Infrared camera control of laser welding of plastic
Laser welding of polymers
Laser welding is an advanced method for joining of polymer materials with many advantages. Transmission welding is the most frequently used configuration for laser welding of polymers, where two parts are clamped together and the welding process occurs in between. The upper part material has to be optically transparent for the wavelength of the laser source. Laser beam is positioned along the defined trajectory; it is goes through the upper part and is absorbed on the surface of the bottom part. The upper part is pressed to the bottom part and thus the heat is transferred to the upper parts from the interface between them. Therefore both parts are melted at their interface and the weld seam can originate along the laser beam trajectory after the solidification and switching off the clamping force.
Results of the process are weld seams with high strength and quality. It is possible to weld even polymers reinforced by glass fibers. The laser process is fast, precise and without visible traces on the component surface. During several seconds a water-tight weld seam can be achieved without needs of further processing.
At the New Technology Research Centre (NTC) of the University of West Bohemia in Pilsen a new infrared camera on-line process control of the transmission laser welding of polymers has been developed. It concerns new algorithms and development of the software for the technology control. The used hardware components are commonly available.
Infrared camera system measures the temperature field of the upper surface of the polymer weldment during the course of the laser welding process. Information contained in the temperature field can be utilized to check, whether the required weld seam has been produced and the surrounding material has not been disturbed and even to control the welding process itself.
The advantage of the infrared camera inspection is its noncontact nondestructive character and especially the ability to perform on-line inspection of all produced parts. The infrared diagnostics is integrated with the welding process. It does not prolong the production time. No additional area of the production line is required. Investment and operating costs are minimized. Immediately after the welding process termination it is possible to automatically evaluate the weld quality, whether in some locations the weld seam is absent because of insufficient heat or clamping or whether the part is not damaged by excessive heating.
The continual measurement of the temperature field can be utilized not only for the inspection of welds but in order to control the whole process. The ability to use temperature information from locations where the laser beam does not directly heat is the advantage. Concerning the spatial definition the welding technology is divided into corresponding laser and infrared sections. It can be utilized to switch off the welding on the relevant section, when the temperature reaches the required level, or it can be utilized to control the power of the laser source or laser beam processing speed in order to the temperature follows the required curve.
Application in industry
The first industrial application of the technology is introduced for welding of plastic covers of central automobile locks at Kiekert. The technology was integrated in the production line, it has been approved by Volkswagen Group and it has been released into serial production of plastic parts. Currently the technology is installed in production in the Czech Republic and in Mexico.