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Does anyone know of a hot end that is sealed? What I meaning is that the hot end has a rubber seal where the filament enters to keep the top airtight (in order to eliminate oozing).

I am looking to build a dual extruder printer but, I do not want any oozing from the hot end which is not in use. I could build a system to retract and 'close' the nozzle but it would be much more elegant if it would work to just seal the top of the hot end. Thus achieving the same effect as when you pull up water with a straw by covering the top with your finger.

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In a true dual-extruder / dual hot-end printer, having a seal isn't necessary.

On my dual-extruder printer, the non-active extruder does an aggressive retraction and then drops the hot-end temp down to a stand by temperature which is lower than the melting point of the filament. Because it had an aggressive retraction ... and it's cooling ... the filament doesn't seem to ever make it out before it has cooled to the point where the filament drops below the melting point. While there is no 'seal' ... it doesn't ooze.

The slicer software (I use Cura) estimates the time remaining before the extruder needs to become active and will start to pre-heat the hot-end so that there wont be much (if any) delay. Once the extruder becomes active, it will re-prime and will usually move to a prime tower to get the filament flowing again ... while the other nozzle (the one that just became inactive) does a retraction and starts cooling down.

With this technique of retracting and lowering the inactive hot-end to a standby temperature, I have never personally experienced an issue with oozing.

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  • $\begingroup$ Is there any noticeable increase in print time using this method? :) $\endgroup$
    – Beacon of Wierd
    Aug 12 '20 at 14:52
  • $\begingroup$ Cura seems to estimate the amount of time needed to raise the hot-end back to print temp. While one toolhead is printing, it seems to work out how far in advance it should start heating the next toolhead -- which (amazingly enough) always seems to reach printing temp just as the active toolhead finishes. I never really experience a delay (except for extruding a little into the prime tower to get the filament flowing nicely again.) $\endgroup$
    – Tim Campbell
    Aug 12 '20 at 16:41
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    $\begingroup$ I should add ... only one of my printers is a dual-extruder. So I don't know if I'm just lucky ... and/or if others do experience delays with their printers. $\endgroup$
    – Tim Campbell
    Aug 12 '20 at 16:42
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It doesn't work that way, or perhaps in some sense it already does.

The "straw effect" you're describing depends on the water not being under any pressure that exceeds the outside atmosphereic pressure. If you squeeze the straw or otherwise apply pressure, it will immediately spill out.

If you're printing at more than a ridiculously slow print speed (slow enough that it would fail for other reasons, like the nozzle melting the already-printed part just by proximity to it), the filament is under very high pressure from the extruder gear. The primary function of retraction is not to move the filament back out of the nozzle orifice, but simply to relieve that pressure. (A little bit more is needed to prevent oozing, however.) When retraction happens, it's exactly like the "straw effect" you're describing. The material pulled back out of the hotend into the heatbreak hardens enough that it makes a nearly air-tight seal, preventing the molten material below from flowing out of the nozzle due to gravity.

If your hotend does not have design problems and if you're using retraction correctly (that means using it everywhere that you're making a move that's not an extrusion one, not skipping it with "Limit Support Retractions" or "Combing"), you should not see oozing, ever. The system already works the way you want it to. But you can't magically get rid of the need for retraction. It's part of how the system you want works.

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    $\begingroup$ @BeaconofWierd: that part of the answer is just explaining why retraction is needed. You're talking about what's a travel move for the inactive nozzle, and as long as it's made retracted everything should apply. The duration of travel for the inactive extruder is so long that you might get some oozing due to imperfection of the system/model but that's independent of the part of the answer you seem to be objecting to. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Aug 10 '20 at 17:09
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    $\begingroup$ @R..GitHubSTOPHELPINGICE I'm not saying it's a bad answer, but from what I've read oozing is an issue with dual extruders for the inactive nozzle and that retraction only temporarily fixes oozing, which is enough for single nozzles. But as you said, there's pretty much a seal already in the hotend itself after the retraction, so adding another seal probably won't do much. $\endgroup$
    – Beacon of Wierd
    Aug 10 '20 at 22:14
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    $\begingroup$ @BeaconofWierd: Indeed. I suspect the problem is that you don't have the surface tension effects you would with water, so the molten plastic eventually runs down. Not a lack of seal. The only real solution I can see, if the "travel time" for the idle nozzle is long enough that this starts happening, is actively cooling it below the temperature that the material can flow. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Aug 11 '20 at 0:08
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    $\begingroup$ @R..GitHubSTOPHELPINGICE Actively cooling the nozzle would probably work, though it can take time and require rather expensive hardware. I guess I would have to investigate further into why oozing happens in an inactive nozzle, the surface tension of PLA around 200 degrees is only about half that of water, so my initial guess is that there's something else going on here. It might simply be that the filament doesn't have a perfect seal and that air is slowly leaking in to equalize the pressure over time. I guess simulating it would give some answers. Fluid Mechanics, we meet again! $\endgroup$
    – Beacon of Wierd
    Aug 11 '20 at 15:28
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    $\begingroup$ I can't actually get my nozzle to steal against the aluminium heat block. No matter how tight I turn it air always leaks through, would you mind testing this too? Maybe I just have cheap, poorly made nozzles, but if that's normally the case it could explain why retraction only prevents oozing temporarily. $\endgroup$
    – Beacon of Wierd
    Aug 11 '20 at 16:13

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