Filament Knots on Spool

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Filament Knots on Spool

Postby runninfarmer » Mon Mar 31, 2014 3:57 am

For some reason my spool of PLA keeps knotting. If I manually push on the filament backwards I can get it untangled, but it's a pain to have to sit at the machine and watch for it every time it does it. Is there a way to alleviate this problem? I even have the spool riding on two 608 bearings, but its like the filament is spooled in a way to maximize knots. Here's a pic:


Knot.jpg



Appreciate an help!
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Re: Filament Knots on Spool

Postby Atreidae » Mon Mar 31, 2014 12:07 pm

First off. Excuse me for being breif. I'm on my phone.

I remember reading something on the reprap wiki that the extruder can twist your filament as it runs through the hobbed bolt. Their solution was to hang the filament from the room in basket things without a spool. Seems like a bit of a basket case personally. (No disrespect to the Rep rap guys. I just had to make a basket joke)

I'd be checking to see if your hobbed bolt/pulley is straight. As well as whatever is applying pressure from the other side. Issues here you cause the filament to attempt to slide out sideways from the extruder assembly. Obviously it can't and this causes a twisting motion on the filament that eventually causes the knot

Just out of curiosity what extruder are you using? I'm running a Bulldog xl and haven't seen any twisting issues yet.

(Edit. Ha. Brief)
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Re: Filament Knots on Spool

Postby fma » Mon Mar 31, 2014 12:35 pm

I think you have several turns entangled... Try to push back filament until you can unroll it for several turns wihtout problems.

I don't see how it could do that, even by twisting it. In this last case, it tends to unroll out of the spool.
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Re: Filament Knots on Spool

Postby runninfarmer » Mon Mar 31, 2014 2:29 pm

I've used a couple different variant's of the same style of extruder, one using a PG35L motor and the other using a geared nema 11 motor. It really seems to come from the factory that way, since I have a spool of PLA from another seller and never had that problem. I will take it off the holder and try to unspool a little later and see if there are more knots underneath or really an extruder issue.
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Re: Filament Knots on Spool

Postby cvoinescu » Mon Mar 31, 2014 3:26 pm

This can happen even with properly wound filament if allowed to get very loose on the spool and subsequently tightened. You may find it helps to add a little friction to keep the spool from unrolling too much. That would reduce the opportunity for the filament to get tangled; however, it would require the extruder to pull with more force, so it could cause other problems. You can also add a piece of tubing (e.g. PTFE) to the inlet of the extruder, to keep the length of filament between the spool and the extruder constant, regardless of extruder motion. Again, this adds friction to the filament, so it can cause other problems, but it should prevent horizontal movement from yanking on the filament and unrolling your spool.

Of course, the filament could also be tangled from the factory, in which case there isn't much you can do, except transfer it to another spool.
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Re: Filament Knots on Spool

Postby orcinus » Tue Apr 01, 2014 1:41 am

Some filament is just like that.

I had a few rolls that would get tangled whatever you did to them.
I normally unroll an estimate of filament length required for a print and snip it off, then wind it loosely in a spiral down a desk lamp.

This works for most filaments, but i recently had some on very small spools that would somehow get entangled and cross-wound even when i unrolled just 5-10 turns of it.
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Re: Filament Knots on Spool

Postby fma » Tue Apr 01, 2014 5:05 am

This is strange... I don't see how this could happen when the filament is put on the spool; to get this problem, this would require to roll the filament *under* previous layers...

When you unroll some layers, make sure you don't reject the problem further: you really need to find where the filament cross the previous layer, et put it back on the right way. This can be tricky, as the crossing section can move back while you unroll!
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Re: Filament Knots on Spool

Postby dave3d » Tue Apr 01, 2014 9:19 am

I think it is the same effect as a fishing reel where tangles occur when the line or filament slackens off on the spool. Back in the factory the filament is wound randomly onto the spool. It is not like a fishing reel with a level wind.

I have built two of Bart Dring's "OrdBot inspired spool holders" and have only had the problem once. I think this was due to the filament springing loose on the spool at the start. It did however pull the spool and spool holder off a shelf with a crash.

I use 1.75mm. Which is the worst for springyness? 3mm or 1.75mm?
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Re: Filament Knots on Spool

Postby runninfarmer » Tue Apr 01, 2014 2:57 pm

I kept unrolling it and unrolling and still would get knot ups. After manually respooling a large portion, it wouldn't jam anymore. I guess it's the price you pay for $13/lb PLA. I use 1.75 mm.
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Re: Filament Knots on Spool

Postby orcinus » Tue Apr 01, 2014 7:24 pm

fma wrote:This is strange... I don't see how this could happen when the filament is put on the spool; to get this problem, this would require to roll the filament *under* previous layers...


Simple - roll one turn from one end to the other, make a few turns there, then roll one turn back to the other side and roll a few turns there (making an X).
Also, what often seems to happen is, when the filament gets loose, a turn will end up with a SMALLER diameter than the turn coming immediately after it off the spool (instead of a bigger diameter). Depending on its path, it's possible for the first turn to go through the next turn, making a tangle.
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