Key principles of laser marking and implications for DPM code quality
(Though broadly applicable across all laser markers, the examples and exhibits in this note are from marks obtained by the Trumpf TruMark 5000 laser unit)
Concept in general
Laser marking is a non-contact, fast method of applying marks permanently on metal or plastic surfaces. While there are other methods of applying semipermanent marks, like chemical etching, ink-jet printing, and dot-peening, lasers are the most popular method because they have the following advantages –
b. No consumables (or operating expenses)
c. Permanent under all conditions
The basic principle of the laser marking is the utilization of a particular and concentrated wavelength of light (i.e. laser light) to focus on a surface and permanently change the contrast at specific areas (which translates into a ‘mark’). This mark can be a barcode, written text, aesthetic patterns, company/product logos etc. for this document, we will focus on barcode marking only.
Variables that have implications on barcode quality
When the goal is to optimize the performance of the laser marker for a DPM code, there are several variables that have an inter-related effect on barcode quality. The most important ones are –
1. Wavelength of light
2. Material (or substrate)
3. Type of marking
4. Laser Settings –Speed, Pulse Frequency and Power Trade-off
Wavelength of light
The utilization of a particular wavelength or more commonly a range of wavelength determines the type of laser marker. The most commonly found laser marker wavelengths are illustrated below. Wavelength distribution (in nm) by laser type on the electromagnetic spectrum.
suited for marking on glass, ceramic and PCBs while Fiber lasers are more suitable for plastics, metal, and other high-power applications. We are increasingly seeing more and more Fiber lasers replace both CO2 and YAG lasers over the past year and expect that trend to continue.
Material (or substrate)
As discussed above, the material that has to be marked has an important consideration of the type of laser used and hence the quality of the mark. CO2 lasers are better for transparent materials like glass, films etc. while YAG Laser Markers Fiber Laser Markers CO2 Laser Markers fiber lasers have higher power needed to mark metals. If CO2 based laser is used to mark say a metal like Titanium or Zirconium, it will lead to poor results.
Type of marking
There are 4 common marking types offered by a laser marker. The general rule of thumb is that Annealing is best suited for metals (Steel, Titanium, Cobalt) and Ablation for Plastic or Aluminum that has a top coating. More specifics of what each type of marking does is illustrated below:
Annealing – Heats the surface material to create contrast. In this method, there is no material removed but heating changes the chemical composition of the metal at the place where the laser beam is pointed which results in variation in color and thus creates the contrast needed for a mark. This works very well on metals (e.g. Steel for auto parts, Titanium for Medical Devices etc.). It is generally recommended to use Annealing as a mark type on metals as it is fast and efficient.
Ablation – Removes the coated top material on a surface that is of a different color than the base material thus creating the contrast needed for the mark. Typically used for Aluminum that has been anodized or plastic that has been painted.
Engraving – Dents the surface and removes material to create the mark. Not typically used for marking codes.
Speed, Pulse Frequency and z-focus Trade-Off Once the type of marking is decided, the last step in producing a good mark will be the settings of the laser marker – particularly the Speed, Pulse – Frequency and z-focus distance. Let’s consider the individual effects of each of the above attributes individually with mark quality before considering the combination.
Speed (usually mm/s)– the speed at which the laser is moved over the surface has a direct correlation with mark quality (all else being constant). The slower the speed, the more time the laser spends on a given portion of the material and hence better the mark.
The example to the left shows an annealed mark at varying speeds when other settings (pulse frequency and z-focal point) are kept constant. You can see that at higher speed, the mark starts to deteriorate as the laser just does not have enough time to make a complete mark.
The type of marking has an implication on the speed. Ablation, in general, can be done at a faster speed than annealing or engraving (again with all other settings being constant). This makes sense intuitively since a thin top layer is removed in Ablation while Annealing and Engraving make changes to the actual substrate material, so the light beam will need more time to create the mark and hence will operate at slower speeds.
Pulse Frequency (usually kHz) – Laser light is not a continuous beam but rather pulses of light. Pulse Frequency (or Pulse Repetition Rate) is inversely related to energy or power output of the light beam. Low frequencies produce high power and usually better (deeper) marks. High frequencies produce lower power and shallower marks.
The trade-off curves between speed, frequency, and type of marking are illustrated for an example below. Please note that this is for a given material for a given laser (Trumpf TruMark 5000) at constant z-focus and constant pulse-width.
Increasing Speed on an Annealed mark z-focus distance (usually mm) – The z-focus distance is used to fine tune the z height from the laser source to the material (or substrate). The more in-focus the beam is, the more concentrated the application and thus better the mark. Changing the z-distance and getting the beam out-of-focus can still produce marks but will only work at low speed and low frequency (i.e. high power) settings.
Based on the above, you will see that there are many possible factors with laser settings that will work for a mark and so the choice of the right combination of laser type, material type, mark type and laser setting (frequency, speed etc.) is often a trial and error. Having a verifier that grades to AIM DPM is often a good way to ensure that the combination of variables that are being chosen results in a mark that is readable down the line. The verifier will also highlight what parts of your code are scoring poorly, thus identifying what changes need to be made to your laser settings to correct and improve your barcode quality.