How to make a mold in Fusion 360, step by step
This guide was drafted with AI assistance.
In Fusion 360 you make a mold by turning the model into a solid body, drawing a box around it, using Combine > Cut with Keep Tools to make the cavity, then Split Body along the parting line, adding keys and a sprue, and saving each half as a mesh. It is precise and parametric, and it shines when you designed the part in Fusion yourself. For a downloaded STL, converting the mesh is the slow part, and a mold generator is usually quicker.
Designed in Fusion, or a downloaded STL?
For an STL: Insert > Insert Mesh, then in the Mesh tab Modify > Convert Mesh to make it a solid body. Faceted conversion is fine for simple, low-poly models. Dense meshes (sculpts, scans, anything with hundreds of thousands of triangles) are slow to convert or fail, so reduce them first with Modify > Reduce in the Mesh tab, and repair holes with Prepare > Repair before converting.
Making the mold, step by step
Position the part
Move the body so its widest outline sits on a plane you can find again, usually the XY plane. That plane will be your parting line. Make sure no surface leans back over it, or the halves will lock onto the cast.
Draw the mold block
Sketch a rectangle on the parting plane around the part and extrude it up and down (Symmetric) past the part on every side. For a rigid mold you pour into, 3 to 5 mm of wall is plenty; for a silicone box, leave 8 to 12 mm around the part. Choose New Body, not Join.
Cut the cavity
Modify > Combine: the block is the Target, the part is the Tool, the operation is Cut, and tick Keep Tools so the part stays for later. Hide the part and check the cavity.
Split into halves
Modify > Split Body, pick the block, and use the parting plane as the splitting tool. You now have two bodies. For a part with a deep undercut, split again with a second plane or a sketch-extruded surface to make a three-part mold.
Add registration keys
Sketch three or four circles on the parting face of one half, extrude them as pins with Join, then cut matching holes in the other half with a small offset (0.2 to 0.3 mm) so printed parts still close. Because it is parametric, you can change the clearance later and everything updates.
Sprue, vents and wick
Revolve a cone into the highest point of the cavity for the pour spout and cut it from both halves. Add 2 to 3 mm vents from other high points. For a candle, cut a thin hole through the bottom for the wick.
Export the halves
Right-click each body in the browser > Save As Mesh, format STL or 3MF, units millimetres. Check both in your slicer before printing.
The personal-use licence, if you plan to sell
Fusion's free personal-use licence exports STL and OBJ, which is all a printed mold needs. It limits you to 10 editable designs at a time, and it is for non-commercial use: Autodesk's terms cap it at home use by people earning under $1,000 a year from what they make with it. If you sell candles or soap cast from a mold you designed in Fusion, check whether you need the paid licence. Read Autodesk's current terms, they have changed several times.
The faster way for an STL you downloaded
If the model already exists and you only want the mold, the mesh conversion, the block, the split, keys, sprue and vents are exactly what Meshcast's rigid box mold does from sliders, working on the mesh directly, so there is no conversion step and no triangle limit to fight. It repairs leaky files, fills undercuts in one click, and puts print settings in the zip. Pick the tool for what you cast:
| You are casting | Use |
|---|---|
| Candles and wax | Rigid box mold: two printed halves, keys, pour spout and wick hole |
| Resin, soap, detailed figures | Silicone mold maker: a printed box to pour silicone into, or a contoured shell that uses less silicone |
| Ice, wax melts, gummies | Tray mold, runs entirely in your browser |
| Concrete planters and vases | Vase and planter mold with a tapered core so the cast comes out hollow |
| Plaster and slip casting | Plaster mold, multi-part housings with clay-shrinkage presets |
| Bath bombs, pressed soap, clay | Press mold, a die and a piston |
Questions
How do you make a mold in Fusion 360?
Make the part a solid body, extrude a box around it as a new body, use Combine > Cut with Keep Tools to make the cavity, split the box with Split Body along the parting line, add keys and a sprue, then Save As Mesh for each half.
Can Fusion 360 make a mold from an STL?
Yes, but the STL has to be converted to a solid first (Mesh tab > Modify > Convert Mesh). Simple models convert fine; dense sculpts and scans are slow or fail and need Reduce and Repair first.
Can I sell what I cast from a mold made with Fusion for personal use?
The personal-use licence is for non-commercial home use, capped at under $1,000 a year in revenue. If you sell, check Autodesk’s current terms; you may need the paid licence.
What clearance should printed mold keys have?
0.2 to 0.3 mm on the diameter works on most FDM printers. Make it a parameter in Fusion so you can loosen it after a test print.
Is there a faster way for a downloaded model?
Yes. A mold generator such as Meshcast works on the mesh directly, so there is no conversion, and does the block, split, keys, spout and vents from sliders in about a minute.
Made by meshcast.app. Menu names are from Fusion (formerly Fusion 360) as of September 2026 and move between releases; licence terms are Autodesk’s, so check theirs before you rely on them. Got stuck? Email a photo and I'll help you debug.