Getting Rid of Blobs With PETG?

[gwhite]

I have a MakerGear M2E, and I'm printing with HatchBox PETG. It's working pretty well, but I'm getting blobs of extra material in places. It is quite repeatable, so it's not a filament issue. I can see in the S3D preview mode exactly where they are occurring. It shows up as a small speed up in the extruder path. It doesn't happen all of the time, but when the extruder has just finished making an outside perimeter "bead", it doesn't retract the filament fast enough, and it ends up extruding material on top of the beginning of the bead, creating a blob.

Here is the sequence of events:

The extruder is coming from the left, just about to finish the outer bead (G-code Line 265441). You can see the red retraction where it had lifted up & then set down before starting the bead:

Next, it completes the loop at normal speed (G-Code Lines 265442-265444):

Here's where it picks up speed, and appears to lay down another bead on top of the existing one (Lines 265445-265449):

This is exactly where I get a blob on the actual print. It starts out big & fades out as it goes around the corner, and you can see in the preview that the extrusion has shrunk to a thin line before it goes off elsewhere. (too bad I'm limited to three images...)

Here is the G-code:
Line Code
265441 G1 X109.398 Y88.423 E7.7658 \
265442 G1 Xll0.249 Y88.594 E7.8044 | Normal extrusion as it approaches corner
265443 G1 Xll0.995 Y88.900 E7.8402 |
265444 G1 X111.732 Y89.389 E7.8795 /
265445 G92 E0 Resets Extruder value to zero
265446 G1 X112.297 Y89.979 E-0.6120 F3200 Speeds up & begins retracting filament
265447 G1 X112.706 Y90.661 E-1.2087 \
265448 G1 X112.940 Y91.407 E-1.7947 | Continues around corner while retracting filament
265449 G1 X112.965 Y91.679 E-2.0000 /
265450 G1 X112.994 Y91.992 F3200 \ Continues on for a bit more without extruding
265451 G1 X112.994 Y94.011 F3200 /
265452 G1 Z43.093 F1200 Lifts extruder (actually drops bed) in preparation to go elsewhere.

So, I think I've pretty much nailed what is going wrong. What I don't know is the best way to fix it. I've got a 5mm wipe turned on, with a 2mm retraction at 2400 mm/min, with 0.6mm vertical lift. I've got "Perform retraction during wipe" turned on as well under Ooze Control.

Should I crank up the speed and/or distance on the retraction? I think I'm already retracting as fast as it can go. I also think I'm retracting enough distance, it's just not occurring soon enough. I almost need to start the retraction BEFORE the bead is complete, but I haven't found an "anticipate blob" parameter to play with.

Should I slow down the wipe? (not sure where to set that). Any other tricks I should try?

Thanks!

[gwhite]

I've done some digging, and it looks like the magic parameter is going to be "Coast", possibly balanced with tweaks to the Restart Distance, both of which I have turned off at the moment.

Does anyone have suggestions for values to start with? I've seen coast numbers for PETG anywhere from 0.2 to 1 mm.

Part of my problem is that I haven't found a good (small) test file that produces this problem. The place I've seen it a lot is on a 5 1/2 hour job, and I'll run out of filament before I nail things down. I'm going to try to chop out a particularly bad section of that & see if I get the same results.

[MikeO]

Or you could try just dropping most of your to-be-printed object down below the build surface, so S3D will be starting the printing at the level just before you get to the problem area.

I don't know your part's structure, so that may not work, but I've done that during testing and it's a big time-saver.

[gwhite]

Ah! I didn't know that was an option. I'm still very new to S3D.

In any event, I chopped off all but the top half inch or so of my model & trimmed it down to one corner where I see a lot of issues. It's working nicely as a test vehicle.

I've cranked the "coast" setting up to 1.5mm. Any higher and I start getting dimples. That helped the blobs just a little. Certainly not the miracle cure I was hoping for.

I'm now experimenting with the Restart Distance parameter

[wirlybird]

Another thing to watch for with PETG is small amounts of filament gathering on the end of the nozzle. If you see this then reduce extrusion multiplier slightly. Possible this is the blobs also.