Use of laser engraving in industrial applications

Cutting
It should be mentioned, though outside the scope of this article, that lasers of high power have the capability of not only engraving, but cutting material. The same basic techniques and considerations are used in fabrication of many cut shapes, whether in wood or in stacked fabric for apparel manufacture, or even metals (plasma-cutting). It is just all done at power levels which allow the laser to penetrate quickly through the piece rather than only at the surface level. Evacuation of released gases is often provided by a forced-air "snout" trained directly on the laser cutting area.


Printing
Direct laser engraving of flexo photopolymer plates or sleeves (which fit over a mandrel) is attracting wider interest following some recent technical developments and mergers of vendors. Up to now the process has been associated with wide-web flexo printing of, for example, film or paper packaging (flexible packaging). Here it competes with rotary gravure, although direct laser engraving is also being introduced. For the less expensive flexo process, the technology is being adapted for smaller formats suitable for engraving flexo plates or sleeves mounted on the actual printing cylinders.

This includes narrow and mid-web flexo presses (up to 20-24 inches wide), which could open up the market for self-adhesive label and packaging converters interested in the digital - that is filmless - route. With this process there is no integral ablation mask as with direct laser imaging (see below). Instead a high-power carbon dioxide laser head burns away, or ablates, unwanted material. The aim is to form sharp, relief images with steep, smooth edges to give a high standard of process color reproduction. A short water wash and dry cycle follows, which is a lot less involved than in the post-processing stages for direct laser imaging or conventional flexo platemaking using photopolymer plates.


Direct laser imaging
Closely related is the direct imaging of a digital flexo plates or sleeves 'in-the-round' on a fast-rotating drum, or cylinder. This is carried out on a platesetter integrated within a digital prepress workflow, that also supports digital proofing. Again, this is a filmless process, which removes one of the variables in obtaining the fine and sharp dots for screened affects, including process color printing.

With this process the electronically-generated image is scanned at speed to a photopolymer plate material that carrys a thin black mask layer on the surface. The infrared laser-imaging head, which runs parallel to the drum axis, ablates the integral mask to reveal the uncured polymer underneath. A main ultraviolet exposure follows to form the image through the mask. The remaining black layer absorbs the ultraviolet radiation, which polymerizes the underlying photopolymer where the black layer has been removed. The exposed digital plate still needs to be processed like a conventional flexo plate. That is, using solvent-based washout with the necessary waste recovery techniques, although some water-washable digital plates are in development. This technology has been used since 1995 and is only now becoming more widely used around the world as more affordable equipment becomes available. Trade sources say there are around 650 digital platesetters installed in label, packaging and trade platemaking houses.

In flexo direct laser engraving can be done using a CO2-laser. This makes it possible to direct ablate the non-printing area. This way steps like UV-exposing, chemical washing and drying are not necessary anymore. Before the year 2000 lasers only produced lower quality in rubber-like materials. In these rubber-like materials, which had a rough structure, higher quality was impossible. At the Drupa 2004 the direct engraving of polymer plates was introduced. This had also an effect on the rubber-developers who, in order to stay competitive, came with new high quality rubber-like materials. Since then direct laser engraving of flexo-printingforms is seen by many as the modern way to make printing-forms for it is the first real digital way.


Sub Surface Laser Engraving (SSLE)
The process of engraving an image below the surface of a solid material, usually glass with an optical clarity to minimize distortion of the laser.

A fairly comprehensive overview of this medium is best described from a designer that specializes in implementing SSLE into custom designs: (following text copied with permission of SharpeAwards [1])


Sub-Surface Laser Engraving or SSLE as it is being termed nowadays, is a technique "discovered" in Russia in the early 1980's. Originally the creation of these "dots" in optical glass was a problem known as "Laser Induced Damage" and was exhaustingly studied.

At the time, the idea was to avoid these problems by selecting specific material compositions and laser optics to improve laboratory use. In the process, a list of material compositions was created that provided good transparency with minimal heat absorption. There was also another list, which included materials that didn’t, and it was somewhere there that laser technicians found it "neat" to write their name in these materials.

Needless to say, someone decided there might be some commercial applications for this and began writing dissertations about the controlled placement of these dots inside of the crystal. Over the years, the science of this process from a purist view has become more of an art form that balances the technical aspects of high-powered lasers and delicate balance of image design within them.

