Mouthplate

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. While no official name exists for these, they are usually called 'mouthplates ' or 'faceplates' (despite a 'regular' face once being described as a faceplate in the TV series). Two of the most famous examples are those of Generation One'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.

Beast Wars
Optimus Primal was shown to have a mouthplate which would slide into place when he entered battle. This was used to reconcile the presence of an actual mouth on his animation model with the lack thereof on his toy. Later retoolings of the toy reflected the animation model.

Cybertron
The Optimus Prime animation model displays a similar setup to Optimus Primal, with a mouthplate that slides into place when entering Super Mode. However in this case the toy reflects this, featuring a gimmick that allows the face to display either configuration.

Dreamwave comics continuity
In The War Within, Bumblebee was depicted as having a removable face mask with mouthplate that resembled the original sculpt of his toy. In The Dark Ages, Jetfire was given a similar treatment with a flip-up faceguard. The Titanium Series release of this character design featured the faceguard, but it was not removable.

IDW comics continuity
In Stormbringer, Jetfire has a removable mask with mouthplate as part of his battle gear. This feature was included in the Classics toy of this character design.

Transformers (2007) film
Optimus Prime is depicted as having a mouthplate which closes over his normal mouth to protect his face. To date, none of the official movie toys display the face beneath the mouthplate.

Trivia

 * Fan commentator Zobovor nicknamed these distinctive items "woobers" (crediting the term to a younger relative), and the name became popular with some fans.


 * Both the Armada Optimus Prime and the 20th Anniversary Optimus Prime toys have mouthplates that can move up and down when a button located on their heads is depressed, even though Armada Prime's mouthplate never moved in the series itself. Additionally, the 20th Anniversary toy has teeth (or possibly a speaker grille) hidden behind the mouthplate, in reference to Optimus Prime's battle-damaged appearance in the Generation 2 comics, though this detail cannot be seen without disassembling the entire head.