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What are the benefits of Silicone Caulk Moulds

GT836 Albion 26 to 1 Barrel Gun 1 sm
18.02.2021

The benefits of Silicone Caulk Moulds

 

There are plenty of different materials to choose from when it comes to making moulds, but there are several good reasons to use silicone caulk moulds.

It’s worth looking into the pros and cons of silicone caulk to see if it’s a good fit for your mould work. A big consideration is what your casting agent is going to be and also how long you want you mould to last for. If you are planning to recast this mould again and again, it’s well worth getting it right!

What are the benefits of Silicone Moulding?

Availability – Silicone is readily available online, making it easy to get your hands on, no matter where you are.

Cost – The caulk generally sells from $5.00 (AUD) making it super cheap for anyone working on a tighter budget. It’s perfect for playing with ideas and perfecting your moulds, since mistakes are cheap and the moulds that you do keep are long lasting.

Flexibility: Silicone moulds are flexible and light, making it useful for a number of mould types and casting materials and easy to dislodge your completed projects (if you are worried about the mould becoming pressed out of shape or losing detail when the cast is poured see my final note about creating a keeper).

Things to keep in mind when using silicone

Layering: The best way to make silicone moulds is with fine layers. Each layer being thin is important for texture as well as drying time, since thick layers take longer to dry before laying the next one down. For perfect results you want to initially lay less than 5 mm thick.

It smells: In some cases, Acetic Acid from the silicone will smell like vinegar. It’s harmless, but some people don’t like it. There are odourless versions available.

It’s not instant: You’ll need to thin the silicone caulk down for the first coats to work well.

Handling: In its un-thinned state it is difficult to spread without leaving imperfections.

In this article I’ll cover some simple and bulletproof ways to get around some of these annoying issues that might be preventing you from using silicone effectively.

What you are working with

There are three basic types of silicone: platinum cure, tin cure and self-cure. The first two are two-part silicones which need to be measured and mixed prior to use.

Platinum cure silicones are pure and these produce extremely accurate moulds that last indefinitely. These are approved safe which makes them perfect for cookware, baby bottle nipples and candy or chocolate moulds. They are also used in the making of strong and precise equipment such as medical and theatrical prosthetics and medical tools and supplies.

Unfortunately, this is also a very sensitive silicone to work with. The cure can be hindered by subtle changes in the atmosphere including moisture, sulphur, latex and tin contaminants meaning a pristine and highly controlled work area is required.

Tin cure silicone moulds are commonly used in the art and craft industry. Please note that these have not been health approved which makes them inappropriate for cooking products, food contact or any long term skin exposure. That said, they have been successfully used for making portable water supply seals. This is a safe and stable compound that can be used across a wide variety of implementations. It is able to cure in just about any situation (including submerged in water), so a controlled atmosphere is not needed. While not as durable as the platinum silicones they are still an accurate mould, lasting several years.

Finally the one-part, self curing silicones, which are actually a variant of the tin cure. These are the aquarium sealant and the caulking silicone types, by far the most common of all hardware store variety silicones. You’ll find them in air-tight tubes ready to use. Typically you’ll be working with the acetoxy (with the vinegar smell) but there is also the less widely available, odourless and more expensive oxime version. Our Neutral Sanitary Silicone is 100% oxime and resists mildew when exposed to prolonged hot and humid environments.

 

How to remedy caulking cure issues

The biggest problem you’ll find when using the silicone caulking for moulds is that it can be hard to thin out. This causes problems in two ways. One, it’s all too easy for air to become trapped in the thick goo, causing holes and other imperfections. And two, when the walls are too thick the mould fails to cure.

Both of these issues can be easily solved by thinning the silicone out.

How this solution works

This common silicone type requires moisture from the air to cure (which is how its tin cure cousin is able to cure underwater). When you get a layer that is too thick it creates an air-tight skin, meaning the caulking underneath is unable to ‘breathe’. So for layers more than 5-6 millimetres thick you can wait days for a cure, only to find the underside is still soft (especially if it’s formed against a moisture resistant material like plasticine clay). In other cases both sides will cure well only to have a wet pocket in the middle. 

To solve this you’ll need to increase the moisture content within the silicone solution so that it doesn’t need to take it from the air, thus making thick mould walls dry consistently.

Adding water is impossible, it simply won’t mix in. Here’s a unique way to add moisture and, at the same time, gain a more workable texture to achieve smooth, thin and flawless applications.

Before you start

It’s important to note that any time you thin a substance you are reducing the chemical qualities that make it work. Be careful only to apply the thinning agents in small doses as too much will weaken the mould.

Thinning Ingredients

For every 2 Tablespoons of 100% clean silicone caulk (approx 30 grams) add four to five drops of glycerine (available at any pharmacy or health supply shop) and one drop of acrylic (craft) paint. Make sure you are using paint that is not oil based.

Directions

Using a large plastic cup for mixing, squeeze out the required amount of silicone for your mould. Using a 2-tablespoon gauge, add in the supporting amount of glycerine and paint.

Mix it gently with a wooden craft stick until you reach a uniform colour. Avoid trapping any excess air as much as possible.

Why the paint?

You don’t really need the paint to make this work, although it does improve the cure and also helps it slide out of the model far more easily when completed. Those small bonuses aside, the main reason for adding it is because it can be hard to determine if the mix has been evenly distributed, since it’s all transparent solutions. Adding the paint is all about the colour, so you can see when you have achieved an even mix without overworking it and risking air bubbles.

Once you get a uniform colour, you have between 15 minutes and an hour before the product hardens up. The time you have will depend mostly on the humidity and temperature of the atmosphere you are working in. A cool, dry environment will give you more working time.

For best results brush a thin coat on first. This is a great way to ensure you get full contact to all surfaces and it also helps remove any air bubbles. Once this first layer has dried you can go for a thicker coat, applied with a spatula. As a rule of thumb, palm sized moulds usually work best with a 5-6 millimetre thickness, while large works can handle thick walls of around three centimetres.

You can expect your cure to be complete and your mould ready to handle in less than two hours. Keep in mind that heat and humidity will speed up the process. With silicone the cure happens evenly, rather than from the outside in like other materials. I prefer to give my moulds a full night of rest before I handle or cast them, but if you need them quickly it’s not a problem to use them straight away.

That just leaves the flexible factor. You want your mould to maintain shape even when a casting agent is pressed in against it. The best way to do this is to provide a support shell that will hold the mould firmly in place during casting. I find that a couple of layers of plaster bandages (i.e. Plaster of Paris bandages) applied underneath do just the trick. A little bit of water and they are able to coat your mould perfectly. You will need to apply a thicker plaster support for larger moulds.

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