Gaps on single walls

I'm pretty happy with my Kossel Mini. My prints generally come out good but I seem to be having trouble with single-walled prints.

There are gaps where the nozzle ends with each layer. The surface finish is not so good either I think I have my extrusion rate dialed-in correctly. It only happens with single walls. What could I be doing wrong?

I'm using 0.4mm nozzle, 0.2mm layers, 0.5 layer width, PLA at 200C, retraction 5mm at 60mm/s, using latest Slic3r

IMG_0002 by Bok, on Flickr

IMG_0003 by Bok, on Flickr

Layer width should be a multiple of your nozzle diameter. if your nozzle is .4mm then you should have a .4mm layer width. Try that and see if it works.

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jaguarking11
Layer width should be a multiple of your nozzle diameter. if your nozzle is .4mm then you should have a .4mm layer width. Try that and see if it works.

I've been following Triffd Hunter's guide regarding extrusion (layer) width. I never had problems with it except with this and I don't think extrusion width has anything to do with my problem.

But I tried your suggestion anyway. I printed the twin cylinder calibration. No dice.

IMG_0006 by Bok, on Flickr

I tried lowering the temperature 5degrees every 5mm up and the surface finish got better at 190~185C and the gap is somewhat reduced but it is still not "watertight".

IMG_0008 by Bok, on Flickr

Edited 1 time(s). Last edit at 08/23/2015 02:05AM by boksbox.

Looks to be a layer switch problem, also looks very familiar but it was some time ago and I don't remember the details. What is your retraction length and speed? I assume you are using a Bowden extruder.

Yes, I'm using a Bowden extruder - retraction length of 5mm at 60mm/s.

A couple of thoughts.
I think retraction is involved - causing a loss of pressurization. Can you try a few things?
- In Slic3r print settings - Extruder 1 - try turning off Retract on Layer Change
- Minimum travel after retraction - change to a larger number like 10 - 15 to test

These will cause less retraction, and if retraction is the cause it should get much better.

Now a couple other things you could try.
- To reduce retraction time Increase the retraction feedrate. There can be a slight blob at the point where movement stops and a retract happens. This is enough to cause pressure loss on the start of the next layer.
- I would suggest reducing the unretract speed, but I don't see that option in Slic3r (I have it in smoothie firmware).
- Add a bit of "Extra length on restart". This will push a bit of extra filament when it unretracts. Try .1mm to .4mm. The filament deforms slightly when fed, and this can make up for that, but also can make up a bit of the loss while retracting.
- Reduce retraction length slightly.

Edited 2 time(s). Last edit at 08/23/2015 11:04AM by Paul Wanamaker.

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My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
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Thanks, Paul.

You are right. It's pressure loss and nothing is coming out for the first few tenths of a mm after unretract.

First, I turned off retract on layer change and upped min travel after retration to 10 as you suggested.

The print went without a single retraction and came out perfectly watertight. However...

IMG_0011 by Bok, on Flickr
IMG_0014 by Bok, on Flickr

So a retraction is definitely necessary so I tweaked the other parameters you suggested. The "extra length on restart" was crucial. It would still need some dialing in but so far the print has greatly improved.

IMG_0029 by Bok, on Flickr
IMG_0030 by Bok, on Flickr

Glad that helped!
With further testing I find that the Extra Length on Restart helps with two things: slight filament deformation, and ooze. The longer the travel distance then the more Extra length is actually needed to compensate for the ooze.

The blob at the end is caused when the retraction happens while the printhead is stationary. This blob is worse with Bowden extruders because the retraction length is longer, and so has more time for the ooze blob to develop.

I have found a solution for the blob. I am using a standalone gcode visualizer, post-processor, and debugger I've written to develop features I'd like to see in Slic3r, and I have just tested something I call Retract while Coasting. The solution is to begin retracting about 1.3mm from the end of a segment (for my printer), then finish the remaining 50% or so of the retract, and no blob is formed. The extrude amount that is not done is added to the un-retract.

You can read about it in this post on Github.

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My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
Can you answer questions about Calibration, Printing issues, Mechanics? Write it up and improve the Wiki!

Awesome work you're doing there, Paul. I'm looking forward to seeing those features in Slic3r soon. Thanks again!