Air Trap (noun)

(also called a gas trap)

Meaning A pocket of air that the incoming plastic cannot push out of the cavity, so it becomes trapped and compressed — usually where two flow fronts close around it, or at the far end of a blind rib or boss.

Think of… An air bubble caught in a syringe — trapped air that burns or blocks the fill.

Etymology Literal: air that is trapped inside the mould as it fills.

DFM Meaning A defect with several symptoms. The trapped air blocks the melt, so the cavity may not fill completely (a short shot); compressed hard enough it heats and scorches the plastic (a burn mark, sometimes called dieseling); and it leaves a weak spot. It is fixed by adding vents so the air can escape, and by steering the fill pattern so air is pushed toward a vent rather than cornered.

Singular - A blind rib left an air trap that scorched the tip on every shot.

Plural - Adding vents at the last-to-fill corners cleared the air traps.

Industry Example - The burn at the rib end was an air trap; a vent slot on the parting line let the gas out and the burn disappeared.

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Part of the Glossary reference.