I’ve noticed over time, that there are certain situations where slicing software has a hard time finding a solution to, or just completely ignores geometry that needs support built under it. This is usually due to a kind of loophole created by rules stated within the slicer. For instance:
There is a setting in every slicer that determines the maximum angle a feature in your model can be relative to the build plane. Imagine you build a standard pyramid shell with all sides 25 degrees from vertical. Most extrusion printers can build surfaces more than a 60 degree angle sans support without issue all the way to the top. Nothing amazing here.
Now take the top of your pyramid and turn it back in on itself. Now you have an inverted pyramid pointing back down towards the build plane. This feature obviously needs support. Otherwise when the printer gets to the tip of the inverted pyramid hanging out in free space, it will have nothing to adhere to, and the print will fail.
But according to the slicer, this feature falls under the standard setting in most slicers of 45-62 degrees from vertical. The slicer ignores the feature, or creates an extremely thin support structure which is sure to move around and break off during printing. Think about the load applied to this structure by the extruder nozzle part way through printing:
This is the point where adjusting your overhang angle settings can solve the issue. But that can waste a whole lot of material, and be tedious if not impossible to remove. Another obvious solution is to rotate your part to print in a different orientation, but this write up is addressing the situations where there seems to be something wrong with every orientation you put your part in. At this point, with a little creativity, the best results are to be had by modeling in your own support.