Laser marking: foaming, engraving, carbonization

Foaming. At low laser intensities, a destruction of molecular structures causes a color change and surface restructuring. This effect is similar to what can be achieved when bending nylon
many times–at the bend you will observe a lighter color.

Engraving. Localized increase in temperatures above the melting point of the material causes melting.
Once the material is resolidified, a modified surface structure occurs in the form of an etch. This effect is typically achieved with CO2 wavelengths in most thermoplastics.

Engraving with color change. At relatively highlaser intensities,, a local evaporation of surface material causes a trench with ridges. As a result of carbonization, a color change occurs.
Laser Marking on Plastic Materials - primross.com.mx
www.primross.com.mx/laser/marking/Laser_Marking_on_Plastics...‎

Recent technological in- novations in the laser indus- try and the formulation of plastic have led to advances that allow a marriage of the two technologies.

A laser engraver uses a highly concentrated beam of light to burn or cut into a variety of materials. Rowmark has developed a complete line of products specifically for the laser engraving process. Most laserable products are 2-ply laminates, the top layer (cap) of which can be burned away to reveal a contrasting substrate below, resulting in one-step signage.

http://www.rowmark.com/mark/laser_guide/pages/intro.asp