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