Prior to laser cutting, apply a masking material to the top surface of the engraving plastic sheet. The masking material protects the top surface from smoke residue created during the laser cutting process. Often a sticky residue is created along the laser cut, and the mask protects the area and achieves a cleaner laser cut. A medium tack paper mask, often called transfer tape, is a great way to go and is easy to peel off after laser cutting of the plastic sheet.
-Mike Fruciano, Coherent