Knitted Appearance of Outer Skin

I have a really odd problem, one I only see with Cura, but not with Print Studio.

I have a Prusa i3 running marlin, and most of my prints are excellent, if I say so myself.

If I have a print that has large sections and smaller sections, I get patches with apparent under-extrusion. In the current example I have a large semi-circular tube printing fine, but two 5x7mm areas are printing the walls with a 'knitted' appearance, it's as if they are made out of foam. The appearance is as if the filament is arranged in angled lines, like chicken wire or knitting.

The attached photo aims to show two fails of this kind.


I have also seen the effect where I have thin walls with a thin layer of fill between them, or where the outer wall has an intricate shape (e.g. imprinted text).

It's as if after Cura has programmed some filling of a small or complex area, the flow takes a while to build up.

Often it's cosmetic, but in this case its seriously weakening come hinge parts.

I have got temperature, flow rate, filament diameter etc. all dialed nicely for this filament. I've just fitted a new tube and nozzle too. Upping the flow percentage just degrades the rest of the print, which is otherwise spankin'. Infiul speed is 80 ms-1, with 60 and 65 for outer and inner layers of skin. Cura 14.12.1, I'm not entirely comfortable with Cura 2.5 yet.

Has anyone else ever seen this?

Edited 1 time(s). Last edit at 07/20/2017 05:25PM by Stub Mandrel.

... this could be caused by overtemperature in small areas (not enough time to cool down) -- try with a cooling fan or a pause loop for the next layer ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I'd say it is retraction related. Everytime after an unretract the chamber pressure isn't there.
What are your settings?
Direct drive or bowden?

Thanks. I think that unlikely, the head is printing four passes or more on about 22 cm of perimeter before moving to that area.

I THINK my problem is localised under-extrusion, its as if the head is over-supplying the previous long section, and then when it retracts for the move to the small area, it isn't properly priming the nozzle?

Quote
o_lampe
I'd say it is retraction related. Everytime after an unretract the chamber pressure isn't there.
What are your settings?
Direct drive or bowden?

It's direct drive Mk 8.

I may have found the problem.

It looks like the old nozzle was worn out (much faster than the previous one), It looks like its >0.6mm but it's marked 0.4 on the side!

I've replaced it with a new one , but I'll have to watch closely for this one wearing too as its from the from the same batch. There's a lot of different types of 'brass' out there; if I can manage to drill it 0.4mm, I may try making my own ones out of phosphor bronze.

Now doing a test print.

... you can "compress" nozzles to smaller bores by hammering on the tip and reopening it with the wished diameter drillbit - so I've "reduced" 0.5mm nozzles to 0.25mm

Shouldn't be a problem to "repair" your worn out one with this methode ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Useful tip, thanks.

Worth keeping handy for when I want to print a really finely detailed part.

BTW it does seem to have been the worn nozzle.

Strange how nozzle wear is never mentioned in fault finding guides. My kit came with advice to change the nozzle every two reels and I should have paid attention!

Quote
o_lampe
I'd say it is retraction related. Everytime after an unretract the chamber pressure isn't there.
What are your settings?
Direct drive or bowden?

If it's not the nozzle, then we are back to square one again.
Did you check your retraction is functional without skipping steps while unretracting?
That would cause underextrusion like I see in your pics.