PA question - mixing hot end - asymetric bowden tubes



  • This is related to a 5 input mixing hot end with 5 extruders.

    For reasons of physical constraints, 2 of my extruders are mounted higher than the others which means that I have Bowden tubes of different lengths. The longest is about 230 mm and the shortest about 170mm (and no, it isn't physically possible to make all the tubes the same length otherwise I would have done so).

    I've been fine tuning pressure advance using single extruders and find that in my usage case, the optimum value is about 0.4 for those extruders which use the shorter tubes and 0.5 for those extruders which use the longer Bowden tubes.

    My question is how does the firmware deal with the situation where I might be mixing? i.e. using a combination of extruders in different proportions which is turn use shorter and longer Bowden tubes and thus different PA values. Specifically say if I used 30% extruder 0 with PA set to 0.5 and 70% of extruder 1 which has PA set to 0.4. Will each extruder be treated as if it were "stand alone" and thus act differently depending on the individual PA value and proportion of the total mix? I'm assuming it will.



  • Can't you insert a pressure advance setting in each tool pre-amble and then set it to zero in the post-amble? (tpre and tpost)



  • @jens55 I don't think that you understand the question. PA is set on a per extruder drive basis. It's largely irrelevant where that is set. I'll reiterate the question in a different way, which is - how does the firmware treat PA when multiple extruders are used concurrently but in different proportions and where each extruder may have a different PA value?


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    @deckingman said in PA question - mixing hot end - asymetric bowden tubes:

    My question is how does the firmware deal with the situation where I might be mixing? i.e. using a combination of extruders in different proportions which is turn use shorter and longer Bowden tubes and thus different PA values. Specifically say if I used 30% extruder 0 with PA set to 0.5 and 70% of extruder 1 which has PA set to 0.4. Will each extruder be treated as if it were "stand alone" and thus act differently depending on the individual PA value and proportion of the total mix? I'm assuming it will.

    Yes, that is how it works - although for a mixing extruder, I am not certain that it is the right way. Suppose you have a 2 input mixing extruder and you are printing with just one filament. When you start extruding that filament, does the pressure in the melt chamber push the other filament out immediately, compressing it in its Bowden tube? If so then pressure advance should be applied to all extruders, so that equal pressure is applied at all inputs of the mixing chamber. In your case PA would be reduced to 0.1 for some extruders and 0.08 for the others, which are much more in line with the normal values for those lengths of Bowden tube.



  • @dc42 said in PA question - mixing hot end - asymetric bowden tubes:

    @deckingman said in PA question - mixing hot end - asymetric bowden tubes:

    My question is how does the firmware deal with the situation where I might be mixing? i.e. using a combination of extruders in different proportions which is turn use shorter and longer Bowden tubes and thus different PA values. Specifically say if I used 30% extruder 0 with PA set to 0.5 and 70% of extruder 1 which has PA set to 0.4. Will each extruder be treated as if it were "stand alone" and thus act differently depending on the individual PA value and proportion of the total mix? I'm assuming it will.

    Yes, that is how it works - although for a mixing extruder, I am not certain that it is the right way. Suppose you have a 2 input mixing extruder and you are printing with just one filament. When you start extruding that filament, does the pressure in the melt chamber push the other filament out immediately, compressing it in its Bowden tube? If so then pressure advance should be applied to all extruders, so that equal pressure is applied at all inputs of the mixing chamber. In your case PA would be reduced to 0.1 for some extruders and 0.08 for the others, which are much more in line with the normal values for those lengths of Bowden tube.

    Thanks. I'm not sure either if that's the right way to do it. But you raise a valid point which might explain why I've always needed to run much higher PA values than is "normal".

    I can say that pressure on one input will definitely try to push filament up any "unused" inputs. And also we know that retraction has to be applied to all inputs otherwise the opposite happens and filament is drawn from "unused" inputs rather than the nozzle tip. But in practice, with a single filament being fed into a mixing hot end, could PA be applied to those extruders that aren't moving? i.e. if PA alters extruder acceleration, can it still work if the acceleration is zero to start with?


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