Under extrusion after retracts

[Nylocke]

I'm having a bit of an issue on my Ultimaker. The stretch of the teflon tube seems to be affecting extrusion after retractions or speed changes (slow perimeters to fast infill). I have the PTC connectors from E3D, and both of them have locks on them so there isn't play in the connector itself. It seems that when its moving slowly, there is less resistance, so it flows nicely and the teflon tube doesn't stretch much, but when it speeds up to do infill or faster perimeters, the change in extrusion velocity stretches the tube because of the back pressure from extrusion, and it extrudes less than it should. I know this is a "feature" of bowden extruders, but is there a way to minimize this?

The issue that seems to be worse is the retractions bit. I notice it a lot more after retracts. I "solved" the issue by adding an extra .4mm of prime for my retraction settings, but I've never had to do that for any other printer I've operated (and thats a long list). This doesn't seem like a thing I should have to do, does anyone have any alternate solutions?

[Eaglezsoar]

Have you tried a little Canola oil to lubricate the inside of the tube as described on many threads for printing PLA?

[Nylocke]

No, but I tend to be on the side of MHackney, I don't really agree with the canola oil thing. Before, I was having jamming issues with the PLA, but I took the nozzle off and shortened the length of the .4mm bore a bit with a 1/8" drill bit (3mm nozzle, bore diameter is 3.2 so 1/8" is actually a bit small) and an 80 degree 2mm drill bit that was custom ground. That helped a considerable amount, but its not perfect.

[Polygonhell]

The issue after a retract is more likely that the extruder is actually skipping, are you using the Ultimaker extruder? It's geared and you can't run as high an acceleration or velocity as you would with a direct drive extruder.
Even direct drive extruders have some hysteresis in extrusion rate change, I assume you are using an Ultimaker hotend? I don't have any experience with them, so I can't comment on back pressure. Generally I try and avoid large speed differences between infill and exterior and if I'm trying to maximize quality I run both at the same speed.

[Batteau62]

geneb mentioned an idea, way back at the beginnings of this forum, that I thought was very good.

"it would be interesting to see if someone could come up with a PTFE bowden tube that's been extruded such that there's a star pattern inside the bore of the tube. It would greatly reduce the filament contact area and still provide a good guide. Kind of the reverse of how Sullivan Gold-n-Rods work for routing control pushrods in R/C airplanes."

It's in this thread: http://forum.seemecnc.com/viewtopic.php ... tar#p29359

I've just never been able to find anything like he proposed. I've always thought the design of a bowden tube could be better, especially the connection points.

[Nylocke]

Its a Greg/Wades style extruder, so its still geared, and an E3D V5 (says in my sig). I have the velocity diminished (I took my 1.75 speed settings (45 and 20), multiplied them by 1.75^2, divided them by 9 (3^2) and got 15 and 7 (15 prime, 7 suck) (in hindsight I don't know why I thought this was ideal)) but I've gone to 10 and 6 with similar results. Retract length is 2mm so its within the threshold that Michael has figured out though lots of printing.

I just put in a beefier motor, and I don't think the extruder is skipping, but its not out of the question.

After I added an extra .4mm to prime I haven't noticed any issues with speed differences and my prints have been perfect, but I still don't like this "fix" to the problem.

[Polygonhell]

It might just be the V5 you have, I have 2 and one of them has what looks like a short delay on restart, the best theory I could come up with is it's some sort of micro jam, where the build up in pressure finally overcomes the small plug leaving a small void after restart.
My other V5 doesn't do this. Neither do it with ABS.
Does the void happen with both PLA and ABS?

[Nylocke]

I can't really test with ABS, I don't have a heated bed on it currently. I've done some Nylon prints that I think came out alright, but its a bit more difficult to tell because of how translucent the nylon can be. I was thinking about making an Ultimaker style nozzle+heaterblock to replace the V5 one, other people have used their stuff from Ultimaker with their E3Ds, that might change things.