How to cast jesmonite in a 3D printed mold

To cast jesmonite from a 3D print, use the print as a master, make a silicone mold from it, then mix jesmonite AC100 at about 2.5 parts powder to 1 part liquid by weight, brush a first coat into the mold, fill it, and demold after roughly half an hour. Here's the full process — from the printed master to a sealed, finished piece.

Step 1 — Know the material

Jesmonite is a water-based composite: a mineral powder (the base) mixed with a water-based acrylic resin (the liquid). It sets fast, is lighter and more chip-resistant than plaster, and takes pigment well — which is why it's popular for trays, coasters, planters, candle holders and terrazzo-style decor. The most common grade for casting is AC100, and the numbers below are for it; other grades differ, so check your product's guidance.

Step 2 — Make a silicone mold

Jesmonite picks up every detail of its mold and releases easily from silicone, so a silicone mold made from a printed master is the reliable route. You can model the mold parts in CAD if you have the time, but I make them with 3DGEN — the printable mold is ready in seconds in the browser, no CAD:

• Any small object: the silicone mold box generator builds a box around your master. After Build the box, its Material estimate panel shows how much silicone to mix.
• Planters and vases: the vase & planter mold generator builds a printed housing with a core, so the silicone forms a jacket around a vessel and the jesmonite pours as a hollow pot.
• Several identical pieces: the casting tray generator in Silicone-pour mold mode makes a multi-cavity silicone tray — handy for coasters and small trays.

Print the master smooth — jesmonite copies layer lines faithfully — then pour the silicone and let it cure fully.

Step 3 — Work out how much to mix

A quick way to size a batch: fill the silicone mold with water, weigh the water and add about 10%. That's roughly the weight of jesmonite you'll need. Split it by the mix ratio: with AC100's 2.5:1, about 29% of the total is liquid and 71% is base. For example, 350 g of mix ≈ 100 g liquid + 250 g base. Dry the mold completely before casting.

Step 4 — Mix

1. Weigh the liquid first into a clean cup on a digital scale.
2. Add the base powder to the liquid — not the other way round — at 2.5 parts base to 1 part liquid by weight.
3. Mix thoroughly until smooth and lump-free. A high-shear mixing blade on a drill gives the smoothest, least bubbly mix for bigger batches.
4. Add pigment if you like — keep it to about 2% of the total weight, or it can affect the set. For a terrazzo look, stir in chips of a contrasting cast.

You have roughly 10–15 minutes of working time for AC100, a little less in a warm room, so have the mold ready before you mix. Wear gloves and a dust mask when handling the powder.

Step 5 — Pour

1. Brush a first coat into the mold — into every corner and detail — or tip and tilt the mold so a thin layer coats the surface. This is what gives a bubble-free face.
2. Fill the mold with a slow, thin stream poured from a little height.
3. Tap or vibrate the mold to bring trapped air to the surface.

Step 6 — Demold and finish

• Demold once the cast is firm and cool — for AC100 usually around 30 minutes, longer in a cold room. Peel the silicone back gently.
• Sand early. Jesmonite is easiest to sand soon after demolding while it's still relatively soft — a sanding block with medium paper takes off seams and sharp edges.
• Let it cure for at least 24 hours for small pieces.
• Seal it once fully cured with thin coats of an acrylic sealer (satin or gloss) — especially for coasters, trays and anything that will see water.

Troubleshooting

• It isn't setting: the ratio was off or the room is cold. Weigh both parts precisely and work somewhere warm.
• Bubbles on the face: brush a thorough first coat, pour slower and more tapping. Mixing too fast also whips in air.
• Blotchy color: too much pigment or incomplete mixing — stay around 2% and mix until fully even.
• Layer lines on the cast: the silicone copied the print surface. Smooth the master and remake the mold.

FAQ

Can I pour jesmonite straight into a 3D printed mold?

It's possible with a well-sealed, release-coated rigid mold, but silicone is far easier: jesmonite releases cleanly and picks up every detail. Use the print as a master and cast a silicone mold.

What is the jesmonite mix ratio?

For AC100, about 2.5 parts base powder to 1 part liquid by weight. Other grades use different ratios — follow the product guidance.

Is jesmonite waterproof?

Seal it for use around water. A cured, sealed piece handles occasional water well; check the sealer's guidance for outdoor use.