Plastic utensils that have absorbed the taste of boiling hot food are koshered via hagala with boiling water, like all other utensils. This is true of all types of metal, such as silver, copper, iron, aluminum, etc., as well as utensils of leather, wood, and bone. It is earthenware alone that the poskim say cannot be koshered, because, due to its unique composition, it is very absorbent, but does not release all that it absorbs. And some poskim say that glassware has the same status as earthenware.
However, Igrot Moshe (OĤ 1:92) states that one should not perform hagala on utensils made of plastic or other synthetic materials not mentioned by the Rishonim, since such materials might be like earthenware, which does not release what it has absorbed. Nonetheless, the overwhelming majority of Aĥaronim agree that hagala is effective on plastic utensils, and this is the halakha. Utensils made of hard plastic are koshered through hagala in a kli rishon on the flame, whereas plastic utensils that are liable to be damaged in a kli rishon on the burner can be koshered at the same level they absorbed the ĥametz.