Some 3D printers use a second material that can be easily disolved in a certain liquid, it’s used to help with printing open spaces or pieces that are supposed to be completely seperated from others.
This second material is used as a little scafold or placeholder so that parts can be kept seperated at times. In the end, when your print is done. It’s washed in a solvent and the secondary material simply disolves and your seperate parts of your prints are free and disconnected from eachother. Ofcourse you’ll need a hole or 2 to make sure your solvent actually get’s to the places it needs to be.
I can’t remember which video explained this, but if you want to get an expert explanation on the matter, try youtubing for 3D printing a working wrench or something similar.
Also sorry to ask an off topic question, but about that support material:
In 3D printing the reality is that you can’t print in mid air, you need support material to make the top part. And the only way that support material comes out afterwards, is that you model some holes in, or drill in it afterwards.
Can’t you also just leave it there in some cases?
At any rate, solidify is probably your best bet, it’s easy to use and it’s also pretty decent.
All you need to do is make sure that your solidify modifier is set up properly.
You’ll want to set it up to go inward (offset -1 usually) and maybe even use “even thickness”