside is it's noisy. If you need ex proof stirrers, your price tag will have likely two extra zeros on it.

SuperOxide - 4-6-2022 at 13:48

Quote: Originally posted by Mateo_swe  
Maybee a regular thermometer adapter will work.
If you are good with DIYíng things together you could make a overhead stirrer.
You already have the stirrer bearing and the rod with the paddle so its just the motor, motor controller and some mecanics.



Quote: Originally posted by Fyndium  
You only need to tighten it so it bears against the shaft and rotates with little resistance. The oil, and replenishment of it is part of keeping a good seal under vacuum. For me, vacuum distillations have been just for lowering temp, not protecting from oxygen, so small leak that is unavoidable is allowed for me. Stuff that boils at 240C will come down to 140C with this setup. If you need vacuum with inert gas, you could add a straight adapter with hose joint and add another bearing and let some inert gas flow into this compartment to leak in. For high vacuum work, the stirrer may not be suitable.

And you might not need actual stirrer motor, I used electric hand mixer for the first half year before I found out actual stirrers were sold cheap. It has got enough torque to crush your fingers and destroy everything around it due to worm gear drive, downside is it's noisy. If you need ex proof stirrers, your price tag will have likely two extra zeros on it.


I actually did use a drill, and held that in place with a clamp, then a zip tie on the trigger. But yesterday I just decided that I may as well buy an overhead mixer, and got this guy.
But still, it would be nice to have a stir plate that could handle a 1" gap. Sometimes you need really strong stirring and heating, which means if I wanted to use a fabric mantle in those situations, I would need to use the overhead stirring. Oh well.

BromicAcid - 4-6-2022 at 14:13

Upgrading the magnet in the hotplate might be a pain but it takes two to Tango. Go to StirBars.com and search Rare Earth and you'll find tons of rare earth stir bars. In my experience they do have less decoupling issues - however sometimes they behave oddly. Our larger stir mantles at work (12+L) all have pneumatically driven stir magnets incorporated into the mantle, they use a very large rare earth magnet (nicknamed finger smashers). However if you try to pair one of them with a rare earth magnet in the flask you end up with an excellent pair, well, until something happens to decouple. Then instead of just remaining immobile the magnet will randomly flip 180° end over end and occasionally smash a hole in the flask. So - yes - one rare earth magnet is good but two not so much.

SuperOxide - 4-6-2022 at 20:17

Quote: Originally posted by BromicAcid  
Upgrading the magnet in the hotplate might be a pain but it takes two to Tango. Go to StirBars.com and search Rare Earth and you'll find tons of rare earth stir bars. In my experience they do have less decoupling issues - however sometimes they behave oddly. Our larger stir mantles at work (12+L) all have pneumatically driven stir magnets incorporated into the mantle, they use a very large rare earth magnet (nicknamed finger smashers). However if you try to pair one of them with a rare earth magnet in the flask you end up with an excellent pair, well, until something happens to decouple. Then instead of just remaining immobile the magnet will randomly flip 180° end over end and occasionally smash a hole in the flask. So - yes - one rare earth magnet is good but two not so much.


Hmm, nice. I purchased a few rare earth oval stir bars. Excited to see how they work out.
Not cheap, but hopefully they're worth it.

Mateo_swe - 4-6-2022 at 23:34

You used a heating mantle placed on a stirrer unit, right?
If the temperature under the mantle where the magnets are placed in the stirrer doesn´t rise above 80°C you can use regular neodymium magnets.
Upgrading the stirrers magnets could work.
Maybe you can just add some of those flat thin neodymium magnets from harddrives (they are pretty strong for their size) to the existing magnets in the stirrer.
Worth a try, you can just remove them if it doesnt work good.

That overhead stirrer you ordered was pretty cheap, 120 dollars with shipping to north Europe.
Anyone use this type of overhead stirrer, and can tell how good or bad they work?
I might also order one of these.

BromicAcid - 5-6-2022 at 06:10

I have never noticed them taking a hit from being heated. Figured they were samarium cobalt instead of the usual neodymium. Since the descriptor is just 'rare earth' it's impossible to tell. Might be worth contacting them. Samarium cobalt magnets are good to about 350°C (some sources say 260°C).

SuperOxide - 5-6-2022 at 09:11

Quote: Originally posted by Mateo_swe  
You used a heating mantle placed on a stirrer unit, right?
If the temperature under the mantle where the magnets are placed in the stirrer doesn´t rise above 80°C you can use regular neodymium magnets.

