How to make a mold from a SketchUp model
This guide was drafted with AI assistance.
To make a mold from a SketchUp model: make it a solid group (Entity Info must show a Volume), fix reversed faces and gaps with the free Solid Inspector extension, export an STL in millimetres, then drop it into a mold generator that builds the parting line, keys, spout and vents. If the file has small holes, the Fix it for me button repairs them.
Drop in an STL and get a printable mold in about a minute. Free, in your browser, nothing to install.
Open the mold generator →Before you start
SketchUp models are built from surfaces, not solids, and that single fact causes nearly every mold problem. A mold generator subtracts your model from a block, so it needs a model with a clear inside and outside. A SketchUp model can look perfect and still have a missing face, a gap or a reversed face that leaves it open. Check these four things first.
- Make it a solid group. Select the model, right-click and choose Make Group. Then open Entity Info (Window > Default Tray > Entity Info): SketchUp lists a Volume only for a closed solid. No volume means the group is not solid.
- Reversed faces. Turn on View > Face Style > Monochrome. The front of a face is white and the back is blue-grey. Any blue-grey face on the outside is reversed: right-click it and choose Reverse Faces.
- Stray edges and hidden geometry. Use View > Hidden Geometry and delete loose edges, guide lines and leftover faces that are not part of the surface.
- Units. Set Window > Model Info > Units to millimetres before exporting, so the numbers in the STL match the size you drew.
Export your model as an STL
Fix the model first
Install the free Solid Inspector extension from the Extension Warehouse (Extensions > Extension Warehouse, search for Solid Inspector). Select your group and run it from the Tools menu. It highlights stray edges, holes and internal faces, and the Fix button repairs the simple ones. Run it until nothing is flagged.
Export with STL
In SketchUp Pro choose File > Export > 3D Model and pick STL file in the format list. In the options set Units to Millimeters and tick Export only current selection if you grouped the model.
SketchUp Free (the web version)
In the browser version use the Download button in the menu and choose STL. It exports the selected geometry, so select your group first.
Check the file
Open the STL in your slicer or in the generator and compare its dimensions to the numbers in SketchUp. If it comes in as 25.4 times too big or too small, the export units were set to inches.
The usual SketchUp problems
| What you see | Why | Fix |
|---|---|---|
| No Volume in Entity Info | The group has a hole, a stray edge or an internal face | Run Solid Inspector, fill the gap with a face using the Line tool, then re-check |
| The mold has no cavity | Faces are reversed, so the inside is read as the outside | Use Monochrome style, then right-click Reverse Faces on the blue-grey ones |
| Model is the wrong size | Export units were inches, or the file was scaled | Set Model Info > Units to millimetres and export in millimetres |
| Smooth surface is faceted or ragged | Curves are exported as flat triangles at low segment counts | Raise the segment count on circles and arcs before extruding |
| File rejected as not watertight | Tiny gaps remain that Solid Inspector cannot see | Use the Fix it for me button in the generator, see below |
| Several loose pieces in the STL | The model is not one group, or the scene has extra objects | Group the model and use Export only current selection |
Turn the STL into a mold
With a clean STL, open Meshcast's two-part box mold and drop it in. The generator builds the block, splits it along the parting line and adds registration keys, a pour spout and vents from sliders, then gives you both halves. It also flags undercuts and can fill them in one click, which matters because SketchUp models of buildings, furniture and props often have overhangs.
If the file is rejected as not watertight, the popup offers Fix it for me. It rebuilds the model as a closed solid and generates in the same click, and it handles most small gaps that SketchUp leaves behind. The full list of rejection messages is in why was my model rejected.
Pick the generator for what you pour:
| You are casting | Use |
|---|---|
| Candles and wax | Candle mold maker: two printed halves, keys and a pour spout |
| 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 |
Can you make the mold inside SketchUp?
Yes, with SketchUp Pro's solid tools: make the model a solid group, draw a larger box around it as another solid group, and use Subtract from the Solid Tools toolbar to cut the model out of the block. That gives you a one-piece open mold. It works for flat-backed shapes. For a split mold you must cut the block in two, model the keys and spout yourself, and then re-check that each half is a solid.
Questions
How do I export an STL from SketchUp?
In SketchUp Pro choose File > Export > 3D Model and select STL, with units set to millimetres. In SketchUp Free use the Download button and choose STL. Select your solid group first so only the model is exported.
Why is my SketchUp model not manifold?
SketchUp models are surfaces, so a missing face, a loose edge or an internal face leaves them open. Group the model, check that Entity Info shows a Volume, and run the free Solid Inspector extension to find and fix the gaps.
Does SketchUp Free export STL?
SketchUp's web version offers STL among its download formats, but SketchUp has changed its export options between plans over time. If the option is missing for you, export from the desktop app or ask for the file in another format.
What units should I use when exporting from SketchUp?
Millimetres. Set Model Info > Units to millimetres and choose Millimeters in the STL export options. If the mold comes out about 25 times too big or too small, the export used inches.
Can Meshcast repair a SketchUp STL with holes?
Often, yes. If a file is rejected as not watertight, the Fix it for me button in the popup rebuilds it as a closed solid and generates straight away. Fixing the gaps in SketchUp first with Solid Inspector gives the cleanest result.
Made by meshcast.app. Menu names are from SketchUp as of November 2026 and can move between releases, and export options differ between the. Got stuck? Email a photo and I'll help you debug.