After a brownout a print failed and it got the whole hotend covered in PLA. I am now in the process of replacing some parts on the hotend (one of the thermistor legs broke of) and also wanted to take the heater cartridge out. The problem is that the bolt that locks the heater cartridge is stuck and I am now afraid to strip the head.

Is there a trick to remove the bolt with the smallest probability of stripping the head? Is it better to apply heat or not? Should I soak it in Acetone or some other solvent? etc.

Don't Panic!

First of all, a printhead caked in PLA is usually not a death sentence, it is often a temporarily setback. Let's start in steps!

Step 1: remove the Extruder

We want to work on the hotend, so we remove the extruder feed first. For direct drive, we unload it, then remove it depending on your printer, so the cooling of the hotend remains. For a Bowden, unload and remove the Bowden tube,

Step 2: remove from carriage

Now that we have the hotend bared, we remove it from the carriage. Usually it is 2 to 3 bolts.

Step 3: remove the cool-end

If you can, remove the cooling fins - we want to have the heatbreak to hold on to.

Step 4: Clamp it up

Take a fire-proof surface (ceramic tile!) and put down the hotend. Rig it up that it can't jump away, best with a small vise. Don't bend the cables!

Step 5: Free the thermosensor

Let's start to clean! If your cartridge still works, get 12V onto the heater and let the stuff melt a little. Use pliers and a scraping tool to clean the thermistor cartridge.

As you have no heat control only heat in short bursts to prevent fire and destroying the cartridge too.

As an alternative and if you can't get the cartridge to work, use a hot air gun or a hot-air soldering station. A soldering iron with a broad tip also works well to scrape off the plastic with controlled heat.
If you use an external heat source, free the heater cartridge first and remove it, hoping that it is not also dead.

As soon as you can, get the thermosensor out. If you run on the heater cartridge, install a fresh Thermosensor, even into the goopy heater block, and wire it to the board to regain temperature control.

Step 6: Finish cleaning

I usually clean up the final stretch under board controlled heat:
Make sure that heater cartridge and thermosensor are installed well and working. Set the hotend 170 to 180°C and do the last cleaning under the use of regulated temperatre.

Step 7: Do steps 1 to 4 in reverse order.

Reassemble, following the steps backwards.

Step 8: Hot Tighten!

Heat the assembled hotend to 240°C, then tighten the nozzle against the heatbreak to ensure tightness. Let cool, done.

Alternative

Replacement/Upgrade!

One could go down to step 3 and replace the hotend assembly (Heaterblock, Thermosensor, heatbreak, nozzle) with fresh parts and reassemble. This is much more expensive than reusing but cleaner and faster - if you need to print now, you might want to keep one hotend assembly on hand as spare and clean the one replaced while the machine runs.

External heat sources

As mentioned, a hot air gun or soldering iron can provide heat to remove the plastic caking.

The soldering iron has the benefit of doubling as a scraping tool and providing localized heat, allowing to possibly free the thermosensor without unsoldering anything in it, and it won't melt the heater block.

A heat gun provides gentle, overall heating of the beater block, but needs extra care where the hot air is going - it can easily char wood and might remove the solder from the heater cartridge.

A gas torch might be used to burn off any residue on a totally stripped heater block, but it also would be able to melt and deform an aluminium heater block! Do not use it on a still assembled heater block, or it will melt any solder in the heater cartridge and destroy it.

In any case, working on a fireproof surface is mandatory!

Chemically cleaning (for non-PLA)

PLA can be removed chemically, but the solvents are rather nasty and take quite some time to work. Very toxic dichloromethane was used to make a solution of PLA to create thin layers used in this paper, and all other solvents the study mentions are at least equally nasty. So chemically cleaning PLA is not an economically viable option. However, it is a viable option to use acetone if you have your hotend caked in ABS.

Acetone will not dissolve PLA, there are other nasty solvents that do that, but it is not recommended to go that way.

You could apply 12 V (if the cartridge is 12 V that is) directly. As the thermistor does not work, you cannot use the printer board. You should take caution not to heat it too far or too hot, but from your experience with your own machine you should know approximately how long it takes to heat up the hotend to working temperature. E.g. start with a minute and try to pry off the lump of plastic with pliers. If you can reach the bolt/worm screw, try heating until you are able to insert the hex key or screw driver.

This video from Josef Prusa shows exactly what you need to do what is described above. The only difference is that you need to connect the heater element directly to the power supply as your thermistor is broken. So don't leave it on too long or you will bake the filament to carbonization, which is much harder to remove. Try to cycle it on/off.

I have just gone through this with an E3D v6 Aero hot end.

Working on the hot end while hot is the best way, but using the internal heater is filled with several problems:

1. The thermister may be broken


2. The heater may be broken


3. The electronics of the heating loop (sense, decide, control heater) may be broken


4. The wires may be too short to make it easy to work on.

It is best to remove the hot end and work on it on your bench -- not inside the printer.

Someone recommended a torch. This is possible, but it is hard to control the heat with a torch. The flame may be hot enough to melt the metals.

Instead, I use a hot air gun. It gets hot enough, but the maximum temperature and the rate of heating is lower. The heat source is more diffuse.

I had an immoveable thermister set screw, which responded well to heat. When reassembling, the heater clamp screw was filled with some plastic which prevented me from clamping the heater while cold. After heating, it screwed in easily.

Don't be afraid to strip the hot end down to the basic parts. Probably you'll be doing it several times over the printer's life, and knowing how to fits together makes everything easier.

If you use a brass heating block (which is a bit heavier than aluminum), you can first use a heat gun to soften the residues to remove the cartridge/sensor.

After removing all electronics, you can use a gas torch and burn the residues off. However, the far best option would be to use a heat block made of stainless steel. I guess heat conductivity is anyway not very important on the hotend as long as the cartridge is powerful enough.

Using a torch makes cleaning very fast. In five minutes you are typically done.