Fix Filament Grinding on Dual-Gear Extruders With Soft 85A TPU
Filament grinding on dual-gear extruders is probably the single most annoying thing about printing ultra-soft 85A TPU. You load the spool, hit print, and ten minutes later you’re staring at a pile of shredded filament bits instead of a part. The root cause is almost always the same: a mismatch between how hard your extruder’s gears are squeezing, how soft the filament actually is, and how convoluted the path is between spool and nozzle. Let’s walk through why this happens and how to actually fix it.
Why 85A TPU Grinds So Easily
PLA and PETG are rigid enough to take a beating. 85A TPU is not. It’s soft enough that the clamping force needed to push it through a hotend can literally squash and deform the filament before it ever gets fed. Dual-gear extruders, the kind you’ll find on a Bambu Lab X1C, a Prusa MK4, or a Creality K1, grip from both sides at once. Great for stiff materials. With 85A TPU, that same squeeze often means the teeth just dig in and chew rather than grab and push.
Telltale signs:
- Gouges or shredded shavings on the filament right where it meets the extruder
- Under-extrusion, or flow that randomly drops mid-print
- A grinding or clicking noise coming from the extruder motor
- Filament jamming right above the gears, never making it to the hotend
Step 1: Reduce Idler Tension
Nine times out of ten, grinding starts with the gears clamped too hard. Most extruders ship pre-tuned for PLA or PETG stiffness, and that setting is way too aggressive for something as soft as 85A TPU.
- Loosen the tensioner screw until there’s just enough bite to move the filament without it slipping.
- Try the pinch test: you should be able to slightly flatten the filament between the gears, not crush it flat.
- If your extruder has adjustable spring tension (Sprite Pro, LGX Lite, that sort of thing), start around 30-40% of whatever you use for PLA, and only dial it up if you see slipping.
Step 2: Shorten the Filament Path
Soft TPU buckles inside long PTFE tubes or sloppy bowden runs. That buckling creates back-pressure, and back-pressure is what makes the gears start grinding instead of feeding.
- Direct drive whenever you can. Bowden setups massively raise the odds of buckling with 85A TPU.
- Stuck with bowden? Keep the tube as short and rigid as possible, and make sure it’s fully seated, no gaps at either end.
- Check the run from spool to extruder entry for kinks or tight bends. Even a small one can be enough.
Step 3: Dial In Temperature and Speed
If the hotend can’t melt filament fast enough to keep pace with the feed rate, you get resistance the gears can’t push through without tearing up the material.
- Print 85A TPU somewhere in the 220-235°C range, tuning based on the specific brand you’re using.
- Slow down to 20-35 mm/s. Soft TPU needs time to actually melt and flow, it won’t cooperate at PLA speeds.
- Bump retraction distance up slightly (1-2mm) and slow the retraction speed down. This avoids sudden tension spikes during travel moves that trigger grinding.
Step 4: Check Gear Type and Condition
The sharp, aggressive teeth on standard dual-gears built for rigid filament will shred a soft TPU surface in seconds.
- If your extruder supports it, swap in gears with a wider tooth profile, or rubber-coated gears made for flexibles.
- Check the teeth for built-up filament debris. That gunk kills grip and causes slippage, which leads straight back to grinding.
- Run a brass brush over the gear teeth regularly, especially when you switch between rigid and flexible materials.
Step 5: Improve Filament Feed Alignment
If the spool, guide tube, and extruder inlet aren’t lined up, you get side-load stress on the filament, and soft TPU has basically zero resistance to that kind of stress.
- Make sure the spool rotates freely, no drag, no snagging.
- Run a guide tube that feeds straight into the extruder inlet.
- Avoid sharp 90-degree bends right before the gears. Even one tight turn can be the difference between smooth feeding and a jam.
Quick Troubleshooting Checklist
- ✅ Idler tension backed off to a light grip only
- ✅ Bowden tube shortened or removed entirely
- ✅ Temperature increased, speed decreased
- ✅ Gear profile inspected and upgraded if needed
- ✅ Filament path checked for straight, low-friction routing
Conclusion
Printing ultra-soft 85A TPU on dual-gear extruders comes down to a balancing act between mechanical pressure and heat. Too much clamping force, or not enough melt, and the gears will shred the filament long before it reaches the nozzle. Back off the tension, shorten the path, get your temperature and speed right, and fix the alignment. Do those four things and most grinding problems vanish, leaving you free to print flexible parts that actually come out looking the way you intended.