How to make a silicone mold from a 3D print
Quick answer: you don't print the silicone. You print the master and the container. Sand and seal the print, because silicone copies every layer line. Fix it inside a printed box or shell. Mix a platinum-cure silicone such as Mold Star 30, 1:1 by volume, and pour it slowly into one corner. Mold Star 30 gives you 45 minutes to work and cures in 6 hours at 23 °C.
- Test first. Smooth-On lists some 3D-printed resins and PLA/PETG among common causes of silicone that stays tacky. Seal the print with clear acrylic, or pour a small test patch.
- A shell (jacket) mold uses far less silicone than a poured box. The free splicestl mold maker prints one from any STL.
Three ways to make the mold
| Method | Best for | Silicone | Effort |
|---|---|---|---|
| One-part box mold | Flat-backed objects: badges, soaps, tiles | Most | Easiest: one pour |
| Two-part box mold | Fully 3D objects and figures | Most | Clay bed, keys, release, two pours |
| Shell (jacket) mold | Large or organic shapes | Least | Print two shell halves; cut the silicone open to demold |
What you need
- Silicone: a platinum-cure rubber such as Mold Star 30. The trial unit suits a first mold.
- Mixing: graduated cups and stir sticks, and a scale that reads 0.1 g if you mix by weight.
- Sealing the print: clear acrylic sealer spray, plus a filler primer and sandpaper.
- Two-part molds: sulfur-free clay and Ease Release 200.
- Optional: a vacuum chamber and pump for bubble-free molds, and nitrile gloves.
Step by step: a poured box mold
1. Prepare the print
Silicone copies everything, including layer lines. Print the master with fine layers, then sand it and coat it with a filler primer. Wash a resin print and cure it fully under UV first.
Then seal the surface with clear acrylic lacquer and let it dry completely. Before the real pour, brush a little mixed silicone on a hidden spot. If it is still tacky after the cure time, seal again.
2. Build the box
Print a box with about 10 mm of space around the master on every side and above it. Thinner silicone tears; thicker silicone is wasted. Hot-glue the master to the floor so it can't float. Seal the box seams with hot glue or a sulfur-free clay, because sulfur stops platinum silicone from curing.
3. Work out how much silicone
You need the box's inner volume minus the master's volume.
grams ≈ cm³ × 1.12 (Mold Star 30)
For example, a 60 × 60 × 40 mm box holds 144 cm³. If the master takes 30 cm³, you need about 114 cm³, or roughly 128 g. Mix about 10 % extra, because some stays on the cup. Smooth-On gives Mold Star 30 a specific volume of 24.7 in³ per pound, which is where the 1.12 g/cm³ comes from.
4. Mix and pour
Mix equal parts by volume, or 100:96 by weight. Scrape the sides and bottom of the cup until there are no streaks. Pour a thin stream into one low corner of the box, not onto the master. Let the silicone rise over the master by itself, which pushes the air out. A vacuum chamber removes the remaining bubbles; a thin, high pour gets most of them.
5. Cure and demold
Leave Mold Star 30 for 6 hours at room temperature (23 °C). A cold room slows the cure, and mild warmth speeds it up. Then open the box and peel the silicone off the master.
A two-part mold for fully 3D objects
A figure, or anything without a flat back, needs a mold that opens in two halves.
- Bed the master halfway into sulfur-free clay inside the box.
- Press a few dimples into the clay as registration keys. Add a pour channel (sprue) and a vent or two.
- Pour the first half and let it cure.
- Remove the clay and brush mold release on the cured silicone. Silicone sticks to silicone.
- Pour the second half. The halves then open and close on the keys.
A shell (jacket) mold uses far less silicone
A shell mold puts an even layer of silicone around the master, inside a thin rigid shell. The silicone follows the shape instead of filling a box, so it takes much less. On our test model, a 54 × 28 × 51 mm creature, the shell needed about 36 cm³ of silicone. A box with the same minimum wall needed about 128 cm³, so the shell used 72 % less. The silicone stretches, so undercuts are fine. To demold, cut the silicone open along one side.
The free splicestl mold maker prints the shell from any model:
- Open the mold maker and drop the master as STL, OBJ, 3MF or GLB. Broken meshes are repaired as they load.
- Choose Silicone shell, then the pull axis: the direction the halves come apart.
- Set the silicone thickness. 5 mm suits most masters up to fist size.
- Generate, read the silicone estimate, and download the two shell halves and the pour pin.
- Print the halves parting face down. Hang the master on the pin, bolt the halves together and pour.
It also makes rigid two-part and open molds, printed in PLA or PETG, for casting plaster, concrete or wax straight into the print.
Problems and fixes
| What you see | Cause | Fix |
|---|---|---|
| Silicone tacky where it touched the print | Cure inhibition | Strip it; it won't cure later. Seal the master, let it dry fully, test, re-pour. |
| Layer lines on every cast | Unfinished master | Sand and prime the print before molding |
| Bubbles on the cast face | Air trapped during the pour | Pour thin into one corner; degas in a vacuum chamber |
| Two halves stuck together | No release between pours | Brush mold release on the first half |
| Silicone tore on demolding | Walls too thin | Leave about 10 mm around the master, or use a shell |
Print a silicone shell for your model
Shell, two-part and open molds from STL, OBJ, 3MF or GLB, with a silicone estimate. Nothing is uploaded.
Open the mold makerFrequently asked questions
Can you 3D print a silicone mold?
Not the silicone itself. You print the master and the box or shell that holds the liquid silicone while it cures. The cured silicone is the mold.
Will silicone cure against PLA or resin prints?
Usually, but not always. Smooth-On lists certain 3D-printed resins and PLA/PETG among common causes of cure inhibition. Wash and fully UV-cure resin prints, seal any print with clear acrylic, and pour a test patch first.
How much silicone do I need for a mold?
The box's inner volume minus the master's volume, plus about 10 % for waste. For Mold Star 30, multiply the cm³ by about 1.12 to get grams. A shell mold needs far less.
How thick should a silicone mold be?
About 10 mm around the master in a box mold. In a shell mold, about 5 mm is enough for masters up to fist size, because the rigid shell supports the silicone.
Do I need mold release for a silicone mold?
Usually not between platinum silicone and a sealed print. You do need it between the two pours of a two-part mold, because silicone sticks to silicone.
How long does a silicone mold take to cure?
It depends on the product. Mold Star 30 has a 45-minute working time and cures in 6 hours at 23 °C. Cold slows the cure, and mild warmth speeds it up.
Sources
- Smooth-On, Mold Star 30 — 1A:1B by volume (100A:96B by weight), 45-minute pot life, 6-hour cure at 23 °C, Shore 30A, specific volume 24.7 in³/lb, acrylic sealer, release usually unnecessary. Read 9 Nov 2026.
- Smooth-On support, My platinum silicone came out tacky / gooey / won't fully cure — inhibitors including certain 3D-printed resins and PLA/PETG; test patch; seal before molding. Read 9 Nov 2026.
- The shell-versus-box figures come from the mold maker's regression test model, described in its documentation.
What changed — November 9, 2026: first published. Same day: added the supplies list.