How can I optimize my print for casting in metal?
Print a two part negative (mold) of your objects.
http://www.thingiverse.com/thing:31581

https://pinshape.com/blog/how-to-generate-a-3d-printed-mold-for-an-object/

Melt and pour wax into the mold. Praffin wax melts at only 37C, not an issue for both PLA and ABS.
Use the wax object for casting, not ABS/PLA/etc.
The mold is also reusable this way.

To prevent the wax from sticking to the mold, something might be applied to the surface. Oil maybe?