I needed a coffee tamper so here we are. If you hack off the the sprue, you can use this 3d print as is, however I wanted a solid pewter coffee tamper so that's what this is for. Obviously you can use this to cast bronze, brass, or aluminum too.
I printed with PLA, 10% infill, 0.7mm walls.
I glued the sprue to the bottom of a container and filled it with a mix of very fine sand and plaster of Paris (2:2:1, sand, plaster, water; save yourself some trouble and pre-mix the sand and plaster before adding the water).
Once the plaster was set, I peeled off the container, and baked it in my oven at 500 degrees Fahrenheit (260 Celsius) for... many hours. I kept the mold sprue-down, elevated over a tray. This is to dry-out the plaster as well as melt-out and PLA. The tray catches the melted PLA so your house doesn't smell like burning PLA every time you use your oven.
You want the mold close to your casting material's liquid temperature--so you can go straight to casting when you take this out of the oven.
I melted my pewter, pulled the mold out of the oven, cast it, and.... realized the mold was broken and got a partial cast with a skewed bottom. But this was my third attempt and I was done trying. I kind of like the top of the skull is missing. Very symbolic. Like... I'm working with a partial brain until I get my coffee.