3D printing gets you one of a thing. Casting gets you thirty, in materials a printer cannot produce — soft, translucent, glow-in-the-dark, metal-filled, food-safe — at a few pence each after the mould exists.
It is also the most unforgiving process in a maker’s toolkit, because almost every failure is invisible until it is permanent. These are the parts that matter.
VanOaksProps on the end-to-end process.
Step 1 — Choose a silicone, and understand the cure chemistry
Two families, and picking the wrong one is the most common way a first mould fails.
Tin-cure (condensation cure). Cheaper, more forgiving about what it touches, shorter library life — it slowly degrades over years and after perhaps 20–40 casts. Shrinks slightly on cure, around 0.1–0.3%, which is irrelevant for most work.
Platinum-cure (addition cure). More expensive, essentially no shrinkage, far longer working life, and extremely fussy about contamination.
That fussiness is the thing to know before you buy any. Platinum silicone suffers cure inhibition: it will refuse to set — staying tacky or liquid at the surface — if it touches sulphur, tin, latex, many plasticines, some 3D-printed resins, and certain adhesives. Plasticine containing sulphur is the classic trap, because plasticine is exactly what everyone reaches for to build a clay wall or block an undercut.
The practical rule: if you are casting against anything you did not test, use tin-cure. If you need platinum, test a 10p-sized blob on the material first and let it cure fully. Twenty minutes of testing against a ruined half-kilo of silicone is a good trade.
Step 2 — Prepare the master
The mould copies everything, including what you did not mean.
Seal porous masters. Wood, plaster, clay, printed PLA — all porous. Silicone will key into the surface and tear out fibres, or refuse to release. A few coats of clear acrylic spray or shellac seals them.
Fix the surface now. Print layer lines will be in every single cast. Sand and fill before moulding, not after.
Check for undercuts. Any feature where the part gets wider before it gets narrower will trap it. Silicone is flexible enough to pull over mild undercuts — this is why it is the material of choice — but a deep one means a two-part mould or a cut.
Step 3 — Block mould or two-part?
A block mould is one solid piece of silicone with the master suspended in it, cut open with a scalpel to remove the part. Fast, simple, and you get a visible seam line where you cut.
A two-part mould is poured in two stages with a parting surface between them, giving a clean seam you controlled. More work, better result, and necessary for anything with undercuts on two axes.
For a two-part mould, cut registration keys — hemispherical dimples in the parting surface of the first half, so the second half moulds matching bumps. Without them the halves shift between casts and every part has a step in it. The back of a round paintbrush pressed into the clay parting layer is the traditional tool.
Build the containment box 10–15 mm larger than the master on all sides. Thinner and the mould distorts under the weight of the casting resin; much thicker and you are paying for silicone you do not need.
Step 4 — Mix properly, and mix by weight
Mix ratios for silicone and resin are almost always by weight, not volume, and the two are not interchangeable because the parts have different densities. Use a scale that reads to 1 g.
Scrape the sides and bottom of the cup. Unmixed material against the wall of the cup is the second most common cause of a soft patch in an otherwise good mould.
Mix slowly. Fast stirring whips air in, and every bubble you introduce you then have to remove.
Watch the pot life. Silicones and resins are specified with a pot life (workable time) and a demould time. A 20-minute pot life sounds generous and is not, once you have weighed, mixed, degassed and poured. Read both numbers before you start, and for a first attempt pick a product with a long pot life even if it means waiting overnight to demould.
Step 5 — Get the air out
This is the step beginners skip and professionals never do.
Degassing — a vacuum chamber pulls dissolved air out of mixed silicone before you pour. The mixture rises dramatically, foams, and collapses; that is correct. Use a container three times the volume of your material.
Pressure casting — a pressure pot compresses bubbles in the resin after pouring, to a size you cannot see. Note the asymmetry: you vacuum the silicone and you pressurise the resin.
If you have neither, these cheats get most of the way:
- Pour in a thin stream from a height, into one corner, and let the material flow across the master rather than dropping onto it. The thin stream breaks bubbles.
- Brush a thin “print coat” of silicone onto the master’s detailed surface first, work it into the detail with a brush, then pour the bulk. Bubbles trapped at the detail surface are the ones that show.
- Tap and vibrate the mould box on the bench for a minute after pouring. An orbital sander held against the side, switched on, works well.
- Pour resin into a tilted mould and let it climb, rather than filling it flat.
Step 6 — Release agent, sometimes
Silicone against silicone needs release agent. Silicone against most other things does not — that is the point of silicone.
Resin against silicone generally does not need it either, but a light release spray extends mould life considerably, and polyurethane resins in particular attack silicone over time.
When in doubt, a mould release that is specified for your resin and silicone combination rather than a generic one. The wrong release agent can inhibit cure just like contamination.
Step 7 — First casts, and what to expect
The first cast out of a new mould is often poor. Fresh silicone is at its least cooperative and there is usually a surface residue. Cast something you do not care about first.
Cure fully before demoulding. The stated demould time is for the stated temperature, and a cold workshop can double it. Demoulding early distorts the part and tears detail out of the mould.
Expect a mould to give you 20–40 casts with tin-cure, considerably more with platinum, and fewer with aggressive resins. Keep the master.
The safety that actually matters
Not a disclaimer — these are the specific risks.
Polyurethane resins release isocyanates. Ventilate properly and wear a respirator with organic vapour cartridges, not a dust mask. This is the one that causes permanent sensitisation.
Epoxy is a sensitiser. Repeated skin contact, not a single splash, is what causes a lifelong allergy. Nitrile gloves, changed when contaminated.
Resin cure is exothermic. A large mass of resin in a confined volume gets genuinely hot — hot enough to deform the mould, crack the casting, or smoke. Respect the maximum pour thickness on the datasheet.
Silicone is the safe part. Both cure systems are comparatively benign, which is why it is a good place to start.
Where to go next
- Rotocasting for hollow parts: pour a small amount, rotate the closed mould until it gels, and you get a shell instead of a solid — lighter, cheaper, and the only sane way to cast anything large.
- Pigments and fillers. Resin takes dyes, pearlescents, metal powders and glow pigment. Metal-filled resin with the surface polished back is convincingly cast metal.
- Flexible casting. Cast silicone into a silicone mould (with release) for soft parts — the route to custom keycaps, buttons, grips and anything that has to squash.
- Pair it with printing. A printed master, cleaned up and moulded, is the standard professional path: print once, cast many, in better material than the printer can produce.