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Trouble printing with flexible filament

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I got some flexible filament recently (https://blueprinted.co/collections/tpu-filament/products/tpu-black-filament?variant=20289791110) but I'm having endless troubles printing with it.

 

I'm using a UM2 with the Tinkergnome firmware. I've upgraded the feeder to https://www.youmagine.com/designs/alternative-um2-feeder-version-two and also applied this mod (https://www.youmagine.com/designs/feeder-bearing-mod-for-flexible-filament) as the flexible filament was slipping off the bearing. It appears to be feeding through the extruder well.

 

The problem is that the material is briefly and intermittently underextruding, leaving blobs instead of lines or sometimes nothing at all. The shape I'm printing is quite a complex one which involves a lot of change of directions and I think that as the print head changes direction it is increasing/decreasing (depending on whether the bowden tube is getting compressed or stretched out) the pressure of the filament in the print head, leading to brief underextruding. I think it's also causing brief overextruding too (big blobs coming out occasionally).

 

The item I am printing is a phone case with part of a clients' logo on the back, as seen here: https://imgur.com/mwif9hr

 

Before I explain the problem in more detail, I will say that the first thing I printed with this filament was a copy of the client's logo, which was about 50% of the size of the logo in the phone case - and it actually came out fine (quite stringy but I know that's always going to be a problem with flexible filaments). Here is the result: https://imgur.com/FTzv4cf (don't mind the uneven edges, it's just the brim that I haven't trimmed off properly). Unfortunately, I have not been able to achieve this result on the phone case even though it's almost an identical design.

 

Here is a photo demonstrating the problem: https://imgur.com/a/JGRhM

 

It prints the brim reasonably well (it leaves some splotches where I'm guessing pressure has momentarily built up and is then released) but once it starts printing the actual part it very quickly starts  intermittently printing blobs instead of lines.

 

The filament, bowden tube, bowden clips, and PTFE coupler are all new. I also have no problem printing with non-flexible filaments.

 

I have searched for help and tried everything that I could find online but I can't get it to improve. The things I have tried include:

 

  • using high feeder spring tension
  • using low feeder spring tension
  • atomic clean of nozzle
  • 0.8mm nozzle and 0.4mm nozzle
  • disabled retraction
  • oiled the inside of the bowden tube
  • uncoiled some filament off the spool to eliminate any friction
  • increased the flow rate on the printer (tried up to 200%)
  • tried printing at low speed (down to 20mm/s) and high speed (up to 150mm/s)
  • tried low and high travel speed
  • tried low and high print head acceleration and jerk
  • tried low and high print head temperature
  • tried low and high bed temperature
  • calibrated the print bed height many times (it seems to be super finicky about the height of the buildplate, much much more so than non-flexible filaments).
  • tried printing it in the back left corner of the print bed and also the front right to try to reduce change in pressure from moving the bowden tube

 

Nothing works though - it always does the same thing. I think I have tried at least 20 times with different settings.I let it keep printing once and even though the bottom kind of smoothed out, the side of the lines were very bulgy and uneven and it eventually stopped extruding half way through the print: https://imgur.com/hMQPCWQ

 

 

Could anyone please recommend me things to try or ideal settings for printing with flexible filament?

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Sorry I cannot help; I went through a similar experience and end up throwing the reel in the bin. BUT there were a lot of posts on the subject a year or so back on this subject. I suggest that if you search the forum for flexible filament you will find them. Even better would be to use the filament name which was very popular but unfortunately I cannot remember it

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