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RepRap 101: Using silicate-based putty to install a thermistor in a hotend.

It may seem obvious; a hotend needs two things to regulate temperature: a heat source to provide the heat, and a temperature sensor to provide feedback as to whether the hotend is too hot, too cool, or in the Goldilocks zone, “Just Right”.

The most commonly used temperature sensor is the thermistor, which is a resistor who’s resistance value changes with temperature.

There are a couple of commonly used methods for installing thermistors. One is to simply insert the glass bead of the thermistor into its pocket on the hotend and hold it in place with a strip of Kapton tape. This works well as long as the glass bead is sized so that it is in contact with most of the circumference of the pocket. If it isn’t, then much of the bead is surrounded by air, which does not have particularly good thermal transfer properties. This means that the thermistor’s reading will not be as accurate as it could be.

Another approach, which I like enough that I have adopted it is as a standard practice, is to pack the thermistor pocket with a silicate-based putty, and insert the thermistor into the paste, which will transfer heat from the hot end more efficiently, and give a more accurate temperature reading.

So, how does one go about acquiring silicate-based putty? In the US, it’s readily available in just about any retail automotive section as “Exhaust” or “Muffler” repair putty. Carefully check the labelling to ensure that it is indeed based around sodium silicate, and you should be fine. This is what I found at the store down the street:

Permatex Muffler and Tailpipe Sealer

This material is a grey putty about the consistency of toothpaste.  The best method I have found for applying it is to use a plunger type syringe.  I load a couple of CCs of putty, and using a 1.5mm dispensing tip, fill the pocket.  I then apply a little more putty to the thermistor tip, then place it into the pocket.  If the thermistor pocket is a through-hole, then I will try to place the thermistor bead into the middle of it.  I then wipe off any excess, and set it aside to cure.  I generally let it sit for at least a couple of hours, but that’s probably more than it needs.

The final step before printing is to “bake” the hotend.  I do this by installing it into the printer, then setting it to about 150 degrees, for somewhere between 15 minutes and an hour.  After that, you’re ready to go!

Finally, if you ever need to remove the thermistor, the putty can easily be chipped away, although the super-fine leads on the thermistor might not survive.

Here’s the process in action:

If you’re having trouble finding them, here are the syringes and dispensing tips that I use:

20mm luer lok syringes

14 guage, 1.5mm dispensing tips

11 comments to RepRap 101: Using silicate-based putty to install a thermistor in a hotend.

  • AbuMaia

    I was starting to plan to get this putty stuff and do this mod, up until I read this bit: “if you ever need to remove the thermistor, the putty can easily be chipped away”. I don’t think I want to do anything that permanent to such an essential part that would make replacement more difficult.

    • Ummm, okay, I’m not quite sure why you felt the need to tell me that, but feel free not to do it, I certainly won’t be mad. Also, there are a number of hotends on the market that are shipped with both the thermistor and the heater cemented in place (Prusa Nozzle, for example), so be careful if you ever upgrade.

      • Mortorojo

        This mod is great idea, just in relation to AbuMaia statement I would be concerned if ever needing to remove the thermistor. Do you know of a possible chemical bath that might break the bond that cements that putty, or a potential different bonding method that has a relatively easy method of removal if was ever necessary.

  • David

    Hey Michael,

    Was wondering if you were still planning on upgrading your makerfarm to a dual extruder yet? I’ve been looking into some options. Was thinking about using two bowden x carriage adapters and feeding the filament from two wade extruders. Not sure if the bowden method is as good though… any thoughts?

    • I’m actually looking into converting a single-sheet Prusa i3 to dual extrusion, and I was actually thinking that the dual Bowden would be the way to go. There are plusses and minuses with any method, but I’m really liking the idea of getting a lot of weight off of the X-carriage! The next obstacle that I’m working on is finding or figuring a solid method to adjust the height of each hotend separately. I’ll certainly be posting as I come up with things!

  • Bruce

    Did you use this method on the E3D? The E3D is such a nice looking hotend, but when you load it up with kapton tape to hold the thermistor, it looks kind of crappy.

    • I did, but with the E3Dv5, there’s no real way to get completely away from Kapton: Since the v5 doesn’t use PTFE sleeves to insulate the thermistor leads, it wraps them in Kapton. It’s possible to keep it pretty neat and clean with a little effort, though, so it’s not that bad. I also used a strip of 5mm Kapton across the top of the block as a stress relief for the leads (that’s important!).

      • Bruce

        Yes, I agree, some Kapton is necessary with the E3Dv5. I just installed mine tonight. I’ve had the hotend since before I received my printer kit around 3 months ago, but have just been lazy to get it put on, plus I had to print a new extruder body and parts. I had assembled the hotend before I saw this article, though, so I used the Kapton tape method. Got it all installed tonight and was ready to start testing, when I noticed the Kapton that was holding the thermistor came loose. I had already purchased the exhaust repair paste from Napa Auto Parts (USA) previously. Mine is called VersaChem exhaust system joint and crack sealer, but it is sodium silicate based. So, I’m about to try this method as we speak. Should be much better. Of course now the E3Dv6 is out and they’ve solved this problem in a totally different way. Arrgh.

        Thanks for the great info!

  • […] found the traditional Kapton retention method to be awfully messy and subject to failure. The putty method is a little better, but the screw is the best of all […]

  • Rastko Miocinovic

    I am in need of a new hot end and settled on an aluhotend v7, any experience with it, and should I go Bowden or direct?

    • I’m a big fan of all the Jolly Grim Reaper’s designs, and I’m looking forward to checking out the v7. As far as Bowden vs. Direct, I’d say do direct if you can. If you’re using a printer with a moving bed design (pretty much any of the Mendels), then a Bowden setup will do very little for you, and cause a great deal of fuss and frustration. On the other hand, of you’re using a printer where all the motion is in the carriage (IE: a delta or ultimaker “box-bot” style), then Bowden is definitely the way to go.

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