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Generating mold…
splicestl/mold is a free in-browser mold maker. Drop in a master model and it builds a mold you print on an ordinary 3D printer: a silicone shell (a thin two-part mother mold that hugs the master, for silicone molds that take a fraction of the silicone), a two-part block you cast straight into, or a one-part open mold with a flat back. Everything runs in your browser — the model is never uploaded.
Which to pick: anything with undercuts, fine detail or many casts → the silicone shell; you pour silicone into the gap around the master, then cast resin, wax, plaster or soap in the cured silicone with the shell holding it. Simple shapes cast once or twice in plaster, wax or resin → the two-part block. Flat-backed pieces such as soap, chocolate, coasters and concrete tiles → the one-part open mold.
What changed — 8 November 2026: version 2. Added OBJ, 3MF, GLB/glTF and PLY import with unit detection; automatic mesh repair and a voxel rebuild; a ray test that finds undercuts the face normals miss, and one-click undercut fill; the silicone shell mold; a one-part open mold that really is open (its top is the plane you choose); the view now shows models Z-up. Earlier: 13 October 2026 — the at-a-glance block and FAQ schema.
Silicone shell mode: print the two shell halves, hang the master on the pour pin, pour platinum-cure silicone into the gap. The cured silicone is the mold; cast clear or pigmented resin through the pin channel. Default 5 mm silicone, 2.4 mm shell.
A silicone shell with the wick hole option puts a wick channel in line with the pour channel, so the wick runs straight through. For flat-backed bars, the one-part open mold with its top at the back face of the master.
Two-part block: pour straight into the printed mold. Use mold release, and fill undercuts first so the halves come apart. Larger pour spout preset for slurry; wax preset leaves clearance for shrinkage.
Open scans, inside-out meshes, parts stacked from separate bodies: the mesh check repairs them on load and says what it fixed. If a model still is not a closed solid it is rebuilt on a voxel grid.
For a two-part block or open mold the steps are the same up to step 4: pick the type, check the undercut readout (fill them if the master is red), set wall and clearance, generate, print, pour.
Comparing tools? Meshcast vs splicestl/mold sets the two side by side, prices and limits included.
Box and two-part silicone molds, how much silicone to mix, and fixes for tacky silicone: How to make a silicone mold from a 3D print.
Not the silicone itself — you print what holds it. In silicone shell mode splicestl/mold prints a thin two-part shell that sits a set distance (5 mm by default) from the master all round. You hang the master inside on the pour pin, pour silicone into the gap, and the cured silicone is your mold. The shell then supports the soft silicone while you cast.
A mold made of two layers: a flexible silicone layer of even thickness against the master, and a rigid outer shell (the mother mold) that keeps it in shape. Because the silicone follows the master instead of filling a box, it takes far less silicone, and because silicone stretches, undercuts that would lock a rigid mold are fine. You cut the silicone open along one side to get the master or the casts out.
It depends on the shape: the more the master differs from its bounding box, the more it saves. The tool prints its estimate next to a poured box mold with the same minimum wall. On the organic test model in our regression tests (a 54 × 28 × 51 mm creature) the shell needs about 36 cm³ against about 128 cm³ for the box — 72% less. A plain cube saves nothing.
STL (binary or ASCII), OBJ, 3MF (plain files and Bambu Studio / Orca Slicer projects with their objects in separate parts), GLB and glTF (drop the .bin with a .gltf) and PLY (ASCII or binary). glTF is in metres, so it is scaled ×1000; OBJ and PLY have no units, so a tiny model is read as metres. You can change the units on the mesh card. Draco-compressed glTF is not supported — export it uncompressed.
Yes, as it loads. It welds duplicate vertices, closes holes (flat ones are capped cleanly, like the cut bottom of a scanned bust), turns flipped faces, drops duplicate and collapsed faces, keeps a hollow part hollow, and merges overlapping shells into one solid. If the mesh still is not closed it is rebuilt on a voxel grid; the button “Rebuild as one solid” does that on demand (for self-intersecting meshes), with undo.
It adds material under every overhang so each column of the master runs unbroken to the parting plane (for the open mold, to the open top). A rigid two-part or open mold then releases. The added material shows in violet and the casts carry it — trim it off, or keep it as a base. It starts from the original master every time, so change the axis or split and fill again; undo puts the master back.
Any FDM printer handles them; PLA or PETG are fine. Print the shell halves parting face down — if your slicer shows overhang at the top of a dome, add tree supports there. For a two-part block cast direct, a resin printer gives a smoother cavity and so smoother casts.
Use silicone shell mode: it makes the two-part mother mold, and you cut the silicone open along one side after it cures. If you want a silicone mold split in two halves instead, pour the first half against a clay bed in the lower shell half, add mold release, then pour the second — the shell, pin and vents work the same way.
No. Reading, repair, booleans, the shell and the STL export all run in your browser with JavaScript and WebAssembly (Manifold). The only network traffic is loading the page and its libraries; after the first visit it works offline.
splicestl cuts large 3D models into printable pieces with joints, so you can print something bigger than your build plate. splicestl/mold builds a mold around a model so you can cast copies of it. Different jobs, same geometry engine.