# Why don't 3D printer heads use ceramic inner walls?

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

However, as Paulster2 states in his answer, there are some technical issues with using it, which make it rather problematic. 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.

From J-head with ceramic body instead of PEEK, specifically this post:

Just to be clear, it's Ceramic Zirconium.

My concern was that Zirconium becomes brittle when it is exposed to heat for consecutive long periods of time. I would stay with PEEK.

MgO or Yttria stabilized grades of Zirconium are very stable.

Pure ZrO2 is known to crack, so additives are used to stabilize it.

Key Properties of Zirconium Oxide

• Use temperatures up to 2400°C
• High density
• Low thermal conductivity (20% that of alumina)
• Chemical inertness
• Resistance to molten metals
• Ionic electrical conduction
• Wear resistance
• High fracture toughness
• High hardness

Typical Uses of ZrO2

• Precision ball valve balls and seats
• High density ball and pebble mill grinding media
• Rollers and guides for metal tube forming
• Thread and wire guides
• Hot metal extrusion dies
• Deep well down-hole valves and seats -Powder compacting dies
• Marine pump seals and shaft guides
• Oxygen sensors
• High temperature induction furnace susceptors
• Fuel cell membranes
• Electric furnace heaters over 2000°C in oxidizing atmospheres

Zirconium oxide

Zirconium oxide is used due to its polymorphism. It exists in three phases: monoclinic, tetragonal, and cubic. Cooling to the monoclinic phase after sintering causes a large volume change, which often causes stress fractures in pure zirconia. Additives such as magnesium, calcium and yttrium are utilized in the manufacture of the knife material to stabilize the high-temperature phases and minimize this volume change. The highest strength and toughness is produced by the addition of 3 mol% yttrium oxide yielding partially stabilized zirconia. This material consists of a mixture of tetragonal and cubic phases with a bending strength of nearly 1200 MPa. Small cracks allow phase transformations to occur, which essentially close the cracks and prevent catastrophic failure, resulting in a relatively tough ceramic material, sometimes known as TTZ (transformation toughened zirconia).

Zirconium dioxide is one of the most studied ceramic materials. Pure ZrO2 has a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at increasing temperatures. The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces very large stresses, and will cause pure ZrO2 to crack upon cooling from high temperatures. Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases: magnesium oxide (MgO), yttrium oxide, (Y2O3), calcium oxide (CaO), and cerium(III) oxide (Ce2O3), amongst others.

In the late 1980s, ceramic engineers learned to stabilize the tetragonal form at room temperature by adding small amounts (3–8 mass%) of calcium and later yttrium or cerium. Although stabilized at room temperature, the tetragonal form is “metastable,” meaning that trapped energy exists within the material to drive it back to the monoclinic state. The highly localized stress ahead of a propagating crack is sufficient to trigger grains of ceramic to transform in the vicinity of that crack tip. In this case, the 4.4% volume increase becomes beneficial, essentially squeezing the crack closed (i.e., transformation decreases the local stress intensity).

and the following post

Thermal conductivity:

• Diamond thermal conductivity: 1000 W/(m·K).
• Copper thermal conductivity: 385 to 401 W/(m·K).
• Aluminum: 205 W/(m·K).
• Stainless steel 16 W/(m·K).

• Granite: 1.7 to 4 W/(m·K).

• Zirconia has a typical thermal conductivity of 1.7 to 2.2 W/(m·K).
• Porcelain has a typical thermal conductivity of 1.5 to 5 W/(m·K).
• Glass thermal conductivity: 1.05 W/(m·K).

### Rulon

As an aside, again from J-head with ceramic body instead of PEEK, specifically this post:

Rulon was one material we used. I think it is a glass filled ptfe. The mechanical strength is far better than solid ptfe and it is easy to machine. There are many grades but Rulon AR for example will withstand 288 deg C.

but there are inconsistencies in quality

Rulon i looked at a while ago, there are plenty of options with it, however the cost of some of these materials can be incredibly high, and in some cases availability is a serious problem, and the difference country to country is borderline criminal in some cases

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.