Laser etching on some plastics can be difficult and various levels of contrast can be achieved depending on the type of plastic and the laser parameters utilized during the marking process.  To achieve optimum contrast, the laser must melt or burn only the surface layer of the plastic.  This type of laser engraving is sometimes referred to as “foaming”.   This is accomplished by lowering the power of the laser, utilizing a low frequency and moving at a high scan speed.  If the laser scanned too slow, used too much power or too high of a frequency, the material would simply melt, as too much heat would be generated by the laser.  In this case it would be difficult to achieve a sharp crisp character with ideal contrast.  In summary, laser marking of plastics is a delicate balance of generating the correct amount of heat on the surface of the material to obtain the best mark.