Definition
A reference line on a sheet metal layout that represents the theoretical intersection of the outside surfaces of a formed part, used as a baseline for calculating bend allowances and laying out flat-pattern dimensions before the metal is bent.
Plain English
An imaginary line on a flat piece of sheet metal that shows where the outer edges of the finished, bent part would meet if the corner were perfectly sharp. Sheet metal workers use it as a measuring reference when planning where to cut and bend.
Context Anchor
Seen in airframe repair, sheet-metal layout, composite work, and aircraft drawings that show the final shape a part must match.
Derivation
From 'mold,' meaning a shape or form, and 'line,' a reference mark. The term comes from the idea of the line that defines the molded or formed shape of the finished part.
Why Pilots Care
Maintenance technicians rely on mold lines to fabricate replacement parts that match original dimensions. An incorrect layout from a misread mold line can produce a part that does not fit, weakening a repair.
Intuition Check
Mold line does not mean a line made by mildew or dirt. In aircraft maintenance, it means the intended finished shape or outline of a part.
Example Sentence 1
Before cutting the blank, the technician marked the mold line on the drawing to calculate the correct bend allowance.
Example Sentence 2
All rivet locations were laid out from the mold line to match the original part dimensions.