The hotend is the most important part of your printer, but something that a lot of new users are afraid to tinker with. In reality, it's just a piece of metal that gets hot and it's really not that complicated as it seems. I'll show you how simple the design is, how you can take it apart if you have to clean it and most importantly, how to change the nozzle the correct way.
There are four main components; the nozzle, the heatblock, the heatbreak and the heatsink.
You also need the heater cartridge that actually heats up everything, and a thermistor that measures the temperature.
I'm showing an E3D hotend as an example, but all of them have the same basic components.
When the cartridge heats up, the heat transfers to the nozzle via the heatblock and that's how the filament melts.
Technically, that's all you need. However, life is never so simple and there is indeed a problem with this approach.
When the plastic melts, it also expands. As it has nowhere else to go, some of it will travel upwards. And because the heatbreak is touching the heatblock, it will heat up as well. This is more commonly known as heat creep.
All of this results in the filament melting higher up than it should. But because the upper part of the hotend is not as hot, the plastic there will solidify and you have no way of melting it again. This will eventually block the new filament coming in and will cause a jam, which can then only be cleaned by taking the hotend apart.
The main job of the heatsink is to prevent this from happening by cooling down the heatbreak.
It has a large surface area which transfers the heat away faster. But even that is not enough, so most hotends have a dedicated heatsink fan as well.
By cooling the area above the heatblock, the filament will only melt near the nozzle, which is what we want.
This also explains why using a larger retraction setting will jam your hotend. As the filament retracts, the tip of it is still melted. This will gradually build up into an obstruction, but because it's so high up, you can't melt it again to remove it, so you've just won another fun day of taking the hotend apart.
Speaking of jams, buy a few of these 0,4mm needles if you ever have to clean it. The rear end works great for cleaning the heatbreak and top of the nozzle.
Now that you know how the hotend works, let's take a look at how to put it together. It's hard to show how it works with the actual one, so I've printed out a large model to help you visualize what's going on inside.
We have the nozzle and the heatblock as before, but you can see the heatbreak better now. The blue part is the heatsink. Most printers will also have a PTFE tube that guides the filament at the top of the heatsink.
Assembling the heatbreak is very easy, just screw it in as far as it will go. You don't have to use a lot of force to tighten it.
You'd think that you can continue by simply attaching the heatblock, but this is where a lot of problems come from.
If the heatbreak isn't low enough, your nozzle will hit the block before reaching the heatbreak. This will leave a gap between them and your hotend will start oozing out plastic.
Sooner or later, it will either jam your hotend or, much more likely, the plastic will start oozing out from above the heatblock. Good luck trying to clean that up.
The correct way is to start with the nozzle first. Screw it in by hand until there's gap of about half a mm.
Now screw in the heatbreak as well until you hit the nozzle. You only have to tighten it slightly for now.
You can mount the hotend to the printer now if you want, but it's a bit harder to tighten it that way.
Next, connect the cartridge and thermistor and heat it up to the maximum. Never tighten the nozzle if the hotend is cold. When it reaches the final temperature, tighten the nozzle, but be careful not to use too much force. If you're using a torque wrench, set it below 1Nm. For most nozzles, you'll need a 7mm hex socket.
You can also print a torque wrench yourself and I was surprised at how good it actually works. Printed in PLA, it should provide about 0.7Nm of torque.
You can now cool down your printer. Again, ensure that you can see a small gap between the nozzle and the heatblock. The top of the nozzle should never touch the heatblock.
And that's about it. Check for any leaks after a few hours of printing, but you really shouldn't have any problems if you've followed these steps. Don't hesitate to get in touch if you have any questions or issues with your hotend.