How to REALLY fix blobbing: Pre-retraction!

[626Pilot]

I posted a thread about this here on the KISSlicer forums.

The theory is this: Retraction causes blobbing because no matter how fast you retract (and you actually want to retract SLOW to reduce stringing), it can never be fast enough to defeat the hot end's tendency to ooze, unless you're printing thin layers at 20mm/sec. (And why have a Delta if that's all you're going to do?)

So, what do we do? First, ramp down the speed as we approach the end of a path, so as to reduce the "static ooze" factor (tendency to ooze even when not pushing filament). Second, start retracting before the tool reaches the end of the path. The way it is now, it gets to the end of a path, stops, and retracts. During the stop/retract phase, the ooze happens, leaving unsightly blobs. A pre-retract option would let us specify the length before end-of-path to begin retracting. Once dialed in, the feed pressure + static ooze would produce a correct extrusion until end-of-path, at which point the filament would be rewound far enough that far less (ideally zero) further oozing would occur until after the retract/wipe/lift cycle is done.

This could also help with printed supports. If you've printed a lot of tall stuff, you may have run into a situation where the support gets knocked over after awhile. The nozzle running into a blob left at the end of a support path may be enough to shear the relatively weak support. No blob means no running into a blob, so perhaps this could enable us to print taller things with support, without having to finagle the settings forever to make it work without falling over.

The physics are pretty simple; we're just removing the cause of the blobbing before the blobbing has a chance to occur.

If you think this is a good idea and you'd like to see it implemented, it will help if you can post in that thread. More different people posting will send the message that people care about this enough that the developer should spend time on it.

[bot]

Interesting. For ABS, I seem to have no trouble at all with retractions at the moment, but I have often thought along similar lines. I have "lift z on retract" enabled (or whatever it's called, I'm using Cura) and noticed that the lift occurs before the retract, when it would actually be better to slow, retract, THEN lift. as you say, without stopping to leave a blob. Cura does a pretty fine job at managing this when it's dialed in, but still there is room for improvement.

[rpress]

Have you seen the JKN algorithm?

http://www.makerbot.com/sailfish/tuning/#5

[jmpreuss]

S3D has a setting called coast where it stops extruding before it reaches a lift. I haven't played with this feature myself but I think it would do what you are thinking. I don't know if kisslicer has as similar feature or not.

[heathenx]

I thought of the same thing in S3D...the coast option. I don't have that feature turned on as I already get a satisfactory amount of blobbing anyway. Plus i print in white color so my eyes dont see the baddies. There is another feature in S3D that I use to control blobbing too. Force retract on layer change. Most of the time i have this off. Default is set to on. Usually makes a big differece when toggled on or off.

[0110-m-p]

I thought of the same thing in S3D...the coast option. I don't have that feature turned on as I already get a satisfactory amount of blobbing anyway. Plus i print in white color so my eyes dont see the baddies. There is another feature in S3D that I use to control blobbing too. Force retract on layer change. Most of the time i have this off. Default is set to on. Usually makes a big differece when toggled on or off.

Would be interesting to try coast on, retract on layer change off.

[JFettig]

S3d doesn't retract until it lifts however, I do use coasting and wiping but unless you want to spend hours calibrating each material, you'll have to deal with a little blobbing.

[rpress]

Have you seen the JKN algorithm?

http://www.makerbot.com/sailfish/tuning/#5

I often talk to myself, no reason to stop now.

Repetier-Firmware supports a similar thing to this algorithm and I'm using it now. There's some kind of noise issue so I've added a small boxcar filter. The algorithm certainly improves things. The idea is that it builds pressure when starting a move, and at the end of the move it reduces the pressure. This helps with blobbing like you talk about (reduces pressure before retraction), the warts on layer transition are much improved, and also sharp corners have less ghosting.

[bot]

Can you describe more about how to use this algorithm in rep firmware?

[rpress]

Can you describe more about how to use this algorithm in rep firmware?

https://github.com/repetier/Repetier-Fi ... ion.h#L255

I'm using a EXT0_ADVANCE_L of 300.

To be honest it's not really usable without hacking the firmware to add the filter, and the filter is just masking the problem somewhere else in the code.

https://github.com/presslab-us/Repetier ... 92#diff-34