There is many things in this world that's made of plastic that we don't even notice it anymore.
For an example That jumper you are wearing.
But I suggest that you make plastic fences. And a plastic door If you are making plastic things how come not an plastic door. And PlasticBlock a buildable block that has no monster spawning on it when dark. And remember that one could use a massive Mould that one had to make in several pieces before one put the 3 pieces in the workbench to make which one could put in the furnace to have it being a big mould then put it into the workbench and add it on top of a furnace. That it would work like the workbench but only with plastic materials.
Now Let's make up some Plastic blocks and let's be simple. Plastic1(like wood.)Plastic2(like stone.)Plastic3(like iron.)Plastic4(Like Gold)(you say gold is bad well it has the same durability like wood then you say okay it's a weak plastic but just wait.)Plastic5(Like Diamond.)Plastic6(yes I thought about obsidian.)(It will be Explosive resistant.)
Now remember the mould we're finally back. Open the interface of the Mould it will need melted Plastic.
It's inteface should look like.
   |  a Plastic block output
   Where you put recipes.
   |  Coal or another fuel.
here I give you one for plastic1= and "smelted Plastic made from a small mould"=
Plastic2= would be made
So to make 1 Diamond valued plastic block one would need 33 smelted plastic. but for a plastic diamond pickaxe one would need 99 smelted plastic.
And the mold you might wonder how it would be made.
Recipe1 left half.
  
Recipe3 right half.