Hi RobotArms24!
The infill is set in the printer settings (not in the model) and it's typically only used for FFM-machines (the typical home desktop 3D printers). Shapeways will always make a solid model, I believe.
I usually print with something called "fast honeycomb". There are lots of alternatives, but I feel that fast honeycomb is a good combination of strength, speed, weight and also the zig-zagging gives less pull on the edges and less warping of ABS prints.
I use a software called Simplify3D. They have a lot of good information on their web site. This is a video on infill:
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https://www.youtube.com/watch?v=BMWTK2ZgJCM
And they also have and this very helpful 3D-printing troubleshooter:
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https://www.simplify3d.com/support/p...oubleshooting/
I don't know exactly the strength of different infill percentages, but to the hand it feels linear, i.e. 20% feels twice as strong as 10%.
Best regards,
Stein :-)