Plastics are commonly injection molded, which is a similar process to metal casting. It would make sense to make molten plastic that foundries can produce and work with.

Some possible recipes:

molten plastic: coal+solid fuel - > molten plastic
plastic bar: from molten plastic
Cannon shell; Molten plastic, Molten iron, explosives
Explosive Cannon shell; Molten plastic, Molten iron, explosives
Advanced Circuit: Molten plastic, molten copper, electronic circuit

I like the idea for the ammo, but I've seen this mod for a similar thing: https://mods.factorio.com/mod/ICastBullets or https://mods.factorio.com/mod/OCs_ammo_casting
About the circuits, I think don't think it fits in the theme and it would better in the EM fab expanded mod.