How are the extruders synced?  

Re: How are the extruders synced?

I looked for an answer to this on the announcement page and embedded vids, but I didn't see any mention (though it's possible I missed something).

My understanding is that there's an extruder on the MMU 2 as well as the MK3. Are these extruders synced, and if so, how?

For example if the MMU extruder is slightly faster than the MK3 extruder, the filament might get compressed.
OTOH, if the MMU extruder is slightly slower than the MK3 extruder, the filament may be stretched. Either situation could be potentially problematic, and cause additional stress on the extruder steppers.

Has anyone experienced this, and if so, what's the solution?

In the handbook you can see that the mmu2 needs a Bowden length calibration, which means that the mmu2 controller knows how far it needs to feed filament until the printer extruder begins to pull on it (autoload function).

As soon this defined Bowden length plus few mm are pushed by the mmu2 extruder motor, the mmu2 idler (revolver) will go into disengaged position so the filament can be pulled freely by the printer extruder.
If the printer fails to detect filament movement it will send a continuing command to the mmu2.

To prevent constant filament friction on the mmu2 extruder gears the filament path inside the mmu2 is slightly bended and in idler disengaged mode the loaded filaments look like a tiny bows.

Hope that helps.

Re: How are the extruders synced?

Motor powers are also "tweaked" during the loading process.

Peter

Re: How are the extruders synced?

So basically, it is a handover thing?
1. The MMU pushes the filament far enough so the printer's extruder can take it.
2. The the MMU extruder disengages and the printer extruder pulls it?

Doesn't that make the MMU more of a bowde-drive rather than a direct drive? 😕
How does that affect flexible materials? (or does it not matter as there is much less resistance as for pushing it into the hot-end?)

Re: How are the extruders synced?

So basically, it is a handover thing?
1. The MMU pushes the filament far enough so the printer's extruder can take it.
2. The the MMU extruder disengages and the printer extruder pulls it?

Doesn't that make the MMU more of a bowde-drive rather than a direct drive? 😕
How does that affect flexible materials? (or does it not matter as there is much less resistance as for pushing it into the hot-end?)

Yes, it is a handover. As for it being a "Bowden drive", it depends on how you define it, but I don't really think so.

In a Bowden drive system (like my Ultimaker), the extruder motor/gear is a full 12 inches of tube away from the hot end. Pushing filament through an empty tube is no big deal (little or no friction), but once the tip of the filament hits the hot end, the drive has to push the filament hard enough to force the filament into the melt zone, enough force to push the previously melted plastic out the tip. If you are running slow enough and hot enough with a rigid filament, it works fine. However, if you start running colder or faster feed it takes more force to push out, and if you have flexi-filament you are pushing on a spring making it harder to get enough force without the filament bending or expanding in the tube and adding friction.

None of that applies here. The MMU2 drive is only loading/unloading the filament in a very low pressure environment. Relatively short tube (looked like 6 inches or so) and it can stop feeding as soon as it gets past the extruder drive (doesn't have to push it into the hot end). Then they swap over and the extruder drive (very close to the hot end) does all the work.

The only place I would see a problem is if the retracted filament doesn't have a nice tip. If the filament has an expanded tip that jams in the tube, the MMU2 motor could have an issue pushing flexi filament. However, since that tip was retracted through the same tube after the previous melt, it should have had an opportunity to mold itself into a correctly sized blob during the retract. I think a critical aspect of the firmware will be the exact timing and speed of the retract to keep the tip well formed.

Re: How are the extruders synced?

Hope that helps.

Somewhat...

In the handbook you can see that the mmu2 needs a Bowden length calibration, which means that the mmu2 controller knows how far it needs to feed filament until the printer extruder begins to pull on it (autoload function).

As soon this defined Bowden length plus few mm are pushed by the mmu2 extruder motor, the mmu2 idler (revolver) will go into disengaged position so the filament can be pulled freely by the printer extruder.
If the printer fails to detect filament movement it will send a continuing command to the mmu2.

To prevent constant filament friction on the mmu2 extruder gears the filament path inside the mmu2 is slightly bended and in idler disengaged mode the loaded filaments look like a tiny bows.

This is where I (think I) can still see some potential problems. We already know that some filaments can cause problems with the filament detector in the printer, necessitating disengaging/turning off the sensor. If/when this needs to be done, (if I understand the above explanation) won't the continuing command sent to the MMU allow/cause both extruders to continue operating, potentially creating tension/compression as I described in the OP?

Not trying to be argumentative; I'm trying to figure out/learn/understand this tech, which is all new to me. 🙂

Re: How are the extruders synced?

