Over the years I’ve seen quite a few diagrams and discussions about how hotends are configured and how to obtain good printed results from them. I have also had lots of experience with clogged nozzles, filament leaks, and re-assembling different kinds of hotends. So i thought it would be helpful to make a blog page explaining what I have discovered based on all that experience. I’ll use the following diagram as a visual aid. Note that this image has it’s own problems, but I’ll explain those below. The last section will explain how to assemble a hotend with assurance that it will function properly.

The parts of a hotend

The key point of this image is the fact that there is no gap between the top of the nozzle (yellow) and the bottom of the heat break (green). This is a critical requirement because it is the only way to prevent filament leaks that will find a way out and creep over the heat block and nozzle.

The diagram shows that the PTFE channel is not the same diameter as the outside diameter of the heat break. In real life it should be the same because the PTFE channel is where you insert a PTFE or Capricorn tube to feed filament into the heat break and then into the nozzle. The PTFE or Capricorn tube, just like the nozzle, needs to have no gap between it and the heat break.

If you have a direct drive extruder it will sit right on top of the hotend, so the PTFE or Capricorn tube will be short – just long enough to reach from the top of the heat break to the bottom of the extruder. But if you have a Bowden tube and remote extruder, the PTFE or Capricorn tube will be a lot longer. In either case the tube feeding your filament to the hotend must not have any gap between it and the top of the heat break.

When you buy your tubing you’ll find that some vendors supply a trimming device with the tubing. This is a good idea to get because it ensures a clean, flat, 90 degree cut on the end of the tube, and that’s what you need for a no-gap fit inside the hotend.

2 Types of hotends

There are 2 basic types: the so-called “all metal” hotends, and the ones that aren’t. The key difference between the 2 is whether or not the hotend has a piece of PTFE tubing inside it. The image I used above is for an all metal hotend, which is why the PTFE channel is empty and leads directly to the top of the heat break. A “regular” hotend has a differently shaped heat break, and the heat exchanger is slightly different too.

I couldn’t find a cutaway diagram of a non-all metal hotend, but here is a photo of the parts that make one. These parts are from an FLSun SuperRacer, which is the kind of printer I have.

The PTFE tube is actually Capricorn, which I think is a better option than plain PTFE. The heat exchanger is a stock item but has been modified by FLSun to have 2 tapped holes on the bottom flange that are used to attach the hotend to the end effector base plate, and 2 more tapped holes (all shown on the next photo) that secure the heat break inside the heat exchanger.

Note that the length of the Capricorn tube is “for illustration purposes only.” Obviously it will be much longer if you have a Bowden tube configuration, and a bit shorter if you have a direct drive extruder.

Here’s how it all looks when put together:

This photo shows the 2 retaining screws at the bottom that hold the hotend onto the base plate, and the other 2 that hold the het break inside the heat exchanger.

How to assemble the hotend

The way to ensure you have no gaps is to assemble the nozzle, heat block, and heat break while cold, make sure the nozzle and heat break are tight against each other, then heat up the entire assembly to printing temperature (or a bit above actually), and re-tighten the nozzle against the heat break. The nozzle should make about a 1/8 – 1/4 turn when you do this. This is important to do because, when you heat up the hotend everything expands, so you have to re-tighten the nozzle to eliminate the expansion gap between it and the heat break.

Note that the flat top of the nozzle should not be touching the bottom of the heat block because you don’t want the heat block to stop the top of the nozzle threads from contacting the bottom of the heat break. Also, note that the bottom flat part of the heat break also does not touch the top of the heat block. This is because you want the minimum amount of contact between the 2 to minimize heat transfer from the heat block to the heat break. (It’s called a heat break for a reason.) The idea is to keep as much heat as possible in the so-called melt zone, which is the space inside the nozzle and the bottom area of the heat block.

Finally, you have to make sure the PTFE tube makes tight contact with either the top of the nozzle (for all metal hotends) or the heat break tube. For FLSun printers (and maybe others) the heat break has a stopping point inside the heat break for the PTFE tube to contact. For other types the csontact point will be the top of the heat break itself. Regardless if which type you have be sure to use enough force when you push the PTFE into the assembled hotend. You should feel a hard stop when the 2 come in contact.

12 Jan 2022