This paper presents a new monitoring principle for laser transmission welding of plastics. The principle uses two
independent detectors: a CCD or CMOS camera to supply an image of the weld seam and a pyrometer to detect heat
radiation from the welding process. In laser transmission welding, the weld seam is usually hidden by the laser
transparent joining partner, which may still be colored in the visible spectrum. Therefore, a special lighting laser will be
used to acquire an image of the weld for CCD or CMOS cameras. If the upper joining partner is not only partly
transparent for the laser wavelength of the welding laser, but also for the laser wavelength of the lighting laser, a direct
visualization of the weld is possible. Factors influencing the weld quality or leading to defects within the weld are kerfs
on the surfaces of the joining partners, moisture uptake of the plastic material and the surface finish of the materials.
Which weld defects result from these factors and how they can be detected by the monitoring system are shown. Not all
of the resulting defects can be detected with a single detector. Therefore, the information from the two detectors,
camera and pyrometer, is correlated.
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