Casting mold

Turn a model into a printable mold for silicone, wax, resin, plaster or soap.

A mold is the negative of your model: a block with the model’s shape hollowed out of it, split so the casting can come back out. Print it once in PLA or PETG and you can pour silicone, wax, resin, plaster, soap or chocolate into it as many times as you like — which is the point, because printing ten copies takes ten prints and casting ten takes one.

This is the only generator that starts from something you already made. Every other one builds a part out of numbers. This one builds a part out of *your model*, so you have to select a body before you click it — the tile will say so if you have not. What it makes is a copy: the mold holds its own private duplicate of the model, so deleting or moving the original afterwards leaves the mold alone. The other side of that bargain is that editing the model afterwards does not update the mold — make the mold last, or make it again.

Two-part box or open face. A two-part box closes right around the model and you pour through a hole in the top: that is the one for wax, resin and silicone. An open face is a single block with the cavity open at the top, which is what you want for clay, soap and bath bombs — things you press in and lift out rather than pour.

The split height is the decision that matters, and the generator makes it for you. The casting has to be able to leave both halves, and a shape is only free to leave a half if it never gets wider further in — so the seam belongs at the model’s widest point. When you make the mold it measures your model and starts the split there. Move it, and a Best split row appears with the number it would rather have; it never applies it for you, because where a seam line is acceptable is a decision about the casting and not only about the shape.

It will tell you when the casting cannot come out. That is what the Release row is for. A face that points downward above the seam is one the top half is hooked under, and no amount of clamping gets the casting past it — so the row either says nothing is undercut, or tells you how much of the surface is and offers the split that fixes it. Where no split fixes it — a handle, an overhanging lip, anything genuinely re-entrant — it says that too, because the answer is a flexible mold or splitting the model first, not another number here.

Registration keys are what make the two halves line up. Bumps on one half, sockets on the other. Two of them sit diagonally opposite, because two on the same edge still let the halves pivot. They are held down to fit inside the side wall, so if you asked for a big key in a thin wall you got a smaller one and the read-out says so.

The pour hole goes on the model’s high point, found by measuring it — so it meets material rather than the middle of a ring. Reach controls how far past the top of the model the funnel’s point goes; it has to break through into the cavity or nothing can get in.

Vents go on the other high points. Air has to leave before the material can fill, and on anything with two summits the pour hole only covers one of them — the far one traps a bubble. So vents are placed on the high points the pour hole does not already take, which is why a simple shape asks for two and gets none: there is nowhere left that needs one, and the read-out says so rather than leaving you to wonder.

Shrink allowance is a percentage, not a gap. Castings shrink in proportion to their size, so a fixed clearance would be wrong at both ends. Silicone and most resins shrink very little; wax and plaster are worth a percent or two. The block grows with the cavity, so raising it does not eat into the wall.

The two halves come out ready to print — the top half is already flipped over so its flat outer face is on the bed, and they are laid out side by side. Run Print check over the result before you commit to it: a mold is a lot of filament, and the wall it is worth checking is the one around the cavity.

Molding an imported STL works the same way — import it first, and if it will not cut cleanly repair the mesh before you mold it.

1. Select the body you want to mold — the mold is built around it, so this comes first.
2. Open the Generate panel from the left sidebar and choose Casting mold (under Molds & casting).
3. The mold drops in as editable bodies, sized to your model. The parameters open in the Properties panel.
4. Pick Two-part box to pour into, or Open face to press into.
5. It measures your model and starts the split where the casting can come out. Watch the Release row: it says whether anything is undercut.
6. Move the split if you want the seam elsewhere — Best split tells you the height it would rather have.
7. Print both halves, clamp them together, and pour.