Xjyuk

Why don't 3D printer heads use ceramic inner walls? PTFE tubes melt with high enough temperatures and all metal ends risk jamming as heat makes its way up the head.

It can be done cheaply, as two different users have proven. See A practical 10 Cents Ceramic tube hotend and Hotend with ceramic parts.

Apparently, in comparison with PTFE, the thermal conductivity of the ceramic in spark plugs is too high, to use (according to nophead - a user on the reprap forums), and there are friction/clogging issues, unless the inner diameter is very well polished.

Synopsis of reference

The RepRap user, hp_, encountered the issues above when attempting a design - from Ceramic Hotend - Part 1

Research

As far as I know there are no ceramic hotends out there, I know
nophead has tried some spark-plugs for nozzle holders but found them
not suitable(thermal conductivity is pretty high). I wanted to give it
a go, confident enough (I hoped), that it would work :)

So in my case, a hotend exists out of 2 main parts, a nozzle holder
and a nozzle.

• The nozzle is the easy part it would stay brass.




• The nozzle holder is the interesting part, here is what I've come-up with

total length should be in the range of 35-40mm, see my first sketch
below:

here are many types of ceramic out there, ie. 95% AI2O3, 99% AI2O3,
Zirconia (see material properties sheet Link)

95% AI2O3 is easy to buy but after a few tests the conclusion was its
to brittle for my taste, second material to try is Zirconia.

I've found a few Chinese ceramic manufactures. Only draw back I had to
order 10 pieces for the first batch.. on something that has never been
tested, well I'd give it a shot.... and ordered the parts.

but the clogging issue mentioned above was encountered:

...after the first layer, it just stopped extruding.. ugh!!! what could
be wrong????

Possible root causes
- Friction coefficient? Meaning after awhile the friction between PLA and the Ceramic became so high it would just jam the nozzle holder.

• Stickiness? Could it be that after awhile PLA would just stick to the Ceramic and would jam because of this?




• PLA thermal expansion( nozzle holder barrel is to small?) so the inner diameter of this nozzle holder is 3.2mm, could it be that the
  3.0mm filament would expand so much because of the heat, that it would start to jam the nozzle holder?




• Connection between nozzle and nozzle holder is insufficient cause the Jam??

The user was forced to return to using PTFE.

From Ceramic hotend part-2, after some rework done by the Chinese manufacturer, the new hotends worked correctly:

Awhile ago i stared working on the ceramic hotend and found out the
first version wouldn't work for 3.0mm fillament,

after some discussion with my chinese counterpart :) i got a new
version of the ceramic piece.

They polished the inside very deep and precise. and i gave it another
go.

and

some more tinkering with the hotend and a new nozzle design, with a smaller Inner diameter, and its longer

Apart from that the details are a little sparse.

Because PTFE doesn't transmit heat very well? The whole idea when using a PTFE tube (and this is just my understanding ... which could be wrong), is for the tubing not to transmit heat, therefore allowing the filament to pass through it without melting or at the very least, collecting a lot of heat along the way (which helps prevent jams). PTFE does a pretty good job of standing up to heat while accomplishing the task at hand. Ceramic does an excellent job of standing up to heat. The problem is, it will pass the heat along to the filament, most likely melting it, thus causing it to deform and jam before it gets to the hot end. This would then become a maintenance nightmare.