Varying nozzle mixing ratio by real time location in a print...

[deckingman]

@CNCModeller said in Varying nozzle mixing ratio by real time location in a print...:

................ I'm only combining different types of PLA ............................

Then here is another little problem that you will encounter............PLA will hydrolyse (become more and more runny) over time when held at print temperature. This will happen with any of the 3 filaments which are not moving forward as the print progresses. So when you do come to use any of those previously unused filaments, if sufficient time has elapsed (a couple of hours or so) the entire contents of that particular melt chamber will get dumped onto the print in one go, then you'll have zero extrusion until that melt chamber gets replenished. PETG does not exhibit the same level of hydrolysis.

Welcome to my world

[CNCModeller]

@deckingman that's worth knowing.

I was thinking of keeping all the filaments at least creeping forward as I thought the filament might degrade if held at temperature for extended periods. Sounds like that's true. I'm not worried about the appearance w.r.t. colour and a trace of the other filaments won't make a major mechanical difference.

Thanks again

[deckingman]

@CNCModeller It depends a bit on the model but generally a couple of percentage points in the mixing ratio is enough to keep the filament moving forward enough to counteract the hydrolysis.

It'll also help with another issue that I haven't yet mentioned, which is that you must use firmware retraction to retract all filaments concurrently - not just the one(s) that is/are "active". If you don't do that and only retract the filaments that you are currently printing with, then when you retract, filaments gets drawn from the other input(s) rather than the nozzle tip effectively giving no retraction. But the issue here is that if you have a lot of retractions, the same portion of those filaments that are not currently in use get retracted and un-retracted and may eventually get worn away. The number of retraction cycles you get before that happens will depend on the extruder type, with a dual hobbed bolt such as the Bondetch BMG doing much better that a single bolt spring loaded roller type such as an E3D Titan. But keeping all filaments moving forward as the print progresses will prevent this from happening.

[CNCModeller]

@deckingman said in Varying nozzle mixing ratio by real time location in a print...:

@CNCModeller It depends a bit on the model but generally a couple of percentage points in the mixing ratio is enough to keep the filament moving forward enough to counteract the hydrolysis.

...
The number of retraction cycles you get before that happens will depend on the extruder type, with a dual hobbed bolt such as the Bondetch BMG doing much better that a single bolt spring loaded roller type such as an E3D Titan. But keeping all filaments moving forward as the print progresses will prevent this from happening.

Sounds like I'm ok on this front too, I'll be creeping the filament and I'm using a BMG.

Just need to figure out proper mixing... I have some ideas but will need to do testing to see if they are any good.

Many thanks again!
Barry M

[deckingman]

@CNCModeller Good luck! (I mean that sincerely, not sarcastically). The most promising method I found was with a split helical micro-mixer which worked quite well in terms of mixing but only at near constant extruder flow rates for the reasons that I've already stated. A driven "paddle" might work too if you can figure out a way to seal the drive shaft, but again, I suspect it will only work at near constant flow rates for those same reasons. One of my YT followers has bought a commercial design which claims to work but thus far, he hasn't commented on whether the claims live up to expectations. When I get back on my PC, I'll see if I can dig out a link.

[o_lampe]

@CNCModeller
I recently proposed to use a Prusa MMU or enraged rabbit to cut filaments according to the extruded length and feed these pieces of filament through a Bowden tube for printing.
I hope with RRF3.5 we could read the gcode twice. First for cutting, second for printing with the right delay (according to the Bowden tube length)

[CNCModeller]

@o_lampe said in Varying nozzle mixing ratio by real time location in a print...:

@CNCModeller
I recently proposed to use a Prusa MMU or enraged rabbit to cut filaments according to the extruded length and feed these pieces of filament through a Bowden tube for printing.
I hope with RRF3.5 we could read the gcode twice. First for cutting, second for printing with the right delay (according to the Bowden tube length)

@o_lampe that'd be interesting, so potentially we could do look ahead to allow for the delay in mix ratio reaching the nozzle throat?

As an aside I'd also like to drive an axis with the look ahead path approx 2mm in front of the current print location, I'm guessing that'd be possible too with multiple reads?

All cool stuff!

[deckingman]

@o_lampe said in Varying nozzle mixing ratio by real time location in a print...:

@CNCModeller
I recently proposed to use a Prusa MMU or enraged rabbit to cut filaments according to the extruded length and feed these pieces of filament through a Bowden tube for printing.
..........................

That's how the Mosaic Pallette claims to work - no need for a separate MMU. It wouldn't be any good for the OP though as he wants to blend two or more filaments together in various different ratios.

[deckingman]

@CNCModeller said in Varying nozzle mixing ratio by real time location in a print...:

............. that'd be interesting, so potentially we could do look ahead to allow for the delay in mix ratio reaching the nozzle throat?................

DC did at one time mention that he might implement my post processing method of advancing the tool chnage command within the gcode, into the firmware but he never did. I guess not enough users to justify the time and effort required.

Don't forget that when using any form of look ahead, to advance the tool or mixing ratio change means segmenting a move such that part of the move will use one tool and the second part the other tool.

That commercial design I mentioned which uses a paddle, is the Rencolor DM-10. I had a quick look and it appears that it only mixes two filaments, and the filament path involves 90 degree bends. I don't see how it would be possible to relieve pressure on the nozzle for non-print moves - retractions just won't cut it. The demo video shows a simple vase with no - non print moves and no fine detail as I suspected. But it might be worth a look for what you want to do.

[CNCModeller]

@deckingman said in Varying nozzle mixing ratio by real time location in a print...:

That commercial design I mentioned which uses a paddle, is the Rencolor DM-10. I had a quick look and it appears that it only mixes two filaments, and the filament path involves 90 degree bends. I don't see how it would be possible to relieve pressure on the nozzle for non-print moves - retractions just won't cut it. The demo video shows a simple vase with no - non print moves and no fine detail as I suspected. But it might be worth a look for what you want to do.

For anyone interested, Rencolor DM-10:-

https://www.kickstarter.com/projects/56949912/rencolor-dm-10-fdm-color-3d-printer