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I doubt this will work with faster printing materials. 40mm/s is suuuuper slow compared to speeds that most modern printers are spitting filament with (not necessarily TPU). Unless you have a coil or something inside the tube heater so it spends longer time in there, but that probably also introduces a lot of resistance so moving the material through is notably harder.
You "just" need a longer distance inside the drying chamber. This could be achieved by coiling up the space where the filament travels through and guide hot and dry air through that space, ideally from the outlet towards the inlet. That air could maybe be pulled from the hotend cooler.
This could be achieved by coiling up the space where the filament travels through
Yes, that's also what I already mentioned in the other comment...
"Unless you have a coil or something inside the tube heater so it spends longer time in there"
but as I also mentioned, pulling filamennt through a coil will also introduce significantly more resistance than pulling (or pushing if using bowden) it straight, which might be an issue at high speeds and cause under extrusion.
Sorry, I completely didn't read all of your comment. You're right about resistance but then again the filament won't need to touch the enclosing coil at a large surface. In the usual bowden tubes, you have a lot of contact surface between tube and filament but this would not need to be the case in the drying coil. In the end it would all depend on the application. I'm not interested in very high speed printing (yet) because my machines are all pretty slow :).
Wonder if you could use some sort of buffer system to extend the time in the dryer
What about a pre-extruder or a set (or pultruder?) that brings it to just below the glass transition temp, but still at the original filament size? Water boils off, plastic is left!