Questions about hot end design

Hello guys.

I'm trying to choose the most suitable hot end for my prusa, and while reading hot ends page I've got some questions.

Why PTFE is so widely used in almost any hot end design? Most hot ends using PTFE as termal insulator, and almost ALL using PTFE inner sleeve. PTFE emits toxic chemicals when heated above ~200 celsius degrees, so I would prefer not to use such material in hot end. Is there any tested hot end design not using PTFE?

Why everyone use brass for heating block? Aluminium has more than twice higher thermal conductivity and about three times less weight, so why not to use it?

And the last question
Hot end from this picture looks very promising, are there any information about it?

I'll appreciate any help.

Edited 1 time(s). Last edit at 09/15/2012 11:04AM by alik.

PTFE is used because it is slippery and a good insulator and it needs to be a lot hotter than 200C before it emits fumes.

There are all metal hot ends that don't use PTFE.

Machinists like brass because it is easier to machine than aluminium, but aluminium works better and is cheaper. There are some hot ends that use aluminium.

The Prusa one isn't released yet. I am interested to see how he gets away with what appears to be a long transition zone. That tends to make it hard to extrude.

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nophead, thanks for the answers.

I've read some articles about PTFE (like this) claiming that it may be toxic even at pretty low temperatures, so this makes me worry.

I've also checked a couple of all metal hot ends (like Arcol hot end) but they are using PTFE tube anyway.

Maybe you could suggest best, in your opinion, hot end?

Well I have run PTFE hot ends at 250C for years and it hasn't killed me yet. I also have used Teflon pans and overheated them and I have overheated PTFE hot ends without suffering any symptoms. I don't have any birds or other animals so I don't know if it poisonous to them.

It is possible to make an all metal hot end without a PTFE liner. It needs to have a short transition zone with a taper and a heatsink with a fan.

I don't know which is the best hot end as I haven't used many commercial ones. I use currently use a mixture of J-heads and my own PEEK / PTFE hybrid design, which I haven't published yet but you can see one here: [funwithelectrons.blogspot.co.uk].

I don't like ones that use a thin walled PTFE pipe as a liner. They fail eventually in my experience.

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[www.hydraraptor.blogspot.com]

I am currently experimenting with an all-metal hot end from E3D. I have run some Polycarbonate through it at up to 300*C and it works great for that. I don't have any 1.75 mm ABS or PLA at the moment though, so I haven't tried it with those.

It is nice because there is no PTFE whatsoever, which means it can be run quite hot. Also, the fan/heat sink means that the top of the hot end doesn't get too hot and keeps the transition zone very short.

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> I don't like ones that use a thin walled PTFE pipe as a liner.
But J-heads use PTFE tube as a liner, or it's not this case?

NewPerfection, I saw this design, and actually like it. It's only seems that the nozzle is not easily exchangeable. And also it's pretty expensive.

To bad they are not selling hot ends for 3mm filament yet

The PTFE liner in the J-head is 8mm in diameter so it seems to hold up. The same in my simplified version but I screw the brass into the PTFE to keep it from being compressed upwards by the pressure.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> The PTFE liner in the J-head is 8mm in diameter so
> it seems to hold up. The same in my simplified
> version but I screw the brass into the PTFE to
> keep it from being compressed upwards by the
> pressure.

Its actually 1/4" x 1/8" in the J-head and is 6.35mm o.d. which is still heavy wall tubing. The standard is 3/16" x 1/8" and is only 4.76mm o.d.

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alik Wrote:
-------------------------------------------------------
> NewPerfection, I saw this design, and actually
> like it. It's only seems that the nozzle is not
> easily exchangeable. And also it's pretty
> expensive.
>
> To bad they are not selling hot ends for 3mm
> filament yet

It is a bit expensive at the moment, simply because this is their first run of the hot end. I actually got the very first finalized version of it. You are right that the nozzle isn't easy to change out. Replacing the brass nozzle insert with a short threaded brass tube and nozzle would probably work well though.

I do wish they had 3 mm hot ends, they did say that 3 mm ones would be available soon.

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nophead, I see, so the problem is that thin walled PTFE liner may be compressed and deformed by filament.

NewPerfection, I've not found information about nozzle sizes available for this all metal hot end, is it 0.5?

The end of the liner presses against the brass to form a seal in a lot of designs. The pressure at that point is quite high so what happens is the liner gets compressed length ways and comes away from the brass forming a small gap that fill with plastic. That jams the extruder. Thicker walled liners seem able to withstand the pressure and maintain the seal.

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nophead, I see, thanks for the explanations.

I have two more questions, if you guys don't mind

Which nozzle size is the best for most reliable and accurate prints? Comparing 0.3mm and 0.5mm I see following pros and cons:
0.3mm
+ Thiner extruded filament - more accurate prints with higher resolution.
- Need more force to extrude.
- Slower printing (I thing approximately should be about 2 times slower)

0.5mm
+ Easier to extrude
+ Faster printing
- Less accurate and detailed prints

So, in general I think 0.3mm is more preferrable, but maybe I'm missing something?

And second question (not directly related to the hot end): is it worth to install cooling fan blowing towards print bed? Is it affects print quality (i.e. print cools quicly and not deformed during printing)?

Thanks in advance!

Depending on your limiting factor - a smaller nozzle can actually print faster (linear speed) because you're melting/extruding less plastic per distance. As long as you keep the length of the tiny orifice very short (<2mm) you shouldn't have any speed issues with a 0.3mm nozzle.

I was very happy with a 0.45mm nozzle because it extruded very smooth and it was very easy to go fast. For larger structural parts it is probably a more appropriate size. Keep in mind too that just because the nozzle is 0.45mm wide doesn't mean you can't position the edge of the extrusion with 0.1mm or better accuracy. As long as your features are relatively large, there's no significant gain in smaller nozzles.

That said - now that I have a 0.35mm nozzle working well with a very short orifice length, I think 0.35 is an excellent size with good compromise between decent print resolution and reasonable speed. Anything smaller would be both very difficult to machine (0.35 is already pretty damn tricky to machine) and would provide diminishing returns on accuracy.

JazzyMT, thanks for the answer. I was just thinking about 0.35 as a perfect nozzle size, and you've confirmed my thoughts.

nophead Wrote:
-------------------------------------------------------
> It is possible to make an all metal hot end
> without a PTFE liner. It needs to have a short
> transition zone with a taper and a heatsink with a
> fan.

Hello nophead,

could you tell something about that taper? If I got you right you made the experience that soft PLA flows better through the zone between the cooler and the heater if the inner diameter decreases there? Or has it to increase towards the nozzle?

How long was your transition zone?

Regards

Detlef

It gets bigger towards the nozzle and acts like a mould draft angle. Details here: [hydraraptor.blogspot.co.uk]

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[www.hydraraptor.blogspot.com]

I see you used a 1:10 reamer. Would a 1:50 reamer do as well?

Both reamers would smoothen the barrel what might have the effect of helping the soft PLA through the transition zone. Hence it is not the taper alone which improves the passage.