there's a guy on youtube who has a whole series about filling 3D prints with concrete.

Perhaps concrete but it can't really dry inside a printed object right?

Concrete doesn't dry; it hydrates and cures. It can harden underwater, let alone inside a container.

If anything, the potential problem with concrete is that curing is exothermic, so it might heat up and distort your part. It'd have to be a pretty big print for that to happen, though.

Concrete doesn't dry, it cures. It is quite possible to fill 3D prints with it, and it has been done.

Clean sand does work. But what you probably want is granite resin for this project. It's very popular with machine tool hobbyists and even expensive manufacturers like Hardinge.

Here is an explanation of how to use the stuff

I’ve used BBs, miscellaneous old nuts/bolts/screws, lead shot. Split shot fishing weights would work as well, if you can source them cheap enough.

I filled a print with pennies, and it worked out better than I expected. They didn’t get everywhere, though, so it might not be what you’re looking for. 🤣

I use casting plaster from a local hobby store. A little messy, but easy cleanup with water.

I use the gyroid infill and leave a 25mm hole in the bottom. Slightly over-fill then sand flat after it dries.

Adds a pleasant heft and is super easy to use.

I sometimes fill the bottoms of smallish boats with hot glue to get them to float well.

Pros: Glue spreads easily, fits crevices, and is pretty cheap and easy to use.

Cons: Bit messy, not particularly heavy (though does make the prints feel more solid), bit of a pain to sort out how much glue to use and at what intervals while printing.

Can't say whether glue would be a good fit for CNC machine parts though.

Ives tried to use resin to fill prints and it ended up leaking all over and making a mess. I'm sure with the right geometry and enough perimeters it can be done, but keeping the walls sealed is an important consideration.

My preferred approach to weighting models is to design in holes for bearing balls and press them in, with optional (and lightly applied) adhesive.

I watched a video a year or two ago about a guy printing a lathe, which he filled with concrete.

It ended up being kinda shit iirc

I've had good results using table salt to add a bit of weight to a chess set.

I have my doubt's that resin would distribute evenly throughout all the sand in the model. I also have some doubt's about the sand, maybe you can shake/vibrate the model so the sand distributes well, but it will probably be hard to get into all the small smaces depending on model shape and how dry the sand is.

Why not just straight resin as filler?