Laser transmission welding is a method of joining plastics, which benefits from the infrared transparency in majority of thermoplastics. During the process, a laser beam passes through the laser transparent part and hits the laser absorbent part, which has been made absorbent using additives such as carbon black. The absorbed laser energy is then converted into heat and in turn welds the interface of the two parts by melting the polymer. In the current work, a new refraction technique of laser transmission welding is used to weld nylon plates to nylon tubes with carbon black. For the laser to refract, an angle was machined into the laser transparent nylon plates adjacent to the weld interface. Effect of different laser properties such as laser speed, number of laser rotations and laser power were studied on the quality of welding in terms of better finish and strength. The strength of these welds was assessed using a new tensile test fixture. Subsequently, the weld seam width was found from the tensile tested samples by analyzing the weld interface using vernier and transmission light microscopy. These tensile test results were then normalized using the weld seam area to obtain the tensile stress. The results showed the samples could withstand more tensile stress with an increase in laser power and rotation, excluding the samples which had decomposition due to excessive laser power. Also, the material property changes at the weld interface due to laser welding were characterized using a nanoindenter, for which small square samples were carved out of the weld interface and cold mounted. The results show that the modulus and hardness of nylon decreases right at the interface of the weld. In order to find the reason why there is a decline in the above mentioned mechanical properties, differential scanning calorimetry (DSC) testing was done to find possible changes in crystallinity, as decreasing modulus in semi-crystalline polymers is usually a result of decreasing crystallinity. The results confirmed that the crystallinity indeed decreased at the weld interface which would explain the decrease in mechanical properties.
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