Most thermoplastics that can be welded – i.e. that have a melt phase and can thus also be used during laser welding – absorb only a small proportion of radiation from solid-state lasers in their usual wavelengths of between 800 and 1,100 nm. To successfully melt and weld these thermoplastics with laser radiation, their absorption has to be brought about by means of additives such as color pigments or soot. During plastic welding using the laser transmission welding method, two different types of thermoplastic are connected to one another: one component is the transparent plastic while the other is the absorbent one.
With this welding method the laser beam penetrates the upper, transparent plastic and reaches the absorbent one beneath it. The laser then melts the surface of the absorbent plastic, and because of the heat transfer the transparent component also melts. The advantage of this overlap welding is that the weld seam is created inside the structural component without any particles of any kind being released. With workpieces that are not transparent to the eye, this creates an esthetic and almost invisible seam. A great advantage of laser processing is the fact that heat is transferred locally – so weld seams could be created in direct proximity to electronic components, for example.
For the welding process, joining pressure is essential: a clamping tool presses the two weld components together, creating a pressure that is necessary for heat transfer between the two of them. Uniform clamping of the components that has been adapted to the shape of the welded plastics is a prerequisite for high processing quality and reproducible welding results.
A distinction is made between four different types of plastic welding:
- contour welding
- quasi-simultaneous welding with scanners
- simultaneous welding
- mask welding
During laser welding of plastics, laser outputs of up to approximately 300 watts are typically used.