Retractable nozzle - how to prevent filament jam?



  • Hello all! 🙂
    I'm thinking about designing a retractable nozzle hotend for my BMG-X2 dual extruder setup. But when thinking about the concept I wondered about the filament path of the retracted nozzle: If the nozzle is being pulled up without the filament also being retracted, it would be forced out of the nozzle (or jam the retracting mechanism). If the filament is being retracted before the nozzle you would create a 'void' inside the melt zone which again could cause problems.
    So in practice you would need to retract the filament and the nozzle at the same time and with the same speed. Since I'm planning on using a servo for the mechanism I'm not sure how to command that. Controlling a servo is a seperate line of g-code which can't be commanded together with E-axis movement as far as I'm concerned.
    Has anyone experience with that kind of setup and how to deal with that problem?

    Thanks in advance! 🙂



  • What are you trying to achieve by retracting the nozzle? Interesting idea but I can't see the purpose.



  • Well, I want the second (idle) nozzle to not drag and ooze all over the print. Right now I tilt the whole extruder assembly for that purpose but there are several problems with that method:

    • the locking mechanism isn't strong enough so it can get knocked out of position (I have tow endstops on the carriage to re-activate the servo and reset the position but I don't like that approach)
    • due to the whole heavy assembly being on bearings, it has some little wobble to it. Not much but coming from a machining background, I can't stand wobble... 😄
    • every time you change the nozzle or just loosen a screw on the hotend you have to set all tool offsets again
    • and lastly I need stuff to keep me occupied (not that I don't have enough projects already lol)


  • @eumldeuml said in Retractable nozzle - how to prevent filament jam?:

    and lastly I need stuff to keep me occupied (not that I don't have enough projects already lol)

    A single nozzle multi filament hotend is a great way to get occupied. 😉

    https://www.aliexpress.com/item/32813067592.html



  • i did it with a linear rail and two solenoids.
    Forget a servo, for a mechanism that has to do that hundreds of thousands of times it is not reliable enough.
    Go check my posts about driving solenoids, you can see how i built it. here is a video

    https://youtu.be/fYOE01SzS9Q

    it uses a 9mm linear rail and a bistable spring mechanism to keep the hotend in the respectable upper/lower position without constantly having to drive the solenoids.



  • @zapta said in Retractable nozzle - how to prevent filament jam?:

    @eumldeuml said in Retractable nozzle - how to prevent filament jam?:

    and lastly I need stuff to keep me occupied (not that I don't have enough projects already lol)

    A single nozzle multi filament hotend is a great way to get occupied. 😉

    https://www.aliexpress.com/item/32813067592.html

    single nozzle solutions are bad because they waste a lot of plastic when purging, create problems with strings from filament when retracting which can jam and are seldom compatible with multiple materials that print at different temperatures.

    the nozzle lifting is important so that the inactive nozzle does not scrape over the build and deposits its ooze there.



  • @nitrofreak said in Retractable nozzle - how to prevent filament jam?:

    the nozzle lifting is important so that the inactive nozzle does not scrape over the build and deposits its ooze there.

    I used to have a dual nozzle printer and could never got any decent two color print. Even with a single material, I had scraping issues as you mention. I ended up removing one extruder+hotend and never looked back.

    I am curious about Prusa's MMU or Mosaic's Pallete 2 which use a single nozzle and may give it a try one day.



  • @nitrofreak said in Retractable nozzle - how to prevent filament jam?:

    single nozzle solutions are bad because they waste a lot of plastic when purging, create problems with strings from filament when retracting which can jam and are seldom compatible with multiple materials that print at different temperatures.

    As a long term user of Diamond multi input/single nozzle hot ends, I have to disagree with every one of those statements.



  • @deckingman said in Retractable nozzle - how to prevent filament jam?:

    @nitrofreak said in Retractable nozzle - how to prevent filament jam?:

    single nozzle solutions are bad because they waste a lot of plastic when purging, create problems with strings from filament when retracting which can jam and are seldom compatible with multiple materials that print at different temperatures.

    As a long term user of Diamond multi input/single nozzle hot ends, I have to disagree with every one of those statements.

    Please explain the handling of multiple materials when printing. How would you forego the rapid deterioration of PVA material when printed in unison with Nylon which likes to print at 250-270?
    You cant lower the temperature of the inactive nozzle.
    As far as i can remember from your blog, you only print multiple types of PLA. As much respect as i have for your research, multi color prints (especially PLA) only count as playing in my book. For engineering applications and real parts, not yoda heads, you need advanced materials like Nylon, PC, PEI coupled with a separate breakaway support or soluble support in order to have good overhang surface finishes and tolerances. A separate nozzle and heat block is the only way that is achievable.

    Sorry if it sounds rude. Not bashing, just telling how it is.



  • @nitrofreak In those situations where filaments of different types and temperatures are required, it is necessary to purge the filament that is held in the hot end at the "wrong" temperature. But it's only the small portion that is actually in the mixing chamber and simply purging about 10 to 15mm into a "bucket" is all that is required. That's hardly wasteful. There is no problem with stringing because only the active filament is being extruded. Retracting filament at the "wrong" temperature has never caused a jam - that only happens if you retract too much up into the heat break.

