Preparation Ear Casts for Silicone and Fiberglass Mould

Hiiii again! Another special effects studio update! This lesson we were preparing for fiberglass and silicone moulds, this involved us ‘bedding in’ a ear cast into water based clay. The reason this is a vital preparation stage it to eliminate any possible undercuts in which may lock the mould. So lets get started!


This is a vital preparation stage.  As I will be using a fibreglass moulding method I need to make sure there is not risk of the mould locking. To ensure this I need to make sure I fill in any undercuts that may prevent the fibreglass from releasing from the cast. To do this I will be placing the ear cast in a bed of clay building up each side. First we made sure to place the positive cast on a wooden board to make it easier to manoeuvre around and build the clay wall. We then secured the ear cast onto the board with clay before building up the sides.


Tip! At first I struggled to get my head around the idea of undercuts however, Becky (our SFX teacher) told me a really good method of figuring out where any undercuts may be hiding. The trick was to look at the cast from a birds eye view – anything that I couldn’t see from this angle (for example the underside of the ear lobe) needed to be filled in with clay.


Once I was happy with the bedding in process I was then able to build up a clay and Mod-Roc wall in preparation for silicone.  We then applied a light layer of Vaseline to the ear cast as silicone can adhere to plaster.


Once prepped, we were then able to begin mixing the silicone.  First we made sure we had all the necessary materials and tools to do this.  We first began by zeroing the scales whilst the plastic bowl was on top to make sure we get an accurate reading as it is vital that the silicone is measured out precisely as it will not vulcanize.  The equation for silicone as are follows:

Part A of silicone:

Part B Silicone:


For the size of the positive face cast I was covering I wanted to mix up 70 grams of part -A 7 grams part -B and 0.7 grams of accelerator to speed up the vulcanisation process.


Before mixing we made sure that the tubs of silicone were very close to the scales as it can drip really easily which can be difficult to clean up when not mixed as it stays in its liquid form. To avoid and drips whilst measuring out the silicone we also used a wooden stick to scrape the excess silicone from the sides of the cups. We first measured out parts A and B and then added the accelerator and mixed them together really well to ensure all the silicone would cure.

Tip! An important thing to remember when mixing up a batch of silicone is to avoid cross contamination of each part as this could ruin the full tub of silicone and its isn’t cheap! To avoid this make sure to use different cups and wooden sticks when measuring out each part.

We then poured the silicone into the centre of the positive to allow it to drip over the entire surface. This is only the first layer so it does not need to fill to the top of the clay wall.

Tip! To avoid air bubbles during this process it is always best to pour the silicone from a height as this eliminates the hair bubbles as it pours.

We then set the positive aside on a flat surface to allow the silicone to level out and cure. Due to the accelerator we would be able to come back and add the next layer in around 2 hours.


The next later of silicone was of a different consistency to the first as we used a different product – Thixo Additive. We carried out the same process as the first layer however we mixed the Thixo at the very end. There was no precise measurement for this ingredient as it was simply to thicken the silicone up to a state I was happy with. The consistency I was looking for thick paste which I would be able to apply generously over the first layer of silicone. If the first layer is still a little tacky that doesn’t matter as it will help the next layer adhere better.  Once the past was mixed we then used another wooden tool to spread the mixture around (Some may fine using a brush easier – but keep in keep in mind it will damage the brush – so think cheap and cheerful!). We then left it to cure for a further 2-3 hours.


Once the second later had cured, we then added another liquid layer over the top (same process as the first layer) this smoothes over any lumps and bumps from the second coat

Then you are ready for the fibreglass outer casing!

I will be posting the next step very soon, so stay tuned ,

Thanks for visiting my blog,
Katy x


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