What part of the heat break should loosen when changing nozzles?
Just swapped a nozzle recently and something occurred that I hadn't expected. Step 1 of the process is to turn the heat block assembly a bit to loosen things.
I'm curious what is meant to happen - should this loosen the heatbreak inside the heatsink, or should it loosen the heatbreak inside the heat block?
I'd expected the latter since we are changing nozzles; I'd (perhaps wrongly) assumed the step was meant to back the heatbreak away from butting up against the back side of the nozzle and thereby loosen it for easy removal.
But to my surprise, instead the end inside the heatsink was what came undone. Was a bit of a brown-pants moment as I thought I'd broken the heatbreak on a nearly-new printer despite applying very little force. Was even more confused when my new nozzle was tight and the heat block was still wobbly/loose.
Then I got to thinking about it and realized there is a counterpoint - if considering the statement that this first loosening is to protect the heatbreak, I could understand wanting the "back" end of it free-floating so it is not torqued while tightening the nozzle (since you are tightening against the wrench you are using to hold the heat block).
But this is a little more troublesome as the heatsink can have a tendency to slip and spin in the extruder, making it difficult to tighten again without over-stressing the hotend wires. (Some connectors would have been nice there for easier hot-end servicing... I'm paranoid enough already I'm going to break a hot end wire without it being fixed in place to boot...)
Ergo, an inquiring mind wants to know - which train of thought is correct? And has anyone added connectors to their Mk3 hotend for easier service?
Re: What part of the heat break should loosen when changing nozzles?
Adding connectors may seem like a good idea. BUT...
(1) The thermistor circuit depends on detecting very subtle changes in resistance (especially near the typical working temperature) so you would have to ensure a very stable connection with milliohms of resistance.
And (2) the heater circuit carries relatively high current, so here you need both low contact resistance and high current-carrying capability.
And these connections have to be accomplished in a rather constrained physical space which may be subject to somewhat elevated temperatures and will definitely be subjected to some vibration. There's a reason Prusa didn't put connectors there, and it's not just saving money; it's increasing safety and reliability.
Once you have the heat break tube and the nozzle "locked" to each other in the heat block, you can use the assembly to tighten the heat break tube into the heat sink. If it needs more than a small fraction of a turn, you didn't assemble it correctly to begin with; it should be firmly seated in the heat sink and stay that way.
When I change a nozzle, I heat the nozzle up to temperature, grab the heater block with a crescent wrench ("spanner" in British English) (or you could use adjustable pliers), and I use a socket wrench on the nozzle. I don't really expect the heat break tube to turn in the heat sink, so I've never really evaluated whether that happens. I just know that after I've tightened the new nozzle against the heat break tube inside the heat block, the whole assembly is stable, i.e, it doesn't turn. That's very important because the nozzle should never have any "wiggle" and its height must be perfectly consistent for the Z first layer adjustment to work.
Re: What part of the heat break should loosen when changing nozzles?
Once you have the heat break tube and the nozzle "locked" to each other in the heat block, you can use the assembly to tighten the heat break tube into the heat sink. If it needs more than a small fraction of a turn, you didn't assemble it correctly to begin with; it should be firmly seated in the heat sink and stay that way.
Fair point on the connectors, I'd expected that was the case.
The instructions in the Mk3 handbook and on the website say to loosen the heatbreak first before attempting to remove the nozzle.
https://help.prusa3d.com/article/fJa7GxLSG6-changing-or-replacing-nozzle
Re: What part of the heat break should loosen when changing nozzles?
Umm... I didn't see that in the instructions you linked. Steps 1 - 5 get you good access to the heater block, and Step 6 is identical to what I said.
Re: What part of the heat break should loosen when changing nozzles?
My bad, wrong link. Page 67-8 of the Print manual, step 5.
https://prusa3d.com/downloads/manual/prusa3d_manual_mk3_en_3_04.pdf
Re: What part of the heat break should loosen when changing nozzles?
Hmm... That's a change from the printed version of the manual that came with my printer. I'm not really sure I understand the concept. When you first screw the heat break tube into the heat sink, you bottom it out but you have no way to apply a great deal of torque.
However, when you tighten the nozzle into the heater block to bottom out against the other end of the heat break tube, you should apply significant torque to guarantee a good seal against plastic leakage. That's because in this zone there is going to be some residual melted plastic that you have to squeeze out of the gap between the nozzle and the heat break tube.
So at the end of the installation process, you have the heat break tube firmly butted against the nozzle, but not as firmly bottomed out against the end of the threads in the heat sink.
Therefore, if you follow the instructions in the new Prusa manual and start by unscrewing the heat block, you will definitely be loosening the heat break tube from the heat sink first as that's where the friction is least.
Maybe the concern is that the user might apply too much counter-torque on the heater block before the nozzle breaks free, and thus drive the stainless steel heat break tube up into the aluminum heatsink and possibly damaging the aluminum?
Re: What part of the heat break should loosen when changing nozzles?
Yes, hence my trying to understand the reason. My printed copy also reflects the online instructions so it's a recent addition as opposed to a removal.
Re: What part of the heat break should loosen when changing nozzles?
If it helps, my nozzle change notes:
* 16 mm spanner, or wrench for the heater block.
* 7 mm socket for the nozzle (on short extension/shank for easy use).
## Changing nozzle
* Heat to 285 C. Avoid getting burnt!
* Move the Z axis up to allow room to work.
* Hold the heater block steady with a spanner, or wrench, do not allow the block to move (best), or:
* Only twist block a very small amount to avoid creep on the heat break thread.
* Use the socket with extension to remove the nozzle.
* **Clean nozzles during and after changing!** The interface between the nozzle and the heat break must be flat and clean!
* Tighten the nozzle full finger tight using the socket and extension.
* After tightening the nozzle: **check that there is still space between the top of the nozzle and the bottom of the heater block.**
I was following the instructions in the Prusa video about changing nozzles but I believe that the the heat break thread migrated in the heater block because of repeated rotations (or the hot end was not assembled correctly in the first place), or not? What happened next is was a plastic leak inside the block and eventually a big mess. I had to clean and rebuild the hot end. I now keep the heater block steady when I change nozzles. I understand that the top of the nozzle tightens against the bottom of the heat break inside the heater block to seal the filament path and prevent leaks.
Cheers
Neil
TANSTAAFL