    But I concede that I have never tried any of the more exotic abrasive filaments because I can't easily change a nozzle and have to change the entire hot end assemble. However, if you have seen my blog recently you'll know that I'm working on a 6 input multi-material hot end with individual heaters for each input. But I'll still combine them into a single nozzle because I've tried the multiple nozzle approach. IMO the downside of oozing from unused nozzles, trying to get and maintain exact relative offsets between nozzles, pausing prints while waiting for nozzles to heat up or cool down, etc are just too much.

    We'll have to agree to disagree on whether a single nozzle output or multiple nozzle outputs is best. Having tried both, I know which I prefer.

    Ohh and I've never printed a Yoda head but I have printed parts using a mixture PLA, Taulman "T Glass" (PET-G), and E3D Scaffold support using a Diamond hot end.



  • @nitrofreak Woah, you're contraption looks really nice! 😄 The idea of using solenoids is very interesting I never thought of that..
    Right now I'm not to concerned about the reliability of the servo (but time might prove me wrong, who knows). The servo won't be active all the time, just for retracting and unretracting the nozzle so it really doesn't get stressed much.


  • administrators

    @nitrofreak said in Retractable nozzle - how to prevent filament jam?:

    single nozzle solutions are bad because they waste a lot of plastic when purging...

    May be true, if you can't arrange the transition to happen during infill.

    ... create problems with strings from filament when retracting...

    All nozzles have that problem if you don't get the temperatures and retraction right.

    ... which can jam...

    Again, not specific to single nozzle multiple input systems.

    ... and are seldom compatible with multiple materials that print at different temperatures.

    That's true.

    the nozzle lifting is important so that the inactive nozzle does not scrape over the build and deposits its ooze there.

    Ooze is not a problem if your hot end allows a reasonable amount of retraction and you use the features provided by RRF to use a lower standby temperature than active temperature. But it's still a problem if the standby nozzle passes over filament freshly-extruded by the active nozzle before it has cooled sufficiently. I had this problem (but none of the other problems you mention) with some prints on my dual-nozzle Ormerod. Lifting nozzles would have solved that. However, that printer had no print cooling fan; and with a fan the filament would mostly have solidified before the inactive nozzle could leave a mark on it.

    IMO, mixing hot ends are great for multi-colour prints. For prints using materials needing different temperatures, I would choose tool changing or IDEX; but a dual-nozzle printer with lifting nozzles would be an interesting alternative.



  • @deckingman said in Retractable nozzle - how to prevent filament jam?:

    @nitrofreak In those situations where filaments of different types and temperatures are required, it is necessary to purge the filament that is held in the hot end at the "wrong" temperature. But it's only the small portion that is actually in the mixing chamber and simply purging about 10 to 15mm into a "bucket" is all that is required. That's hardly wasteful.

    Sorry in this regard i actually meant systems like the Prusa MMU2 and the Prometheus that have a single melt chamber and have to purge the filament all the way. This is especially bad if you are changing from black to white filament, sometimes needing 100mm or more purge length. With regards to the mixing chamber, it is not possible to use that with material combinations like Nylon/PVA because while the Nylon needs like 270C the PVA will break down, crystallize and plug the nozzle. This already happens at standard print temperatures.

    There is no problem with stringing because only the active filament is being extruded. Retracting filament at the "wrong" temperature has never caused a jam - that only happens if you retract too much up into the heat break.

    Yes what i meant was again the single melt chamber method like the MMU2 where you retract the filament all the way out of the bowden tube and insert a new one. What happens is that it produces a little string that can get caught up in the filament sensor and bowden tube and produce blockages. 0Calvin has made an extensive problem summary of that method. Warning, it´s long but still interesting.
    https://www.youtube.com/watch?v=SAg1vDW5VNo

    ... the downside of oozing from unused nozzles, trying to get and maintain exact relative offsets between nozzles, pausing prints while waiting for nozzles to heat up or cool down, etc are just too much.

    In my experience with machined components the offset calibration is a set it and forget it type deal. The same as I´ve never had to relevel my bed once after tightening my screws from the Z plate sideways to the 15mm bed.
    The oozing is mitigated by the lifting hotends, the pausing of print while waiting to heat up is also at least a possibility in RRF (that does not have to be used).

    Just about every professional printer uses this lifting setup- Stratasys, Makerbot Method, Zortrax, Ultimaker (3 and S5), and now the Lulzbot TAZ Pro. Most of them use a mechanical lifter that actuates when you run into the end of travels (Stratasys uPrint, Dimension, Makerbot, Zortrax, Ultimaker) The downside is that you loose time because you have to travel to the EOT´s. Some machines use an electrical system (Fortus printers use an emotor with a leadscrew and endstops. I use solenoids with a bistable mechanism to keep it in place because it is lighter weight). This has the advantage that you can change nozzles anywhere on the build platform. It is not the biggest issue because you still mostly have to travel to the purge bucket and nozzle brush or prime pillar. The lulzbot uses a servo i believe. I dont think this solution will last. e3d also planned on using a servo in their tool changer but ditched that idea due to longevity concerns as it failed after 400.000 actuations.

