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3DAI
Free · runs in your browser · files stay on your device

Mold Box Generator

Drop a master model (STL, 3MF, OBJ, GLB or PLY) and get a printable mold. Pick a pour box to cast silicone around your printed master, or a two-part mold: a block with the master as a hollow, split at its widest point, with registration keys, a pour sprue and air vents. It all runs on your device.

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Safety: resin, silicone and other casting materials come with a safety data sheet (SDS) from their maker. Read it, wear the protection it lists and work in a well-ventilated room.

How to make a mold from an STL

  1. 1

    Open your master

    Drag an STL, 3MF, OBJ, GLB or PLY onto the tool, or press Try a sample rook. Set the up axis, turn it and scale it if needed.

  2. 2

    Pick the mold type

    Pour box: walls and a floor to stand your printed master in for a silicone pour. Two-part mold: a printed block split in two, with the master as the hollow inside.

  3. 3

    Set the split, keys and vents

    The parting plane goes through the widest cross-section by default. Choose the key shape, size and clearance, and the sprue and vent sizes.

  4. 4

    Download and print

    Download each half as its own STL, or both side by side in one 3MF. The halves are laid parting face up, so the hollow prints like an open bowl.

Pour box or two-part mold

Pour boxA box with no top. You print it and the master, stand the master in the box, and pour silicone over it. When it sets you cut or peel the silicone to free the master, and the silicone is your mold.
Two-part moldA solid block with the master taken out of it as a hollow, cut in two at the parting plane. You print both halves, clamp them together and pour your casting material in through the sprue.

What each setting does

Wall and floorHow much mold surrounds the master on the sides and top (wall) and underneath (floor). 8 mm is the default; thicker walls are stiffer but use more material.
Split and parting planeAuto picks the axis (horizontal for flat pieces, otherwise across the thinner side) and puts the plane where the master is widest, so each half lets go of it. Choose Manual to type the position in mm.
KeysCones or half spheres stick out of half A, and matching sockets are cut into half B, so the halves line up. Three keys make the halves fit only one way round.
Key clearanceThe gap added to each socket so the printed key slides in. 0.2 to 0.4 mm suits most printers; go larger if your printer prints holes small.
Sprue and ventsThe sprue is a pour hole from the top of the mold down to the highest point of the master. Vents are thin holes over the other high points, where air would be trapped.
Silicone around (pour box)The space between the master and the box walls, and above the master. This sets how thick the silicone mold will be.

Cast your own AI figure: turn a photo into a figurine

Upload a photo of a pet, a toy or any object and get a 3D figurine you can download as STL, then make a mold of it here and cast as many copies as you like. The AI generators use credits; this mold maker does not.

Turn a photo into a figurine →Image to 3D

More free 3D printing tools

STL repairScale STLSTL viewer3D print cost calculatorImage to 3D

Mold box generator FAQ

Open the STL in the tool at the top of this page and pick Two-part mold. It builds a block around the model, takes the model out as a hollow, splits the block at the widest point and adds keys, a sprue and vents. Download half A and half B as STL files, or both in one 3MF, and print them.
A mold box is an open-topped box you stand a master model in before pouring liquid silicone over it. When the silicone has set you take it out of the box, free the master, and use the silicone as a flexible mold. Pick Pour box here to print one sized to your master.
A pour box makes a flexible silicone mold, which lets go of shapes with undercuts and gives a smooth surface. A two-part printed mold needs no silicone and is ready as soon as it is printed, but it is rigid, so it suits shapes that come out cleanly from both sides of the parting plane.
The stats show it. For a two-part mold, Cast volume is the volume of the master in millilitres, which is how much material fills the hollow (plus a little for the sprue). For a pour box, Silicone needed is the box filled to the fill line minus the master. Add some extra for mixing losses.
Start with 0.3 mm. The sockets are made bigger than the keys by exactly the clearance you set, so the halves close without force. If they are tight, print again with 0.4 mm; if they wobble, use 0.2 mm. Three keys make the halves fit only one way.
A mold needs a model with a clear inside and outside. If the file has holes or loose edges, the tool first tries to close and join it. If that still fails, open it in an STL repair tool, save the fixed file and open it here again.
STL (binary or text), 3MF, OBJ, GLB and PLY, up to 100 MB and 1.5 million triangles. GLB and OBJ files from AI 3D generators work too; they are usually Y up, so the tool sets Y up for them. The downloads are binary STL files and a 3MF with both halves, which open in Bambu Studio, PrusaSlicer, OrcaSlicer and Cura.
No. The file is read, turned into a mold and saved in your browser. Only the geometry engine (the open-source manifold-3d library) is downloaded once, from a public CDN. No account, no credits, no watermark.
Follow the safety data sheet of the resin, silicone or plaster you use, wear the gloves it asks for and work with good ventilation. Some resins get hot as they cure and can soften thin printed plastic, so test a small pour first. This tool makes no food or medical claims for any mold.

Bambu Studio, PrusaSlicer, OrcaSlicer and Cura are named only to describe compatibility; The3DAI is not affiliated with or endorsed by them or by any silicone or resin maker.