This is where I (think I) can still see some potential problems. We already know that some filaments can cause problems with the filament detector in the printer, necessitating disengaging/turning off the sensor. If/when this needs to be done, (if I understand the above explanation) won't the continuing command sent to the MMU allow/cause both extruders to continue operating, potentially creating tension/compression as I described in the OP?

Not trying to be argumentative; I'm trying to figure out/learn/understand this tech, which is all new to me. 🙂

That's a good question. I'd _hope_ that if the filament sensor is turned off, the MMU would push the filament the right distance and assume that the filament made it to the extruder.

It's also thought provoking that for filament detection in the MMU 2 that they use a PINDA and a metal ball pushed up by the filament, so they know filament is there based on physical displacement rather than by a camera seeing motion, so it shouldn't be sensitive to differences in filament. Which makes me wonder if the filament sensor in the hot end should do the same? Though then it wouldn't be able to sense when the filament stops moving (jams, tangles), so there would be a loss of functionality. But the filament load/out detection should be more predictable. I wonder if jams could be detected by back-pressure on the stepper motor somehow?

Re: How are the extruders synced?

In the handbook you can see that the mmu2 needs a Bowden length calibration, which means that the mmu2 controller knows how far it needs to feed filament until the printer extruder begins to pull on it (autoload function).

As soon this defined Bowden length plus few mm are pushed by the mmu2 extruder motor, the mmu2 idler (revolver) will go into disengaged position so the filament can be pulled freely by the printer extruder.

I had logged off and gone on with my Sunday, but being a lifelong designer, I couldn't (easily) disengage my brain, and another thought occurred to me about this explanation....

I'm not sure how "free", the "freely" above actually is/will be. The filament will now be pulled through (essentially) a Bowden tube, and a whole 'nother filament path. Disengaged idlers notwithstanding, this has GOT to add additional drag/load on the printer extruder motor. There've been many threads/posts about "hot" printer extruder motors. I've noticed on a few multi-hour prints that my own printer extruder motor got quite warm--never so hot that I couldn't keep my fingers on it--but warmer than any of the other stepper motors (probably to be expected). So far this has not concerned me.

But it got me to wondering if, when originally sizing the printer extruder motor, was the additional drag of the MMU2 filament path figured in to the calculations? IOW, is the MK3 extruder motor up to the task of the additional workload that will be required over time or will it need to be upgraded? I understand that the MMU2 has gone through a number of design changes; was the original printer extruder motor sizing considered all along the way?

My own way of designing always includes an over-design factor for unforeseen contingencies. However I also understand that in business, cost is a significant factor, so the smallest motor that will do the job is often spec'ed in lieu of overdesign. That's fine. I have no idea how Prusa approached this, nor am I making any assumptions--it's just a question (Will the MK3 extruder motor handle the additional load?) that occurred to me.

Re: How are the extruders synced?

I'm not sure how "free", the "freely" above actually is/will be. The filament will now be pulled through (essentially) a Bowden tube, and a whole 'nother filament path. Disengaged idlers notwithstanding, this has GOT to add additional drag/load on the printer extruder motor. There've been many threads/posts about "hot" printer extruder motors. I've noticed on a few multi-hour prints that my own printer extruder motor got quite warm--never so hot that I couldn't keep my fingers on it--but warmer than any of the other stepper motors (probably to be expected). So far this has not concerned me.

This is a good thought, but the force required to pull the filament through even a fairly long and bendy length of PTFE tube is much less than the force required to extrude the melted filament through a small hole in the nozzle.

So I'm not expecting that any changes to the extruder stepper will be required.

Re: How are the extruders synced?

Forget the Mk3 filament sensor (for now), that is not used for MMU2.

Currently the FINDA senses the filament load, the filament is then fed the calibrated distance to the top of the Bondtech drive pulley and then the filament is fed a further 15mm as the Bondtech takes over (seen my earlier comment), with varying speed and power.

It's actually a very good system, except for one problem, and that is stringing on unload which can cause tips to be elongated, strings can cause the FINDA to fail to un-trigger (which leads to over-unloaded filament which will not then reload) and strings can bet into the FINDA ball chamber (preventing the ball from proper movement).

Stringing is dealt with in my "Troubleshooting" thread.

Peter

Re: How are the extruders synced?

Doesn't the MMU 2 basically become a reverse Bowden setup once filament is loaded?

Re: How are the extruders synced?

Doesn't the MMU 2 basically become a reverse Bowden setup once filament is loaded?

As I understand it, once the filament is loaded, the MMU is out of the picture: it disconnects and lets the filament pass through, pulled by the printer's extruder.