Laser transmission welding is a well-known technique for joining in the industrial environment, e.g. automotive sector. The principle is that the upper welding partner has a high transmittance for the laser wavelength, while the lower partner has to be absorbent. The laser energy is transformed into heat in the absorbing part and during heat conduction the transparent part is also heated. The materials are melted in the intersection and after cooling down a strong weld is formed.

Often, the absorbing part is filled with carbon black and the transparent part is the natural thermoplastic. In addition, there are additives to change the color of the upper partner without affecting the transmissivity or to increase the absorption of a colored lower partner.

In this paper, an approach is shown to weld automotive hidden components, such as clips and pins, to larger elements, e.g. for repairs or maximum color freedom for design, without the need of absorbance for the lower partner. Therefore, the upper partner has been further developed. Now the laser energy is converted into heat inside the upper partner, near to the contact point to the lower partner. The advanced upper partners were welded to colored and transparent parts in overlap configuration and lap shear tests were performed with the specimen. The results were compared with conventional welds between black and natural thermoplastic parts. A maximum weld seam strength of over 24 MPa was achieved in both the conventional and the newly developed laser transmission welding process.