Mouthplate
From MediaWiki
Instead of regular human facial features, some Transformers have what looks like a plate affixed to their lower jaw, usually covering up to their nose, called a mouthplate. Two of the most famous mouthplates are those of Generation 1's Soundwave and Optimus Prime.
In earlier series, a Transformer's mouthplate moved when they spoke, as if there were a working jaw underneath. In Robots in Disguise and the Unicron Trilogy, however, it remains static.
Some Optimus Prime toys have moving mouthplates.
