Laser Light Transmission Through Thermoplastics as a Function of Thickness and Laser Incidence Angle: Experimental and Modeling

Abstract

It is important to accurately measure and predict the laser light transmission through unreinforced and reinforced thermoplastics if candidate materials are to be assessed for laser transmission welding (LTW) applications. This paper presents the results of laser transmission measurements through unreinforced polyamide 6 (PA6) and 10% glass fiber reinforced polycarbonate of various thicknesses and corresponding to various laser incidence angles (angle between the incident laser beam and the normal to the transparent part). A novel transmission measurement method, developed by the authors, was employed. A model, utilizing the Fresnel specular surface reflection conditions as well as accounting for refraction, absorption and reflection of the laser light through the bulk material, was used to predict transmission as a function of thickness and laser incidence angle. Results of transmission tests on both materials showed that, for a given thickness, the transmission decreases as the laser angle of incidence increases. In addition, at any given laser incidence angle, the transmission decreases as the thickness increases. The advantage of the model is that it requires only one experimentally determined constant for a given material. Good agreement existed between the experimentally measured transmission and the model prediction for the range of thicknesses and laser incidence angles studied.