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:
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: