Filament knotting in extruder

I am plagued by an intractable problem, many prints are destroyed by the filament getting knotted in the extruder, just after the hobbed bolt. I have checked the forum for likely answers and eliminated them all as problems.

• Temperature at the hobbed bolt is normally 31 degtrees C, never more than 35 degrees
• Alignment of the hobbed area on the bolt is pretty much central (see photos attached)
• The path from close to the extruder down to the beginning of the hot end is PTFE and very smooth
• The extruder can easily feed 50mm of 1.75mm ABS at 360mm/min without showing signs of kinking.
• The hobbing on the bolts is clean and even.

The photos show the view down the guide holes on the standard Wade extruder. One bolt is normal, the other is hobbed on narrow shoulders with a gutter so that plastic shavings don't block it.




The only things that I have not been able to eliminate are:

• Temprorary blockage of the extruder nozzle (0.25mm), but this seems to go away when I remove the kinked bit of filament.
• Weak spots in the filament, but I have had the same behaviour on 3 different samples of ABS.
• Malicious daemons bent on making my life miserable.
• Something I have not thought of.

Any thoughts greatfully received.

Mike

Edited 2 time(s). Last edit at 12/22/2012 09:08AM by leadinglights.

Wades is designed for 3mm filament. 1.75mm filament is (1.75/3)^ 4 less stiff., i.e. about 9 times but it still requires 1/3 of the force.

You also have a small nozzle and again the pressure required goes up as a forth law.

ABS is not very stiff compared to PLA.

So I think you are just pushing up against the limits what force the plastic can withstand without buckling. You probably need to reduce the diameter of the hole through the Wades block as keeping the force straight along it length will greatly reduce the tendency to buckle.

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Hi nophead,

Thanks for the quick answer. I have done some quick modifications to see if I can knock solve this. I have a PTFE adaptor between the heatsink of the hot end with a 4mm dia spiggot which extends through the Wades block. I have extended this spiggot to almost touch the hobbed bolt - cut away to just miss the idler bearing.

I had previously dropped the temperature of the hot end from 230 to 220 degrees as it seemed to improve the strength of the produced bits, but now I have pushed it up to 240 degrees to try to knock this forth law thingy on the head.

Thanks again,

Mike.

p.s., I purchased my Prusa Mendel plastic from you in July last year - but got delayed in the building of it until the last month or so. Lots of Yoad-Lite, Owls and screwing jewlery boxes have come out really good, but every time I try to make bits to upgrade my printer it goes on strike with the extruder problem or a communication problem.

I run a small piece of brass tubing sleeve, available at most hardware/hobby stores, with an i.d. of 2mm in my extruder below the hobbed bolt to help guide 1.75mm filament. Running a .25mm nozzle. The hobbed bolt will start to tear the ABS on my setup if there is a nozzle clog. Never get a kink.

Hi Dirty Steve,

That was pretty much what I meant about the spiggot
Sorry the photo isn't clear, but PTFE is too white to photo well. The spiggot has a 2mm hole for the 1.75mm filament.

Mike

Interesting information. I too had problems with those bolts because I always thought that it was easy to install but it isn't after all. Thanks a lot for sharing this.

Hi Lawrence,

Although the problem hasn't ever completely gone away, it was due to back pressure from the 0.25mm nozzle. Main cure was increasing the temperature to 230 degrees although 235 was needed on one print recently.
I am waiting for some 0.3mm carbide PCB drill bits to make a slightly bigger nozzle - already made nozzles of 0.2mm and 0.15mm will have to wait for me to really come to grips with other problems on the extruder.

Mike