Mould making

A mould is a negative of an object, made in something flexible enough to release it and stable enough to do it repeatedly. Making one is a design problem before it is a chemistry problem: you look at the original, work out which parts of it would trap the mould, and decide how the mould will come apart.

That word — undercut — is most of the subject. Any feature where the object gets wider before it gets narrower will lock a rigid mould in place. Rubber solves some of it by stretching, and beyond a certain point it does not, and then the mould has to split into two or more parts with a parting line chosen where the seam will least matter. Reading an object for that, before mixing anything, is the skill.

The rest is choosing the right rubber for what you intend to pour. Silicone is not one material: condensation-cure and addition-cure behave differently, cost differently, and — crucially — addition-cure silicone is inhibited by a list of common contaminants, which is why a mould sometimes stays sticky forever for no visible reason.

The actual problem

Undercuts and the parting line

A first mould

Under £30

Where the budget goes

Wasted silicone

Not optional

Scales, and ventilation for urethanes

1. 1

   Step 1: Pick an object with no undercuts at all

   A coin, a tile, a simple relief. A one-part block mould of something that lifts straight out teaches the whole workflow with none of the design problem, and it will work first time.

   You will know it worked when: A working mould and a clean cast.

2. 2

   Step 2: Buy condensation-cure silicone and scales

   Tin-cure for the first several projects, because it forgives contamination that would stop addition-cure dead. Under £50 for both.

   You will know it worked when: Materials that will not fail for reasons you cannot diagnose.

3. 3

   Step 3: Build the box tight to the object

   Ten to fifteen millimetres of clearance all round. This single habit is the difference between using 200g of rubber and 600g, and rubber is the entire budget.

   You will know it worked when: A box that wastes almost nothing.

4. 4

   Step 4: Mix by weight, slowly, and pour in a thin stream

   Scrape the sides and bottom of the mixing pot properly — unmixed material in the corners is the classic uncured patch. Pour from height into one corner and let the silicone flow across the object rather than onto it, which chases air out instead of trapping it.

   You will know it worked when: A bubble-free mould without a vacuum chamber.

5. 5

   Step 5: Now do something with one undercut

   A figure, a handle, anything with a feature that traps. Decide where it splits before you mix, and cut or key the mould accordingly. This is the step where the subject actually begins.

   You will know it worked when: A two-part mould that opens and registers.

6. 6

   Step 6: Cast the material you actually want, not the material you have

   Plaster, resin, concrete, wax and chocolate all demand different things of a mould. Test the intended material early rather than perfecting a mould for a material you will not use.

   You will know it worked when: A mould matched to its job.

7. 7

   Step 7: Only then move to addition-cure

   Once you can produce a good mould reliably, platinum silicone gives better detail and a mould that lasts years — and it will punish you for a contaminant you did not know was there, which is much easier to diagnose when everything else is already under control.

   You will know it worked when: Moulds worth keeping.

• Condensation-cure silicone, the forgiving one — Tin-catalysed, £25–£45 a kilogram, tolerant of almost any contaminant, and it will cure against nearly anything. Its costs are a shorter library life — it shrinks slightly and degrades over a few years — and less fine detail. The right first purchase.
• Addition-cure silicone, the accurate one — Platinum-catalysed, £35–£70 a kilogram, essentially no shrinkage, superb detail and a mould that lasts. The catch is inhibition: sulphur-containing clays, latex, some tapes, tin-cure residue and certain adhesives will stop it curing entirely, and the mould stays tacky forever. Test on a scrap before committing.
• Polyurethane rubber, the tough one — Cheaper and far more tear-resistant than silicone, which makes it the industrial choice for concrete and plaster production work. It is isocyanate chemistry — see above — and it needs a release agent, which silicone generally does not.
• Shore hardness, which decides demoulding — The number after the product name. Softer rubber releases deeper undercuts and distorts more; harder rubber holds shape and traps things. Around Shore 20–30A is the general-purpose range and where to start.
• Digital scales reading to a gram — £15–£25 and genuinely mandatory. These are ratio products, most now measured by weight rather than volume, and mixing by eye is the commonest cause of a batch that never sets.
• Mould box material, which can be free — Lego, foam board, a 3D print, a plastic tub. What matters is that it is close to the object — every millimetre of gap is silicone you are buying and throwing away.
• Release agent, for the cases that need it — Silicone against silicone always needs one. Silicone against most other things does not, which is a large part of why silicone is the beginner's material.
• What can wait — A vacuum chamber, a pressure pot, thixotropic additives for brush-on moulds and mother-mould materials. All are genuinely useful and none of them fixes a mould whose parting line was chosen badly.

The first stage is the workflow: box, mix, pour, demould, cast. A few simple one-part moulds get you there in an afternoon each, and there is very little to go wrong if the object has no undercuts and the silicone is tin-cure.

The second is the design problem, and it is the whole craft. Reading an object for undercuts before touching it, choosing a parting line where the seam will not matter, keying two halves so they can only close one way, and knowing when to go to a brush-on mould instead of a block. This is judgement about a specific object rather than a procedure, which is why it is not written as technique here — the useful thing a page can give is the vocabulary and the failure modes.

Beyond that it becomes a service to whatever you actually make. Miniature casters chase detail and go to pressure pots and addition-cure silicone. Prop makers go large, with mother moulds and lightweight materials. Jewellers meet it from the other direction through wax. And a fair number end up making moulds for other people, because it is a genuinely scarce skill in every making community and the person who can solve an undercut is always in demand.

The smaller ones, which are ordinary bench hazards:

• Exotherm — Curing resin generates heat, and a thick section can get hot enough to smoke, warp its mould or crack. Pour deep sections in layers rather than in one go.
• Water and polyurethane do not mix — Moisture reacts with urethane resins and produces gas, which is why a damp mould gives a foamed, bubbled cast. It is a quality problem that occasionally becomes a pressure one in a sealed mould.
• Tin-cure silicone releases a little alcohol — Condensation-cure silicone gives off a small amount of alcohol as it sets. Not dangerous in a ventilated room and a reason not to cure a large batch in a cupboard.
• Keep it out of the kitchen — Separate mixing vessels, separate scales, and nothing that returns to food use. The simplest rule and the one most often broken because the scales were handy.

• Model and miniature casting — Small resin parts in quantity, usually with a pressure pot to eliminate bubbles. The largest hobby use and the one with the most exacting detail requirement.
• Prop and costume making — Large pieces in flexible foam and lightweight resin, frequently brush-on moulds with mother moulds because the objects are too big to block-mould.
• Jewellery and small metal casting — Where mould making meets lost-wax: a rubber mould produces wax originals, which are then cast in metal. The link between this page and the metalworking side of the catalogue.
• Concrete and plaster production — Planters, tiles, architectural detail. Polyurethane rubber territory rather than silicone, because the moulds take abuse and are used hundreds of times.
• Food moulding — Chocolate and sugar work, using food-grade silicone specifically certified for it. A genuinely separate materials question and not a place to improvise.
• Life casting — Moulding directly from a living body with skin-safe materials. A distinct discipline with real airway risks when casting a face.