This is a quick and simple method for making your own general-purpose plastic. The constituent ingredients are milk and vinegar. That's it. The total cost is less than $10, possibly less than $5 if you can get a good deal.

The plastic is moldable, and has a consistency of soggy cheese (I certainly hope I never encounter cheese that's anything like this!). When all is said and done, it should take you about 10-15 minutes to make the plastic (less if you make a small amount), 10 minutes of cleanup, plus 2+ days to allow the plastic to dry. As always, your milage may vary.

As for the final product's strength and whatnot, I would classify it as "okay." If you roll it thin (as I did in this instructable), it can easily be snapped in half, though it will probably survive a small drop on to carpet. Thicker pieces seem to be more resilient against average abuses - no problem dropping on to carpet, and if it's thick enough, you wouldn't be able to snap it in half. A blow from a hammer or other such object would quickly shatter it, though.

All in all, I would say this is mostly useful for folks looking for a fast and dirt cheap alternative to much better products available to the average joe. It's definitely not for something that will be handled on a daily basis - good enough for light decor, not much else.

This is my first Instructable, so bear with. Constructive criticism is always welcome!

Step 1: Ingredients & Utensils

This doesn't require anything fancy.

Essential Items:
Milk
Vinegar
Large Pot
Larger Pot (needs to be the same size as the other, or larger).
Spoon, preferably plastic or metal
Strainer or Colander, the finer the better
Stove or other item with which to simmer milk.
Paper Towels, lots and lots of paper towels

Handy, but not necessarily essential, items:
Wax Paper (good for rolling and shaping on. Water and other liquids bead up on it for easy clean-up; also non-stick with the casein)
Aluminum Foil (good for certain types of molds)
Rolling pin (if you're going to make a flat sheet of plastic)

Before you begin making the plastic, it's important to know what you're intending to make. By knowing what you're final goal is, you can make the preparations for the mold before you have a sloppy wad of plastic on your counter.

For this Instructable, I am making a curved piece that will fit on the front bezel of my computer case. Since this is beyond the scope of this particular instructable, I'm going to gloss over what I'm using. My mold is going to be the front bezel of the computer case, covered with aluminum foil. The plastic will dry nicely on top of it, and the aluminum won't shrivel or wrinkle when it gets wet as the plastic cures.

How much milk and vinegar will you need? The basic ratio that I follow is 1 TBSP of vinegar for every cup of milk. (16:1 ratio, milk to vinegar)

1 cup of milk will produce a puck of plastic about 2" in diameter, 1/8" thick.

To cover my case bezel, which is about 6 inches wide and 8 inches tall, I am going to use 8 cups of milk (and therefore 8 Tbsp of vinegar).

Step 2: Heat It Up

If you've never heated milk before, it's important to know that if you heat it too fast, it will start to burn on the bottom of the pot. To bring it to a simmer, it's best to keep the heat down to about 50-60%.

I don't follow my own advice, and use full heat, because I don't want to wait.

While the milk is heating, be sure to stir it occasionally, so it doesn't burn. Also, if you're heating more than one or two cups of milk, measure the vinegar out into the cup while the milk is warming. This is personal preference, as it makes it easier to pour the vinegar in all at once, rather than measuring it out in a hurry.

During this time, you may need to pander to a needy cat, if one happens to be around.

When the milk nears boiling, you will notice a foam forming on top of the milk, as well as a little noise coming from the pot. Steam will start to appear too.

Once you reach this point, turn off the heat, pour in the vinegar, and stir. You will immediately notice chunks of casein forming. Stir it for another thirty seconds or so, for good measure.

Step 3: Strain

Slowly pour the heated liquid through the colander and into the larger pot. Most of the chunks of casein will get caught. We pour slowly to avoid splashing the plastic-loaded liquid into the sink. Something tells me that chunks of this stuff in the drain is not a Good Thing(tm).

Once you've poured it out (don't worry about what's left in the bottom, we'll get to that), gently shake the strainer, and swirl it around a little. Most of the casein will lump up in the bottom, pulling itself out of the holes. Make sure a majority of the liquid is strained out, but don't press it out -- the casein will just get stuck in the holes, and make it harder to get out.

Now that it's all clumped together, dump it out on some wax paper.

If you have a super-porous strainer like mine, you may want to pour the liquid through the strainer several times. Doing it a second time pulled a chunk about 1/4 as large as the original mass; that's a nice addition.

Step 4: Sop Up the Excess

After you've strained all of the casein out of the liquid, you'll want to soak up some of the remaining juice in the mass on the wax paper.

Use a few paper towels, and press gently on the casein. The liquid will spill out like squeezing a sponge. Be careful not to make it too dry, because it will become difficult to mold if it's too dry.

Step 5: Mold!

At this point, you're ready to do whatever you want with the plastic.

It will take about two days to cure, but if you're using a mold where it cannot easily breathe, it will take longer.

One thing to watch out for is that the casein will warp when it's drying, especially if you have it rolled out into a sheet. It's best to put a weight on it. Watch out, though, because it will seep a milky-vinegary scented liquid into whatever is holding it in place. I used a heavy programming book...and it now has a funny smell to it. Awesome.

For my project, I just need to roll the casein out and let it dry.

Lay another sheet of wax paper over the casein. Use a rolling pin to spread it out. If you didn't dry it too much in the previous step, it should roll nicely, without many cracks or chunks.

Finally, I pressed it between two sheets of aluminum foil, and curved it across the bevel I'm using.

Give it a few days to dry, and it'll be ready to go. This is critical. Depending on how you wrap/mold your casein, you may find it takes more than a week to dry. Wrapping mine in foil took two weeks to get remotely dry. As is expected, the thicker it is, the longer it will take to dry.

I should also note that if you're making a flat piece like I am, the plastic will curl if you remove it from its mold before its dry. I made this mistake with this project, and ended up with a piece that was unusable because it curled.

The final product is quite rigid when it's thick (1/8 inch or thicker), moderately pliable when it's a little thinner, and brittle if it's paper thin. It's also sandable and paintable.

Go forth and have homemade plasticy fun!