miriam,
Liquid Wrench isn't that bad on plastic gears. Certain solvents such as acetone, toluene, and others are nasty on plastic.
I don't use solvents on plastic geared machines anyway. Reason = so far I haven't had to unfreeze a plastic geared machine. Just replace the busted gears. Arghhhhhhh!!! Not saying I would anyway.
Next machine I get with a busted plastic gear I'll use the gear as a test medium and hose it with something and see what happens.
Joe