Most industrial laser marking applications are small and precise. But every now and then, an application comes along requiring a large area mark – with fill - and that generally means long process times, which no manufacturer likes to hear. Fortunately, there are several tricks of the trade that can greatly reduce process time, and, in some cases, the optimized process time can be significantly improved.
In this application note, we'll consider a large area mark of the Miyachi Unitek logo on stainless steel and will compare process times based on tool selection, overlap, and path. We will also briefly discuss the limitations associated with each of these choices.
Selecting the right tool
Not unlike painting, to cover a large area (a wall, for example), you want to select the broadest brush possible to maximize coverage and decrease the time needed to complete the job. For laser marking, the "brush” is the laser spot size, determined by the optics (lens focal length) and laser parameters (input beam diameter on the lens). The figure below shows the different spot sizes for a given input diameter and focal length; the table shows calculated focal spot sizes for a given input beam diameter (7mm):
Let’s consider using each of the spot diameters to fill a square as shown below. Multiple parallel lines are used to create the fill and the spacing between adjacent lines is equivalent to the diameter of the spot size, i.e., the edges of the spots just touch.
So, a marking speed of 300mm/s results in the following mark times for the Miyachi Unitek logo. You'll see that correct lens selection can reduce the process time by a factor of 4!
Selecting the right overlap
The overlap between adjacent rows may also negatively impact process time. Fill spacing can range from 0-99% overlap. Typically, a solid fill will have at least a 25% overlap. The figure below shows 0% overlap (left) and 50% overlap (right):
Let's compare three different overlaps. The mark time is just about doubled by increasing the overlap from 0% overlap (0.1 mm) to 50% overlap (0.05 mm spacing). The optimal fill in this case was found to be 25%.
Selecting the right path
How the object is filled also affects total process time; the laser beam must be steered around the object and it does take time to jump from one line to the next.
In the simple case above, filling the square is rather straightforward, and using parallel lines (also parallel to the square edge) is the easiest and fastest way to get it done. In the first order approximation, however, we neglected to account for the jump times in the calculation of the total process time.
Limiting jumps naturally decreases process time. In the case of the square, the best marking time will occur with a bidirectional fill since the jump paths (dashed lines in the figure) will be shorter.
This difference is magnified for more complicated text. The process time to fill the Miyachi Unitek logo with unidirectional and bidirectional fill is shown below. The bidirectional fill is 15% faster than the unidirectional fill.
For more complex geometries, such as text, the fill orientation that will result in the fastest mark may not be obvious. In these cases, the mark time versus orientation must be tested. Consider these three orientations – 0°, 45°, and 90°. For the Miyachi Unitek logo example, there was only a slight decrease in time for the different angles:
Large spots sizes do have some limitations. For one, the large diameter of the laser beam prevents production of smaller marks. Therefore, it's important to select a configuration that will provide adequate resolution for the desired mark.
Consider also the type of mark that is to be made. The large beam diameter equates to lower energy density, which in turn will limit the processes that are possible. You may not be able to etch steel, for example (depending on energy level). For our logo mark, the 254mm lens was the longest that could be used to get the desired mark.
In summary, process time can be minimized by balancing the spot size, overlap, and fill path with the limitations set by the material and type of mark.
In our example, a 20 W laser marker was used to mark a stainless steel plate with the Miyachi Unitek logo. The total mark size is 1” x 3.4” (25.4 mm x 86.4 mm). The optimal configuration was found to be 254mm f-theta lens, 25% overlap, and a vertical (90°), bidirectional fill.
The optimal process time was found to be 31.3 seconds.