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7 October 2005 Optical monitoring of thin oil film thickness in extrusion processes
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We have used reflectance spectroscopy for the in-situ, non-invasive monitoring of a thin oil film thickness during extrusion process of ceramic paste in capillary rheometer. Investigated pastes are disperse solid liquid systems prepared from the silicone oil AK106 (Wacker) and ceramic powder AlOOH. The thin oil film, extracted from the extruded paste, appears on walls of the rheometer die. A borosilicate view-port-glass provides optical access to the thin film inside the die. Reflectance spectroscopy enables the thin film thickness measurements by wideband spectral analysis of light back reflected from the sample. This spectrum includes extremes, which results from interference between beams reflected from glass-oil boundary and oil-paste boundary. Position and intensity of this extremes were determined by thickness of the thin film as well as refractive indices of the oil and the paste. Optoelectronic system dedicated for process monitoring by means of reflectance spectroscopy had been designed and built. The system comprises tungsten halogen lamp and fiber optic spectrometer. Optical signals are transmitted through bifurcated fibers, focusing optics and the view-port-window. Spectroscopic monitoring was carried out in VIS-NIR range from 400 to 900 nm as a function of extrusion velocity (0.01-5mm/s) and paste particle granulation (5-20 μm). Computer calculation, performed using dedicated software, enables fast determination of thickness even for reflectance spectra interfered by high noise level. Fast development of ceramic components technology requires detailed description of complex rheometric processes. Monitoring of the most important process parameter - oil layer thickness - enables pre-determination of rheometric factors required for proper paste extrusion and accurate shape filling.
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Robert Bogdanowicz, Piotr Wroczyński, Jan Graczyk, and Marcin Gnyba "Optical monitoring of thin oil film thickness in extrusion processes", Proc. SPIE 5948, Photonics Applications in Industry and Research IV, 59482G (7 October 2005);

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