The method for making objects such as badges, plaques, jewellery items or pendants which have flat backs is basically the same. Silicone resin jewellery moulds can either be manufactured by pouring the silicone over the original model or by brushing it on. Pouring is probably the easiest method for all items with a flat back.
WHAT MOULD CONTAINER TO USE
We are going to look at the pouring method. For this method we need a container which prevents the silicone rubber from running away from the model and leaking out all over our work bench.
There are many readily available plastic food containers which are perfect for smaller sized objects. Butter containers, takeaway food containers and even ice cream tubs are a few examples. When finding an ideal sized container for your model you need to leave 1-2cm around all sides of the model. The height of the container needs to be at least 5cm taller than the model.
If your item is extremely small you may not even need a container. You can fix the small model to a small board with a non hardening clay or glue if you prefer and build up a wall around the model with clay or even lego bricks.
For larger items you can stick the model directly to the plastic container. It is important to fix the model securely to the base so that it won't float to the top when silicone is poured in.
We will now focus on the method which uses a plastic food container to make the mould.
USING A PLASTIC FOOD CONTAINER AS A MOULD DAM
1) Make sure the container you want to use has a lid. Close the lid securely. Turn it upside down and cut the bottom of the container out of it with a hobby knife.
2) Now secure the model to the base (which will be the lid) of the container with glue or clay.
3) We don't really need a release agent when working with silicones as it sticks to very few things. Glass is one item which silicone tends to stick to. If you are unsure conduct a small test with the material before making the mould. If you find a release agent is needed you can make your own with petroleum jelly dissolved in a solvent such as white spirit. We use a ratio of 10% petroleum jelly. You can then spray or brush the solution onto the model and let it dry. A fine film of petroleum jelly will be left on the model when the white spirit evapourates.
4) The silicone we use to pour over the model comes in two parts and needs to be mixed together to start the curing process. Mixing ratios vary from product to product so make sure you read your label. We find waxed disposable paper cups to be ideal as mixing containers and wooden stirrers or paddle pop sticks for mixing.
5) Measure out the right quantities of parts A and B into their own cups. Then pour these two quantities into a new cup and stir well. Try not to beat air into the mix by aggressive mixing as this will create many air bubbles in the mix. Try to use a folding action.
6) When you have finished mixing pour the mixture into a new mixing cup. This ensures that no unmixed material remains in the cup when pouring it onto the model.
7) If there are areas on the model which have very fine detail or undercuts it is advisable to brush some silicone onto those parts before pouring. This will ensure that the silicone will flow onto each and every surface and minimize the chance of air bubbles on these high detailed areas.
8) You can now start pouring the silicone onto the model. Use a very slow and fine pouring stream from a high point over the model. The thin stream will have the effect of popping any bubbles which made its way into the mixture on the way down. Aim your pouring stream onto the lowest point in the mould box. It is best not to pour directly onto the model so aim for the floor of the mould container. Pour all the material into the mould box until the material covers the model with at least 1cm of material higher than the highest point on the model.
9) Let the silicone cure overnight. You can remove the model from the silicone the following day by first taking the lid off the container and pushing the new silicone mould out. It should slide out the bottom. The model should easily pop out from the silicone and leave a cavity ready for casting. It is a good idea to allow the silicone to cure for a few days before casting to ensure maximum mould life.
