Anyone who has tried to mark aluminium with a CO2 laser, then you have most likely experienced the reflectivity challenge of the CO2 laser beam and metals. Alas, there is one product, AlumaMark, that has overcome these challenges and has made marking aluminium possible using a CO2 laser. It uses a proprietary coating on aluminium that absorbs the CO2 laser energy resulting in a black mark on any of its three coloured aluminium coated materials. Typical applications include trophies, plaques, data plates, and flow diagrams, but the one application that is becoming very popular is label marking for UID (2D) barcodes.
The objective of this article is to provide some hopefully helpful insight in how changes in machine parameters affect the results and give some general tips for adjusting individual laser machines.
It’s all about settings.
Results with AlumaMark can achieve “A” grades in contrast. However, there are definite laser formulas that should be followed in order to achieve best results. This is mainly due to the sensitivity of the coating. Finding the right process parameters can be complicated by the fact that even lasers with the same power rating from the same manufacturer can vary in output power. Additional variations come from the actual mechanical design of a laser vendor’s machine including working table flatness and accuracy of the motion system. Other factors that play a role are laser beam quality and beam divergence. All up, quite an array of parameters that can cause inconsistencies in the quality of the result especially when marking a sensitive material such as AlumaMark.
Identifying the issues in the application lab.
Below are the interesting results of three different AlumaMark application tests that demonstrate quite well the issues with sensitive material such as AlumaMark.
The testing process and findings
Artwork:
1) Artwork 1: 200 x 112mm. A graphic with vertical, horizontal and diagonal lines, text as small as 3.5 pt and a logo.
2) Artwork 2: 65 x 20mm. A data plate with fine text and bold borders.
3) Artwork 3: a 2D Matrix and traditional 3 of 9 barcode.
Materials used: Satin gold, bronze and silver and matte silver AlumaMark.
Laser System used: a 75W Trotec Speedy 300 running at 90W +/- 1.5W output power at the lens over the entire 726 x 432mm working area. For comparative purposes this laser system has a maximum speed of 3.55m / sec.
The first tests were done on the satin gold product since it seems to be one of the easiest colours to process. Past experiences dictated the starting parameters. Test number one was done with the large graphical artwork and resulted in an excellent, black marking using the following parameters:
- 13% Power, 20% Speed
- 1000 DPI, 1000 PPI
- 1.5” Lens, 1mm out of focus
- High Quality Engraving feature turned on
- Total processing time: about 45 minutes
Regarding the choice of lens, the 1.5” was selected for this job due to the small characters and fine lines in the artwork. Some of the other experiments were done with lenses with longer focal lengths and showed certainly interesting results that will be discussed later.
Same settings - different results.
Using the same artwork on the satin bronze, silver and matte silver finishes showed that the settings for the gold worked equally well with the bronze. The marks were dark black and the quality of the mark was excellent, however, it was immediately clear that some adjustments were required for both the satin and matte silver. As it turned out reducing the power to 8% while leaving all the other settings the same would do the trick. The marks produced were certainly satisfactory though not as black as the ones the gold and bronze material.
Finding the most productive set of parameters.
The next series of tests (done with the data plate artwork) focused on identifying the influence of parameters (other than power) such as pulsing (PPI), speed and focal length. Based on the results from above the best results seem to achievable using the highest possible resolution (DPI), highest pulse setting (PPI) and rather long dwell time (slower speeds) as well as running the job about 1mm out of focus.
What does it take to reduce the overall processing time without sacrificing any marking quality?
Increasing speed and power proportionally resulted in rapidly deteriorating quality and thus is not an option. After a series of more tests an interesting relationship came up: when the speed was doubled a better result was achieved by increasing the power by only 25% rather than doubling it as well. Ultimately acceptable results could be achieved with speeds up to twice as high as the ideal setting reducing the total processing time by about 45%. Adjusting other parameters such as PPI, DPI or focal length showed that the quality of the mark leaned towards unacceptable with only nominal reductions in processing time.
How can the results of the Trotec Laser Application Lab be used to find the perfect AlumaMark settings on virtually any laser system fairly quickly?
