VARIABLES TO CONSIDER
Despite lasers being specified as having a specific output power, the reality is that many lasers are not actually delivering the exact power advertised. This variation is not normally extreme, but some lasers vary as much as 5-10% from what their manufacturers describe. In most cases this is easily adjusted for by increasing or decreasing the power of the laser during the marking process. However, for our customers with lower power lasers, this can be a potential problem if their laser cannot deliver the minimum power needed to create a mark. For more information on required power for marking, please see our section on The process of establishing the correct laser settings to make TherMark laser marking materials successfully bond can be time consuming because of the number of variables involved. To expedite this process, it is possible to quickly determine the optimum settings using what is known as a “power grid”. In the laser bonding process, the two most important variables are the laser power and the speed with which the beam passes over the material.
A power grid is a graphic file constructed to apply different speeds and powers to different parts of the image; this allows the user to test multiple settings with a single pass of the laser, greatly reducing the time taken to find the best setting. In the picture below, each square in the grid has a different combination of power and speed settings as illustrated in the axes. The Y axis decreases power in increments of 10% from top to bottom (100% power at the top, 30% power at the bottom), while the X axis increases speed in increments of 10% from left to right (10% speed at the left, 100% speed at the right).
As you can see, optimal marking will only take place when the right amount of energy is transferred by the laser. Section (1) shows that when power is too low and speed is too high there is not enough energy to create a bond.
Likewise, section (2) shows that too much power at a low speed causes the material to be removed, possibly causing damage to the substrate surface.
The ideal “process window” lies in section (3) of the photo. Any of these settings will produce excellent marks on the substrate. Picking a setting in the middle will allow maximum latitude for variability between machines and material. In other cases such as a production environment, the fastest speed possible to create a mark may be preferred.
In summary, when using a TherMark product for the first time on a new substrate, it is advisable to run a power grid on a scrap part in order to determine the optimal settings for your laser/substrate/material/coating combination.
The laser beam
The laser beam in laser marking systems is focused through a lens which focuses the beam on to the surface to be marked. The focal spot size of the laser beam directly relates to the energy density of the beam: the tighter the focus, the higher the energy density and vice versa. Different lenses will focus the same beam differently resulting in a large variation of spot sizes from laser to laser, and thus of energy density, for the same output powers. Moreover, the laser beam quality (M2 value in technical terms) will strongly affect its focusing properties, the focal spot size, and consequently the energy density at the focus. Hence, proper adjustment of laser settings will be needed for successful marking.
Variation in substrate composition is something to be aware of when marking metals with CerMark laser marking materials. If your parts to be marked do not have a consistent metallurgy or composition they may each react differently with our materials. When doing large production runs it is important to use parts that have the same composition in order to achieve consistent marks.
The optimum marking settings
The optimum marking settings will also depend on the thermal conductivity of the substrate being marked. The reason for this is that to successfully bond to the substrate the laser beam should heat the frit in the ink to certain high temperatures locally. When marking materials with high thermal conductivity, heat can dissipate due to thermal conduction. Therefore, higher settings may be required. With the same token, different materials have different heat capacities, which is the amount of energy required to increase the temperature by one degree. These two parameters combined will define the optimum settings required for each substrate. Usually metals require higher powers than plastics or glass materials.