A much more technical overview

The creation of 3D laser crystals utilize high-energy laser beams to produce a phenomena known as “Multi-photon Absorption” within optically perfect crystal. This phenomena which use the electromagnetic wave of the laser beam known as coherent light creates an electric field greater than 10 million volts per centimeter. When the laser beam is focused within the interior of the subject crystal the energy creates unattached electrons also known as “free” electrons. These “free” electrons, accelerated by the electric field created by the laser beam causes the high energy electrons to collide with atoms and ions in the focus area. As the process continues it causes a chain reaction and produces about 1 million trillion free electrons per cubic centimeter in about 1 trillionth of a second. The laser then emits a short pulse beam of a few billionths per second and produces a tiny micro crack. The laser head then align and position tens of thousands of additional micro cracks to create 2 or 3 dimensional images. Although, the laser generates power densities of 10 billion watts per square centimeter, the surface of the crystal is not damaged due to the highly transparent nature of optically perfect crystal. The resulting images appear to be suspended within the crystal.

Techniques used in SSLE

There are basically three different techniques in use by sub-surface engraving houses, each of which vary in their approach but all fall into similar categories. Regardless of the technique, the palette is the same - white or shades of white. Another important thing to know is the objects in the design will rarely ever look like a solid white object because they cast no shadow on themselves. With white being the primary "color" and no ability for shadows to enforce contours, the designer should have a working knowledge of negative space, spatial juxtaposition and a basic understanding of good design. Here is an example of a BAD use of SSLE Too Many Layers of White are Confusing – to see what the designer is referring to 3D WHAT?

NOTE: Color SSLE has been developed using furnaces to reheat the crystal that has be engraved (changing the color of the points). After cooling, the crystal is engraved in other areas to create a "contrast" with its original white palette. Heating and annealing is a time intensive process with larger blocks taken as long as 70 days to cool properly. Therefore, this process is only cost effective in small sized crystal done in batch run designs used exclusively for limited editions or collectors.

3d modeling - A scene, logo, or product is designed completely in 3D cad system - ideally, different components may have differing shades of white. This approach works well with simple shapes that are easily identifiable for the viewer. Complex shapes that create numerous overlapping surfaces will confuse the viewer with dense white regions and hide the actual shape. Best used sparingly by a designer that understands "less is more" for a better result - cad file drops from the client rarely provide satisfactory results out of the box.

2d bitmap - One of the most common methods of laser engraving for both sub-surface and surface engravers. Images are converted into a "halftone" which is a pattern of dots spaced to evoke a sense of tone across the varying intensity of the image. The brighter (whiter) the area, the closer the dots are placed to one another. Less dense areas appear less bright creating tonal changes in the image. Look at newspaper print of an image - same idea - different medium. This approach works well with photographs that have good contrast in them as well as single or two color logos or text. There is some size limitations though since images, symbols and text need enough dots to be recognizable.

2.5d bitmap - This is an interesting approach which combines the advantages of 2D bitmaps and some of the dimensionality of 3D modeling. This techniques builds as many as seven to eight layers of points over one another to create a whiter "white" than is available with the basic 2D approach. This approach works really well with logos and a certain amount of text - photographs of heads are commonly done but tend to look "spiky" when viewed from subtle angles and quickly loose the visual effect desired if not viewed from the front.

One additional note regarding SSLE should be made regarding these "points". Points generated by the laser are relatively small usually .1mm and slightly eggshapped. Perhaps more importantly, they are spaced typically no closer than 1.5X their size (across x/y/z planes). This is due largely by the fact that the entire lasered image is held together by the internal stress of the glass itself - tighter spacing increases the probability of creating a fissure between points resulting in damaging the crystal and/or design. This enforced spacing also reveals why SSLE objects lack the ability to cast shadows on themselves.

Since its inception in the late 90's, SSLE has become more cost effective with a number of different sized machines ranging from small (~$35-60KUS) to large production sized tables (>$250KUS) - still although these machines are becoming more available there is estimated only a few hundred in operation worldwide. Many machines require very expensive cooling, maintenance and calibration for proper use. The primary component being the laser diode can easily cost 1/3rd of the machine cost itself that has a limited number of hours.

In past 5 years the use of SSLE has become more cost effective to produce 3D images in souvenir 'crystal' or promotional items with only a few designers concentrating on designs incorporating large or monolithic sized crystal. A number of companies offer custom made souvenirs by taking 3D pictures or photos and engraving them into the crystal. Quality of the designs and images varies greatly between vendors in the promotional and personal services sector (photo engravers) - the mass producing curio vendors have the habit of reducing resolution of the points and lowering output to maximize their laser diodes lifespan. When properly produced, designs in this medium can be striking and mesmerizing.

Source: Wikipedia.

All text is available under the terms of the GNU Free Documentation License.