STL to Mold Generator – Free 2-Part Mold Maker

How the STL to mold generator works

The tool builds a solid block around your model, carves the model out of it, and splits the block into two halves at the parting line. Round alignment keys in the corners lock the halves together, a pour hole is drilled at the highest point of the model, and thin air vents are added at other high points so trapped air can escape.

Everything runs in your browser using Manifold, the same geometry library used by professional CAD tools, so every half comes out as a clean closed solid your slicer accepts.

Choosing the parting line

The parting line is where the two halves meet. The tool checks every part of the model’s surface and paints the areas that would lock the cast in the mold red — undercuts, where part of the model curls back under itself. The automatic setting picks the height with the least locking, and “Find the best direction” tries about 80 directions to find the one that releases cleanly.

For tilted or uneven shapes, whose edge is widest at different heights in different places, the tool switches to a curved parting line: instead of a flat cut, the two halves meet along a surface that follows the model’s edge, so each half still lifts straight off.

Some shapes — figurines, faces, anything with deep detail — lock in every direction. For those, switch to the silicone mold box: silicone flexes over undercuts and peels off the cast.

What to cast and which filament to print with

Match the mold material to the heat of what you pour:

• Epoxy and polyurethane resin, plaster, concrete, soap — PLA works well.
• Warm wax, chocolate or anything above about 50 °C — print in PETG or ABS, since PLA softens around 55 °C.
• Always use a release agent, print with 3+ walls, and seal the layer lines with a thin coat of epoxy for a smoother cast.