Yeah, that matches what I’ve heard too—those star-shaped PTFE tubes usually create more friction instead of less. For TPU and PPA, a smooth, high-quality PTFE tube is still the best option. A little dry PTFE lube inside can also help reduce drag without needing special tubing.
I hate it when people down vote others but then add zero to the discussion on why they down vote or respond at all… Like wtf? @mackins7000
I’m loving this physics discussion… Please continue.
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7000 hours on x1c and 600 on h2d running pa6cf/gf and ppa-cf almost exclusively, through AMS for PA6, through the buffer for PPA. Only replaced the buffer on the x1c at 5500 hours trying to diagnose an issue. Wasn’t even worn out. Rather have it be easier to run than worry about a theoretical issue which is so far in the future it’s not even a concern.
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Indeed PPA-CF (and all other non TPU filaments) were designed to run through the buffer. Even when you use the AMT HT bypass hole or external spool, its supposed to go to the hot end though the buffer.
H2D is not like P1/X1 where the external spool feed bypasses the buffer. On H2D the external spool feed goes through one of the two buffers, maybe thats where the confusion for some people occurs.
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just do what we used to do in the old days of 3d printing:
put a couple drops of machine oil/lube on a cotton round and an aligator clip and allow your tpe/tpu to run across it so it picks up a little of the oil.
i print tpu through ptfe tubes all the time by doing this. as long as the tube isn’t damaged or have a sharp bend it usually works cromulently.
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Thanks for the suggestion but I mainly wanted to try the star shaped tubes to see if there is any advantage, not trying to solve any problems. 95A and 90A run just fine through the PTFE tubes without adding any lube.
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That seems reasonable. You provide less surface area, nothing to reduce drag coefficient, so now there is a smaller area of contact but more mass per area of contact. As such, a smaller bit of surface area must now carry the load. The idea should be to reduce the amount of mass or the friction. The TPU is naturally sticky so that means you need a slippery tube — something coated in teflon perhaps.
Thanks for sharing this information.
For the drag on the AMS-HT, I used this :
Maybe you could adujst and fit it for your need ?
My need was from one shelf to the other underneath. Once I put my original tubes into this, it’s not moving anymore and the TPU go right throught it without problem.
I use Polymaker Polyflex-tpu95 and will probably check for tpu for ams next time 
It seems that the star tubes might wear faster with abrasive filaments.
And relative to friction and surface area, the shafts (staffs) in watches have their ends turned down to a small diameter and polished to minimize friction. The smaller the better, with the lower limit being due to shock resistance. Too small and they can break off when the watch is dropped.
I print a motorcycle flag bracket out of PPA (https://flag.3dcreationsofri.shop) I’ve tried multiple materials from PA6 (not rigid enough) to pa612-CF (too brittle) to PPA-CF (crazy strong, but lacking impact resistance) to finally settling on unfilled PPA (great impact resistance and tensile strength, with just enough rigidity to resist to torsional forces of a flag flapping at highway speeds).
Ummm…you want something coated in Teflon? Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer, famously known by the brand name Teflon. The whole tube is made of it…
Well you literally just hampered yourself by using this stuff.
Bambu uses 4mm OD x 2.5mm ID, with the inside as loose as it is to try and help create as loose of filament path as possible.
Sadly, like everything else, there a a multitude of PTFE sizes out there for 1.75mm filament and only one that Bambi uses / recommends to work properly.
It sounds like the stuff you were using is almost an entire millimeter smaller.
Yeah. I bet it was much worse using that stuff.