    Ohh and I've never printed a Yoda head but I have printed parts using a mixture PLA, Taulman "T Glass" (PET-G), and E3D Scaffold support using a Diamond hot end.

    That sounds interesting, do you have a picture?

    @dc42 said in Retractable nozzle - how to prevent filament jam?:

    @nitrofreak said in Retractable nozzle - how to prevent filament jam?:

    single nozzle solutions are bad because they waste a lot of plastic when purging...

    May be true, if you can't arrange the transition to happen during infill.

    This is not an option when printing with soluble support. I don´t care really about multiple color printing. I only care about printable geometries, tolerances and technical materials because of the application of the part in its respective environment. In the rare case i need a pretty part, i would still prefer sanding, acetone smoothing and then painting in the required colors.

    ... create problems with strings from filament when retracting...

    All nozzles have that problem if you don't get the temperatures and retraction right. ...not specific to single nozzle multiple input systems.

    Sorry that i wasnt being clear. This does not apply to diamond hotends, but to the fully retracting types like the MMU2. When the filament is pulled out of the nozzle, it usually creates a string that can get caught in the sensor, see my reply to deckingman above

    Ooze is not a problem if your hot end allows a reasonable amount of retraction

    Even with a higher amount of retraction you will still have a bit of oozing. This is due to the thermal degradation and offgassing of the plastic in the nozzle producing pressure or maybe a tiny rest of moisture in the filament. maybe even gravity if it is a particularly runny thermoplastic. You can´t always avoid it, and those oozings will then get wiped into the shell surface. Ooze shields only work on smaller models where the second nozzle always resides on the outside of the ooze shield while the first nozzle is printing.

    IMO, mixing hot ends are great for multi-colour prints. For prints using materials needing different temperatures

    I agree a mixing diamond hotend is probably the best when printing with multiple colors of the same filament type.

    I would choose tool changing or IDEX

    They are good systems however again lack the practicability of "real" materials. Tool changing printers are completely incompatible with high chamber temperatures where one would seal the top of the chamber with expanding bellows so as to not 1. let the heat escape 2. damage precision linear rails/ belts.
    IDEX printers are compatible to some extent. However they waste a lot of space due to the needed extra bellow between the print heads which makes the gantry longer than necessary and would decrease stiffness and therefore increase ringing.

    This whole debate about the style of dual extrusion is very much a question of application and reliability. In printers that get used to manufacture real parts from engineering grade Materials like Nylon, PC or PPSU you can not afford to loose a print because the PVA plugged again due to the too high standby temperature, can not have the second nozzle drag over the fresh print causing defects and can not not have a separate breakaway/soluble support because a heated chamber will permanently fuse the support to your model if it is made from the same material.



  • @nitrofreak I think we have found some common ground on which we can agree. I just had to disagree with the sweeping, generalised statements.

    So yes, for sure the Prusa style MMU and Y splitter arrangements are wasteful in filaments with purge tower that use more filament than the part being printed. But mixing hot ends like the Diamond are not. So the generalised statement that "single nozzle solutions are bad because they waste a lot of plastic" is only true in certain cases and not every case of single nozzle solutions.

    Likewise the statement that single nozzle solutions are not compatible with materials that print at different temperatures. For sure, you wouldn't want to heat PET-G above 260 deg C for example because it will cross link and get very nasty. But printing PLA at say 195, scaffold support at around 215 and PET-G at around 230 are all perfectly doable with a single nozzle solution. Hell I've even mixed PLA and PET-G and just about got away with it. So as a sweeping, all encompassing statement, it is not true but in certain cases, it is true.

    But yes, it's horses for courses. I have 6 extruders and currently run with 5 connected to a Diamond mixing hot end. But I also swap between 0.5 and 0.9m nozzles simply buy exchanging hot ends. I'd hate to try that and change 5 or 6 different nozzles and expect the offset between them to remain the same. Conversely, I could not print a part that is a combination of say PETG and PC.



  • @nitrofreak Regarding the purge/wipe after the nozzle change: In my current design with the tilting extruder I already used a small piece of stainless steel sheet metal which blocks the nozzle so no filament comes out. It worked quite good and it eliminates the need for purging since no filament can ooze out and create an empty space in the melt chamber.
    In theory the filament degrades over time in the hot nozzle but I haven't found it to be a problem. Most of the time the infill will print first anyway so a potential degradation doesn't deteriorate the strength very much as the infill doesn't really contribute much to it anyways.
    But regarding my initial question: Your extruder is bowden drive, right? I'm still not sure about the need for simultanoeus retraction of nozzle and filament when using a direct drive setup. But I guess I'll have to do a little bit of pioneering here 🤔

    @deckingman As I already told you in another thread I am really impressed by your mixing hotend solution 🙂 It's just not the best way for my needs (apart from being waaay over my budget lol) since I don't print with more than one color/material very often. But when I do it's mostly for a combination of a rigid and a soft-ish filament (nylon for example). In the future I also want to experiment with different nozzle sizes in one setup.


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