Laser Marking Systems : Engraving Technologies
High Speed Moving Mirrors ("Galvo/Mirror')
Laser is light, and it is reflected by mirrors much the same as any other
(incoherent) beam of light is reflected. Moving mirror laser delivery
(steered-beam) exploits this property through the use of a pair of mirrors
oriented at right angles to one another.
Each mirror is mounted directly to the output shaft of an ultra high
speed servo motor, which is itself under computer control. The beam of
coherent laser light is reflected, or "folded" from the first
mirror onto the second. From the second mirror it is reflected outward
to the target. The relative angles of the two mirrors determines the beam's
final position on the target.
The mirrors used in steered-beam systems are usually small, typically
less than one inch across. They are of light weight and low inertia design.
The high-speed motors can turn them in less than 1/1000th of a second.
This is the reason that laser delivery to the target by steered-beam is
so fast. Steered beam is 100X to 1000X faster than gantry systems and
10X - 100X faster than plotter delivery.
Other Methods for Applying Laser
Gantrys
Gantry systems are common for sheetmetal cutting and punching, and in
the automotive world as a platform for robots and CNC tools. Gantry systems
can support powerful lasers and address large table areas, but they are
too slow for economical engraving and intricate die-cutting.
Plotter-type systems
Plotter systems "pipe" the laser beam with mirrors along the
axes of a mechanical XY plotter. They are driven by light-weight, stepper-motor
control systems pioneered for use in engineering pen-plotters. They can
address areas up to 1 meter2 although the effective working area is usually
about half that size. Pen plotters are relatively inexpensive. Throughput
is low to moderate. They are well suited to small jobshops with low volumes,
such as retail engraving stores.
Two ways that high-speed galvo-mirror technology writes on materials:
Fixed-Field with "F-Theta" Optics
In Fixed-Field systems the coherent laser beam passes through a passive
focusing lensafter it leaves the second mirror. This lens focuses the
beam to a tightly concentrated spot of energy on the target, generally
from 0.002" (0.050 µmeters) to 0.008" (0.200 µmeters)
in diameter.
It is this enormous concentration of the laser's energy which does the
work of heating, melting and vaporizing a path on the target's surface,
even when that surface is steel, stone, tungsten, carbide, ceramic or
other high melting-point material. The lens has a special design which
brings the laser beam to a precise focus everywhere on the flat surface
of the target within a prescribed area, or "field size." Field
sizes range from 2" to 12".
Varifield™ with Dynamic Focus Optics
In Varifield™ systems the output lens is replaced by an optical telescope
similar to an old-fashioned spyglass. The "eyepiece" of this
focusing telescope moves back and forth under dynamic computer control
to focus the beam at different points on the target's surface.
It takes a third ultra high-speed servomotor in the system to drive the
moving lens. Because of the extra servo-control axis, Varifield™
technology is more complex and costly to implement than Fixed Field. Its
principal advantage is its ability to engrave much larger fields, up to
24" x 24" (600mm x 600mm), than F-Theta lenses. Varifield also
offers a smaller focused spot size and a higher graphics resolution than
an F-Theta system of comparable field size.
Beam Steering: Fixed-Field vs. Varifield™
Fixed-Field laser systems address specific engraving areas whose sizes
are determined by the lens design. Lenses are interchangable in the field,
but once a lens is installed the engraving area is fixed until it is changed.
Varifield™ technology, on the other hand, can be adjusted by the
operator from 24" x 24" (600mm x 600mm) engraving area to a
2.5" x 2.5" (60mm x 60mm) engraving area with a few simple adjustments.
Varifield™ technology is more complex than Fixed Field (it incorporates
a "third servo axis) and is somewhat more costly. Varifield™
can address considerably larger field sizes than are practical with Fixed
Field output lenses.
Spot Size and Field Size
When you buy any laser tool you are purchasing two essential functional
ingredients: raw laser power packed into a tightly concentrated spot size.
The combination of these two factors dictates the ultimate capability
of the system to engrave, cut, weld or drill into materials.
A large, diffuse spot smears the expensive laser energy across a broad
area of the target, barely heating it up. By contrast, a tightly focused
spot has sufficient "energy density" to overcome physical properties
of the target's material and vaporize, weld or anneal it. Since the focused
laser spot covers an area, its energy density decreases in proportion
to the square of its diameter.
A general rule of steered-beam laser engraving and surface modification
systems is that as field size increases, focused spot size increases as
well. A tradeoff is at work in selecting the proper system for a particular
application: larger field sizes may seem desirable, but they also imply
larger focused spot sizes and reduced energy density on the target.
For hard-to-penetrate materials such as steels and ceramics this imposes
a practical limit on useful field size. For Fixed Field systems this limit
is usually between 4" (100mm) and 8" (200mm) square fields.
Varifield™ spot size
The optics of Varifield™ systems are superior to those of Fixed Field
systems. At a given field size-for instance 8" (200mm)-a Varifield™
system will focus its spot between 15% (for small fields) and 30% (for
large fields) more tightly than can be realized with the equivalent Fixed
Field system. Since concentration of the laser's energy is the most important
single specification for an industrial laser system, this represents a
distinct advantage for Varifield™ technology.
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