That's what I was thinking. I mean I haven't cranked the temperature up all the way, but I boiled water (getting the inside temp to like 200C) and honestly never even felt that warm on the bottom, at least not very much.
Quote: Originally posted by Mateo_swe  
That overhead stirrer you ordered was pretty cheap, 120 dollars with shipping to north Europe.
Anyone use this type of overhead stirrer, and can tell how good or bad they work?
I might also order one of these.

It should be delivered today. I can test it out and send you pictures, maybe a video (I can't promise I'll have the pictures/video today, but I will get them soon).

Also - I ordered 3 different rare earth stir bars from the website referenced above (stirbars.com), I may share how those work out as well.

SuperOxide - 5-6-2022 at 14:54

Quote: Originally posted by Mush  
PTFE stirrer bearing.
Mine stirrer shaft (ptfe coated steel) had been chewed up by the o -ring, marking the shaft in multiple place only after about 10 uses.
Using under vacuum don't seem to be a good idea after this.

I haven't done a long enough test yet, but I did hook up the PTFE coated stirrer shaft and used some vacuum grease - It has a very noticeable groove in the shaft now from where the o-ring was. I don't even see this lasting past 10 uses. I think glass will have to be the way to go.

BromicAcid - 5-6-2022 at 15:44

This is always the King stirrer bearing for me

https://www.sigmaaldrich.com/US/en/product/aldrich/z154407

They can run for hundreds of hours on one set of gaskets, can get a decent vacuum. I have had to in the past distill very air sensitive materials and I just put a bag around the bearing and flush it with nitrogen while I do my vacuum distillations.

Fyndium - 6-6-2022 at 00:52

Quote: Originally posted by BromicAcid  
the magnet will randomly flip 180° end over end and occasionally smash a hole in the flask.


Randomly imploding multi-liter flasks full with 150C boiling highly flammable liquids under vacuum is not something that is part of my lab plan. Not even when they are ntp and non-reactive materials, because I value my flasks and reagents even when the damage is less than burning my whole building to the ground.
Quote: Originally posted by Mateo_swe  
That overhead stirrer you ordered was pretty cheap, 120 dollars with shipping to north Europe.
Anyone use this type of overhead stirrer, and can tell how good or bad they work?


I have two of those and got mine for 60 bucks + shipping and it works like a charm. The housing and mounting support are very rigid, made from solid 16mm steel stock. I've ran it several hundred hours at least, multiple times 24-48 hours non-stop with zero issues, and it's very quiet and the speed control works well also on low RPM. You seldom need to run overheads on higher rpm anyway, but these things go way faster than I have ever needed by far. I mostly stick to the low quarter of the range. The included stand is also of excellent quality, very rigid and heavy, made from solid steel, and while you may not use it with the stirrer as it is not tall enough for most purposes except those demo pics, but it works perfect as standalone stand for many other purposes, like holding sep funnels, filters, and as a flask holder. The included SS stirrer shaft is not good, so get yourself a proper PTFE coated one.

I used a 3D printer flexible aluminum coupling to attach the shaft, and made delrin mount to clamp it into a permanently installed 16mm support. If you use wobbly mounts, it will likely vibrate and cause more noise and issues.

While Chinese stirplates and mantles are generally garbage, this is top tier quality. At least the units I got. The price is way unmatched, some professional supplier could stamp their logos on it, tag it for $600 and people would pay that and be happy.

[Edited on 6-6-2022 by Fyndium]

SuperOxide - 11-6-2022 at 13:01

For any of those wondering about the rare earth stirbars, I got them in today. I tested them in a 3L RBF with a fabric heating mantle. The smaller one did excellent. The large one was a total disappointment. It wouldn't even stir at 200 RPMs.

I took some videos: https://imgur.com/a/pYpRhNd

P.S. The larger "rare earth" magnet - I couldn't get it out with the magnetic stirbar retriever. It wouldn't stick to it well enough to let me pull it out. Thing is about as strong as a refrigerator magnet. Ridiculous..

[Edited on 12-6-2022 by SuperOxide]

Sulaiman - 11-6-2022 at 22:11

Although I have abandoned adding a magnetic stirrer to my diy heating mantle in favour of a diy overhead stirrer,
(based on ptfe stirrer gland like shown above)
during my experimenting/prototyping I did notice that the most stable coupling of rotating magnet(s) was when the length of the motorised magnet(s)
(large AlNiCo horseshoe, various configurations of ferrite and NdFeB magnets etc.)
was similar to the length of the stirbar magnet.