Laser Radiation Impacts on Polypropylene Molded Pieces Depending on Their Surfaces

The increasing demands regarding the quality of the marking of polymer products are attributed to an increased need for durability of the graphic record, as well as a tendency to personalize products. A permanent marking on products is closely tied to the development of methods to monitor production lines, as well as to the development of the industry 4.0 concept. Laserbeschriftung Kunststoff is currently used on packaging, cables, medical devices, and other everyday-use products.

Environmental protection is another important argument for using this method. Laser marking of products reduces the amount of material used in traditional methods of decoration of product surface layers, such as printing, painting, tamponing (a specific type of paint), and in IML (in-mold labeling). In the sorting stage, this makes it easier to identify the polymers. According to Kremer and Hahn, laser marking of polymer parts has its advantages and disadvantages, including an increased susceptibility to recycling. With the interdisciplinary collaboration of laser manufacturers, producers of plastics and additives, and selected groups of plastics processors, substantial progress has been made in the development of laser marking technology.  A good example of such a cooperation is the Alaska Project, developed by RWTH Aachen University and Fraunhofer Institut.

An element is marked with a laser by discoloring its surface. Focused laser radiation causes changes in surface features in a small, local area of a sample. The internal stresses generated by this process can cause cracking and deformation of products, depending on the type of material. In composite materials, where it is often necessary to add auxiliary materials (additives) to increase laser sensitivity, the process of modification is more complex. Żenkiewicz et al. extensively described the goals and methods of using laser energy to modify the surface layer of polymers. In addition, Tan and et al. described the use of a laser to form polymer microelements through precise laser melting. A numerical analysis of thermal effects in the laser operation area supported these studies. The paper presented results of research on the use of a laser to support the modification of polylactide (PLA) surfaces with copper. However, there have only been a few studies exploring the relationship between tool surface quality and laser record quality.

A color change can be achieved both in the pigment and in the polymer. The interaction of atmospheric oxygen with molten material can produce oxides whose colors enhance the contrast and readability of graphic symbols. Laser marking, however, has a very limited range of colors and depends on the materials that are being used. Polymer processing types, such as co-extrusion or two-component injection molding, allow for the creation of additional color effects by removing a layer of one of the polymers. Control of the geometrical features of the obtained sign (size, shape, orientation, brightness, etc.) can be achieved by adjusting the focal field, wavelength, duration, and energy of laser pulses, as well as by altering the temperature of the marked objects. Gnatyuk et al. describe how laser radiation can be absorbed depending on its intensity.