Definition
A method of forming composite or laminated aircraft parts in which layers of resin-impregnated material are placed over a mold and sealed inside a flexible bag. Air is then drawn out of the bag, and atmospheric pressure (sometimes combined with heat in an autoclave) presses the layers tightly against the mold while the resin cures, producing a smooth, void-free part.
Plain English
A way of making aircraft parts by stacking layers of fiber and resin on a mold, sealing them inside a flexible bag, and sucking the air out so outside air pressure squeezes everything together evenly while it hardens.
Context Anchor
Seen in aircraft composite manufacturing and composite repair discussions, especially when a part is made or repaired using a vacuum bag.
Derivation
Named directly for the technique: the part is molded inside a sealed bag. The bag is the key element because it allows atmospheric pressure to do the clamping work that rigid molds or mechanical presses would otherwise have to do.
Why Pilots Care
Proper execution produces void-free, high-strength components that maintain airframe integrity and flight safety.
Analogy
It is a little like sealing layered material in a heavy plastic bag and pulling the air out so the outside air presses the layers flat and tight against the shape underneath.
Intuition Check
Bag does not mean a storage bag here. It means a flexible airtight cover used to apply pressure during the shaping and hardening process.
Example Sentence 1
The composite wingtips were produced by bag molding, which gave them a smooth outer surface and consistent thickness.
Example Sentence 2
Bag molding ensures uniform pressure across the layup so the finished part meets structural requirements.