There are two methods used for laser marking on curved surfaces, such as the example shown. The first is to use a rotary indexer that rotates the part keeping the marking position at the focal point all the time. Commercial issue with this is that additional tooling is required and the process does require additional time which adds cost. The second method is to use the single position guidelines which, as a rule means that you can maintain the mark integrity if you contain the mark within a 20 – 25 degree segment on the circumference. This principle works for all diameters. The focal position is then set for the midway point between the upper mark position and the lower mark position. 20 degrees tends to be for the harder more reflective materials and 25 degrees for softer less reflective materials. This focal positioning reduces any potential image distortion caused by the curved surface, while also averaging out any spot change which effects power density across the mark area.
In the image shown, the part is located using the slot, while the body sits in a v-block jig. This ensures part mark alignment every time. The marking takes place in a single hit with no component movement during the process, resulting in the most economic per unit price.
The process for achieving the dark black mark, is referred to as annealing or CW marking. During the annealing process, no material is removed. A colour change occurs through heating up the material surface. Stainless steel for example, changes colour when it reaches temperatures between 200 and 300°C. During this range, the material surface can achieve different colours, though, the goal is usually a \ black\. These black markings are possible on stainless steel and Titanium, but not on other metals. Blue is a common colour on Titanium as well. Annealing is also possible on any alloys which contain iron.
When creating this type of marking, it is important to ensure that vaporisation temperatures are not reached at the surface defeating the objective. This can be done in a number of ways, including running the laser out of focus which changes the energy density characteristics and removes the peak energy. With fibre lasers and the higher pulsing capability, the annealing mark is readily available, due to the nature of the tuned wavelength and pulse rates.
To anneal metal, it is important to only heat it instead of vaporizing it. This can be achieved by reducing the energy density of the laser impact on the steel. The best way to do so, is to go out of focus to increase the laser spot size, as the same energy on a much bigger surface will reduce the energy density. Heating up metal takes time; therefore, slow marking speeds should be used.
If you would like further information on this laser marking application, or any other application, please request a call back or talk to one of our laser marking specialists on 01737 826902
