Parting Lines and Parting Planes Explained

Parting plane, parting line, parting direction

Three related terms that get used interchangeably and should not be.

• Parting direction: the axis along which the two halves separate. In this tool it is the Z axis of the oriented part.
• Parting plane: the flat surface, perpendicular to the parting direction, where the block is split into core and cavity.
• Parting line: the visible seam left on the finished part where the two halves met. It follows the silhouette of the part at the parting plane.

Move the plane and all three change together. A plane placed at the widest cross-section of the part usually gives the cleanest release, because everything above it draws upward and everything below draws downward.

How automatic analysis scores a plane

The tool slices the model along the parting direction at a fine interval, 0.1 mm by default. In Custom Size mode, parts wider than 400 mm switch to a 0.5 mm interval so the analysis stays responsive. Each candidate plane is then evaluated.

• Undercut area: geometry that overhangs the slice, which would lock the part into a half.
• Floating sections: disconnected islands in a slice, which produce sections of cavity with no path out.
• Cross-section size: wider cross-sections at the plane generally mean easier release.
• Symmetry: when the part is detected as symmetric, the center plane is preferred so the seam lands on the natural mirror line.

Candidates are ranked and the best is preselected, but every scored plane stays visible in the 3D view. Valid planes are outlined in green, invalid ones in red, and clicking one previews the split.

When to override the recommendation

The score optimizes for demoldability. You may care about something else.

• Cosmetics: the parting line is visible on the finished part. Moving it onto a hidden edge is often worth a slightly harder release.
• Flat reference face: putting a large flat face on the parting plane makes the mold easier to clamp and less prone to flash.
• Fill behavior: material has to travel from the sprue to the far end of the cavity. A plane that produces one deep half and one shallow one can fill unevenly.
• Post-processing: a seam across a surface you intend to sand anyway costs you nothing.

Parts that have no good plane

Some geometry cannot be split into two halves that both release. A sphere with a through-hole across the parting direction, a threaded shaft, an enclosed handle, a part with re-entrant detail on both sides: no single plane works.

Your options, in order of effort:

1. Rotate the part. Most apparent undercuts are the result of a bad orientation and disappear at a different angle.
2. Accept a flexible mold. Silicone flexes over shallow undercuts that a rigid mold cannot release.
3. Redesign the part. Add draft, split a feature, or move a boss so it draws in one direction.
4. Split the part into pieces and cast each separately, then assemble.

Reading the plane preview in 3D

The plane preview shows the split before any geometry is generated. The selected plane is highlighted with an emissive green fill; other candidates are wireframed. Rotating the model with the plane visible is the fastest way to see what will be trapped, because you are looking at the same silhouette the mold will see.