We use the Microwave to heat and reheat plastic. Occasionally we run into microbubbles, sometimes major bubbles, or even the dreaded Foam Monster. There are many sources for moisture, just about all of which are mentioned here at the CCM Forum. I believe I may have discovered another source of moisture. The Microwave, itself.
Is there anything else heated in the Microwave You're using to heat plastic? If so, nearly every food (and of course every beverage) is loaded with moisture that turns to vapor in the microwave, almost 100% of the time. That moisture will sit in the microwave and get reheated again and again...then comes the Tackle Guy with a cup of plastic and probably a grin...until that moisture gets vaporized.
Just a thought, Bro's!!!