Definition
A thin separating layer (often plastic film, wax, or release agent) placed between a wet composite repair lay-up and the surrounding tooling, bagging materials, or surfaces during cure, to prevent the resin from bonding to anything it should not stick to. It allows the repair to cure cleanly and the bagging materials to be peeled away afterward without damaging the part.
Plain English
A non-stick layer placed between a wet composite repair and the materials around it, so the resin doesn't glue everything together during cure.
Context Anchor
Seen in composite structure repairs, especially when a wet repair area is covered before curing under pressure or a vacuum bag.
Derivation
From 'parting,' meaning separating, and 'film,' a thin sheet. The name describes its job: a thin sheet that keeps things from sticking together when the resin cures.
Why Pilots Care
If parting film is missing or the wrong type is used, the repair materials can stick together incorrectly or the repair may not cure as intended. That can lead to a rejected repair or an unsafe structure.
Analogy
Like the parchment paper between a baking sheet and a sticky cookie -- it lets you lift the finished item away cleanly without tearing it apart.
Intuition Check
Parting film does not mean a film that is peeling apart. Here, parting means separating: it is a thin layer that keeps one repair material from bonding to another.
Example Sentence 1
Before applying the vacuum bag, the technician laid a sheet of parting film over the wet repair so the bleeder cloth wouldn't bond to the laminate.
Example Sentence 2
After cure the parting film peeled away cleanly, leaving the part surface undamaged.