Air entrapment occurs when the sheet seals around a region before the air inside it has an escape path. The trapped pocket prevents full tool contact and may leave a soft area, dull spot, blister-like depression, or incomplete texture.
Deep recesses, engraved details, broad polished surfaces, and geometric shadow zones are common locations. Early contact from sag, plug motion, or uneven heating can close the available path even when vents are present.
Diagnose the seal sequence
The first-contact pattern should be observed or inferred from witness marks and thickness distribution. If the pocket remains in exactly the same location, the tool needs a better evacuation path or a different forming sequence. If it moves with sag or temperature, sheet control is contributing.
Vents or shallow channels should connect the pocket to a low-resistance manifold without leaving unacceptable witness marks. A controlled matte tool finish can provide micro-paths for air, but surface blasting should not be used blindly on a tool that must reproduce high gloss. Pressure and plug timing may need adjustment so the sheet does not seal the region too early.