Optical properties of plastics and their role for the modelling of the laser transmission welding process

Optical properties of plastics and their role for the modelling of the laser transmission welding process

Abstract

Laser transmission welding of plastics is a joining technique which permits the welding of plastic parts with low process inherent thermal and mechanical stresses. In order to compute the temperature fields during the welding process a simulation model of the process is established. Especially the optical and thermal properties of the welded materials are necessary as input parameters in order to model the process properly. While the thermal properties of polymers are well-investigated, there is little knowledge about the optical properties of plastics. The optical properties of different amorphous and semi-crys-talline thermoplastics are therefore investigated in dependence on material temperature and additive concen-tration. Based on these data it is possible to analytically describe the absorption of laser energy in plastics. The computation of temperature fields during welding is carried out for polypropylene and the results are then compared with experimentally accomplished welding tests. The calculated results show good accordance with the experi-mentally determined melt pool geometries of welded specimens and prove the compatibility of the computation model with the experimental analysis.

Introduction

There are a number of established welding processes for joining plastics, such as vibration welding, ultrasonic welding or hot plate welding [1–3]. Another process has increasingly been gaining in importance over the last years: laser transmission welding. One apparent advantage of this process is that the transmission of energy can be achieved without contact by means of laser radiation. It is advisable to use this process, when there must not be any welding debris, the optical requirements towards the weld seam are very high or if three-dimensional weld seams cannot be avoided. The spectrum for application of laser plastic welding ranges from thin-walled and filigree components, like foils or membranes, for instance, to large thick-walled components from thermoplastics or thermoplastic elasto-mers. One major application area is joining of housings in all sizes for temperature or vibration sensitive parts, such as electronic components. The mechanical and thermal load on the components inside the housing is very low during laser welding. At the same time, the quality of the weld seam is very good and it resists high temperatures and aggressive ambient media. Laser plastic welding is there-fore a frequently applied process in this area [4].

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