3DPrinting

The hardware store had 100 fender washers, or 1kg, for 1€, so I'll be using them for a while.

My solution has been pennies. They're cheap and and easy to get. Since they are a fixed size, it's really easy to model a cavity for them in your slicer. You can also leave them in their rolls for bigger projects.

Maybe bolts you can screw into your print, if you don't want to mess with it while it is getting printed

Used it as a pattern to cast it in metal

Last weighted thing I printed was a phone stand. Two 1"/25mm rusty old flat washers turned out to be a fine solution. Lots of used items you can "recycle" into you designs.

When wanting more weight, but not wanting to add anything extra to the print, I use modifiers in Prusaslicer to add an internal shape that I set to 100% infill in contrast to the rest of the print. So if I want a weighted bottom to prevent a printed object from tipping over, just throw the modifier in with 100% infill at the bottom of the model before printing.

I've been a fan of making false bottom pieces and using steel wheel weights in them.

Here's a lamp I made as an example.

Drop a penny or two. Or a nut. Stainless steel ones seem heavier. But cost more.

Just make a cavity in your print. Pause before it closes up. Drop the weight in. Also I pause and put in some glue when I drop it in so it doesn't roll around inside.

You could also make cutouts for pennies or nuts on the bottom and glue them in after the print finishes.

There is a Lego phone holder that does the cavity.

https://makerworld.com/models/660273

Hope that helps.

I made this. Used nuts cause I didn't have any pennies. Worked pretty well.

There are pictures in the description. So check those out.

I don't like pouring in sand during the print. It's too easy for the fans to blow it all over.

I use gyroid infill and leave a hole in the bottom. Then pour in casting plaster after the print is removed from the bed. Let dry and sand smooth.

I don't know a general approach, but in one particular print with low infill I paused the print to pour in some sand just to make it more bottom heavy. While it worked great in that case, I don't know how well it'd work in other circumstances. Great cost/weight ratio, though.

I used wheel balancing weights (tiny 5g iron bars) and built them into my prints.

They are pretty cheap and the included adhesive makes it so they don't rattle around inside if the tolerances of the cavity are not perfect.

Used them in the base of a tall slim model to make it less prone to tipping over.

Not exactly the densest material out there, but pennies are cheap and easily procured. May not be quite what you're looking for for your use case. (You asked about "cost/weight ratio" and "weight to space" which makes it sound like you're looking to add a lot of weight.)

I've been known to make a fully-enclosed cylindrical cavity and set my slicer to pause at exactly the right layer to where I can drop a few stacks of pennies into the print before upper layers seal the cavity closed.

I’ve used both plaster of Paris and Portland cement. But both have exothermic curing processes so you have to be careful with large cavities. I have also used some of the very low temp alloys for smaller prints with mixed success. Stuff like Field’s Metal, Wood’s Metal, or Rose’s Metal.

If you're using cheap filament, also consider cranking up the infill percentage. I recently printed something for a friend that is almost a 15cm diameter sphere at something like 85% infill. Every time I hold it I'm surprised by how heavy it is. I haven't tried it, but I think many slicers will let you change the infill percentage after reaching a particular layer, so you can make something bottom-heavy to prevent it from tipping over.

Leave cavities for steel things like big bolts, shaped so the bolts won’t rattle around. Add a pause to the print before covering the cavities. Disable support for the cavities.

When the printer stops, stuff the cavities with bolts, then resume the print.

Tire balancing weights as others have said.

I have also used sand by pausing the print then filling up some of the voids left by infill. But you have to be really careful because you don't want it in you belts or motors. I usually put the sand in a reusable condiment squeeze bottle from the dollar store and also use a small funnel to make extra sure its going where I want. I don't fill all the way to the top and make sure all the fans around the printer are off while filling so they don' blow it around... use tire weights if you can.