Gaps at layer start

[bpolczyn 0]

Hello!

 

I've been trying to dial in a 0.6 nozzle I just installed for days. I have everything nice except I am getting a gap when the layer starts. I have "print outside walls first" turned on so I notice it, I know I could just flip that setting and hide the issue, but I really want to fix the issue now that I have noticed it.

 

I've been messing with retraction towers & cubes, combing, and coasting... but can't seem to figure out anything to help the issue. See some attached pictures from attempting to tweak those settings. 1mm retraction distance and 50mms retraction speed were the best results for stringing when doing earlier retraction tower tests.

 

I'm using this retracting test article and test print file:

https://www.matterhackers.com/articles/retraction-just-say-no-to-oozing

 

 

Anything else I can try?

 

Thanks!

 

Edited November 1 by bpolczyn

[gr5 2,265]

Make sure "retract at layer change" is turned off.  Look at it in PREVIEW mode and make sure there is no retraction happening at the layer change for other reasons (make sure no light blue move at layer change). 

 

Also maybe experiment with more acceleration on the z axis?  The faster the better until it loses steps so this is a whole different calibration thing.

[Slashee_the_Cow 480]

Also, forget not the prime directive: If in doubt, slow down. This applies to both speed and acceleration. Especially with larger nozzles. Many printers these days can go far faster than is advisable (or a good idea in general).

 

Most filament spools will list on the side the recommended speed they should be printed at. Don't go above that. Feel free to go below that, within reason, especially if you're using a bigger nozzle. Bear in mind that the "Print Speed" setting in Cura is extremely misleading: it will print infill at that speed (quality is less important) and everything else at half that, so double the speed you actually want (i.e. if you want to print at 60mm/s, put in 120mm/s):

 

Acceleration these days is a %#$! and could be your problem. Especially when your printer ignores acceleration rates in gcode and instead just accelerates as fast as the maximum in its firmware (not naming names, other than Creality). Unless you have specialty "high speed" filament, at the acceleration rates printers these days can go they'll zoom off before the filament they started printing actually sets and drag it behind the nozzle a bit. This can easily cause holes since the start of the layer has been pulled away from where it was printed. I've actually experienced it warp parts of models where they finish a section and do a travel move. In Cura, go to Speed > Enable Acceleration Control and turn it on:

500mm/s² might seem pretty paltry compared to what a printer might be able to do (4000mm/s², for mine... probably) but don't forget Slashee's Golden Rule™: Slow print > bad print. If you have a newer printer from almost any Chinese manufacturer (they're in a pissing contest to print a Benchy the faster) and it is or isn't made by Creality it may ignore the limits Cura sets and do whatever it wants. I used the control panel on mine to manually limit it to 1000mm/s².

 

Why this matters more with bigger nozzles:

Flow rate. Hot end power. Flow rate is roughly layer height * line width * print speed. Using a layer height or line width greater than what you can do with a smaller nozzle (and if you're not exceeding the limits of a smaller nozzle, you should use the smaller nozzle) results in a higher flow rate. The hot end of your printer has to be powerful enough to heat the filament going through it all the way through as it passes through. Higher flow means a bigger stream of filament to reach the middle of. If your filament isn't nice and molten all the way through, it won't form the intended shape, nor will it adhere nearly as well.

 

You can counteract this by lowering the print speed so that the flow rate is much closer to printing with a smaller nozzle (to a degree - no pun intended - you can also increase the hot end temperature, but that requires experimentation).

 

But Slashee, I only bought a bigger nozzle so I could print things faster - if I lower the speed it isn't any faster than the smaller nozzle?!?!

Well you're saving time in other ways - like how you can get the same wall thickness with fewer walls. But any decent printer should be able to keep up with the requirements of a 0.6mm nozzle as long as you're not trying to go at warp speed. It's when you get into a 0.8mm or 1mm nozzle territory that things get difficult.

[gr5 2,265]

If you go from 0.4mm to 0.8mm nozzle, that's double the diameter and 4 times the area so you should be able to print 4X as fast with the same nozzle pressure.

 

The best way to print faster is to then double the line width to .8mm (which everyone does) and double the layer height.  Not to double the "speed".

 

I have some 0.3mm layer height prints that I ship to customers and need to look "good".  goodish.  They look pretty damn good actually.  They are printed with a 0.8mm nozzle and print 4X faster than if I had used a 0.4mm nozzle.

 

If you are using a big nozzle, try a thick layer height.  It won't look as bad as you expect.

 

[bpolczyn 0]

Thank you both for your insights! I appreciate you taking the time to reply. Sorry to get back to this after so long but never got a chance this week to try some prints out. I guess I should've probably had some more specific in my original post. I'm printing on a Crealty Ender 3 s1, and I've attached some settings screenshots 

 

@gr5 - I did already have "retract at layer change" turned off. But the travel distance might be causing a retraction anyways (if I understand retraction correctly... The more I read the more I get confused 😅) The final nozzle move of the layer is going back from far bottom right to top left, which is where the next layer starts. On the retraction cube test this is around 100 mm move. This distance I believe should trigger a retraction. I took a screenshot and you can see the light blue travel  line coming back.

 

@Slashee_the_Cow In my testing of different settings and using the view ALL settings, I did learn about the misleading "print speed" I've only got it turned up to 90mms (so 45mms on the walls) 
I did already have acceleration control enabled. Never messed with any of the speeds though (that I can remember doing at least)

So I've attached the speed settings I had running. Looks about the same except 2000mms at travel acceleration. I turned that down to 500 like your screenshot but no change noticed.

 

Also regarding the flow rate discussion I did a flow tower and 105 looked the best so my flow rate has been slightly increased. 

 

See attached two tests I just printed. One was with the settings I posted, and another with the travel acceleration turned down to 500mms.

 

 

Any other ideas after seeing my settings??

 

Thanks!

[Slashee_the_Cow 480]

Those misprints are too similar to be random bad prints, and they seem to vaguely line up with the infill:

(obviously they'll zigzag a bit as the pattern changes as you go up, however cubic with a density that low won't change greatly at that size).

 

Technically this is where I have to ask if you have Infill > Infill Before Walls turned on. I say "technically" because with that many walls it shouldn't make a difference either way.

1. Please post the Cura project file (.3mf, get it set up then go to File > Save Project) for this test if you can so we can dig through all the settings.
2. If you do have Infill Before Walls turned on, try turning it off.
3. Try using a predictable infill pattern like Lines and set the density higher (say 20%) and see what the results are with that.
4. If you have a way to get a clean cut of one of your tests, I'd be interested in seeing a cross section.
   Disclaimer: Always be careful when using sharp things. Consider using cut-resistant gloves.
   Cleanly cutting can be hard. No power tools, they'll melt the plastic. No bladed tools unless they have a single flat blade (like a guillotine), scissors and similar will crush the area around where you're cutting. A craft/hobby or utility knife might be able to pull it off, especially on something that small. If you run the blade under warm water to heat it up a bit, that will help, but with the layer adhesion it looks like you're (not) getting it shouldn't be too hard.

[gr5 2,265]

If you are in a rush to get going, try cutting the speed in 1/4.  That will probably fix all the issues. I'm not saying that's a permanent solution (or maybe it is).  So try like 25mm/sec for all speeds.