Welding processes like hot plate welding enable the energy which is necessary for the plastification of the material to be inserted directly in the welding zone. In contrast the laser beam has to penetrate through the plastics in order to reach the welding interface. Thus, specific material properties play a decisive role in laser transmission welding. Especially the optical properties have a large influence on the welding process. Since they depend both on the plastic, additives and production conditions, attention has to be paid for the selection of a suitable material. Moreover, this welding process poses certain requirements on the design of the welding parts, which have to be taken into account.