I thought I would put this quick little guide up so that people who might not have thought of this method can see how easy it is.
What you will need:
1- Two part silicone (I used OOMOO 30 from Smooth-on for this)
2- Two part plastic (I used Smooth-Cast 310 again from Smooth-on)
3- Universal mold releaser (any brand will work, just ask for mold release from wherever you buy the other stuff)
4- Something to make your mold box (explained later)
5- Polymer clay (the stuff you bake to harden)
6- Sealer spray for clay
7- Misc stuff like stir sticks, disposable cups, paper towels, a container to put your finished seals in, play-doh.
Now on to making the seals;
1) First you need to make your seals out of your clay. This is completely up to your own artistic freedom and you can put whatever you want on them to suit the costume you are doing. Once you are done that simply follow the instructions that are most likely on the package of clay to bake and harden your seals. Once they have baked and cooled completely you spray them with a sealant spray. Make sure it is a hard finish, at the craft stores there are both hard and soft finishes so make sure to read the label as to which one it is. Here is a picture of the two seals we made for my build I'm in the middle of.
2) Next is making your mold. Once the sealant has had plenty of time to harden it's time to start making your mold box. You can do this any number of ways and there are tutorial videos all over the internet on making these molds so feel free to look around for ideas. Since I didn't have a lot of stuff around the house I ended up using a carton of milk the I cut apart and hot glued down to a piece of plasticard. Don't be shy with the glue here since it will stop the silicone from leaking all over and wrecking your mold. Make sure that the box extends well past the top of your seal since you want plenty of silicone to give your mold strength. Here is a picture of my mold box with the seal inside it, as well as the silicone ready to be poured into my cups.
3) Once your box is secure it's time to mix your silicone. Give your mold box and the seal inside a spray of mold release at this point as well as make sure the surface you have it on is level. Directions should be included with the product you get so make sure to read and understand them well so you don't mess up your mix ratios. The stuff I used was a simple 1:1 mix. I don't use any fancy way to estimate how much silicone to use so I just eyeballed it and started mixing. You want to work quickly but smoothly since you don't usually have a lot of pot time before it starts to harden but don't slack on the mixing! Make sure your parts are VERY well mixed. When you are confident you have mixed enough start pouring the silicone. Choose a low spot in the corner of the box away from your actual piece and slowly let it flow over the piece and fill all the gaps. Don't rush this pour, you will have enough time so just let it flow and you will be less likely to end up with air bubbles. You want to pour enough silicone in there to give yourseld a good half an inch above the highest point of your seal since you want your mold to be strong enough for multiple casts and for handling. At this point you will find any leaks in your box that may be there, no worries just grab the play-doh (or whatever else you want to use) and plug them up. Here's a picture of the silicone after pouring.
4) Once the mold has cured for the required time (it should specify in the instructions for the product you use) it's time to take it apart. I used a good new exacto blade to cut all the hot glue away from the base then made a slit along the side of the box (being careful not to cut the silicone) and just pulled it carefully away. If you remembered to spray the mold release on everything it should come off pretty easily. Once you have the mold in your hand it should be easy to flex it around a bit to loosen the clay piece inside and take it out. Be careful with this part so you don't ruin and small pieces that might stick. After you have it removed you can take a look at it and clean up any portions with you knife that you think need it. Use a very sharp blade and take your time since this stuff cuts easily but is flexible so can be annoying to trim. Here is a picture of the mold after being removed.
5) It's finally time to make some plastic! Once again make sure to read the instructions on your product carefully. Make sure your mold is on a level surface. Give your mold a spray of mold release and get your cups out to start mixing. Pour out your parts and give them a good mix together. Same as with the silicone you have limited pot time so moving quickly is good but don't rush and make mistakes, you can always mix another batch. Once mixed well pick a low spot on your mold and slowly pour in your plastic. Let it flow over and into the mold by itself, just keep pouring where you started from. Again I didn't use any fancy way to calculate how much to use and I just fine tuned it over the first couple I made. Once the mold is full let it sit and don't touch! Here is a picture of it poured as well as all the plastic leftover when I mixed WAY too much my first time.
6) After you have let it cure for the amount of time specified for the product you're using it's time to demold. Just like when you were making the box flex the mold a bit to loosen it up and take the piece out. Again watch to make sure there are no pieces that are sticking so that you don't tear the mold. Here is what you end up with.
That's all there is to making silicone molds for your seals. This process works for any piece that is flat on the one side. If you want to make a piece that has detail on all sides then you have to start making two piece molds. Still not a hard process but takes a bit more time and I won't go into that here. This mold will hold up well to many castings but if you plan to use it for a long time make sure you get a silicone that has a good tear strength. If you buy from a good dealer they should know the products well enough to give you some advice.
A last note, this was just to show people a way to possibly think about making some of their costume pieces. There are tutorials for this same process all over the net and a lot of them are very good videos (in fact the company I order from has a very good reference page http://www.sculpturesupply.com/media.php
, the TECH INFO page is also very helpful) on this method and other methode for making pieces for your costumes.