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.

Draft Angles and Undercuts in Mold Design

Draft is the slight taper on a mold wall that lets a part slide out. Undercuts are geometry that hooks behind the parting plane and physically prevents release. Together they explain almost every stuck, torn, or broken cast part, and both are visible in the demoldability heatmap before you print anything.

What draft angle means

A wall perpendicular to the parting plane has zero draft. As the part moves out of the mold, that wall slides along the cavity for its entire length, with full surface contact the whole way. Friction and vacuum fight you, and on a soft cast the surface scuffs.

Add one or two degrees of taper and the wall separates from the cavity the instant motion begins. Contact drops to nothing, and the part lifts out.

Draft angle	Behavior
0 degrees	Full-length sliding contact. Sticking, scuffing, vacuum lock.
0.5 to 1 degree	Workable on smooth, polished, well-released surfaces.
1 to 2 degrees	The practical default for most cast and molded parts.
3 degrees or more	Comfortable release, needed on textured surfaces and deep walls.
Negative	An undercut. The part cannot leave the mold in that direction.

Deeper walls need more draft, because a fixed angle produces more clearance the further the part has to travel.

Reading the demoldability heatmap

The heatmap colors every surface by how it will behave during release, computed against the selected parting plane.

Green: easy. Comfortable draft, releases without help.
Yellow: medium. Releases with release agent and clean mold surfaces.
Orange: difficult. Marginal draft. Expect sticking, scuffing, and short mold life.
Red: impossible. Negative draft, an undercut. The part is mechanically locked in a rigid mold.

Red does not always mean stop. A flexible silicone mold stretches over shallow undercuts. But in a rigid printed or machined mold, red means the part will not come out without breaking something.

Kinds of undercut

Side holes and bosses

A hole or pin running perpendicular to the parting direction locks both halves. The standard industrial fix is a side action or slide; in a printed two-part mold, the practical fix is to rotate the part so the feature runs along the parting direction, or to cast the part solid and drill afterwards.

Overhanging lips and snap features

Any lip that hooks back under itself. Often solvable by moving the parting plane so the lip sits at the split rather than below it.

Enclosed volumes

A closed loop such as a handle traps mold material inside it. No two-part rigid mold releases this. Use a flexible mold, cast in two pieces, or redesign.

Texture and lettering

Engraved detail perpendicular to the wall behaves as a miniature undercut. Recess depth should stay shallow and every wall of the recess should carry draft.

Fixing draft problems without redesigning

Rotate the part. Orientation is free and fixes more problems than any other change.
Move the parting plane to the widest cross-section, so opposing surfaces draw in opposite directions.
Switch to a flexible silicone mold for shallow undercuts.
Polish the cavity. Surface finish materially reduces the draft you need.
Use a release agent, and reapply it. Most stuck parts on marginal draft are release-agent failures.

If none of these clear the red areas, the geometry genuinely needs a design change. Better to learn that from a heatmap in the browser than from a ruined print and a stuck cast.

Keep reading

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.
3D Printed Molds for Resin Casting
Print settings, materials, and release strategy for casting resin directly in a 3D printed two-part mold.