Mould making by application: which material to choose for what

Which silicone and which casting material should you choose for your application? Below is what we recommend per project, and why. Still unsure about the type of silicone in general? Use our selection guide.

A silicone bracelet

Master model. The result is determined by the master model: the finer the model, the finer the bracelet. Synthetic clay, plasticine and polymer clay are very soft, which makes sharp cuts and fine detail difficult. We recommend hard modelling wax.

Production mould. Cast the mould the bracelet is made in from polyurethane casting resin. The silicone then cannot bond to that mould, so a release agent is unnecessary.

The bracelet itself. It goes on skin. So take a platinum silicone, or better still Siliconen Culinair, which is food- and skin-safe. Colour it with silicone colourant. Bear in mind the silicone is white as standard, which makes colours look pastel.

Moulding a wooden hand or ornament

Two routes:

Alginate. Very easy, mould ready in 5 minutes, flexible like very thick custard. If it tears during demoulding it falls back into place by itself. Downside: single use, and it must be used within 12 hours, after which the mould loses water and therefore shape. Not suitable for undercuts.

Silicone. Around 12 hours demould time, strong, reusable many times, and it can be cut to make demoulding easier — including with undercuts.

In both cases: treat unvarnished wood beforehand. Varnish it, or rub it with petroleum jelly, colourless shoe polish or beeswax.

Two things to watch with an old wooden frame: the silicone may bond if the wood is porous or unvarnished, and the B component can attack the frame. Test on an invisible spot. All silicones can leave a little oiliness while curing, which may leave permanent stains in the wood.

A copy of an existing sculpture

A mould can be made of almost any object. Shape and detail determine the difficulty: a flat tile is simple, a sculpture with undercuts is not.

For the mould you use a 2-component silicone. For the cast you have a choice: porcelain plaster, A1 Acrylic Resin, casting wax or a polyurethane casting system. See the guides in the Information Centre for one-part and multi-part moulds.

Moulds for decorative stones and jewellery

You can make decorative stones from polymer clay through to resins such as polyurethane, epoxy or polyester. The choice of mould depends on how aggressive that material is and how difficult the original.

Condensation silicones are less demanding during curing and cheaper, but cope less well with epoxy and other resins — particularly with bulky shapes that generate a lot of heat. Platinum silicones stand up better to aggressive resins and hold their shape better, but they cost more and are sensitive to contamination before and during curing.

Moulds for miniatures

That depends on the casting material:

• Pewter miniatures require a high-temperature silicone. Dust the mould with talcum powder before use. Get in touch for the current option — this is a specialist application.
• Polyurethane, Acrylic One, wax or plaster: Condensation Silicone 20 will do.
• More aggressive or more sensitive materials such as crystal-clear PU: use platinum silicones.
• Edible miniatures should only be cast in Siliconen Culinair.

Moulds for soap, candles, cement and concrete

For all three, Condensation Silicone 20 is the standard choice.

Soap: with very aggressive ingredients or strongly alkaline soaps you are better off with platinum silicones.

Candles: if the candles are large and you don't want to cast the whole model inside formwork, use Thixo C to make the silicone spreadable.

Cement and concrete: this is where Condensation 20 really shows its strength. This silicone is highly tear-resistant, and if it does tear, the tear zigzags rather than running straight through — less damage, and sometimes it stops by itself. Repairs are simple: apply fresh Condensation 20. For very large moulds, make it spreadable with Thixo C.

Realistic skin and fat pads

Silicone Soft Flex 00-35 was designed to imitate skin, so it comes close.

Adding oil lowers the hardness and often increases elongation, but it also reduces strength. Where the limit lies depends on the application: if the silicone will be stretched often, you don't want to lose much strength. If it will only be squeezed occasionally, plenty of oil is less of an issue.

Important: oil does not cross-link into the silicone system. Over time, and especially under pressure such as its own weight or prolonged bending, an oil film forms on the skin. That can also discolour the stone, paper or card it stands on.

The mould can be alginate, plaster, acrylic resin, wax or polyurethane. Always run a small test on two points: is the surface not too porous, and is the mould material or one of the varnishes or adhesives not inhibiting to the silicone? The advantage of Soft Flex is its high platinum content, so it barely suffers from cure inhibition. Note: Soft Flex is quite fast. Don't mix too much, stir calmly and pour quickly in a thin stream.

A reusable mould of a hand

If the mould has to be kept, use silicone rather than alginate. A popular choice is Wacker Elastosil Vario 15 Fast: around 10 to 12 minutes cure time, safe on skin and fairly fluid.

You can also brush the silicone on to form a kind of glove mould. Thicken it for that with Thixo A. For brushing three hands, a 1.1 kg set plus 50 grams of Thixo A is enough — depending on the layer thickness.

Turning a plaster hand into an epoxy hand

This is possible, but it takes a different approach than you probably expect. You cannot use your existing cast in an alginate mould, for two reasons: you need to be able to move the hand to get it out of the alginate without tearing it, and epoxy is difficult to work with in a damp alginate mould.

So you need a silicone mould. Three routes, from best to easiest:

1. Two-part brush-on mould with an epoxy support shell. You need: Condensation Silicone DC 3481 R, Thixo C to make the silicone spreadable like butter, and plaster bandage as an inexpensive support shell.
2. Two-part poured mould. Only the silicone, no thickener — though it uses more material.
3. Transparent silicone. With a little practice the easiest route, because you can see where you're cutting and won't damage your existing piece. Note: transparent platinum silicones can be contaminated in liquid form by substances in varnishes, adhesives and latex gloves, leaving them tacky. So test everything you plan to use first.

For the cast, choose a clear epoxy with a long working time so it comes out bubble-free and finishes well — for example River & Art Epoxy Resin. Transparent colouring is possible with transparent colourant.

Casting a butterfly or flower in epoxy

This has to work first time. Practise with something less precious first, such as a dried leaf.

You need three things:

• A flexible silicone mould. If you're making it yourself, use for example Wacker Elastosil Vario 15 Normal.
• A dry, moisture-free butterfly.
• Epoxy with a UV blocker, preferably one that cures very slowly, such as River & Art Epoxy Resin.

Tip: fix the butterfly in the right position in the mould so it can't float up on the resin.

Coating a worktop with epoxy

We recommend UltraCast XT, heat resistant to 95 degrees.

1. The worktop doesn't need priming, but it must be extremely clean and thoroughly degreased with isopropanol.
2. At raised edges the epoxy runs off somewhat and the layer thins. If your top is light and you want a dark colour, paint or prime first — certainly the front edge.
3. Count on roughly 0.5 mm layer thickness, so 0.5 litres per square metre. Pouring thicker is possible up to a maximum of 1 mm.
4. Follow the instructions in the data sheet exactly.

Further:

• The resin needs up to 21 days to reach its final hardness and heat resistance.
• Don't use too much colour paste, silicone or effect solution; they lower heat resistance.
• In particular, use no alcohol-based colour solution.
• Colour with powders where possible.
• Apply an extra uncoloured layer of UltraCast XT over the coloured layer to raise heat resistance.
• Keep the worktop out of direct sunlight; mask windows if necessary.
• Work at 20 to 25 degrees. Too warm leaves too little working time; too cold exposes the epoxy to temperature and humidity swings for too long, causing amine blush or orange peel.
• Avoid air currents, including from air conditioning. Larger fluctuations cause wrinkles, orange peel or fish eyes.

Always test on a similar surface first.

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