From the application lab: How far out of focus is readability?

In the successor model you will find some considerations and a series of images for marking moving parts. When marking moving parts - also known as "marking on the fly" - in addition to the optical effect of defocusing, effects from the synchronization between material movement and beam deflection also come into play. These effects will be dealt with in a separate article. Related to this topic are the articles: Product marking in motion - How does it work? and the video tutorial: "Tips and tricks for shaft encoders". We will limit ourselves to optical imaging in the following.

If products of different heights are manufactured on a line or the marking station is mobile and used on different lines, problems can arise when focusing the laser beam.

In any case, it is common practice to adjust and verify the working distance. And that is a good thing. However, you can "leave the church in the village" during the process. Because with a clever choice of flat field lens, adjusting the focus position is not rocket science - and in some cases not even necessary.

The example shown here involves PVC profiles of different geometries. As soft PVC is sometimes used, the shape accuracy at the marking position is not always constant.

In our application laboratory, the first step was therefore to determine the maximum marking speed with the laser of the desired power class (in this case a 10 W_{CO2 Laser}).

The result can be seen in Figure 1 - the maximum belt speed of the extruder could be 14 m/min with a 100 mm focal length. There is virtually no difference in marking quality between the setting in focus and 5 mm outside the focus (Fig. 2).

If, at the maximum speed of 14 m/min, a full 10 percent of the focal length, i.e. 10 mm, is moved out of focus, the marking quality is reduced and the process window is exited. (Fig. 3)

In this case, the "distance performance" is no longer sufficient to reliably mark all characters.

However, as the customer's actual process speed is significantly lower and even 5 m/min still offers some reserve, the same test sequence was created again at 5 m/min. This much can be said in advance: the process became significantly more tolerant of defocusing.

In this case, the focus was off by 15 mm. As the distance performance is still sufficient, the marking result is very good - with a deviation from the focus position of 15 percent of the focal length.

In fact, the threshold of the process window is now just under 20 mm away from the focus with the same power, the same focal length and a speed reduced to 5 m/min. It has therefore almost doubled.

Finally, the focal length was changed to 200 mm while retaining the 10 W and 5 m/min.

The line width of the marking in the focus increased slightly and even at 50 mm deviation from the focus position still produced an acceptable marking result (Figs. 6 and 7).

What constitutes acceptable marking quality is therefore a matter of subjective judgment, as long as machine-readable marking is not required. The test results should also not be taken to mean that focusing in the laser process is irrelevant. It is important to take the necessary care. Marking processes with moving products are more demanding in this respect than markings at a standstill.

What is striking, however, is the wide range of adjustment that can be achieved in this application simply by selecting the focal length and adjusting the marking speed.

A wide selection of flat field lenses makes it possible to compensate for deviations due to positioning errors or different product heights within the limits shown without the need for further technological or financial expenditure.

However, the extent to which this is possible needs to be clarified on a case-by-case basis. This is because the material, wavelength, marking speed and, last but not least, the customer's expectations also have a significant influence here.

If you have a corresponding application, we will be happy to verify with you how the process can be optimally designed for cost efficiency and quality. Talk to our experts.