I was playing around with some inverted geometry (inward normals) as a means of creating “hollows” more simply inside an otherwise solid mesh in Preform.
As far as I can tell, the slicer does a fairly sane job of subtracting “inverted” objects from the inside of a solid object.
I have had success (as far as Preform shows) both with placing the “hollow object” inside my solid object in the modeler, and by loading up my goal object and ‘positioning’ stl’s that I load that represent the hollowed sections (meaning, the stl itself IS the inverted object).
In the latter case, it works the same, but admittedly Preform is not made for moving individual STL parts around vertically. If we could just move objects up/down, this approach would allow you to have some stock “hollows” that you bring in, scale, and position as needed. Better to do in the modeler though.
Overall, This may seem a bit hacky, but it does have some advantages - namely that you do not absolutely need to keep strictly manifold geometry if you can place one or more ‘negative volumes’ inside your object mesh WITHOUT intersecting the inner walls of your object.
I suspect that based on what I’ve read on the forum, there is still a potential need to connect the inner surface with the outer one to let pressure escape?.. not sure how critical that is, if the structure is generally not too thin. Maybe it only matters in really thin walls where the pressure builds up?
One other big advantage of this approach is that “hollows” would not need the same number of triangles for the inner shell that you need in the outer shell. Meaning, you could have a 1 million triangle outer mesh, and have one or more negative inner shapes that are represented by significantly fewer polygons. This would lighten the load on the modeler and the file transfer/load/print process (or so I would think).
I’m running a basic test print now, but I was wondering if anyone else has tried this?
Here’s a picture of what the slicer is showing me for a simple cube-within-a-cube. The inner cube has normals flipped. I have also verified that if the inner cube overlaps the outer wall, things do get funky in the slicer… so you do need to avoid touching the outer wall. There is otherwise no connection between the outer and inner shape.
Here’s a pick of a different shape cube with overlapping negative ‘hollows’ inside. The hollow objects are overlapping, but they do not touch the sides of the outer shell. As you can see, this does cause the slicer to get a bit confused on the inside, but I am not yet sure how bad that is… less resin is good and if some parts generate more solids, maybe that is not so terrible? (still a bit hacky I admit).
Note to Formlabs Preform folks… if the slicer was able to ignore negative geometry that ventured outside the external wall - that opens up a LOT of great short cuts for punching holes in objects, because we could really treat the negative shapes as true boolean differences. Now THAT would be cool.