It doesn't have to be made of silicone either. A regular Ice tray will work too. Any thing that has a flat and smooth surface should work fine. I read a suggestion, by a Mom on some recipe site, to pull the rack out of your toaster oven, put a piece of foil on it, and push the foil down through the slats. She made a gummy worm mold that way.
This stuff is very forgiving.
Pour the candy in a flat pan and after it has gelled, then just slice it into squares, rectangles and triangles with a knife. You could cut shapes out with cookie cutters. You could even peel the sheet off in one piece, and cut it up with scissors.
Whatever you do, I'd recommend you keep them pretty small. If you are using an ice tray, or a butter mold, or any mold with large compartments, don't fill it to the top. Just fill them up to around 1/4 or 1/2 an inch.
You can make Mega-Gummies if you really want to. Ever wanted a gummy the size of an Ice Cube? Go ahead. It will work fine. Want one shaped like a Banana? Go ahead - just don't tell me what you plan on doing with it.
HOW I MADE THE SILICONE MOLD
I purchased Food Grade Silicone from Douglas & Sturgess. They are a Sculpture Tools, Materials and Supplies store in San Francisco. They are really great folks and have some incredible casting materials that you can't find anywhere else. I bet you MythBusters buys their Ballistics Gel there. Well, I guess I shouldn't bet. Where they bought it is probably on the web, and I've already spent enough money on this project.
The number I used is FGS-2237 Food Grade Silicone. (There is a new number MC-1287) Here is a link
You need two parts. Part A is the silicone gel and part B is the catalyst. They sell it as a set. This is a 1 lb. set.
The silicone must be weighed to make sure you put in the right amount of catalyst. It's 10 parts Silicone (part A) to 1 part Catalyst (part B). Buy yourself a gram scale to do this. You really want to be accurate. You don't want to waste this stuff because you measured wrong.
I made my positive (the part that I made the mold from) out of - you guessed it - Legos!
When I was a kid, way back in the last Century, Legos were still this strange toy from Scandinavia. That was where all the "adult" films and Anita Eckberg came from - she was our Paris Hilton. She was beautiful, classy, spoke with a sultry Swedish accent, and went to the best parties. Well I guess she wasn't very Paris Hiltonish at all.
The only kids who had Legos were the ones who had Moms, or Grandparents, from Europe, or a Dad that we never saw, because he was always "away on business". Those kids got cool gifts. You could tell some of them came from the Airport gift store though.
I had an Erector set and was very happy with it.
Woops Sorry I got sidetracked -
HOW TO MAKE THE MOLD
As I was saying, I never had Legos and I still didn't have any when I decided to make this project. I visited the Lego website and found a set that I really wanted that had enough pieces for my mold and bought that. I planned what I needed with the Lego Digital Designer. It's a free software program you can download from Lego and it lets you build Lego stuff in VR. It will even count up the pieces for you and place an order.
I wanted my tray to not waste any space. If I left more than a single peg between each candy piece I'd be wasting the silicone.
When I got the Legos - I built the Taj Mahal - I couldn't resist! Have you seen it? It's amazing. 5,922 pieces.
So three days later, after I finished the Taj, I started taking it apart to get my candy mold pieces.
I put the pieces on a tray, and ringed the tray edge with a double stack of pieces to act as a wall and hold the silicone in (there is a photo of it below). Legos are made so well, that they really fit tightly together. I found that I didn't have to seal any of the cracks between the pieces inside.
Silicone will not stick to Legos. So you don't need to put any kind of mold release on the positive.
Just stick the pieces in there and make sure you stick them down really good.
Weigh the Silicone
To measure how much silicone you are going to need to fill the mold, you can measure it a couple of ways. The standard way is to fill the positive with water, pour the water into the plastic cup you are going to mix it in, and just draw a line on the cup at the water line. That is how much silicone you will need to fill the mold.
You can't let any water get anywhere near your silicone before you cast it though. If you measure your mold with water, make sure you allow plenty of time for it to get bone dry before you try to make your mold.
To avoid the water problem I used Lentils - yeah like in the soup - but dry ones. They have some really small black ones at the bulk food store and they don't put out any dust, so they are great for this. Just pour the lentils over your positive, then pour them into your mixing container. They don't mess your positive up at all. Then you don't have to worry about liquid spoiling your mold.
Put your plastic container - with the line marked on it, on your gram scale and set the scale to zero. That way you won't be weighing your plastic cup, just the silicone. Now pour enough of Part A to get up to your marked line. Don't just glop it in there, try to avoid causing any air pockets. Check to see how much the silicone weighs. Let's say it measured 350g. Because you use 10 parts A and 1 part B, you will need 10% of that weight in Catalyst or 35g. Zero (sometimes the button you use to do this is labeled "Tare") your scale out, with the silicone still on it. Then add the 35g of Part B right into the same container.
Mix it up
Mix your Part A and your Part B with something plastic. The catalyst is blue and the silicone is white so you can tell if you have it mixed. Scrape the sides and bottom as you do it. You want to be careful not to mix any extra air into it. Does that sound familiar? Just like the Gummies!
If you really want to make a perfect mold then you have to "de-air" the silicone. As this involves putting it in a vacuum chamber and most of us don't have one, you can just skip that step. There are lots of tips on the web on how to get the least bubbly silicone castings.
Pour it in your tray
You will pour the mixed silicone over the positive. Fill the tray up until you cover the tops of the pieces.
Pour slowly and don't move the Silicone back and forth like you are drizzling on frosting. That will leave air bubbles in the corners around your pieces. Pour it in a stream that hits the bottom of the tray - not over the Lego pieces, and let it flow around the tray and fill it up. This will help you avoid trapping any air.
Silicone is pretty good about releasing bubbles so you probably will get a pretty good mold.
I'd tap the tray a few times, and I've even held a back massager against it to help it release any air. You need to be very careful, that any of this tapping, or vibrating doesn't loosen any of the pieces inside the mold. I did a test piece and when I was tapping it, one of them floated to the top! I just grabbed my plastic spoon and pushed it back down. The only problem it caused was I wasted silicone, because the inside filled up. Avoid that by making sure your pieces are stuck down really well before you pour in the Silicone.
Let it set up
Don't try to clean up the cup and tools right now. Let them sit overnight. Most of it will set up and you can just peel it off. If you find you have any sticky spots of silicone gel, you can clean those off by dabbing them with a little Catalyst. That will make them set up and you should be able to peel it off.
Let the silicone set at least overnight, then just pull the Legos out of it. They will come out very easily. Let it set up for 24 hours before you use it. I put mine in the oven at 175 degrees for about 15 minutes after it had already been curing for 24 hours, just to make sure. It might not have been the right thing to do (I should have asked Douglas and Sturgess) but I knew they did that with some of the other silicones.
After you get the Legos out, just trim off any of the little bits that squeezed between the pieces. Wash the mold and you are ready to go.
Now if you will excuse me, I have to go rebuild the Taj Mahal.