I was amazed to see in Business Week that someone has invented a plastic recycling machine that produces 3D print filament. Says it can recycle the plastic 3 to 30 times, and handle other plastics besides failed 3D prints.
Considering that many plastics are a mix of plastics, and old plastic has some oxidation to it (discolouring), I wonder how well that would actually work.
You might go to http://hackaday.com and search for filament. There have been several projects over the years on making your own filament.