Is wanting to chew on plastic or wanting to chew something all the time a disorder ?

Maybe: Pica is a behavior that may be in response to a medical problem - specifically anemia as related to iron deficiency. Other vitamin and mineral deficiencies may give rise to pica as well as psychiatric disorders like anxiety.

Called 'Pica': It may fall into the disorder that is called 'pica' (pike-ah). Pica is the practice of craving non-food substances and affects a minority of pregnant women. Doctors believe it can be triggered by low vitamin/iron levels. The most 'off-the-wall' cravings are: soap, dirt, cement, charcoal, cigarette ashes, & paint. You should get seen to discuss possible lab tests & management.