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.

STEP vs STL vs OBJ vs 3MF: Which Mold File Format to Use

Every mold exports in four formats, and they are not interchangeable. Three are meshes and one is a real solid model. Picking the wrong one costs you either detail on the print or a rejection from the machine shop.

The short version

Format	Type	Use it for	Avoid when
STL	Triangle mesh	Any 3D printer, universal compatibility	You need to edit the mold in CAD
OBJ	Triangle mesh	3D software pipelines, rendering, sculpting apps	The receiving tool prefers STL
3MF	Mesh + metadata	Modern slicers, unambiguous units and print settings	Older software that cannot read it
STEP (.stp)	Solid B-rep	CNC machining, CAD editing, professional handoff	You only ever 3D print

Why a mesh is not enough for machining

STL, OBJ, and 3MF all describe a surface as a bag of triangles. A curved wall becomes hundreds of flat facets. A printer does not care, because it slices everything into layers anyway. A CNC toolpath does care: it will faithfully machine the faceting, and the machinist has to either accept it or rebuild the geometry.

STEP stores the actual mathematical surfaces: a cylinder is a cylinder, a fillet is a fillet. That means exact toolpaths, editable features, and no reverse engineering. It is the format a machine shop asks for.

How STEP export works here

STEP export runs through OpenCascade compiled to WebAssembly, in your browser. The mold is rebuilt as boundary-representation solids rather than converted from the mesh, so the result is true solid B-rep geometry, not a triangulated shell wrapped in a STEP wrapper. It carries no feature history, so it is a solid you can machine and edit, not a parametric model tree.

That reconstruction is heavy. It can take a few minutes on complex parts and retries automatically if a boolean operation fails. STEP export is included on the Pro plan; the mesh formats are available on every paid export.

3MF: the quiet upgrade over STL

STL has no unit definition. Every scale mishap where a mold arrives ten times too large or 25.4 times too small comes from that gap. 3MF stores units explicitly, along with color and print metadata, in a compressed package that is usually smaller than the equivalent STL.

If your slicer reads 3MF, use it. It removes an entire category of error for free.

Which one should you download?

Printing on your own machine: 3MF, or STL if your slicer is older.
Sending to a print service: STL, because it is what every service accepts.
Sending to a CNC shop: STEP.
Editing the mold in Fusion, SolidWorks, Onshape, or FreeCAD: STEP.
Rendering or further sculpting: OBJ.

There is no penalty for taking all of them. Every export includes both mold halves in each available format, plus your original source file.

Keep reading

STL to Mold: Convert a 3D Model into a Printable Two-Part Mold
What actually happens between uploading an STL and downloading two printable mold halves, and how to prepare a model that converts cleanly.
3D Printed Molds for Resin Casting
Print settings, materials, and release strategy for casting resin directly in a 3D printed two-part mold.
Designing Molds for the INJEKTO 3 Desktop Injection Molder
The exact size envelope, shot volume limit, and channel geometry for desktop injection molding with the INJEKTO 3.