Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

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exalt
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Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

Post by exalt » Tue Feb 21, 2017 6:56 pm

Rostock Max v2. At seemingly random intervals I get this issue where the hobbed gear grinds into PLA filament and forms large burrs. The larger burrs then get stuck in the 4mm Quick Connect PTC Fitting connectors. Once this starts happening on a print it tends to recur,the burr gets stuck on one or both of the PTC brass connectors, leading to the filament getting slowed down or stuck at the cold end too, causing even more burrs. The choke causes extremely under-extruded portions or even entire layers with nothing more than wisps of plastic in between. Occasionally, after a while, the burr gets pushed through and the print can resume, but with extremely weak layers in between, the part integrity is severely compromised.

In extreme cases the hobbed drive roller can even eat through so much filament that it simply cannot grip any more and the printer just airprints the rest of the job, or the part becomes so weak it snaps from just the movement of the hot end. Other times the shredded filament bits get forced into the bowden tube and contribute to the jam near the PTC connectors.

At first I tried cleaning the hobbed roller but the amount of shredded filament around it is quite little, if anything most of the shreds drop into the bowden tube anyway.

Testing to try to reproduce the problem is difficult, sometimes I get entire 24hour prints where this is no issue at all. Cranking up the speed and temperature seems to reduce the issue(or at least reduce the likelihood of it happening, this could simply be because there's less time for things to screw up) but anything above 50mm/s causes unacceptable layer shifting for me due to belts slipping or other play in the mechanical system. It also reduces the surface quality and so I do not consider increasing the speed a valid solution. I have already gone through over 1kg of filament printing incomplete or poor-strength parts, spent over 1 week, with no clear resolution in sight. Frustration is very high.

Slicer: Simplify3D
Temperatures tested: 180 -230
Print speeds tested 40-60mm/s
Acceleration: 2000mm/s^2
Retraction:5mm, 100mm/s
Filament: Honeycomb(most persistent), ESUN and Reprapper(less persistent) PLA

7.jpg

Unscrewed the hot end PTC and had to use pliers to pull this out because the burrs jammed at the top PTC.

4.jpg

If it was just 1 or 2 burrs it probably wouldn't be an issue but once it starts happening the frequency rapidly rises, practically the entire length of filament is covered in burrs.

11.jpg

Huge burr. The thickness is easily 2mm if not 2.5mm.

10.jpg

Part ruined due to underextrusion from burr. First, small parts start being underextruded. Then entire layers become wisps of plastic. Finally the part simply broke due to not enough strength.

Possible solutions:
1. Replace the cold end with a proven effective design
2. Replace the spring on the EZstruder(I don't know much about the design/engineering, or what would be a good sprint size/weight, or where to source for the right spring)
3. Find some way to restrict the burrs from forming(maybe a 2mm PTFE tube just below the roller might help prevent the burrs from forming?)
4. Find some way to reduce the pressure the EZstruder spring is placing on the filament
5. Buy a new SeemeCNC hobbed gear(probably won't change anything)
6. Buy a new printer: unpalatable, but increasingly plausible

This issue has cost me a lot of time. Please advise.

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CRPerryJr
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Re: Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

Post by CRPerryJr » Wed Feb 22, 2017 12:43 am

I'm just throwing out a guess here, but I'd say your hotend is not hot enough. Perhaps your thermistor is giving bad readings? Other place I'd look is to ensure that the filament path is clear and easy to move through from spool to print head. Any resistance is one more thing the EZstruder has to push through to get the filament down to your print head. The last thing I'd look after mechanical issues is the slicer itself. Have you tried other slicers and do you get the same results?
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Glacian22
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Re: Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

Post by Glacian22 » Wed Feb 22, 2017 1:27 am

For PLA you generally need to lower retraction speed significantly, it should be around 25mm/s.

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geneb
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Re: Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

Post by geneb » Wed Feb 22, 2017 7:55 am

It could also be that the bowden tube isn't fully seated in the hot end, resulting in a partial clog that could result in the filament being harder to drive. The EZR is doing exactly what it's designed to do - push real hard. :)

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exalt
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Re: Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

Post by exalt » Wed Feb 22, 2017 8:55 am

CRPerryJr wrote:I'm just throwing out a guess here, but I'd say your hotend is not hot enough. Perhaps your thermistor is giving bad readings? Other place I'd look is to ensure that the filament path is clear and easy to move through from spool to print head. Any resistance is one more thing the EZstruder has to push through to get the filament down to your print head. The last thing I'd look after mechanical issues is the slicer itself. Have you tried other slicers and do you get the same results?


definitely not. there's a huge amount of molten dribbling filament even at 180, much less 230.

Glacian22 wrote:For PLA you generally need to lower retraction speed significantly, it should be around 25mm/s.


The burrs are forming below the ground-out portion though, not above. So I don't think the retracting motion is what is causing the filament to grind.

geneb wrote:It could also be that the bowden tube isn't fully seated in the hot end, resulting in a partial clog that could result in the filament being harder to drive. The EZR is doing exactly what it's designed to do - push real hard. :)

g.


I am using a V2, with the EZstruder instead of EZR. The hot end is using the older PTFE liner and PEEK section. I did dig the PTFE out today and the bottom portion(around 1cm or so) was somewhat burnt, covered in soot and/or discoloured, and some melted filament in there but otherwise there wasn't any filament stuck under the liner itself. How do you ensure the liner is properly seated? Could it be the case that 230 is too high for PLA?

burnt.jpg

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Re: Hobbed Drive Roller Causing Rose Thorn Filament, Failed Prints

Post by geneb » Thu Feb 23, 2017 9:23 am

And that's your problem. :) When PTFE is over-temped for a long period of time, it will shrink. What's happening is that the interior diameter of the tubing has shrunk, and is now making the extruder work very hard to push through that restriction. Just replace the PTFE tubing and you'll be fine.

230C is WAY too hot for typical PLA material. 230 is more commonly seen with ABS. At 230C, PLA should practically pour out of the hot end like water.

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