IANAE (I am not an engineer), but I've been wondering: wouldn't it make more sense to limit the length of the melt-zone and hot-end as much as possible? I've been having a lot of problems with nylon causing jams inside the heat-block, this being the region where the filament is no longer rigid, but becomes squishy then liquid as it is heated.
In my extruder (E3d v6) there is a cold section, then a heat-break then about 20mm of hot-end.
If the heating of the filament took place at the last possible moment, say the last couple of mm before the orifice, wouldn't this not only alleviate jamming problems caused by soft filament deforming, but also allow for much more precision with extrusion - less hysteresis due to the reduced volume of semi-liquid material.
I'd imagine that this would work by having a heated nozzle tip, say a nichrome ring around the nozzle orifice, and as the filament hits the inside of the bottom of the nozzle it melts and is forced out the hole.
There is a trade-off between the length of the melt zone and the speed at which you can print. The filament itself is somewhat of an insulator, so as the outside of the filament is heated up by being in contact with the melt zone, the inside stays cold. Therefore, the filament needs a certain amount of time inside the melt zone for the inside to fully melt. If you have a shorter melt zone, you need to print more slowly to give the filament enough time to melt. This is precisely the reason why the E3D volcano exists. It has a longer melt zone, so you can print more quickly.
You can partially compensate for a shorter melt zone by heating the nozzle up more, but there is an upper limit to how hot you can go (you don't want the outside of the filament to burn before the inside is melted).