This is a super bad idea. Without knowing the type of plastic and its appropriate softening temperature range, you’re likely overheating the material. That results in the release of noxious and sometimes carcinogenic vapors.
Depends on the plastic, you can safely heat most polypropylene and polyethylene based plastics. If it's putting off noxious fumes then it probably has urethane, styrene, or vinyl in it.
The worst plastic to overheat that I've worked with is kydex. Even though it's most common application is as a thermoplastic, if you over heat it the stuff off gasses hydrogen chloride.