Your first question should be: Do I need a mould? Don’t get me wrong, but for example when you’re making maquettes or one-offs, then you might not need to mould at all. There are heat curing materials such as Sculpey or Fimo, there are two part clays such as Miliput and then you could sculpt with NSP or wax-based clays that go reasonably hard once cooled and stiffen up after a while when left alone, there is winterstone culpting compound or you can easily carve into cured plaster if it is kept moist. All of the above can be coloured and could also be moulded at a later date, should you want to. Just saying – if you don’t need to make a mould, then don’t! A copy will never be as good as the original, it will take you much longer and cost you to reproduce your original.
Next question is: What will I be casting in? Well, there a as many answers to that as there are materials, you may even want to cast in porridge – fine, as long as you know before you start moulding. Basically you will have to know: Is my cast going to be hard or flexible? If your cast is going to be hard, you may want a flexible mould – if its flexible, you can get away with a solid mould (normally quicker and cheaper)
Then: You need to keep an eye on the properties of your chosen casting material.
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What does it stick to?
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Do I need to see inside the mould while casting?
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What contaminants are there?
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How runny is my casting material?
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Will it be laminated, solid, injected or swivelled?
(the places that sell your materials may know and I recommend calling them, describe what your planning to do and they may be able to advise you on the best material/release agent/pigment/special needs of that material)
If you are moulding for money, also keep an eye on efficiency – the most expensive part of the mould is your time! Yes, most moulds I make could be done so much nicer, but they don’t need to be, they need to be super accurate, on few occasions the need to last for quite a number of casts, but thats it!
Then you need to keep an eye on the size of your object – this has quite an impact on how to mould it.
for most small items (up to 30cm) I would only use one moulding material – make a blockmould either from rubber or resin or plaster, if you need extra stiffness use some durable tubing or box that you can put your mould back into for casting. Superfast to make, for bigger items too material-consuming
For most regular sized items (up to 2m) I recommend brush on rubber moulds – fewest possible seams and the greatest range of materials for casting, good accuracy and fast to make, material-efficient. If you know you will ONLY cast in rubber or want to produce a waste mould (mould destroyed for demoulding) you can make stiff moulds for regular sized items – they are considerably faster to make, a lot lighter(if not made from plaster), cheaper, and (if not made from plaster) more durable.
For most large items (larger than life) I would try and laminate a stiff mould, due to weight and normally the fact that large items have large undercuts and you may need to do many pieces anyways and will also very likely have a laminated positive. One little trick for more intricate pieces is the fact that polyurethane rubber will bond to resin if used as gel-coat, so does silicone if you laminate with epoxy. This enables you to make resin reinforced slates that can be very light since you need very little rubber. This however would be far too time consuming and inaccurate for smaller items.
