In laser Transmission Welding (LTW), the laser beam passes through the transparent part and is dissipated as heat in the absorbent material through the use of laser-absorbing pigments such as carbon black (CB). This energy is then conducted further into both parts. Melting and subsequent solidification occur at the interface causing a weld to form between the two parts.

Gluing or welding structures to the back of automotive Class-A panels often results in the appearance of undesirable surface deformations on the Class-A side. Through control of the laser welding and material parameters, it may be possible to use contour LTW as a means of joining structures to the back of absorbent Class-A panels without creating these unwanted surface defects.

A series of lap welds was made using a range of CB levels, laser powers and polypropylene part thicknesses. A profilometer was used to measure the size and shape of the defects generated on the surface of the black part. Two types of defects were observed:​ ribs and sink marks. It was observed that lower powers combined with higher carbon black levels generally resulted in smaller defects. The type of defect depended on the boundary conditions between the two parts and the flow of polymer that had thermally expanded during welding (flash). If weld flash flowed into gaps between the two plates, rib defects were always observed. If flash flowed elsewhere and no gaps existed between the plates, sink marks occurred. Finite element modeling was used to qualitatively validate these observations.