2nd Trimester - Once the sculpt is finished and has been approved by Pete it is now time to make a silicone mould of the bear. Firstly we make a strong box around the bear using scrap pieces of plastic tacked together with superglue.
The box is then sealed along the seams using a hot melt glue gun to prevent any leaks when we come to pour silicone rubber into it. We pour the right amount of silicone into a bucket and then add the catalyst which will “cure” the silicone rubber. The silicone and catalyst are thoroughly mixed together using a large drill.
The bucket is now placed into a vacuum chamber for a couple of minutes to ensure there is no air in the mix. This stage is very important as any air bubbles in the silicone rubber will be evident on any finished resin casts. We now pour the mix into the box mould’s making sure they are fully covered by at least 30mm. The moulds are now placed into the vacuum chamber as pouring the silicone into the moulds can cause small pockets of air to get trapped against the bear. After another couple of minutes the moulds are removed and placed somewhere flat to cure. This can take anywhere from between 1 – 8 hours de pending on the ratio of mix and room temperature etc.
When fully cured we remove the outer box and think about where we are going to cut the mould to remove the wax bear. At this stage it is important to cut the mould in the most suitable place as we do not want to have a mould line running across anywhere obvious like the face. A sharp scalpel blade and a pair of circlip pliers are used
We now hold the mould together using elastic bands and wrap some masking tape around the top to create a “well” which stops any excess resin coming out of the mould when the resin is poured. The resin we use is a two part polyurethane which is mixed together in equal parts then poured into the mould. The mould is again put into the vacuum chamber to extract any air bubbles, this time from the resin. After removal we leave the mould for about ten minutes while a chemical reaction in the resin causes it to get very hot and set hard. We can now remove the resin from the mould and think about finishing the bear.
At this stage the bear finish is quite rough as the sculptor’s tool marks can be seen on any smooth surfaces. This is when we turn the rough cast into what we call the final “grey”. The cast is sprayed with a grey primer them rubbed down using different grades of wet & dry paper until a smooth finish is achieved. We now add things like the legal wording and individual number to the reverse of the bear.
Sometimes at this stage it is necessary to separate some parts of the model to make it easier to mould and produce on the Far East, the example shown is quite basic so this is not needed. This picture shows the final grey which will now have the silicone mould process repeated giving us a “master grey” mould.
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