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tried searching but couldn't find anything. I do not have a 3d printer so can't really experiment on my own, which means that when I am going to order a 3d print I want to get it as good as possible. So, my question:
Do quality of geometry matters when 3d printing? Will 3d printer only print quads, or ngons are fine? Are there shapes to avoid?
Cheers :) M.
Welcome to the site!
In regards to quads, vs polygon. People will often reduce the overall detail to make it easier to print. But so long as after you export it to a STL and verify that your Manifold edges were done correctly and what you though was solid is solid, you should be good to go. As near as I can tell so long as you can export it to STL then it doesn't matter what meshing you use. That said I see Polygons more than anything.
Worst case you can run it through a STL repair program and it will make the required changes for you. Usually I used these tools to fix poorly rendered files. My favorite is Nettfab. which is now part of microsoft.
You can verify if it will print by downloading slic3r, then "slicing" the file. After that you should be able to view a layer by later output.
Article talking about quads vs triangles from design
Shapeways article on preparing blender files for 3d printing
If you're talking about the shape of the end result, rather than the constituent elements of the model - the answer is no, there is no simple geometric restriction. Have a look at 3D benchy for an example of how print quality can be affected by different aspects.
One obvious issue is overhangs, so the orientation of the part is important for printing. A flat circle will (on a cartesian printer) come out smooth as X any Y move in sync, and have good support. A vertical circle will have steps introduced by the slicing which quantises X and Y from layer to latyer.
Sharp, un-supported corners may be the weakest aspect to resolve in the print - extruded filament tends to shrink as it cools, but as far as I know the errors like this can be reduced by printing more slowly (and reducing the dynamic flexing of the frame too).
Talking about a top surface of an extruded n-gon, if you look at the slicing output, you'll see infill in the bulk, with a layer filling only the top 3 or 4 layers. There are several infill patern options, but yes, there is scope for a scenario where the top layer needs to bridge a long way. However, since the alternate layers fill in orthogonal directions this should be a minimal effect. Tweaking the infill percentage, or chamfering the corners can fix these small defects.
