Prosthetics - Master mould, part 1! Silicone mould

Master moulds are very useful tools. They are made with the intention of being able to use them numerous times and make countless copies of whatever has been moulded. In this case, a master mould is being made from a life cast. This will allow many casts to be made without having to do the full life casting process again, but also lets you get a cast of just the nose, or the mouth etc. This will be discussed more in detail later on. 

Once the life cast has been prepped and the wall and keys have been built, the next step is to apply a spray release agent. The artist used Eject-it spray, also called Supralease CLR (petrolease), which is a very good mould release. This will make it easier to remove the mould from the cast when it is time to do so.

After this, it is time to mix the silicone. Silicone (or siloxane) is like a hybrid between a synthetic rubber and a synthetic plastic material. It has many properties, such as being very malleable (depending on its hardness), and is temperature and water resistant. It can be used for different things from shoe soles, to cooking utensils, to car tyres. (Plamondon, C. 2020). This will depend on its hardness, which is measured on the Shore Scale of rubber. 

Figure 1 - The Shore Scale of rubber (Smooth-on, no date).

Shore 00 is used to measure rubbers and gels that are very soft, Shore A measures the hardness of flexible mould rubber to semi-rigid plastics, and Shore D measures hard rubbers, semi-rigid and hard plastics. (Smooth-on, no date). The moulding silicone that will be used is measured on Shore Scale A25. (NeillsMaterials, no date).

For this mould, Platsil Gel 25 was used, but there are others such as Key22. It comes as part A and part B, which need to be mixed together in equal amounts. The silicone can also be coloured, using silicone pigments. "They are highly concentrated and a drop of pigment goes a long way". (NeillsMaterials, no date). This is added to the silicone when mixing or can be added to each A and B before mixing together.

The process was done in three layers. This was to ensure that the cast itself had been moulded thick enough, as the silicone will drip down and pool at the bottom. Each layer was done in a different colour, which made it easier to determine that each layer had set before starting the next. Each layer was poured from height and in a small stream to release any air bubbles.

The first layer was a total of 300g of silicone, so 150g of part A and part B. The second and third layers were 200g of silicone, so 100g each of part A and B. This was to make sure that the silicone was not wasted and that too much was not being made if it did not need to be.

Figure 2 - Orange first layer of moulding silicone (Brown, J. 2020)

Figure 3 - Blue second layer of moulding silicone (Brown, J. 2020)

Figure 4 - Yellow third layer of moulding silicone (Brown, J. 2020)

After each of these layers have set (up to 60 minutes per layer, though this could change depending on the temperature of the room), two more layers of silicone was added, however this was mixed with poly fibre. Poly fibre is a "thickening product designed for use with polyurethane plastics and rubbers". (Neillsmaterials, no date). This ensures that the mould around the cast was definitely thick enough. 100g of silicone was used for each of these layers (50g of each part). Poly fibre is added to this mix until the desired thickness is achieved. The artist definitely did not add enough poly fibre, so it was still quite a runny consistency. This is something to keep in mind for future attempts at this. It should be a lot thicker, however, it did still do the job it was intended to do. 

Figure 5 - Pink first layer of poly fibre (Brown, J. 2020)

Figure 6 - Purple second layer of poly fibre (Brown, J. 2020)

Once this process is complete, the next step is to create the supporting jacket, which will be discussed on the next blog.

The artist found this process to be fairly simple, the only tedious thing about it was having to manipulate the silicone while it was setting to ensure that some of it actually stayed on the cast. Other than this, and not adding enough poly fibre, there was not much else that they struggled with here.

-Jess

References:

• Durometer Shore Hardness Scale. (no date) Available from https://www.smooth-on.com/page/durometer-shore-hardness-scale/#:~:text=The%20Shore%2000%20Hardness%20Scale,almost%20no%20flexibility%20at%20all. [Accessed 18/12/2020].
• Plamondon, C. (2020) Silicone vs. Plastic: What's the difference & is one safer? Available from https://www.mindbodygreen.com/articles/is-silicone-toxic [Accessed 18/12/2020].
• Platsil Gel 0020. (no date) Available from https://www.neillsmaterials.co.uk/product/platsil-gel-0020/ [Accessed 18/12/2020].
• Poly Fibre II. (no date) Available from https://www.neillsmaterials.co.uk/product/poly-fibre-ii/ [Accessed 18/12/2020].
• Silicone Pigments. (no date) Available from https://www.neillsmaterials.co.uk/product/silicone-pigments/ [Accessed 18/12/2020].

Bibliography:

• Bray, S. (2016) Master Moulding. Available from http://www.learnmakeupeffects.com/mastermoulds/ [Accessed 17/12/2020].
• Creating Permanent Masters for Mold Making. (no date) Available from https://www.polytek.com/tutorial/creating-permanent-masters-mold-making [Accessed 14/12/2020].
• Master moulds. (no date) Available from http://www.tegulas.ch/en/Master%20moulds-8 [Accessed 17/12/2020].

Figures:

• Figure 1 - Durometer Shore Hardness Scale. (no date) Available from https://www.smooth-on.com/page/durometer-shore-hardness-scale/#:~:text=The%20Shore%2000%20Hardness%20Scale,almost%20no%20flexibility%20at%20all. [Accessed 18/12/2020].
• Figure 2, 3, 4, 5, 6 - Brown, J. (2020) York College.