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Making Molds from Silicone

As some of you know I have built a roof for my tower project and needed some ridge tiles to cover the corner joints between my roof casts. The first idea that popped into my head was of course scratchbuilding them and so I started thinking how was I suppose to make small 0,4-0,5mm plaster cones that could be made into ridge tiles. After around 15 minutes I decided to use plasticine (nonhardening modeling clay) to make small moulds based on the tip of my pen, which is luckily cone shaped. So I cast 18 small tip cut cones, added texture to them and made a small dry fitted piece, which looked like the first image on the right.

After asking for feedback on the cone master I thought that it must look ok and decided to make a silicone mould of the 18 cone shaped pieces. I had my camera nearby and also decided to take in-progress shots and post them here to help anyone who just starts at making molds. So here we go:)

Step By Step

Step 1
Make sure that the object you are trying to copy is clean and protected from sticking to the silicone stuff. So anything which is porous should be varnished or protected with a sealer, or the gooish silicone stuff will stick to it and make it harder to get out of the mold. I used 18 cone shaped dental plaster pieces and covered them with a product that seals the plaster and protects it from soaking water and stuff like that. Attach the object to a flat nonporous base, see the second image.

Step 2
The next step is to build a small box to surround the object. The box will hold the silicone until it cures. I built mine using the wonderful LEGO blocks. I also used plasticine to attach the box to the base and make sure that the box doesn't leak:

Step 3
Now you need to calculate how much silicone you need. You do this by measuring the inside of the LEGO box and calculating its capacity. You also need to calculate the volume of the object. Then you subtract the object volume from the box capacity and you have your desired silicone amount. Take a medium sized container and pour the silicone into it. Now weigh the whole thing using an electronic scale, see the image or a normal kitchen scale.
(Remember to first weigh the empty container and subtract this from the total weight of the container and the silicone)

Step 4
The next step is to measure the amount of hardener you need. Basically the hardener is the stuff which makes the whole mix hard:) If you add too much, the mix will start to cure too fast making it impossible to pour from the container to the box. If you add to little you will be left with you object covered in goo and completely destroyed. This step is really important so please make sure you read the label of your silicone product well and that you measure everything precisely. My product label says you need to add 3 parts of hardener to 100 parts of silicone, so basically it's 3% of the silicone weight. I used a syringe to measure it and here you can see both parts ready to be mixed:

Step 5
Now all you need to do is mix it WELL. And I mean really put your heart into it:) if you think it's mixed already, mix it some more. Than it's ready to be poured, but before that happens, wait around 15 minutes for the bubbles to come out onto the surface of the mix. You can help them by tapping the bottom of the container with your hand. If you think it's ready, pour it:)

Step 6
Wait for the mixture to get into all the places of the box, if it's not cooperating use a small spatula or a toothpick and make it move, also tap the whole base to get rid of bubbles from the mixture.

Step 7
Wait a day or so, it depends how fast your silicone stuff cures.
If everything went well you'll be left with an undamaged object and a fully usable mold:)

Step 8
After waiting a day or so to harden you will be ready to move on. Before disassembling the box touch the surface of the mold with you finger. If it's hard and you mixed it well before pouring, you can remove the box

Step 9
When the box is gone you have to separate the base and the mold, do it gently so that the mold doesn't get damaged. In the images you can see the objects partially removed from the mold and the mold finally free from all the elements

Step 10
Wash the mold in some warm soapy water to get rid of any nasty chemicals from its surface. Now you are ready to make your first cast. You can use resin (I've read that you should wait another day for the mold to finally cure, so that it's harder and can take the resin, which can be quite nasty and destroy a mold) or plaster. I used the second option as you can see.

Step 11
Don't worry if the first cast isn't successful, keep on trying and try to use different methods for casting. Below you can see a good quality cast, and some worse looking elements from a previous cast: