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ALE glossary

Send mail to: sales@appliedlaser.co.uk with questions or comments about this web site. Copyright © Applied Laser Engineering Ltd 2002

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External beam modulation

There are two main ways that the power amplitude of a laser beam can be controlled: internally or externally.  Internal power modulation involves control of the excitation means internal to the laser generator. This can have the disadvantages of slow speed or undesirable effects at turn on/off.  External modulation can input a constant laser beam amplitude and uses some mechanism to vary the amplitude of the useful transmitted beams.  Examples of external modulation devices include both acousto-optic modulators and electro-optic modulators.

Focus control

In order to achieve the desired end result, it is often necessary to control the focus of the engraving laser beam(s) during the engraving process.  Examples of this would include the following:

Dynamic focus control, defined as a rapid change compared of the order of a few roll rotations or less, where an anilox cell or BEF cell is made up of several hits each at potentially different focus position to give the desired cell shape profile.

Passive focus control, defined as a less rapid change compared of the order of a few roll rotations or more, where the focus of the engraving beam(s) is changed for each pass of a laminar engraving to maintain sharp focus for each strata.

Either Passive or Dynamic focus control can be enhanced by measurement of the engraved result to provide feedback to correct the subsequent focus changes.  When focus control feedback is applied to an engraving such as a Laminar engraving, it may be necessary to re-stratify the subsequent data for optimum results.

The detection of optimum focus of engraving beam(s) can be automated to eliminate manual trial and error techniques and/or to provide feedback.  This can be achieved in a number of ways including spectrometry of the plume, depth scanning, microscopy etc.

Hokey-cokey

This term refers to problems sometimes observed with the motion of the linear axis of a machine.  Due to worn or stiff bearings the carriage can be seen to exhibit an undesirable small rotation in the horizontal plane, or crabbing, when moving axially, especially in a non-continuous motion suck as clunk-clunk multi-hit or at the start of a smooth traverse.

Interpass cleaning

Depending on the substrate material and the laser parameters there is often some debris left by the engraving of a single pass in the form of ash, dust or burrs.  In order to achieve suitable quality and productivity it is usually necessary to either minimise the debris or to remove it between passes.  Many cleaning processes can be employed including ultrasonic, dry-ice blasting and snow-jet.

Laminar Engraving

The ALE Laminar engraving process allows a textured engraving to be created by the accumulation of a number of passes of engraving.  This allows a fine resolution 2.5D or 3D texture to be produced in a multitude of fine resolution layers.  Thus a texture can be produced at a total depth and quality which would not be possible in a single pass.  The 3D texture data, where grey level typically represents desire depth, can be divided into multiple strata, with one strata for each pass of engraving.  Each pass can engrave this grey strata data manitining the variation in depth through the strata, hence achieving a true 3D result, or each strata of data can be thresholded giving a final 2.5D result.  In order to retain the required symmetry of the engraved details it is often necessary to vary the engraving and/or cleaning directions from one layer to the next.  Also, to maintain the desired laser focus from layer to layer it is usually necessary to apply focus control to the laser spot(s) for each pass.

Laser glazing/ cleaning/ hardening

By selecting the type of laser, the beam delivery and the laser parameters it is possible to subject a substrate to a thermal malting process which removes roughness by melting the surface to a ‘glazed’ finish.  This can be useful before or after an engraving pass.  By optimising of the parameters it is also possible to clean debris from a surface or to harden a surface either post- or pre-engraving.

Laser wavelengths

Depending on the type of engraving required, the substrate material and the required productivity a wide range of laser wavelengths have to be considered including deep UV (typically 193nm) all the way up to deep infra-red (10600nm).  Each of these lasers has different capabilities including volumetric removal rate and material interaction.

Maestro Engraving

Maestro refers to a way of engraving textures, often into hard substrate materials at high resolution using a laminar technique.  In order to produce a cumulative and ‘dry’ engraving with minimal molten material recast it is often necessary to use pulsed lasers and/or modulated CW lasers and/or lasers of different wavelengths.  Similarly, in order to achieve the required finish and detail resolution lasers with high beam quality together with special beam delivery are employed.  In order to achieve target productivities and surface finish it is often necessary to employ multiple lasers of either the same or different type working together.  It is usually necessary to clean each engraving between passes with interpass cleaning.  For some types of textures such as BEF/ anilox engravings some type of surface finishing is often required.  The Maestro parameters can be optimised to also produce engravings suitable for Intaglio printing forms or for holographics.

There can be advantage in Maestro engraving into a roll already coated with a resist coating, thereby achieving an end result which is a combination of direct Maestro engraving and Mask Ablation.  As with most Mask Ablation applications the engraved coated roll is usually subjected to a further etching or other chemical process.

 

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ALE GLOSSARY - Page 2