Laser marking is a great way to produce dark marks on stainless steels; by precisely controlling the laser and heat input, you can get an oxide layer to build up on the part, which, in turn, produces the characteristic appearance without melting the material.
For parts with good thermal sinking, this is quite straightforward. As the part reduces in size and volume, however, it can be more challenging. Take, for example, the stainless steel tubing used in the medical industry. Because creation of a dark mark requires the part to be at the right temperature for a specified amount of time, you run the risk of overheating under thermal runaway, leading to distortion or even collapse. The implementation of a suitable "banding" strategy solves this problem with single or multiple passes completed at high speeds with tuned laser parameters.
Here are three different banding techniques that can be used on tubing:
- Mark and move – mark a rectangular segment and then rotate the tube, repeating until the circumference has completed one full turn.
- Spiral marking – rotate the tube while moving in the parallel axis, so that the laser path is a spiral that has just a small amount of overlap, ensuring mark continuity.
- Marking around the circumference – Raster the laser across the entire width of the band while continuously moving the tubing.
Again, any of these methods may use multiple laser passes to build up the mark. Multiple pass processing is especially recommended for fixed length text or logos/graphics. If the physical length of the mark changes, however, the time gap for the laser to return to the same part of the mark varies and may not produce the required effect. This issue can be alleviated by employing double or multiple pass cross hatching on every letter before the next one is marked.
In the following example, which features banding of thin-walled tubes, the spiral marking method (#2) was used. The combination of tube rotation and linear axis speed enabled the necessary control using just a single pass. See video of rotary laser marking here.
Banding on 0.05” diameter tubing, wall thickness 0.002”
with fiber laser marker