UM2 retracts after print way too far

[Nicolinux 288]

Hi,

I noticed that after a finished print the filament is retracted quite a bit. This is actually a problem because the filament is transported very far up, past the teflon piece. On the next print, the filament is pushed down again, through the teflon piece. This will certainly lead to increased wear because the filament tip tends to form a significant buldge which is squished through the teflon for every print.

Depending on the reason why the filament tip forms such a buldge or how pronounced it is, this might explain the underextrusion problems of some users where the teflon piece gets deformed.

What do you think?

Tested with firmware 14.09.

 

[solid-print-3d 30]

I've noticed the extreme retraction as well. I've noticed myself checking more and more lately on the beginning layer of prints to make sure the nozzle is sufficiently primed, and the first layer is going down fine. I'm having to do this becasue there are times where it just doesn't prime enough, or for whatever reason the filament binds in the hotend/teflon, causing a jam. I used to be able to just hit print and walk away. Not 100% sure it's this long retraction, but it's definitely a strong candidate for being the culprit.

 

[Dim3nsioneer 558]

It's 20mm: https://github.com/Ultimaker/Ultimaker2Marlin/blob/7e5d0013cd0c36b09aad8a0edaa6a456e3cb2c72/Marlin/Configuration.h#L572

 

[Nicolinux 288]

By the way, why does it have to retract at all?

 

[gr5 2,265]

By the way, why does it have to retract at all?

 

Not sure but maybe "filament change" wouldn't work if you didn't retract?

I wish it would wait until temp is 100C and then do a "cold pull".

 

[Dim3nsioneer 558]

@Nicolinux: If someone is printing very hot it might drop onto the finished print -> not funny. But a standard retraction would be enough IMHO.

@gr5: Thought of that too. But does the motor have the strength to do a cold pull or would it just result in skipping?

 

[Nicolinux 288]

Hm, right after the print, the bed moves down a bit (but fast) and then the head moves to the side. I don't think there would be a big chance of excess filament falling down. And even if, this lil strand of filament would be no problem to remove. I think teflon piece wear is by far more serious than that.

 

[pm_dude 27]

Yeah I have the same issue.

Its not for change material because the process heat the nozzle before retracting it.

It might just be to prevent materials to burn in the hot nozzle like it can happen for woodfill.

[eldrick 65]

Yep - it's to keep PLA from baking onto the nozzle while it cools down, and so you can more easily switch filament between prints.

 

[Nicolinux 288]

But are you happy with the status quo? The prospect of the filament bulge passing through the teflon piece every time isn't something I'd would trade. I mean, this retraction after print is a somewhat recent change if I remember correctly (since 14.x). Even if PLA bakes onto the nozzle, the next time it is heated up it will soften again so it can be pulled out easily.

I'd love to hear Daid's thoughts about it.

 

[DidierKlein 729]

I agree with you Nico i think i sometimes have problems with that retraction as you said it can probably damage the Teflon, i had one or twice the filament stuck in the Teflon because it was too expanded.

I had to remove the filament by taking the Bowden off like in the Atomic procedure to get it out (was to hard to pull out from the feeder).

But indeed for things like woodfill it's probably a very good thing, anyway i would recommend cleaning the nozzle after every woodfill or bronzefill print to prevent blockage. Maybe it could be a parameter of the material?

 

[pm_dude 27]

Maybe it could be a parameter of the material?

 

+1

Is it retracting when aborting a print? I don't have the impression it does (I'm not home to test it).

 

[Daid 306]

+1

Is it retracting when aborting a print? I don't have the impression it does (I'm not home to test it).

 

It does in later firmware versions.

The idea of this retraction is to remove the filament from the hot-zone so it does not degrade while the hotend is cooling down. And heat from the hotend "creeps" up trough the filament melting more filament inside the teflon, causing problems there.

 

[pm_dude 27]

Could it be a good idea to keep pushing some filament out while starting to cool down (at least a few degrees) and then retract? It could prevent the blockage in the teflon like it does now.

I can already see cases where that would potentially grind the filament :(

 

[gr5 2,265]

The feeder motor is impressively strong. Mine pulls 5kg. That should be enough for a cold pull. Then if you change colors later you don't have to wait for the color to change. It changes instantly.

http://umforum.ultimaker.com/index.php?/topic/4222-pulling-force-of-um-extruder/?p=34887

 

[eldrick 65]

Daid: It seems like it would be good idea to have the fans run for a minute after Abort or Ending a print, to reduce that heat soak from the still-hot hotend (and from pulling the hot filament up into the spacer).

 

[dirkdirk 5]

Same problem for me.

Is there a way to change the retract lenght after print?