Is it possible to take an existing stl file and reverse the dimensions and 3D print a mold?
Something like, make a box slightly larger than the object, then have the box “hollowed out” using exact dimensions of the object inside the box, then split that box in half to use as a mold?
Your expertise is appreciated!
The concept @DaveR shows is sound. But I’d think a lot would depend on the stl. Many sources create stl’s that are a horrible spider web of skinny triangles. In such cases you are likely to have issues with small edges unless you put a lot of effort into cleaning up the imported stl. It might be easier to redraw a clean version in SketchUp.
I already created the files and they aren’t very complex. But my expertise isn’t quite up to being able to do this quickly. I’m slowly figuring it out, but I’m sure there is someone who could do it in seconds. 
Fireplace 4. Cap.stl (17.2 KB) Fireplace 4.1 Case Bottom.stl (50.8 KB) Fireplace 4.1 Case Top.stl (76.9 KB)
I’m hoping I can make a silicone 3D print and use for a mold. I can have these made with an aluminum injection mold using PET plastics, but it’s very expensive. I’m exploring the option of silicone molds as it appears to be a cheaper process.
I invented a device that lets you control your gas fireplace via an app on your phone and trying to figure out mass production. So I only have the beta version working.
There is also a mold function in Cura.
I never used it, so can’t tell you more about it…
Thank you! I’ll give this a try.
I found that the importer (or maybe the stl originals) left some holes and other flaws that kept the objects from being solids in SketchUp. For example, the green triangles in this image are places where you are seeing through to the inside (I use a bright green for the back side color on faces to make it easy to see reversals and holes).
If you plan to create molds that a 3D printer will accept, you should find and repair these flaws first. I did the work for you in the attached, but you should give it a try yourself so you learn how.
Untitled.skp (211.8 KB)
Steve quite likely did it but if the STL needs repair, it often pays to make your STL import into a component (if it isn’t already) then scale up a copy by 1000 and use metres as if they were millimetres - it avoids problems that SU has creating small edges.
When you’ve fixed the scaled up copy, you can delete it, and the origin component will be able to keep even very short scaled down edges without deleting them, and the faces they bound.
Search the forum for The Dave Method, for a longer explanation.
