How to Make a Simple Silicone Mould

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Introduction: How to Make a Simple Silicone Mould

About: I love making and creating things - and I'm lucky enough to do it for my job and as a hobby!

I'm often asked how to make simple silicone moulds that can be used to produce high quality resin castings so this Instructable shares the basic process and hopefully shows how straightforward it can be to produce a quality mould which can be used to produce lots of resin cast replicas, even with fine surface detail.

A silicone mould like this is suitable for use with lots of different casting products and can be produced with or without more sophisticated equipment like a degassing chamber.

The process is broken down into an easy to understand step by step guide as follows:

  • Making the Mould Box
  • Measuring, Mixing, Pouring and Curing the Mould
  • Using the Mould

The process is really simple and can be used by beginners as well as experienced mould makers!

Step 1: Materials

You will need:

  • An original part to create a mould from - I've chosen this 3D printed 'Xencast' coin with detail on one side and a flat underside - This example piece is the size of a large medal or commemorative coin.
  • Silicone rubber and catalyst - I used the CS25 Condensation Cure Silicone Rubber and Catalyst
  • Digital scales
  • Nitrile gloves
  • Measuring cups and mixing sticks
  • A container to act as a mould box (or you can create your own)
  • Hot melt gun
  • Cocktail stick
  • Tools for cutting and sticking

You will also need to prepare your work area and yourself according to the product instructions.

Step 2: Mould Container

  1. Create or choose a suitable mould container - the bottom of this small bucket was a perfect size for this project, you could use any plastic container of a suitable size or make a box using signboard and a hot-melt glue-gun
  2. There needs to be a 10 mm gap all around the original, and it needs to be watertight - if you test it with water remember to dry it thoroughly
  3. Then using a glue gun, secure the original to the centre of the container
  4. Don't forget to allow any glue to dry before pouring the silicone
  5. Then prepare the silicone!

Step 3: Measure and Mix Silicone

Following the manufacturer's instructions:

  1. Make sure your work surface is protected and you're wearing protective clothing and gloves
  2. First weigh out the required amount of silicone into a mixing cup using digital scales.
  3. Now shake the catalyst really well (most have a pigment in so you can see when it's properly mixed-in)
  4. Measure out the required amount of catalyst - I used a syringe which made measuring out a small amount much easier but if you're measuring a larger amount you can use digital scales.
  5. Mix the silicone and catalyst together slowly and steadily - this is really important so you don't add in unwanted air bubbles - you may still get some so I'll show you how to deal with those in the next step*.
  6. Scrape the sides and bottom of the pot to ensure a thorough mix and keep going until the silicone is a consistent colour.
  7. Time to pour!


* Some information about degassing chambers

People who work with silicones and casting resins regularly will often use a piece of equipment called a degassing chamber which is an airtight chamber that is connected to a vacuum pump so that all the air can be removed. Silicones and casting resins can be loaded into the chamber (whilst still liquid) and put under vacuum to expand and expel trapped air. Although a degassing chamber is great for guaranteeing all trapped air is removed, many silicones (such as the Easy Composites CS2 I use in this tutorial) and casting resins will still produce great results when used without a degassing chamber.

If you have access to a degassing chamber then you would use it at this point to remove any trapped air before pouring the silicone into the mould - however it doesn't matter if you don't have one.

Step 4: Pour and Cure the Silicone

  1. RTV silicones really don't stick to much at all (apart from themselves) and so don't require a release agent to be applied to your original part - unless you are making a mould from a silicone master (as it will stick to itself) or a porous material which would need to be sealed first.
  2. Pour the silicone into the lowest point of the container (don't pour it on to the original part) and allow it to flow naturally around the original - this should reduce the chance of introducing trapped air or causing bubbles.
  3. Pour in a long thin stream - this will help to force out air bubbles from the mix too. This is sometimes called 'stretch pouring'.
  4. Then finally if you can see any persistent bubbles, use a cocktail stick to pop them.
  5. Remember to cover the original by 10 mm or more, then leave to cure for around 24 hours.

Step 5: Remove the Original

24 hours later:

  1. The silicone will now be cured so you can gently remove the mould box - be careful not to damage the silicone.
  2. If you need to, trim and tidy up any edges and remove the glue.
  3. Loosen the mould from around the edges of the original and peel apart carefully.

Step 6: The Finished Silicone Mould

There you have the finished silicone mould ready to begin casting!

This silicone produces great surface detail on moulds, and has a high tear strength and excellent flexibility. It can be used many, many times to produce your replica resin castings.

The silicone used to create this mould is suitable for use with a wide range of casting resins and materials including:

  • Polyurethane resin (often referred to as 'Fast Cast') such as the Xencast P2 I used for the coins.
  • Polyester resin
  • Epoxy castings resins (such as GlassCastĀ® clear epoxy resin range)
  • Two part polyurethane foam
  • Wax (for candles etc.)
  • Casting plaster
  • Jesmonite
  • Concrete

The images show cast coins using the Xencast P2 Fast Cast Polyurethane Resin coloured with some Translucent Tinting Pigments.

Timescale:

The timescale for the project working time was under 30 minutes plus the curing time of 24 hours!

The silicone and casting resin came from Easy Composites where I work as my day job. Other suppliers are available but I know how to use these products and get great results. If you use different products then remember that mix ratios, cure times and appearance could all be different so make sure you read the accompanying datasheets.

Have a look at my other Instructables to see more moulding and casting projects, including cold casting with metal powders and if you have any questions please post them in the comments!

Thanks for reading :-)

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