(August 11, 2019) It's fairly easy to melt down plastic bags, of the right type, and in a mold make a strong material that's like marble. Basically one gets a large quantity of plastic bags, or other plastic, puts them in an oven for melting, then pours the "dough" into a mold. After the plastic has cooled and hardened in the mold, it can be processed using metal or wood working tools like sanders and polishers.
(August 10, 2019) We have lots of plastic around us, and just throwing it "away" creates problems. It is better to reuse things, and it seems HDPE plastic is incredibly easy to melt down and form into new thingies.