Different plastic brands, different hardnesses of the plastics and differences in microwaves in power and minor little glitches make it imperative that a thermometer is used with any of the plastics. Timers and rheostats are only for ballpark guessing. Use the thermometer and you'll be a much happier camper.

The hollow heads mentioned are not a result of plastic that is too cool, but rather from air in the injector. Those built in plunger jobs are borderline useless because getting he air inside purged out before injecting is a mess.