Laser Cutting Theory
These are considerations with which an operator must become concerned when cutting results are below expectations, and all primary considerations (listed above) have been checked.
Choice of lens
As a general rule, the most sharply defined focal point is produced by the shortest focal length lens (5″). Thus, the 5” lens is used when maximum intensity is important – that is, cutting materials with high intrinsic reflectivity (metals). In practice, there is only a slight (but usually noticeable) difference between a 5” lens and a 7.5 lens in this respect.
The longer focal length is required, however, to achieve parallel sided cuts in some materials when the material is reasonably thick. For example, to cut 1” thick acrylic, it is found virtually impossible to keep the sides of the cut parallel with the 5” lens, whereas the 7.5” lens makes this quite easy. Note that the choice of laser power, assist gas pressure, and feedrate all combine to influence the cut quality in this respect, apart from the lens itself.
Condition of the lens
Cleanliness of the lens is of major importance, since any contaminants on its surfaces will cause it to absorb energy and become warm. Thermal distortion in the lens inevitably produces fuzziness in the focal point of the beam, and consequent reduction in cutting performance. Eventually, if a lens becomes excessively heated, thermal stress and gas pressure will cause it to shatter.
The operator should inspect the lens regularly (and clean as necessary). In fact, common sense is the rule here; the source of contamination is virtually always airborne particles produced by the cutting. Therefore, if material being cut produces contaminants (eg. sheetmetal often has oil on the surface; rubber produces black smog when cut; etc.), the lens should be inspected as often as convenient. The assist gas greatly helps in keeping contaminants away from the lens, but the operator must be aware that this is by no means total protection. Lifting the focal height while piercing will also help protect the lens.