Slip casting with a 3D printed plaster mold

Slip casting turns liquid clay into a hollow ceramic piece using a plaster mold — and you can 3D print the form that makes that mold. You print the form, pour plaster into it to create the mold, then pour liquid clay "slip" into the cured plaster. The plaster pulls water out of the slip, building a clay wall against its surface; you drain the rest, let it firm, and demold. Here's the full workflow.

What slip casting is

Slip casting is how a lot of identical ceramics — cups, vases, figurines — are made. Slip is clay suspended in water with a little deflocculant so it stays fluid without excess water. Poured into a plaster mold, the porous plaster draws water out of the slip by capillary action, and a solid clay skin forms against the mold wall. The longer you leave it, the thicker that wall grows.

How the plaster mold generator works

The plaster slip-casting mold generator doesn't print your final mold — it prints the form you pour plaster into. You load your vessel model, and it wraps it with a plaster gap (the thickness of the plaster layer between the model and the wall) and adds a pour opening. Print that form, pour in plaster to cast the actual mold, and once the plaster sets and dries you have a reusable slip-casting mold — no plaster carving or coddling required.

Print the form and cast the plaster

• Print the form in PLA or PETG with watertight walls so plaster can't seep — the same leak-free habits as a silicone mold box.
• Mix casting plaster (e.g. pottery plaster) to its instructions, pour it into the printed form, tap out air bubbles, and let it set.
• Dry the plaster thoroughly before casting — a damp mold won't absorb water from the slip. Fresh molds often need days to dry fully.

Cast the slip

1. Use a clean, dry plaster mold. Its water absorption is the whole mechanism.
2. Mix your slip smooth and pour it into the mold in one steady stream, topping up as the plaster rapidly drinks the first water.
3. Let the wall build. Over several minutes (often in the ~5–20 minute range, depending on how thick you want the wall) a clay layer forms against the plaster.
4. Drain the excess. Pour the remaining slip back into your bucket and invert the mold to drain — this is what makes the piece hollow.
5. Let it firm up. The piece releases moisture and shrinks slightly as it reaches leather-hard.

Demold, dry, and fire

Once the clay is leather-hard and has pulled away from the plaster, open the mold and lift out the greenware. Trim the seam and pour-hole with a fettling knife and smooth them. Let the piece dry completely, then bisque fire it like any pottery, and glaze fire if you wish. Let the plaster mold dry out again between casts.

Troubleshooting

• No wall forms / very slow: the plaster mold is still damp — dry it fully before casting.
• Cracks in the greenware: slip too watery, or drying too fast; use properly deflocculated slip and dry slowly and evenly.
• Piece won't release: let it reach leather-hard so it shrinks away from the plaster before demolding.
• Plaster seeps out of the printed form: add perimeters and seal seams so the form is watertight.

FAQ

Do I pour clay into the 3D print?

No — the print is a form for plaster. Pour plaster into it to make the mold, dry it, then slip-cast liquid clay into the plaster.

Why does the plaster mold have to be dry?

Slip casting works because dry plaster absorbs water from the slip to form the clay wall. A damp mold can't absorb, so no wall builds.

What clay do I use?

A ready-made casting slip (deflocculated clay) rather than plastic throwing clay. Follow your slip supplier's mixing guidance.