STL to mold: turn any 3D model into a printable mold
Drop any STL into the free Meshcast mold generator and it builds a print-ready two-part mold around your model automatically — parting line, alignment keys and pour hole included. Download the STL, print both halves, clamp them together, and pour wax, soap, resin or plaster. No CAD, no signup, and nothing is uploaded — it runs entirely in your browser.
STL to mold in 5 steps
Making a mold used to mean an hour of boolean operations in CAD. A mold generator does the same job in about a minute: it wraps a box around your model, splits it into two mating halves, and cuts the plumbing you need to pour.
Upload your STL
Open the generator and drag in any watertight STL. Everything runs locally via WebAssembly, so your file never leaves your device. No model yet? Load one of the built-in examples to test-drive the workflow.
Set the parting line
The mold splits along the orange plane. Auto-orient gives a solid first pass; then drag the seam so the model's widest silhouette sits exactly on it and both halves pull free. If a surface leans back over the seam, that's an undercut — see draft angles & undercuts.
Tune seal, pour hole & walls
Choose a tongue-and-groove seal for liquids (wax, resin) or registration pins for rigid casts, size the pour hole, and adjust wall thickness and fit clearance if your printer runs tight. The defaults are sensible — most models need zero tweaking.
Download & print both halves
Export the print-ready STL and slice with 0.2 mm layers, 3–4 walls, PLA or PETG, both halves flat-face-down on the same plate. Full settings in best print settings for molds.
Clamp, pour, demold
Mist the cavity with mold release, clamp the halves with rubber bands, pour through the hole, let it set, and pry the seam open. New to casting? Start with the beginner's casting guide.
Which mold tool do you need?
Meshcast has one generator per job. All of them are free, run in your browser, and export print-ready STL/3MF files.
| You want to make | Use this tool |
|---|---|
| A rigid two-part mold for almost anything — wax, soap, resin, plaster | /mold (the flagship) |
| A flexible silicone mold — pour silicone around your model in a printed housing | /silicone-mold |
| A candle — two-part box mold with a clean pour spout | /candle-mold |
| Custom ice cubes — food-safe silicone tray workflow | /ice-cube-tray-mold |
| Bath bombs — two-part press mold | /bath-bomb-mold |
| Slip casting — plaster mold housings for ceramics | /plaster-mold |
Not sure whether you need a rigid printed mold or a silicone one? The decision guide settles it in two minutes.
The two things that trip people up
Watertight meshes. The generator subtracts your model from a solid block, so the STL must be manifold — no holes, no self-intersecting shells. Most downloaded models are fine; scans and sculpts sometimes aren't. If generation fails, repair the mesh in your slicer or a mesh-repair tool and try again.
Undercuts. A rigid two-part mold can only release shapes that taper away from the parting line in both directions. Slide the seam first; if the shape genuinely locks in (deep overhangs, wrapped details), switch to a silicone mold — flexible rubber stretches over undercuts that would trap a rigid mold.
FAQ
Is there a free STL mold generator?
Yes. The Meshcast mold generator at meshcast.app/mold is free for personal use with no signup. It runs entirely in your browser and exports a print-ready two-part mold from any STL you drop in.
Can I make a mold from any STL?
Almost. The mesh must be watertight (manifold) so the generator can subtract it cleanly from the mold block. If your STL has holes or self-intersections, repair it in your slicer or a mesh-repair tool first, then upload it.
What filament should I print molds in?
PLA is the easy default for wax, soap, plaster and most resins. Use PETG if the mold sees heat or repeated use, and polypropylene if you want the casting material to release without any coating.
How do I avoid undercuts in a two-part mold?
Orient the model so its widest silhouette lies exactly on the parting plane, then drag the seam until every surface slopes away from it. Shapes with severe overhangs cast better in a flexible silicone mold instead of a rigid printed one.
Made by @meshminds3d. Got stuck? Email a photo and I'll help you debug.