What are the requirements for using laser welding technology for thermoplastic materials?

Thermoplastics are plastics that can be melted. They can therefore be welded together using the laser. Below, two thermoplastics and the principle of weldability of the two materials are explained in detail.

Thermoplastics can be divided into amorphous and semi-crystalline thermoplastics. Amorphous thermoplastics are transparent because they have no visible additives. Semi-crystalline thermoplastics, on the other hand, appear opaque or milky to the naked eye. In principle, identical thermoplastics can be welded together using the laser. However, the optical properties of the thermoplastics must be taken into account.

The table lists the material combinations that can be laser welded. In addition to these combinations, modified mixtures can also be used to expand the spectrum.

Optical properties

The optical properties of plastics influence the welding results of laser welding. On the one hand, laser welding requires a transparent welding partner. Without additives, every thermoplastic is transparent to laser radiation. But there are amorphous and semi-crystalline thermoplastics. With amorphous thermoplastics, the radiation is transmitted almost perfectly even through thicker materials. With semi-crystalline thermoplastics, the radiation is refracted and reflected at the crystallites. This results in radiation scattering, which depends mainly on the degree of crystallization and the thickness of the material through which the radiation passes.

The figure below shows a spectrum analysis of transparent polypropylene (PP). Plastics in the wavelength range between 800-1100nm are even more transparent in the visible range (400-700nm).

Optical penetration depth

Optical penetration depth is a measure of the properties of an absorptive addition. It shows how far the radiation penetrates the plastic surface before heat is generated. Ideally, the optical penetration depth is in the 渭m range, see the figure above. If the absorption is insufficient, volume absorption is more likely to occur. This heats up the entire thickness of the material, see middle case. The third case describes a surface that is too reflective. In this case, the radiation cannot penetrate the surface at all. The last two cases are therefore quite unfavorable for the process.

The heat generated during welding forms a heat-affected zone, which can be seen under a microscope by microscopic sections or microsections. The design of the weld can be handled very simply. To put it bluntly, the components in the weld zone need to be in physical contact. But it is not that simple either. The components should be designed for laser use.