How to Make a Silicone Mold with a 3D Printer (Complete Walkthrough)

From a 3D model to a finished cast — print the mold shell, pour silicone, demold. A step-by-step guide with the supplies you need.

Silicone molds are traditionally made by placing a master in a box and pouring silicone around it. If your master only exists as a 3D model, you can flip the process: 3D print the mold shell itself and pour silicone into it. Printing the master, sanding it and building a box all disappear.

This article is the whole process end to end. The other guide articles go deeper on each step.

What you need

Step 1 — Generate the mold STL

A mold shell is a two-piece box wrapped around your model. Designing one by hand means modelling the parting plane, seal ridge, locator pins, sprue and vents one by one — half a day. The mold generator does all of that automatically:

1. Open your model file. Everything is computed in your browser; the file never leaves your device.
2. Set the cavity gap. 0 for a solid cast; 1–1.5 mm if you want a hollow skin (squishies, doll skin).
3. Pick a casting method: side pour (upright) for viscous silicone, top pour (flat) for thin resin or wax, open casting for skins.
4. If parts of the model turn red (undercuts), try another split height. When a "Splitting at N % cuts undercuts" hint appears, click it. The reasoning is in two-piece mold design.
5. Download the bottom and top mold STLs.

Step 2 — Print the shell

PLA, 0.2 mm layers, 3–4 walls, 15 % infill is plenty. Print both halves parting face up, exactly as exported. The inside of the cavity becomes the surface of your cast, so the one rule is: no support material touching it. Shells from the generator are designed to print without supports. Details in printing the mold.

After printing:

• Scrape any bumps off the parting faces with a fingernail or blade so the seal ridge seats fully
• Dry-fit the halves and check the locator pins go all the way in
• If layer lines inside the cavity bother you, a light pass with 600–1000 grit or a thin coat of primer

Step 3 — Pour the silicone

1. Apply a thin coat of release agent to the cavity and parting faces, let it dry 5 minutes. Silicone does not bond to PLA, but release makes demolding far easier.
2. Close the halves and clamp them with rubber bands or clips. If you printed the generator's frame STL, just slide it on.
3. Weigh parts A and B by weight at the manufacturer's ratio. Never eyeball it — a wrong ratio never cures.
4. Stir slowly for 2–3 minutes, scraping the sides and bottom. Fast stirring only adds bubbles.
5. Pour in a thin, high stream into the sprue funnel. A thin stream pops bubbles on the way in. When silicone rises in the air vents, the cavity is full.
6. Finish within the pot life, then wait the cure time (typically 4–12 h). Gentle warmth (around 40 °C) speeds it up.

Step 4 — Demold and finish

• Insert a flat screwdriver into the pry notches at the corners and work all four corners a little at a time. Forcing one corner cracks PLA.
• Cut the cured sprue and vent stubs with a blade; peel the thin flash along the parting line with a fingernail or scissors.
• The same shell can be reused many times — PLA shells typically last dozens of pours. Reprint when the seal ridge wears or warps.

Common mistakes

• Eyeballing the A:B ratio — the number one failure. Use a scale.
• Platinum silicone near sulfur clay or latex — cure inhibition leaves a sticky surface. See choosing silicone.
• Demolding too early — the surface cures first; thick sections lag. Add two hours to the stated cure time.
• Support scars inside the cavity — orient the shell so no supports are needed.

When something goes wrong, the troubleshooting article is organised by symptom.