How to Make a Silicone Mold from a 3D Model
If you have a 3D model, you are most of the way to a silicone mold. This guide covers both routes: printing a rigid two-part mold you cast directly in, and printing a mold master you pour RTV silicone around to get a flexible two-part silicone mold. The Mold Maker tool generates the two-part geometry for either route from an STL or OBJ file.
- 1
Upload your 3D model
Upload an STL or OBJ file of the object you want to reproduce. The tool reads the mesh, measures the bounding box and volume, and scales the model to fit the selected mold size.
- 2
Choose Custom Size or Mold Backer Compatible
Custom Size grows the mold block around your part with no size limit set by the tool, which is what most silicone casting workflows want. In practice your print bed and browser memory are the ceiling. Mold Backer Compatible locks the block to the INJEKTO 3 cavity size.
- 3
Run parting plane analysis
The tool slices the model along the parting direction and scores each candidate plane for undercuts and floating sections, then recommends the cleanest split. Symmetrical parts are split at the center.
- 4
Generate the two mold halves
A core and a cavity are generated with a sprue channel for pouring, a vent channel for trapped air, and optional alignment pins so the halves register to each other.
- 5
Check the demoldability heatmap
The heatmap colors each surface by draft angle. Green releases easily, red is likely to stick or tear. Fix red areas by rotating the part or choosing a different parting plane before you print.
- 6
Export and 3D print the halves
Export both halves as STL, OBJ, or 3MF and print them. Use a small layer height on the cavity faces so print lines do not transfer into the cast surface.
- 7
Cast directly or pour silicone around the master
For a rigid mold, clamp the printed halves, apply release agent, and pour resin, wax, plaster, or soap. For a flexible silicone mold, use the printed halves as a master, pour RTV silicone around it, cure, and demold.
Two ways to get from a 3D model to a cast part
People say silicone mold when they mean two different things, and the difference decides how you use the tool.
Route A: a rigid printed mold you cast in directly
You print the two halves, clamp them, and pour material straight into the sprue. This is fast, dimensionally accurate, and repeatable. It suits low-viscosity resin, wax, plaster, and soap, and parts with generous draft and no undercuts. The mold does not flex, so anything that locks itself into the cavity stays there.
Route B: a printed master, then RTV silicone
You print the part shape (or the mold halves) and use it as a master. Pour two-part RTV silicone around it, cure, and pull the master out. The resulting mold flexes, so it releases shallow undercuts and fine surface detail that a rigid mold would trap. This is the standard approach for miniatures, jewelry, and detailed decorative casts.
Both routes start with the same geometry decisions: where the mold splits, whether the part has undercuts, and how material gets in and air gets out. That is exactly what the tool computes for you.
Preparing your 3D model
A mold is only as good as the mesh you feed it. Before uploading, check the following.
- The mesh should be watertight. Holes and non-manifold edges make the boolean subtraction that carves the cavity unreliable. The tool runs repair passes, but a clean mesh gives a cleaner cavity.
- Model at real-world scale in millimeters. The tool scales down to fit the mold envelope but never scales up, so an oversized model quietly shrinks and a tiny one stays tiny.
- Remove interior geometry. Anything hidden inside the part still gets carved into the cavity.
- Orient the part so its flattest, largest face sits perpendicular to the parting direction. This alone eliminates most undercuts.
If the tool suggests rotating the part, it is because the Z dimension is much larger than the smallest dimension. Tall thin parts split badly and trap air at the top.
Choosing silicone and casting material
For Route B, platinum-cure (addition-cure) RTV silicone gives better detail and dimensional stability, but it is inhibited by sulfur, some resins, latex, and certain 3D printing photopolymers. Tin-cure (condensation-cure) silicone is more forgiving of contamination and cheaper, but shrinks slightly and has a shorter library life.
Test cure a small amount against a scrap print from the same material and settings before committing to a full pour. Inhibition shows up as a permanently tacky surface layer, and it ruins the mold.
Shore hardness matters too. A softer silicone (Shore 10-20A) releases undercuts easily but distorts under the weight of a heavy cast. A firmer silicone (Shore 30-40A) holds dimensions better on larger parts.
Print settings that matter for mold surfaces
- Layer height: 0.1 mm or finer on cavity faces. Every layer line transfers into the cast part and, on Route B, into the silicone.
- Walls: at least four perimeters so the cavity face does not flex or leak under clamping pressure.
- Infill: 30 percent or higher. Molds take clamping force and thermal cycling.
- Orientation: print each half with the cavity face up so the flat parting face sits on the bed.
- Material: PLA and PETG are fine for silicone pours and room-temperature resin. Use a higher-temperature material for anything hot.
Smoothing the printed cavity with a light sand and a coat of sanding sealer or clear spray removes layer texture completely, at the cost of a small amount of detail.
Sprues, vents, and clamping
The generated mold includes a sprue channel where material enters and a vent channel where displaced air escapes. In Custom Size mode both scale with the part; on Mold Backer Compatible molds the sprue is 4.8 mm and the vent 2 mm to match the INJEKTO 3.
Pour into the sprue slowly and in a single continuous stream. Stopping mid-pour traps a seam in the cast. Watch the vent: material appearing there means the cavity is full.
Clamp the halves firmly but evenly. Uneven pressure opens one edge of the parting line and you get flash, the thin fin of material that squeezes into the gap. Alignment pins, available on the Pro plan, keep the halves registered so the two sides of the cast line up.
Common problems and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Part will not release | Undercut or zero draft | Check the heatmap, rotate the part or move the parting plane, or switch to a flexible silicone mold |
| Bubbles in the cast | Trapped air, no vent path | Pour slower, tilt the mold while filling, degas or pressure-cast |
| Flash along the seam | Halves not clamped evenly | Add alignment pins, clamp across the center, reduce pour pressure |
| Cast surface is ridged | Print layer lines transferred | Reduce layer height, sand and seal the cavity face |
| Silicone stays tacky | Cure inhibition from the print material | Test cure first, seal the master, or switch to tin-cure silicone |
| Short fill | Material set before filling | Warm the mold, use a longer pot-life mix, widen the sprue |
Keep reading
- 3D Printed Molds for Resin Casting
Print settings, materials, and release strategy for casting resin directly in a 3D printed two-part mold.
- Draft Angles and Undercuts in Mold Design
Why parts stick, tear, or trap themselves in a mold, and how to spot every problem surface using the color-coded demoldability heatmap.
- Parting Lines and Parting Planes Explained
The single decision that determines whether a mold works. How to read the scored plane list and when to override the recommendation.