printer extrudes beautifully but jams/blocks when retracting

Hi folks,
I'm just at the stage of calibrating the extruder on my i3 rework. The extruder is a
bulldog lite with stepper replaced with one I know the info on. The hotend is a custom
build based on the ebay e3d style heatsink, a 30mm stainless/ptfe throat that is
completely flush with 0.4mm nozzle. The DRV8825 is set to provide 1.1A out of 1.2A
max to the extruder

I'm working with 1.75mm ABS with extrusion temp set to 235^c. Pronterface is feeding the
filament at 100mm/min.

When I extrude 5,10 or 100+ mm of filament it comes out smoothly,constant size,
going straight down to the bed. But if I retract filament, with temp tried at 230 to 245c,
it will for a bit before jamming with stepper trying to turn before the tensioner on the
bulldog keeps popping loose. I have to manually pull the filament out with pliers and
after disassembling the hotend the nozzle is always blocked with ABS. I have tried
4 different nozzles with the same results.

The tensioner on the bulldog is already at the point where it is impossible to manually feed
filament into the extruder. I have also tried with minimal tension, same results.

Can anyone suggest a solution and explanation?


Many thanks,
Andrew

I don't know how much you retract but I do know that if you retract already hot filament that it will be of greater diameter.
So if you pull this semi-molten filament back into the "cold" part of the nozzle you have to make sure there is enough room.
I polished the stainless steel heatbreak on the inside wil fine sandpaper followed by a few minutes of electro-polishing.
I also increased the inner diameter of the heatbreak from 3.0 to 3.2mm (using 3mm filament here).

Turn the temperature down prior to retracting all the filament (I use 120C for PLA, not sure what is right for ABS), and retract slowly until the filament has been pulled out of the nozzle.

Edited 1 time(s). Last edit at 10/25/2015 07:53AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

What retract distance are you using? Try using 1mm or less. Keep in mind that with a retract you move hot filament in an area in th ehotend that should remain cool. The further up you transport it the higher the chances for failure.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

Hi folks,

Tried reducing retract speed to 20mm/min 5mm or less length. Tried lowering the nozzle
to 215c. Still all five nozzles jamming. Odd thing is I managed to pull the plug out of one
nozzle and the filament in the nozzle was in practically perfect condition but the plug had
spread out into the campher of the nozzle and the throat and appears to adhered there
only.

I'm assuming the nozzle temps are accurate, I recalibrated it recently using the thermocouple
on the multimeter. Haven't yet tried holding the thermocouple against the nozzle in situ to
double check yet.

I think it is jamming at the junction of the throat and nozzle but I don't know why. I tried turning
the throat around so ptfe at the top and reverse but no joy.

Throw the ptfe/metal throat in the bin and switch to metal only? Can filament be really really fussy
about nozzle temps with regards to retracting filament?

Is getting it up to temp and step down 2c at a time,trying to retract a valid but monotonous
solution?


Andrew

Again, what length are you using for retract? What you describe can happen due to overly long retract.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

How are you screwing the nozzle and heat break into the heater block?

It's important that the heat break be screwed up hard against the nozzle inside the heater block. If there's a gap, the filament can expand there and jam.

The proper way to do this is
1. Heat the block up to normal temperature
2. Screw the nozzle all the way in, then back it out a quarter turn or so.
3. Screw the heat break into the heat sink.
4. Screw the heat break into the heater block, finger tight. The heat sink is convenient for this.
5. Tighten the nozzle against the heater block using a spanner.

I agree with Srek... make sure your retract distance is < 1mm -- mine is at 0.5mm.

Sorry, can't comment on ABS. But do NOT switch to all-metal for PLA -- I had never-ending problems with all-metal, which almost all went away when I switched to a PTFE-lined heat break. Maybe try printing with PLA?

With installing a nozzle, I first wind the heat block down a 1/2 turn or so.
Fit the nozzle flush to the block then turn the heat block back up so the throat stops
against the nozzle.
Preheat to 150-200, depends on how sensitive my fingers are, give a final tighten
with a hex set.

With the retract distance, I think last time I tried 1-2mm. I suspect, obviously, the
retract speed plays a part. Try, as you suggest frankvdh and srek, <1mm at 10mm/min?
So the general rule with retracting filament to do it very very slowly to begin with
for say the first 10-15mm or so?

As for trying PLA, I would prefer to leave that till absolutely last. I tend to make functional
designs and I know the issue of which is better, PLA or ABS, can be politely described as
contentious, but I think ABS has the benefit of longevity.

Thanks for all everyone. All my nozzles should have finished cleaning so I'll give the
latest round of advice a whirl.


Many thanks,
Andrew

Quote
friarfish
With the retract distance, I think last time I tried 1-2mm. I suspect, obviously, the
retract speed plays a part. Try, as you suggest frankvdh and srek, <1mm at 10mm/min?
So the general rule with retracting filament to do it very very slowly to begin with
for say the first 10-15mm or so?

I retract as fast as possible to minimize the time in which ooze might happen. I never had any reason to retract slow.
I take it you mean 10mm/s?

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

Hi Srek,
okay, a bit of extra thinking and rereading of your reply. I think I might have the
retract problem completely wrong way around. A basic assumption about my setup
is probably utterly wrong.

The default speed in Pronterface on my system is 100mm/min and from what
you are saying Srek, that is woefully on the slow side. I never had a reason to
question the speed.

I have been working on the belief that fast extracts would lead to the filament ripping/ shearing/
leaving detritus behind. This is avoided by extracting at rates below 100mm/min,
the default on my system.

Now it appears I should reset the default speed to 300mm/min and higher.

Does that seem a better option Srek?


Andrew

Pronterface should offer speed settings in mm/s, not mm/min if i remember correctly.
If you are trying to extrude to fast with a powerfull extruder the bolt can rip into the filament and grind it to dust, instead of pushing it.
I can force this with my old printer and wade extruder, but the extrusion speed is much to high then for the rest of the mechanics of my printer to keep up.
Usually you only have a problem with the bolt grinding the filament if the hotend has a blockage.
The upper speed limit is usually determined by the maximum movement each axis can support and your expectations regarding the quality of the print.
If you like check out this video of my secondary printer (OrdBot) doing a test print at the maximum speed i was able to achieve with flexible filament.
In this case the quality achieved wasn't realy good, but all components kept up
[www.youtube.com]
If your printer is half as fast in movement at top speed it would be still ok. For outer perimeters and top infill it should actually be a lot slower.

Sadly speed is given in very different units and ways, some talk about over all movement speed of the nozzle, others count the speed of the filament going into the extruder. It realy is stupid sometimes.
Personaly i always and only talk about the movement speed of the nozzle in space.
Another issue when talking about speed is that many people forget about acceleration. The shorter the distances the nozzle has to move are, the more important acceleration becomes, since the nozzle seldomly reaches top speed over those short distances.
You can see what difference high acceleration makes in this video
[www.youtube.com]

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]