Why does TPU jam up my multi-material unit so badly?

Yeah, the box and soft flex don't mix. The problem is the feed path, not really the gears themselves. Those long stretches of tube plus the rollers grip the filament so hard that anything squishy just deforms instead of pushing through. Once it buckles in the tube you get exactly what you described, a chewed up tangle in the feeder housing. I did the same shredding job my first week and swore off routing anything soft through it ever again.

The external holder thing is real and it works. There's a spool holder on the back of the printer itself, and most of these printers have a bypass port so the filament goes from that holder straight down to the print head without ever entering the box. That short direct path is what makes flex printable. Fewer bends, no feeder grabbing it, and the extruder alone can push 95A fine at slow speeds. Some people print a taller riser or a side mount holder that sits above the printer so the filament hangs even straighter, and opening the door plus taking the top lid off helps the path stay smooth. If you've got the add-on cooling unit on the head, that helps too because it lets you keep temps down on slower flex prints.

One more thing worth knowing, hardness matters alot. The softer shore ratings like 95A are a lost cause in the box. But the stiffer flexes, think 70D or even lower numbers on the D scale, have gone through the box fine for me with the normal dryer settings and a slower print speed. So if your project doesn't need true rubber softness, a stiffer blend might let you keep using the box. For actual soft stuff, yes I swap spools manually. It takes two minutes to unload whatever's in there and hang the flex on the rear holder. Annoying but way better than picking rubber out of gear teeth again. Slow the print way down, maybe 20 or 30 mm/s, and you'll get good parts.

Drying made the difference for me. I had a couple jam episodes even on the rear holder before I figured out my flex spool had been sitting open for months. Now it lives in a sealed bin with desiccant packs and gets a few hours in the food dehydrator at low temp before a print. Wet TPU foams and gets inconsistent, which makes the extruder struggle way more than it shoud. Since then my flex prints just work at slow speeds. Worth checking if yours came from a spool that's been sitting around a while.

That swap-every-time thing sounds like a pain if you bounce back and forth a lot. I only needed soft squishy parts once so far, a bunch of little bumper pads for a cabinet project, and honestly I almost gave up on the whole idea untill I found this thread. Ended up printing them on a friend's machine instead. Now I'm wondering if it's even worth learning the rear holder routine for somthing I need once or twice a year. How often do you all actually end up printing with the rubbery stuff? Is it a regular thing for most people or just occasional?

I leave a cheap spare spool core on the rear holder permanently with my flex wound onto it, so the swap is literally just lift the old tube, hang the new one, feed it down. Takes under a minute now. The first few times felt fiddly but it gets quick fast. Only thing I'd add is keep the lid off for the whole print, not just during loading, since that stuff seems to kink at the slightest downward angle.

Another thing that bit me early on was retraction settings. I ran my first flex print with the same profile I use for PLA and it strung like crazy, looked like spider webs everywhere. Dropped the retraction way down, almost nothing, and the parts came out clean. Soft stuff just keeps moving when you pull it back, so the usual numbers don't apply. Also don't panic if the first layer sticks too well on flex. I tore a corner off a print once yanking it off the plate. Let the bed cool fully before you flex the part off, it peels way easier.