Plastic recycling is certainly a great challenge and we had a Challenge on that last year in Berlin: [CHALLENGE] Documentation Jam on CubeFactory's Recycle Bin
There were also these topics last year:
Plastic Shredder
[SOLUTION] Design a shredder to recycle plastic
[DOCUMENTATION 2015] - Circular Economy Cosmetic Bottles
[CHALLENGE] Extruder Prototyping
And there was mention recently of this OS plalstic recycling tool series:
NEWS_STREAM _ for quick news
This year there is one Challenge on the topic: Upcycling Plastic Bottles -> To Build A Vacuum Cleaner!