I have a similar 80w laser from WK Laser. I have only cut paper a few times but with the focus set correctly and the air assist on, I don't get any burnt edges at all. I'm using a 2", or 54mm FL lens which puts the air cone pretty close to the material for better results. Maybe that's the difference.
Are you running air also? Are your lenses and mirrors due for a cleaning by any chance? Are you using more power than is necessary?
Sorry I can't be of more help.