Good question. “Off” is a preposition here because “the plastic” needs a preposition before it in this sentence. Alternatively, we could say “cut the top off from the plastic” or “cut off the top from the plastic” (by the way, I’m having trouble picturing exactly what this is about; what is “the plastic?), in which case “off” would be an adverbial particle, and “from” would be the preposition. Edited: I provided a clearer example.

I not know