What does “glass-filled plastic” mean in camera bodies?

“Glass-filled plastic” usually means an engineering thermoplastic—commonly polycarbonate or sometimes nylon—reinforced with glass fibers. In camera construction, this is often used for outer shells or panels over a metal internal chassis.

It’s different from cheap consumer plastic because the glass reinforcement improves stiffness, toughness, dimensional stability, and resistance to impact, moisture, and chemicals. That’s why it feels denser and more rigid than typical molded plastic parts.

It is broadly similar in concept to fiberglass in that glass is used to reinforce a polymer, but in camera parts it’s typically a thermoplastic composite made for injection molding rather than a resin-laminated fiberglass structure. The “glass” is usually short fibers mixed into the plastic, not large visible strands.

Many cameras combine materials: glass-filled polycarbonate or similar engineering plastics for exterior parts, plus an internal aluminum or magnesium frame, and sometimes magnesium alloy panels on higher-end bodies.