Laser transmission welding of thermoplastics

The goal of this thesis project is to develop a design tool for laser transmission welding of thermoplastics. The majority of previous work in laser transmission welding of thermoplastics relied on a trial and error approach to developing operating parameters, primarily due to current models not accounting for complex issues relating to the material properties. The research in this thesis, focusing primarily on poly-vinyl chloride, is divided into three main categories: devising and utilizing methods to measure optical material properties, developing a two-dimension numerical model of the welding process, and experimental validation of the model.; Optical properties were measured primarily using a custom black box. Methods were developed to measure both the light transmitted through a thermoplastic sample and the light reflected from the surface simultaneously. Additionally measuring the light transmission at elevated temperatures revealed that the absorption coefficient greatly increases through the melting range.; The two-dimensional numerical model of laser transmission welding was developed from first principles of heat transfer using the energy balance method. The model utilizes the measured optical properties in determining the absorption distribution of a high-power laser source. This model also incorporates many advanced features such as material properties that change as a function of temperature and the thermal contact conduction between the two parts changing with temperature and pressure. The primary outputs of the model are the temperature and pressure distributions within the two joined parts as at each timestep; this information is compared with work hot-pin welding samples, to predict the weld quality.; A diode laser welding system was used to experimentally validate the model. Comparing the weld width predicted by the model and the measured weld width resulted in an average error of 5.6%, indicating that the model is quite accurate. An exploration of the role of pressure in the weld zone indicated the weld quality improves with increasing clamping pressure up to 2.5 MPa, with inconclusive results above this level. Finally, the model predicted the conditions necessary to form welds across gaps of 12.7 mum and 25.4 mum.