Parting line

What the line controls#

The parting line divides the regions formed by different mold sections. Faces on opposite sides normally need draft in opposing directions so the tool can open and the part can leave the cavity or core.

Autodesk Fusion's Draft Analysis uses a selected pull direction and a draft-angle gradient to help locate zero-draft faces, undercuts, and possible parting-line positions. Onshape's Draft feature can apply draft from a selected parting line, allowing the taper to change direction across the split.

Why placement matters#

Parting-line placement affects more than whether two mold halves can separate. It determines where flash, mismatch, and a visible witness may appear. It also shapes shutoff surfaces and can turn a feature into an undercut that needs another mold action.

Protolabs connects difficult parting-line and shutoff design with flash risk and recommends reviewing ejection and undercuts before tooling. Keep the line away from sealing surfaces, critical fits, and prominent cosmetic faces when the product and mold construction permit it.

CAD representation and release#

A parting line can be represented by split faces, surface boundaries, sketches, or native mold-design features. The visible edge alone does not capture the full tool decision. The release record should also state pull direction, core and cavity sides, accepted shutoffs, side actions, and the model revision that was reviewed.

A STEP export can preserve the final face geometry while losing the native analysis and feature history. Include the proposed split and its assumptions in the DFM package rather than expecting the recipient to infer them from the solid.