Laser Weldable Materials

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

The table lists laser-weldable material combinations. In addition to these combinations, it is also possible to expand the spectrum with modified blends.

Optical properties

The optical properties of the plastic influences the welding result in laser welding. On the one hand, a transparent welding partner is required for laser welding.

Without additives, every thermoplastic is transparent to laser radiation. However, a distinction is made between amorphous and semi-crystalline thermoplastics. With amorphous thermoplastics, the radiation transmits almost perfectly, even with thicker materials. With semi-crystalline thermoplastics, on the other hand, the radiation is refracted and reflected at the crystallites. This leads to the radiation scattering which mainly depends on the degree of crystallites and the thickness of the material to be radiated through.

The following figure shows the spectral analysis of transparent polypropylene (PP). The wavelength range between 800-1100 nm the plastic is even more transparent than in the visible range (400 - 700 nm).

Optical penetration depth

The optical penetration depth is a measurement of the properties in the absorbing joining partner. It shows how deep the radiation penetrates the surface of the plastic before heat is generated.

Ideally, the optical penetration depth is in the µm range, see the graph above. If there is not enough absorption, volume absorption is more likely to occur. This heats the entire thickness of the material, see middle case. The third case describes a surface reflection that is too large . In this instance, the radiation cannot penetrate the surface at all. Therefore, the last two cases are rather negative for the process.

The heat generated during welding creates a heat-affected zone which can be seen under a microscope by microtome sections or microsections.

The design of the weld seam can be handled very simply. To put it bluntly, the components in the welding zone need to have physical contact. But it is not quite as simple as that: The components should be designed for laser use. Guidelines for this can be requested from us or directly via the German Association for Welding and Allied Processes e. V. (DVS) can be requested. Ask for DVS Guideline 2243 on laser welding thermoplastics.