I was thinking about building the machines last night and I thought why not make the grinder blades of the shredder out of ABS soda bottle lids. Also make as much as possible of the machines out of plastic. I don’t know if it will work but hopefully someone can try and let us know.
I would recommend against making the machines themselves out of plastic, firstly the issue is that plastics material properties simply do not meet the requirements of the machines, they are too soft, so they would break after a very short time. secondly another issue is that you would need quite a lot of machines to make the things you need out of plastic. If you need more answers please ask