Actually, the tricorder isn't supposed be metal. In the "Star Trek The Next Generation Technical Manual" on page 119 it says that the case is constructed of "micromilled duranium foam". The manual was written by Rick Sternbach and Michael Okuda... who had a lot to do with the technology for the show.
Lot's of real world science and engineering equipment is plastic. However, metal props are cooler. The EFX Next Generation movie tricorder is metal, which is really nice.
Lot's of real world science and engineering equipment is plastic. However, metal props are cooler. The EFX Next Generation movie tricorder is metal, which is really nice.