That’s because Acetone is a solvent that attacks Acrylic. Which is why it can be used to “weld” Acrylic. You’re not hosing the entire sheet with Acetone, you’re just dosing the area of the joint. The plastic melts and the joint flows together and the Acetone evaporates and you’re left with strong bond.
Here’s a random YouTube video from 2011 where someone explains how to use Acetone to bond Acrylic.
