Making a slip-cast mold

From a 3D model to a plaster mold you can print and pour: bring the model, set its fired size, pick the pieces, generate, and get the print files.

A slip-cast mold is made of plaster pieces. You pour liquid clay (slip) in, the plaster draws water out of it so that a wall of clay builds on the inside, and you pour the rest back out. Castworthy designs the plaster pieces around your model and gives you the cases to print and pour them in.

1. Bring a model

Open the slip-cast mold tool and drop in your model: STL, OBJ, GLB, PLY or 3MF, in millimetres, standing the way it will be cast. Or start from one of the examples. The model is checked first, and a mesh with holes or other faults is repaired for you.

2. Set the size it fires to

Clay shrinks as it dries and fires. Press Size beside the 3D view and type the height the finished piece should be, and the clay's shrinkage. The mold is made bigger by that much: with 12 % shrinkage, a piece that should fire to 200 mm is cast at about 227 mm. Picking your clay and cone fills in its shrinkage from its data sheet; type your own if you measured it.

3. Pick the pieces

The mold is planned on the model before anything is built. Locks colours what would lock in the plaster (an undercut that would hold a piece in place), and the plan shows where air would be caught. The settings beside the view:

• Pour opening: where the slip goes in and comes back out. Vessels pour through their own rim, closed forms such as figures through the base, plates through the rim edge.
• Side pieces: how many plaster pieces stand around the model. More pieces free more undercuts but add seams to clean up and cases to print. Auto tries each number at its best turn and keeps the one that locks least.
• Turn: moves the seams around the model, to keep one off a face or a handle.
• Drop-out base: a separate piece under the model, so no seam crosses the foot.
• Plaster wall: how thick the plaster is. A thicker wall draws water for longer and lasts more casts; a thinner one saves plaster and weight.

4. Generate

Press Generate mold. When it is done, look at the pieces in the 3D view and at the plaster they take. Change a setting and generate again as often as you like: making and looking at molds is free.

5. Get the print files

Press Get print files to lay the cases out for your printer. Downloading them needs an account: see Plans and downloads. Then print the cases and follow the pour sheet.

Read next

• Printing the cases: How the print files are laid out for your printer, what to print them in, and how the clamps, edge frames and printed natches go together.
• Reading the pour sheet: What the pour sheet tells you: the plaster and water for each piece, the slip to have ready, what you need, and the steps with their times.
• Draining and demolding a slip cast: How long to leave the slip in, how to drain the mold, and when the cast is ready to come out.

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