The Plax melted down to their basic component. Art by Richard Elson.
Plastic-X is a highly malleable "living" plastic that is produced on Shanazar.
The Plax are made of this substance and require the mineral magnaz to power the factory that manufactures it. Being made of the living plastic makes the creatures invulnerable to any attack or outside damage. It can, however, be melted down, but even scrambling the Plax's molecules is not enough to separate their hive-mind from the plastic. It is possible that the Plax can stretch their plastic arms to attack enemies slightly further away.