A reasonable starting point is having a look at some factors which are related to laser processing in general and to the processing of AlumaMark specifically. This material is sensitive but being aware of its idiosyncrasies and laser tendencies correct parameters can be found fairly quickly.
The effect of PPIs.
CO2 lasers pulse a beam of light energy. Some lasers are equipped with a PPI (pulses per inch) feature that allows the laser to fire at different intervals as the laser head moves along the X axis. Depending upon speed and resolution of the processing, it may make sense to increase or decrease the PPI in order to achieve a better result. AlumaMark is a sensitive material that requires a long dwell time or heat period for a proper reaction to take place. Since slower speeds are defined as longer dwell times, it makes sense to increase the PPI to its maximal value to maximize the heat effect.
The right choice of lens.
Continuing the thought of dwelling the heat, larger lenses have a longer focal distance and a longer depth of focus and for the benefit of this case – a larger spot size. The larger spot size equates to a larger heat zone. Since the laser beam diameter is wider the energy density of the beam decreases. This effect needs to be compensated with extra power when using a larger lens or marking slightly out of focus in order to achieve similar results.
Marking intricate designs.
Using a shorter focal lens (smaller spot size) is required for marking small characters down to 4 point and sometimes smaller. This is especially important for marking intricate designs such as bar codes where vertical lines are very close to each other. Using the 1.5” lens is critical for those thin, fine lines. In addition, those lines must be perfectly straight for readers to make a proper read. The mechanical and software design of Trotec lasers automatically compensates for potential distortions ensuring the lines are always marked perfectly straight. Other types of lasers may need to process those types of jobs by turning the material 90 degrees to achieve similar quality.
Getting the focal distance right.
With the aim to find the perfect focal distance the Trotec test system’s interactive Z axis was exercised in increments as small as 0.025mm. Using this feature eliminated guessing focus distance and allowed us to confidently choose1 mm as a general rule, if an out of focus mark is desired.
Other factors that influence the marking quality
Besides the already mentioned factors the actual laser beam quality, the laser’s beam divergence and the flatness of the working table are potential culprits if inconsistencies in the mark are evident. This is especially true if the processing is done using a large portion of the table. The flatness of the work table in relationship to the motion system and beam divergence are all interrelated in determining how much of the work table can actually be used to process AlumaMark.
Beam divergence and working table.
Beam divergence is the tendency for the beam to increase in size the farther it travels from the source. Focus point and energy densities change dynamically and can cause different results especially when processing sensitive materials. This difference will become more prevalent on larger files since more of the working table is used. If the system has a hardware component called a telescope or sometimes called a beam expander or collimator similar to the test system used the effect of beam divergence can be minimized.
The magnetic working table of the test machine was within 0.025mm from one end of the table to the other and combined with the rigid construction of the chassis designed for flatness over the entire work surface produced consistent results in all four quadrants without any adjustments of machine parameters.
Even if you experience inconsistencies on your specific equipment there is usually an area of the working table which produces reliable high quality results. However, if divergence is an issue the entire process area will be affected. This may limit the size of the item or batch that can be done at once.
Flatness of material.
Another factor to consider is the flatness of the material. Especially larger pieces might be slightly warped. If your machine has a ferromagnetic table the simplest solution is using magnets to hold the material down. Alternatively, a strong vacuum table will do the job.
Beam quality.
Regarding the actual laser beam itself lower power systems are recommended. Higher powered lasers can become unstable when operating at the lower end of their rated output. Certainly below 5% of the rated output can start to cause instabilities in the laser beam performance - sometimes even below 10%. In the test case some colour consistency problems came up especially with large bold letters on the satin silver material using the 1.5” lens in focus. Defocusing while leaving the DPI and PPI at 1000 helped resolve the issue.
Lessons learned
The major lesson learned from the extensive tests in the lab certainly was that virtually any CO2 laser should be just fine to mark AlumaMark as long as the mechanical and optical aspects of the specific laser used are well known and the operator is aware of the system’s limitations.
Generally speaking, marking AlumaMark effectively and robustly requires a relatively large, slowly heated and critically focused laser beam. A correctly marked piece should be black in colour. If the focus is off the mark or changes across the work piece due to beam divergence issues the resulting colour will most likely be different shades of brown. This may also cause line thicknesses to differ even within a job.