5D-D600-D800

OK, well rather than rewrite it all allow me to just paste something I wrote on this subject some time ago in another thread:

Plastic in reference to materials can refer to a wide range of materials. Now some might label the material that the 5DIII's chassis is made from as 'plastic' in a derogatory sense. However the material used in the 5DIII is one a raft of materials that are described as Engineering Plastics.

Typically, an engineering plastic is chosen for its range of enhanced physical properties and other properties exhibited by various grades of engineering plastics include high heat resistance, mechanical strength, rigidity and chemical stability.

The reinforced plastic that is used in the 5DIII is carbon fibre reinforced plastic (CFRP). CFRP materials are distinguished by their extremely high strength and rigidity. Low density, excellent damping properties and a high resistance to impacting combined with exactly modifiable thermal expansion to complement the complex characteristics profile. Unlike glass fibre reinforced plastics (GRP), CFRP exhibit considerably greater rigidity, sharply enhanced electrical and thermal conductivity and a lower density. Their positive characteristics (relative to the weight) mean that CFRP materials are typically used for applications in aerospace engineering (the wings of the Airbus A350), in the automotive industry, in motor racing (monocoque chassis in Formula 1), sport equipment subject to high levels of stress (bicycle frames) and high-strength and high-rigidity parts in industrial applications, such as robot arms, reinforcement and sleeves in turbomolecular pumps or drive shafts. The positive chemical resistance pays off in the case of CFRP vanes in sliding vane rotary pumps used for aggressive media. CFRP material consists of a polymer (usually duroplastics, thermoplastics) employed as a matrix material in which carbon fibres with a diameter of a few micrometers are embedded.

You may associate 'plastic' with toys but I certainly wouldn't consider an F-1 car, for example, as a toy.  Nor would any manufacturer consider making the chassis of one of these out of metal these days - not if they wanted to pass crash regulations or be competitive.