You mix 10 % clay with 90 % quartz sand, fill a wooden box 1/3, then insert your wax model which includes a funnel, then fill the rest of the box with more clay and quartz sand. After heating to about 90 °C, the wax will melt and flow out of a hole in the bottom. Then you can cast aluminum from the top.
Except I want to do this with 3D printed plastic, not wax. What material would be suited for this?