Gap following a move without extrusion

Willl

Hi, I fitted a D3DWifi to my Ender3 and I don't know if I have it configured correctly, but I have a weird issue printing.

I'm printing with Cura4.2.1 but I've tried 3.6.0 and have the same issue (but less clearly as the profile is for the CR10 and a little messy).

My printer is modified with "direct drive" (really a short bowden - https://www.thingiverse.com/thing:3373107)

If I print a stringing test https://www.thingiverse.com/thing:2219103 then the result looks pretty good.

I have retraction set to 1mm@25mm/s, Prime 50mm/s, but i still get this if i turn retraction off.

Coasting is switched off (makes no odds)

I have Pressure Advance set with M572 D0 S0.250, but this seems to make no odds.

At the start of a extrusion there just appears to be a section where the extrusion doesn't happen, or if it does it grossly under extrudes. I can move this one around the print by changing the z-seam
underextruded side

Here you can see it on the top of left starting the external wall, and not so obviously at the bottom right starting the inner wall.

Finally, here you can see it so pronounced that you can peer through all layers of the wall and into the infill

I'm stumped, any ideas?

burtoogle

You will get those kind of under-extrusions if you have combing enabled and you don't have the max comb distance without retract setting set to something non-zero. This setting is fairly new and defaults to zero which gives poor results but is compatible with the old days. Personally, unless printing PETG or similar, I will use the no-skin combing mode with, say, 20 for the max distance without retract. For PETG, I don't use combing at all. Hope this helps.

Willl

setting: combing mode = off

so i don't think its that

Sethipus

That seems to be a very high pressure advance for a direct drive. Have you tried it with something more like .1 or thereabouts?

grizewald

Interesting!

I noticed a very similar thing on a print I just completed. It's printed on my big core-xy printer with RRF 2.03 using PLA and Cura 4.2.1. My extruder is almost direct, I'd say there's about 35mm of tube between the BMG and the E3D V6. Pressure advance is enabled at 0.11.

There are four large holes on this print and on one of them only is a very obvious group of missing layers in the skin. The object was printed at 80mm/s and 40mm/s on outer walls. There are no similar defects anywhere else on the print. Combing was on and "Max comb distance with no retract" is set to 1mm.

Please ignore the rough filing marks on the hole, they only look bad on a macro photo.

alt text

I can't see that this is a problem with the model or the filament. I printed over 500g of this PLA over the last few days without any other problems.

Willl

@sethipus Thank you This appears to be it. I thought I had done a set of tests to isolate that, but something must have been wrong with them, my test print seems more solid now!