Finish on the underside of a supported section

[jens3 10]

I am fresh out of ideas as to what causes what I am seeing and more importantly, how to fix it.

The attached picture shows the underside of a part that is printed with full support, a roof in the support. There is  0.4 mm gap between support and the model.

There is a narrow strip on the far left that is on the glass build surface. The next section is maybe ten times the width of the part that sits on the glass. It is flat and when cura prints it it prints alternating 45 degree right lines and 45 degree left lines. It produces what I would call a perfect finish on the underside as you can see. The next / remainder section is where the problem shows. The model is no longer flat so Cura can no longer do the alternating 45 degree lay downs but it forced into printing individual lines and as you can see the entire thing goes to pot with a horrible and unacceptable finish. Not only that, but if you look at the area where the extruded filament changes from horizontal to vertical travel, the lines are not necessarily laid down right between each other.

 

So, why do I get a perfect roof and an extremely messy roof section in the same print? What is going on and how can I fix it.

It almost looks like the support is somehow too far away from the model but reducing the gap makes pulling the support impossible.

BTW, this is printed in PETG, 250C extruder, 60 mm/sec speed. I can't see the temperature or speed to be the cause because I get a section of perfect finish which probably indicates that the parameters are correct. It seems to me that I must be missing a fundamental parameter on the roof support that shows up when the roof is not flat.

 

 

Edited December 2, 2018 by jens3
fix typos

[jens3 10]

33 views and no thoughts ? Anyone ?

[IwanBeentjes 1]

maybe it is the print order? walls before infill would mean that your wall will be printed in air ??

 

just giving you some ideas!(A)

 

[P3D 46]

Walls before infill shouldn't have an effect on this, as far as I know. I also struggle with supports in Cura, and sometimes I get similarly messy results, even with the support roof (or especially with a support roof?). If you have a dual extruder setup, trying breakaway support would be worthwhile I think. Otherwise, I would experiment with different support strategies, densities and gaps - and maybe also printing slower.

[jens3 10]

Thank you for the suggestions. I will try them all out tomorrow.

This is driving me nuts, especially since the flat overhang prints perfectly in the same print!

Unfortunately I only have a single extruder system ... but again, why do I get a perfect result on the flat portion of the print and then garbage on the sloped part. I would be happy (but disappointed) if I would get similar results on the entire underside, flat and curved - but the way it is printing one section fine one one section terribly bad has me feeling like somebody is sticking their tongue out at me and daring me to figure out why this is happening.

I will design a smaller test object tomorrow as the original print takes about 16 hours 🙂

Going through all the support options tonight, the only possible issue I can see would be x/y support distance priority over Z support distance and I will check that out as well.

Again, thanks for the suggestions!

[jens3 10]

I tried printing slower, both increase and decrease in extruder temperature, more of a gap between support and model, less of a gap ... all without appreciable improvement.

Early on in my tries I managed ts switch the support roof from parallel lines to zig-zag which improved the support surface by a very large amount but unfortunately did not help any with the finish on the model side.

The model side is laid down as single parallel lines. I believe that the lines sag to meet the support and in some cases lightly stick to the support. Because we are talking very thin lines that have sagged and are not really held up by other lines higher up (or at least not much), these lines might stick to the support or the model causing a terrible finish. This is by no means the only issue ... the other issue is that the lines are not necessarily laid down parallel, even with a speed of only 20 mm/sec and because the lines are not supported they can move sideways causing more of a mess. This is all conjecture of course.

Anyway, as I have mentioned, I managed to get the support structure roof to print in a zig zag pattern. The zig zag covers two parallel lines but it does give structure. I have been unable to produce a zig zag pattern on the model side ... it always comes out as parallel lines. I suspect that if I can get the mating surface to the support roof to also print in zig zag fashion then possibly I would see a good surface finish improvement.

 

If anybody has a brilliant (actually even if it is not brilliant) idea as to how to force a zig zag, I would really appreciate it if you could share the method.

Currently I have the support interface enabled with a thickness of 0.8 mm, a resolution of 0.1 mm, a density of 100% and with a zig zag pattern.

The model has a top and bottom thickness of 0.6 with top/bottom pattern being zig zag and bottom pattern initial layer being zig zag as well. I do not know if I am missing any other settings to force a zigzag. Heck, I don't even know if is possible to force a zig zag because there is a slope and each line laid down would be laid at a slightly different height which probably prevents zig zag.

 

I am fresh out of ideas as to what to try next 😞

 

Hmmm, I think I will try increasing the bottom thickness y a couple more layers just for the heck of it and while I am at it go from 0.2 mm / layer to 0.1 mm / layer

 

 

 

[jens3 10]

The 0.1 mm resolution print came out quite nice ... on all surfaces except the surface that was giving me issues. It still prints as single lines instead of zig zag and the lines are quite messy. The support interface layer came out quite smooth and solid but as long as the lines are laid down parallel on the support, this thing isn't going to fly 😞

I don't know if the geometry is forcing individual lines or if I am missing some magic setting somewhere .

[jens3 10]

Just a followup - it turns out that the lines I am seeing are wall lines of the model and at my knowledge level they are always straight lines that depend on proper support to squish into each other. Substantial improvement was obtained when printing without walls and 100% infill. Of course this had some negative effects on the outside of the model.

In a wild leap of faith I completely changed the way the model was printed and I used tree support and got a very acceptable result.

[Smithy 1,141]

Thanks for the follow-up, please keep us informed.