In-Line Countercurrent Filament Drier

I recently needed to 3D print something out of PETG. When left exposed to the atmosphere, PETG absorbs water from humidity in the air. If this is used, prints can look ugly or fail completely. The solution is to dry the filament, for this I have a filament drier. Its essentially a heated box and for PETG the protocol is 4-6 hrs at 60°C. This is annoying.

My 3D printer has something called an AMS <link>, essentially a box on top that holds 4 rolls of filament. These can be any combination of different filament colors/types and the device will load/unload the desired filament automatically, so you can change filaments between or even within print projects. It's very good. Except I can't leave PETG in it for very long, I have to manually unload it, put it in the drier for 4-6hrs, then manually re-load it, a very poor use of my time.

What's needed is a way of drying the air in the AMS. There are slots for desiccant, but they're tiny, rapidly saturated and irritating to change. Heating the entire AMS interior would lower the relative humidity, but a good drying temp for one filament is going to soften another, Plus I don't know if the motors gears etc. are compatible with >60°C. What's needed is a recirculating system to dehumidify the air. This could be through cooling and condensing out the water, or pumping air through a substantial volume of desiccant. I'll probably do this myself, an aquarium air pump, 5 gallon bucket 1/2 full of desiccant and some glue/tubing/fittings. Crude, but it will work.

This leaves both a problem and opportunity. Between the AMS and the nozzle the 1.75mm diameter filament feeds through 3-4'/1-1.25m of 2mm internal diameter PTFE tubing. Adding a T-piece above the extruder, dry air could be fed in, travelling back along the PTFE tubing in the small gap between filament and PTFE guide tubing, ultimately ending up back in the AMS. This would mean the driest possible air is in contact with the filament at the most important point; just before it enters the extruder. Turning the filament feed system into a countercurrent dryer. This is an efficient set up that's drying the filament we care about and not the stuff in the middle of the roll that won't get used for months.