I’ve been having trouble with TPU stringing that has really reduced the quality of my prints. I’m using dried Bambu TPU 90A on a P1S with the top removed and direct feeding into the extruder via the AMS HT. I have had no issues with stringing when the filament is straight out of the box, but am having problems with filament that’s been unsealed for a while. Identical prints with this older filament have terrible stringing across small gaps with identical print settings, wheras brand new filament has zero issues.
My top theory has to do with filament moisture, and I’ve taken a number of steps to try and dry the thing out.
Multiple sequential rounds (4 so far) of drying using recommended settings in the Bambu AMS HT (18H @ 75C) with spool rotation. I have indicating silica beads in multiple places in the dryer - a brand new pack in the small chamber under the rollers and a container of beads in the spool hole. The room where the dryer is has a stable temperature between 18-20C and stable RH of ~50%. Interestingly, the room temperature filament clocks in at around 13% RH, and when drying the RH drops only to ~8% @ 75C. Using the Arden buck equation based on the room air I’d expect that number to drop to ~3% RH when the chamber is heated. The filament dryer seems to be behaving as expected, with the vent ports clicking open once the dryer reaches temperature.
I’ve also tried printing direct from the AMS HT from the bypass outlet with the filament dryer on. I get similar results with the dryer off and on during printing.
Have I done something stupid here? What else can I test or measure?