this is probably one of the most common cries of anguish heard from new RepRap owners. The picture on the right shows two common modes of extruder failure. In each case, the symptom is that the filament cannot be pushed through the extruder, or requires enough force to damage the filament even when the nozzle has been removed and the barrel is hot enough for molten plastic to dribble out of it.To find out what is going on, take the cooled extruder apart and look at the filament inside. If you can't get the filament out of the PTFE or PEEK spacer, the chances are that your spacer has got hot and the filament has bulged inside it. Use hose clamps or a metal sheath to stop the spacer from deforming. You will probably need a new one.
If you see a blob of plastic sitting directly on top of the heater barrel, you have a gap. Any space between the top of the barrel and the end of the hole that it fits into will accumulate plastic. This plastic will not all be molten, and will cause a lot of friction on the filament as it enters the barrel. Typically the extruder will run for a while, but mysteriously clog up if you let it cool and restart it. Or it may just jam solid.
The cure is to taper the top of the barrel to ensure it is driven into the PTFE or PEEK spacer, and to remove the thread from the top 1mm or so of the heater barrel. If the hole has not been tapped all the way to the end, this will let the heater barrel reach the bottom of it. It is important to clear any remaining melted filament before fitting the barrel back!
Vik :v)
My general feeling on why extruders would fail, is because the heater barrel isn't screwed into the PTFE hard enough, the taper is important, on both ends, but if you can't screw it real tight, like ~1.5Nm then you'll get a nasty air gap.
ReplyDeleteI've attempted to fix this by defining flats on my version of thermal barrier, and hopefully i'll be sorting out acquiring the week after next, and before my birthday.
I posted the designs up on Thingiverse for people to try out
http://www.thingiverse.com/thing:1043
Ps. Keeping in mind that domed nut nozzles will cost less to produce than the machined ones.
Pps. Vik, i've uploaded on Thingiverse the domed nut nozzles with the second taper on the inside that you mentioned.
The simple explanation is that PTFE creeps at the temperatures we use.
ReplyDeleteIt does not matter how tight it is to start with, or whether you clamp it with pipe clip, it will always shrink away from the clip and the thread eventually.
I have a collection of about half a dozen failed PTFE insulators. Since using PEEK or stainless steel instead I have had no failures.
You could just specify a heavier walled PTFE thermal barrier. I used a 1x1 inch PTFE block and never had any trouble.
ReplyDeletei was wondering if anyone ever thought of trying to use a parts of a hot glue gun in there projects.?
ReplyDeleteIf there is that much heat there, wouldn't a heatsink on the thermal barrier be enough to stop it? I dunno yet, so i'm just guessing.
ReplyDeleteIf there is that much thermal movement in the insulator, a heatsink (hose clamp) should fix it.
The hose clamp does not fix it because PTFE creeps. What this means is not a true solid. Think of it more like a super viscous fluid. From wikipedia:
ReplyDelete"Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no "memory," and is subject to creep, also known as "cold flow" and "compression set". A little bit of creep allows PTFE seals to conform to mating surfaces better than most other plastic seals. Too much creep, however, and the seal can be compromised. Compounding fillers control unwanted creep and improve wear, friction, and other properties. Sometimes metal springs apply continuous force to PTFE seals to give good contact, while permitting some creep."
You can tighten the clamp and it makes a very tight seal but come back to it a week later after some heat cycles and it will be lose again. It would need a spring loaded clamp to maintain pressure but then I think you would also need to constrain the length, otherwise it will just get thinner and longer over time.
I don't think making it thicker helps either. The outside can be constrained but the thread can still expand away from the heater barrel due to the pressure.
I guess that the fact that I was using a long copper extruder barrel with the heating at the bottom saved me there. The temperature at the thermal break rarely exceeded 80-100 C.
ReplyDeleteFantastic post - it's all consistent with my experiments in extruders, and the failure modes.
ReplyDeletePEEK has been good to me, much more reliable and stronger than PTFE. It's also able to hold a screw thread.
Verry pleased from all the comments that I have opted for PEEK instead of PTFE my main reason was the price diffrence was not that bad and I didnt like the Idea of the fumes from PTFE if the teprature controller messed up. I have made two peek Insulators now and the Welding tip nozeles I just need to make the heaters and perfect the ABS drive mechanisim having made 4 diffrent types so far I think I have an easy to make version now.
ReplyDeleteSo next week with any luck I will have at least one working extruder.
This is a great thread! I had never really considered that the air gap between the heater barrel and the PEEK/PTFE barrel would have been an issue. I figured that the plastic would simply be displaced. I am almost certain that this is what is causing me headaches! I will be fixing my PEEK insulator today! Thank you Vik and thank you RepRap community!
ReplyDelete(Me gots a Makerbot)
Wow! My Makerbot never printed so beautifully! Since i fixed the heater and made it mate with the PEEK insulator well, the printed come out much more consistent.
ReplyDeleteThank you!