Parting Line — Where the Mold Halves Meet

A diamond-marked phantom line on casting, forging and molded-part drawings showing where the mold or die halves meet — the seam the process will leave on every part unless the drawing says otherwise.

Definition

A parting line is the separation between mold or die segments — the trace left on the part where the halves of the mold, die or forge tooling came together. It is depicted on casting, forging and molded-part drawings as a phantom line extending beyond the part in the applicable views, with the parting line symbol added. The convention is governed by ASME Y14.8, the castings/forgings/molded-parts companion to Y14.5; Y14.5 itself delegates all such requirements to Y14.8.

Why the drawing marks it

The tooling will part somewhere; unmarked, the location is the toolmaker's discretion and it moves with every design iteration. Marking the line commits the split to a known seam — ideally a corner or flange edge where flash is trimmed away and the witness mark disappears into the geometry. If the parting line is not dimensioned on the drawing, its location is up to the manufacturer — which is precisely why drawings that care mark it explicitly.

What the line commits

• Flash and witness-mark location — the seam, and the trim operation that cleans it, land where the line is drawn
• Draft direction — faces drafted away from the split on each side; relocating the line flips the draft on the affected faces
• Tolerance authority — tolerances are held more easily on the same side of a split than across it, so the line placement is a tolerancing decision, not just a tooling one
• Profile controls — Y14.8's all-around-this-side / all-over-this-side-of-parting-line symbols extend a profile control to only one side of the marked seam

Reading the notation

The line itself is a phantom line — long dash, alternating short dashes — drawn past the part outline, with the diamond-shaped parting line symbol placed on it. Drawings often abbreviate the label as PL; on molded parts the abbreviation is common where the symbol is impractical at drawing scale. Either way the meaning is identical: the mold opens along this trace, everything above is tool half one, everything below is tool half two.