What is a two-part mold?

A two-part mold consists of a top half (cavity) and bottom half (core) that fit together to form a hollow space in the shape of your part. Material is injected through a sprue channel, fills the cavity, and solidifies into the desired shape. Two-part molds are widely used in plastic injection molding, resin casting, and educational prototyping.

How do I convert an STL file to a mold?

Upload your STL or OBJ file to the Online Two-Part Mold Tool. The tool auto-scales your model to fit mold constraints, analyzes possible parting planes, and generates a two-part mold design with injection and vent channels. You can then export the mold halves in STL, OBJ, 3MF, or STEP format for 3D printing or CNC machining.

What file formats can I export molds to?

The tool supports exporting mold halves in four formats: STL and OBJ for 3D printing, 3MF for advanced 3D printing with color and material data, and STEP (.stp) for professional CAD and CNC machining workflows.

What is parting plane analysis?

Parting plane analysis determines the optimal split line where the two mold halves separate. The tool evaluates multiple planes along the parting direction, scoring each for undercuts and floating sections, then recommends the best plane for clean mold separation and part ejection.

What is demoldability analysis?

Demoldability analysis evaluates the draft angles of your part relative to the parting plane to determine how easily it can be removed from the mold. The tool generates a color-coded heatmap overlay on your 3D model — green for easy, yellow for medium, orange for difficult, and red for impossible draft angles — helping you identify areas that may stick, deform, or break during demolding before you commit to manufacturing.

Can I use this tool for 3D printed molds?

Yes. Export your mold halves as STL, OBJ, or 3MF files and print them on any FDM or SLA 3D printer. 3D printed molds work well for resin casting, silicone mold making, and low-volume injection molding with desktop machines like the INJEKTO 3. The tool automatically generates proper mold geometry including sprue channels and venting.

What 3D file formats can I upload?

You can upload STL and OBJ 3D model files. The tool automatically scales your model to fit within the mold cavity constraints (up to 69×80×48mm) and validates dimensions before analysis. Models are never scaled up — only scaled down if they exceed the maximum mold dimensions.

Can I use this tool to design silicone casting molds?

Yes. Export the two mold halves as STL, OBJ, or 3MF and 3D print them. You can use the printed halves directly as a two-part mold for low-viscosity resin, wax, plaster, or soap casting, or use them as a master to pour RTV silicone around and produce a flexible two-part silicone casting mold. The same parting plane analysis and demoldability heatmap apply to silicone casting workflows.

What is the difference between an injection mold and a silicone casting mold made with this tool?

The geometry is the same two-part core-and-cavity design. For plastic injection molding, the halves are 3D printed, CNC-machined, or ordered as PPA-CF, resin, or aluminum molds and used with a machine like INJEKTO 3. For silicone casting, the printed halves are used either directly as a rigid two-part casting mold or as a master to pour RTV silicone around, then resin, wax, plaster, chocolate, or soap is cast in the resulting silicone mold.

STL to Mold: Convert a 3D Model into a Printable Two-Part Mold

An STL describes a surface, not a mold. Turning one into a mold means deciding where the block splits, subtracting the part from both halves, and adding the channels that let material in and air out. Here is what happens between uploading a file and downloading two printable halves.

1
Upload the STL or OBJ
Drop the file into the tool. It parses the mesh, computes the bounding box, volume, and triangle count, and repairs common mesh defects.
2
Set scale and mold mode
Pick Custom Size for a mold that grows around the part, or Mold Backer Compatible for the fixed INJEKTO 3 block. Auto-fit scales the model down to fit, never up.
3
Analyze parting planes
The tool slices along the parting direction and scores each plane for undercuts and floating sections, then ranks the candidates.
4
Generate the mold
Core and cavity halves are produced with sprue and vent channels, plus optional alignment pins.
5
Export
Download both halves in STL, OBJ, 3MF, or STEP.

What the tool does to your mesh

The conversion runs in your browser in four stages.

Repair and preprocessing. Duplicate vertices are welded, degenerate triangles dropped, normals made consistent, and small holes filled so the mesh is watertight enough for boolean operations.
Fit and scale. The bounding box is compared to the mold envelope. In Mold Backer Compatible mode the part is scaled down to fit within 69 x 80 x 48 mm and 45 mL; in Custom Size mode the block grows around the part instead.
Parting plane analysis. The geometry is sliced at a fixed interval along the parting direction. Each slice is scored on undercut area and disconnected islands. Symmetry detection biases toward the center plane when the part is symmetric.
Mold generation. A solid block is built, the part is subtracted from it at the chosen plane, and the sprue, vent, and alignment features are added to the two halves.

STL vs OBJ: does it matter?

Not much, for this purpose. Both are triangle meshes. OBJ can carry material and UV data that a mold does not use. STL is smaller and more universally exported. Use whichever your CAD or sculpting software produces most reliably.

What does matter is export resolution. A coarse STL with visible faceting on curved surfaces produces a faceted cavity, and that faceting shows up on every cast part. Export with a chord tolerance around 0.01 mm for parts of this size.

Getting a clean cavity

Watertight mesh. Boolean subtraction on an open mesh produces stray surfaces inside the block.
No self-intersections. Overlapping geometry from a careless boolean in your CAD tool carries straight through.
Reasonable triangle count. Very dense meshes slow analysis without improving the mold; very coarse ones facet the cavity.
Single connected body. Multiple loose shells confuse the parting analysis, which reads them as floating sections.

Custom Size versus Mold Backer Compatible

	Custom Size	Mold Backer Compatible
Block dimensions	Grow to fit the part	Fixed 75.6 x 86 x 25.88 mm per half
Max part size	No limit in the tool (print bed and browser memory apply)	69 x 80 x 48 mm
Max volume	No limit in the tool	45 mL
Sprue	Scales with the part	4.8 mm
Vent	Scales with the part, removable	2 mm
Best for	Silicone casting, resin casting, larger prototypes	INJEKTO 3 injection molding, ordering a physical mold

Switching modes clears the current analysis and generated mold, because the geometry depends on the block size. Pick your mode before you run the analysis.

Export formats at a glance

STL and OBJ suit any 3D printer. 3MF carries units and print metadata that keep scale unambiguous. STEP is a real solid model rather than a mesh, which is what a CNC shop or a CAD package needs to modify the mold. STEP export is a Pro feature and is generated with OpenCascade, so the result is a proper B-rep solid rather than a triangulated shell.

Keep reading

STEP vs STL vs OBJ vs 3MF: Which Mold File Format to Use
A practical comparison of the four export formats and the workflows each one belongs in.
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.
How to Make a Silicone Mold from a 3D Model
The full workflow from 3D model to a flexible two-part silicone mold, including which route to take when you want a rigid printed mold instead.

STL to Mold: Convert a 3D Model into a Printable Two-Part Mold

Upload the STL or OBJ

Set scale and mold mode

Analyze parting planes

Generate the mold

Export