<p>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.</p>

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.