best of all, this method involves no fumes!
I'm gonna show you how to make plastic wheels for your robot!
I first heard about stewing plastic bags to make new things from Dave Huebsch's book " Village Assignment " about interesting adventures had while running a charity/NGO (" Common Hope ") in Guatemala. He, amazingly, repaired the bottom weight-carrying main bearing of a washing machine with a big plastic disc made of stewed plastic bags, which actually was such a good stand-in replacement that it held up for several years. (and here are some more Guatemalan Handy Tricks)
I had to try it right away, and the first couple times I just burnt plastic, until I came up with this method.
By the end of this, you'll be able to make yourself a knife sheath, mold around your shoes and make DIY hard-toe sneakers, wheels, bearings, bushings, or any kind of plastic part! Take pictures and post them in the comments!
Step 1: Materials
Shredding them will make your final texture finer.
Use an old pot that you aren't going to use for food any more, or get one secondhand for really cheap at a thrift store.
Find a stick or something else you can use for a spatula.
I liked the clothespin a lot because I could pick things up as well as stir.
Oh, and oil! (I used canola because it was right there next to my stove. If you're, for example, making a bearing you can impregnate it with your personal favorite grease.)
So grab your favorite oil, here's why:
Plastic bags are made of LDPE (low density polyethylene) which melts at 248 F.
I got the temperature wrong and tried doing this in a homemade double boiler filled with water (to regulate the temperature to water's boiling point), which, of course, just made hot plastic bags.
After the double-boiler mistake I re-read the LDPE melt-point and discovered my folly! So I came up with melting them in oil.
Cooking oil boils around 350, which is far too hot for plastic, and which you don't want anyway (splashing boiling oil = no fun unless you're a hun), so I keep it to a nice low-viscosity canola oil heat and things work just fine.
If you wanted a smooth, non-oily finished surface, consider using wax instead.
ALSO: oil is not required per se --
if you had a constant temperature hot surface that you could guarantee to keep down around the melting point, you could do this just fine with no oil or wax!
Step 2: Add Oil, and Stew.
That is, Resin Identification Code #2 and #4 are both good.
Polyethylene Terephthalate, that is PETE/PET or #1, may also work.
EDIT: Thanks to reader concern, I will state what may not be obvious from the pictures: there are no fumes. There's no smoke, no fans, no inhalation hazard. If there's smoke or fumes, you're doing something wrong and you're burning the plastic. That's why you use oil for temperature regulation.
The plastic bags do this alive-shrinky thing that looks really neat, so I took a video!
Step 3: Mash It
Step 4: Get Moldy
Since I'm just experimenting, I grabbed a salsa container that looked about the right shape for a wheel mold, and a wine bottle to make the hole in the middle.
I don't know any special techniques for molding and casting, but matt, rachel, bilal, paul, freemanmfg and smooth-on do!
If you were a molding ninja, you could make a plastic positive of your own face!
Step 5: Freeze
Step 6: Enjoy!
This shows the finished product, the wheel, as well as another disc I made, and a video of just how surprisingly bouncy homemade plastic is.
Improvements to make:
I'd love to try using wax, instead of oil, so that the final surface is less greasy.
Shredding the bags beforehand would probably give a more homogeneous texture.
Also exciting! If you machined down a brick like this into large-ish chunks, you could feed them to your homemade injection molder! DIY action figures, hooray!