Design your part with at least one flat side for adhesion to the print surface.
Avoid unsupported overhangs less than 45 degrees from horizontal of the print surface.
Unsupported bridges should be no longer than 3-4cm.
Fused filament 3D printed objects can not have floating geometries or features that are not attached to the model or print surface.
Support is automatically generated by slicing software and does not need to be designed into the part. Clearance for supported features is important to consider for removal of support material.
Minimum feature size 1mm
Holes print best that are 2mm or larger
Tolerances (degrees of variation) are +/- .2mm depending on part or feature
When models are exported for 3D printing, water tight features (completely enclosed) become hollow. So, a box enclosed on all sides will be a hollow shell. Features like this can be given density or made completely solid by changing the % of infill or shell thickness used on the part. Infill and shell thickness is not designed into the part, but adjusted in slicing software.
OnShape is free online CAD software.
Autodesk Fusion 360.