Well, here's a very DIY, cheap an recyclable method to make molds for relatively low temperature casting.
Step 1: Materials
Gelatin -- I'm using Jello
A vessel to pour in that will hold your positive item
A refrigerator with space for your vessel
You may need:
Skewers or other support device
You probably want:
Chap Stick or Vaseline (chap stick being "edible")
Step 2: How Much "Stuff" Do I Need?
1. No undercuts - that is, if the mold material goes over (or into) your positive - you won't be able to get it out (without some surgical maneuvers).
2. Avoid super fine detailing - I haven't experimented with this yet, but it seems like it would be bad.
3. Don't let the positive rest on the "floor" **
So how much gelatin do we need? Lets do a "dry" run.
First, place your positive into your vessel. Now, fill with water (you may need to hold your positive under). Next, remove the positive and measure how much water is left in the container (you'll probably be pouring off into a measuring cup).
You will need one 8oz serving package of Jello/gelatin for every 3/4 cups of water. I have not tried higher concentration (yet), but feel free to experiment as a less "jiggly" mold would be better.
** There is an exception to this rule. That is, if your positive has a flat side that doesn't matter (like the back side of a chocolate bar) - it is beneficial for that back side to rest on the "floor" of your vessel. This was the case for the Tetris Ice Cube Tray.
Step 3: Positive Preparation
Now, chicken poop does not come from the backside of any fowl. It's actually a brand similar to chap stick that uses soybean oil among other things. Technically, it's edible "lip junk." So smear it over your positive and rub it in.
Now it's time to place in your vessel. If your positive is to rest on the floor AND it floats -- you're probably going to want to use a dab of hot glue or other adhesive to keep it down. If not, you can build a stand with wood skewers or make a little suspension structure as not to interfere with the mold.
Hot glue being my temporary adhesive of choice, I fired up the manual hot glue applicator and secured my positive (an apple) in place.
Step 4: Mixing and the Pour
Once fully mixed and your positive ready -- pour your mold material into your vessel. Then move into your refrigerator and allow it to cool/set.
Step 5: Demolding
Should your mold tear beyond recognition -- see the last step.
Something I have not tried yet... Gelatin will freeze (at a cost). So if you're having a lot of problems, you could theoretically freeze everything -- then try again. Removing your positive from a solid structure is much easier.
The cost: Gelatin looses a significant amount of structure when thawed. It maintains shape (for the most part), but looses a lot of structural integrity (as much as gelatin could ever have). My advice for those that freeze their molds -- pour your cast in the frozen mold. This is a better alternative over Ice as it seems that gelatin does not expand as much as water does when frozen (strange, I know) - this makes removing the positive much easier.
Step 6: Casting and Thing That Go Wrong
I'm pouring chocolate as it seems to be the only non melting cast material I can find around Instructables HQ. Plus, chocolate is tasty :) Going too hot is dangerous given the low melting point of gelatin - so be aware and experiment first ;) I allowed my chocolate to cool off a lot before pouring - probably overkill, but better safe than waste a half day's worth of work.
Things that go wrong -- Don't do these things....
1. Don't use olive oil as a release agent when making your mold -- the oil will float and you won't get a good separation. But, by melting and setting -- you can remove the excess oil.
2. Pouring gelatin into your gelatin mold probably won't work -- other than for non separable decoration.
3. I personally wouldn't eat my mold material...Mostly because of the high concentration of gelatin. It's also labeled "Not people food" in the Instructables HQ fridge -- so no one gets the idea it's up for grabs :)
Step 7: The Last Step -- Recycle
This has awesome implications - you can re-use your mold making supply over and over again. Gelatin does have a finite cycle life, but I have yet to hit it (it seems rather high). Just throw it in the microwave on medium power and stir once a minute until completely liquid again - be sure to check for small bits that need a little more time to melt down.
So, if your mold tore apart when you tried to release (at any point).... Melt it down and try again -- perhaps with a little more gelatin powder? Then give it another shot.
What about contaminants?
Oil based mold releases float on top of the gelatin, so when your melt down and refrigerate -- you should have a film of release agent on the top of your solid gelatin -- you can simply wipe it out with a paper towel.
Happy mold making :)
Let me know what you make -- I'm considering casting a plastic soldier or hood ornament just to see how well the gelatin takes to it -- and how the details show up :p