15.2.6 Object design and model file problems

Some object designs simply aren’t very printable.

Sometimes that’s because the shape of the object just doesn’t easily lend itself to 3D printing. An object may include features or geometries that are challenging to print successfully. (For instance, keep in mind that often OBJ file models were created with no consideration of 3D printing.) Rotating the orientation of an object on the build plate, printing with support, using extra adhesion, using extra cooling, or sometimes re-designing an object to print in separate pieces, are approaches that can solve some print challenges.

Other times, a 3D model may benefit from some “repair” to its underlying .stl file or OBJ file. (This may be encountered more frequently when uploading object files generated by other designers using CAD or modelling software oriented primarily towards purposes other than 3D printing.) When the designer or CAD/design software that created a model left a few problems (holes in the mesh,³ volumes that intersect along a line, volume intersections where the boolean choice of what’s removed or retained doesn’t really make sense, etc. – in general, anything that leaves it unclear what is the “inside” vs. the “outside” of the model), then running the model through “STL repair” or “mesh repair” software may help. If you see that infill material is getting printed on what is intended as the “outside” of your object, while being omitted from what is intended as the “inside” of your object, this is a clear tip-off – likely corresponding to a spectacular print failure! – indicating that the object files need repair. Other symptoms could be: expected parts of the object are entirely missing, or the object “falls apart” into separate component volumes instead of being bonded as a single volume. Inspect the object’s rendered image before you attempt to print it: rotate your view of it on the build plate or via the object detail screen’s 3D visualization overlay. Oddities in the object’s rendered image – such as for instance if “inside” and “outside” bizarrely switch, or if some rotated orientations seem to show you an “x-ray view” of the object – are another tip-off. Some free mesh repair software is available – try an Internet search. Or quality professional CAD software, such as Autodesk, can have quality mesh repair features.

Note that models that use few polygons allow for less detail; models that use very large numbers of polygons and hence more detail may have huge underlying files and hence be slow to manipulate (i.e., render an image, or slice for printing). Note also that .stl files come in two possible formats, ASCII format vs. binary format; an ASCII format file can be many times larger than a binary format file for the same object, so you can’t necessarily judge the resolution/detail level of an object file simply by the raw file size. When viewed outside the Polar Cloud, OBJ file models can look deceptively detailed: even when the underlying model shape is not itself especially detailed, the additional display of surface texturing and coloring from an OBJ file’s companion MTL file can make it appear that the model is quite detailed. However, while that appearance of detail works fine in online visualizations or animation, for 3D printing, it’s the actual detail in the shape of the model that really matters (unless you plan to paint on details after printing); you can see how detailed an OBJ file object really is by inspecting its rendered image on the Polar Cloud build plate screen.

Also note that your printer’s “Nozzle Diameter” limits how fine of detail your printer can effectively reproduce.

So, to recapitulate:

• Objects from external sources designed without consideration of 3D printing feasibility may include features (lots of “bridging” or protrusions, tiny details, small bottom of print compared to increased volume higher up) that will be challenging to print.
• 3D model files from external sources may not be quite “3D print ready”, if the model file has “holes” or non-manifold surfaces. Using “mesh repair” software on such a 3D model file may solve the file problems and allow it to be successfully printed.
• 3D model files comprising few polygons (hence “small” in overall size) will tend to be easier to print, but include less detail. 3D model files comprising very large numbers of polygons can include more detail, but will be slower to process.