Making a silicone mould from a 3D print

Tin-cure or platinum?

Pourable mould silicones come in two chemistries. Tin-cure (condensation) silicone is mixed at roughly 100 parts base to 3–10 parts catalyst by weight, cures in about 24 hours and is the cheaper of the two. Platinum-cure (addition) silicone is mixed 1:1, cures in 30 minutes to 6 hours depending on the grade, shrinks almost nothing and keeps its properties for decades. It is also the one that can fail to cure at all.

For a first kit, a tin-cure silicone of Shore A 15 to 25 is the pragmatic choice: it tolerates printed masters, sealed surfaces and small mistakes. The best-selling mould kits in this niche are poured with exactly that. Move to platinum when you cast the same vessel dozens of times and want a mould that stays dimensionally stable for years.

Cure inhibition: the one failure you cannot fix

Platinum silicone that stays gummy against the master is lost — it never cures, and the master has to be cleaned down to bare plastic. The usual culprits are sulphur (modelling clay, latex gloves), residues of tin silicone, and some 3D-printed surfaces. SLA resins inhibit most often; PLA and PETG are reported both ways, usually fine but not guaranteed.

The remedies are simple. Wash resin masters in isopropanol, post-cure them under UV for at least 30 minutes and let them rest 24 hours. Seal any printed master with spray paint, acrylic lacquer or shellac and let it dry completely. Wear vinyl gloves. And always run a contact test: pour a spoonful of mixed silicone onto a scrap print of the same material and wait the full cure time before committing the real pour.

Preparing the printed master

Silicone copies everything, including layer lines. Print the master upright with the cavity facing up, at 0.12 mm layers with three perimeters, then sand it from 240 to 600 grit. Where a relief or a sharp edge must stay crisp, sand lightly around it rather than over it. Primer and a second sanding pass give a near-injection finish; an epoxy coating such as XTC-3D does the same faster but needs a release agent afterwards because rubbers bond to cured epoxy.

Apply a thin coat of non-silicone release agent to the master, the inside of the formwork and the core. On the first pour this is what keeps the frame from cracking when you open it. Vaseline rubbed thin and buffed works where you have no spray.

How much silicone to mix

The volume is the inner volume of the formwork minus the master, plus the cavity volume minus the core for the inner sleeve. Add 5 to 10 % for what stays in the cup and on the stick, then multiply by the density on the datasheet (1.10 to 1.18) to get the mass to weigh.

Mouldery kits do this arithmetic from the actual geometry and print it in the README. For an 80 mm candle jar the result is about 430 ml computed, 470 ml to buy, around 520 g at a density of 1.10. The defaults keep 9 mm of silicone around the master and 9 mm above its rim: enough for a mould that will be used inside the frame, not so much that you pay for rubber you do not need.

• 6 mm is the minimum wall for a mould that works inside a rigid frame.
• 8–12 mm is the usual range, and what the kits generate.
• Above 20 mm you are buying weight, not strength.

Mixing, degassing, pouring

1. Work at 20–25 °C, never below 18 °C. A warm room speeds the cure; a cold garage can double it.
2. Weigh both parts on a scale. Stir 3 to 5 minutes, scraping the cup walls and the bottom, without whipping air in.
3. Degas if you can: 25–30 mbar (−80 to −100 kPa) for 3 to 5 minutes, let the foam rise and fall. Without a chamber, pour from 30–40 cm in a thread as thin as a pencil.
4. Pour into one corner at the lowest point and let the silicone find its own level. It rises around the master, covers its rim and fills the cavity around the core.
5. Stop at the engraved pour line. The top of the core stays 4 mm proud so nothing flows inside it.
6. Leave it alone for the cure time on the datasheet: 6 to 24 hours for tin-cure, less for fast platinum grades. Demoulding early is the second most common way to tear a mould.

Demoulding without tearing

Wash the mould with soap, dry it, and store it assembled in the frame, flat. Shore A 15–25 is right for relief and tight corners when the frame serves as the counter-mould during casting; a free-standing mould wants 25–30 A. Concrete is abrasive: expect fine detail to soften after dozens of casts, and a light coat of release on each pour to roughly double that.

1. Undo the 4 mm screw, lift the bridge, pull the core out of the inner sleeve with a slight twist.
2. Remove the clips, open the split corner and slide the frame off the block.
3. Turn the block over, undo the M5 screw and lift the base plate away.
4. Push the master out through the foot opening. The silicone stretches to let the wider rim pass; a few drops of soapy water help.
5. Trim the thin flash left by the split with a sharp blade.