So why would anyone want a plastic lower? I work plastics daily. There are no plastics that can compete with metals across the board. Some take heat up to aaround 1100f, some are inert, some are light, wear resistant or pliable.
You could invest in a $100k+ mold and make lowers from glass or fiber reinforced nylon like glock or H&K, etc.. Price would go down after the first 50000 or so pieces were sold.
Anybody with a CNC mill and some machining knowledge can download 3-D solids of AR lowers and run them out of 7075 Alum properly. Could be worth while at a few hundred pieces.
The problem with part is getting a proper FFl to manufacture. If your looking to have one to use for criminal purposes you certainly wouldnt go through the trouble. Just go steel one.
Printed lowers are a novelty at best.
Concentrated power is not rendered harmless by the good intentions of those who create it.