OPTICAL MEASUREMENT OF TEMPERATURE OF EXTRUDED ALUMINIUM THIN WALL TUBES

摘要

Temperature control is of great importance in the aluminium extrusion, in order to obtain both, high productivity and high quality. Extrusion velocity and metallurgical characteristics of the products are in fact strongly related to temperature, which must remain within a narrow control band. Non-contact measurement must be performed, but the accurate optical measurement of the aluminium temperature is a difficult task because of aluminium physical properties and extrusion process peculiarities. The aluminium is not a grey body, with emissivity varying with temperature and wavelength. In addition emissivity value is low and the extrusion temperature is not very high: the measured signal to noise ratio is therefore low. The shape difference among various extruded profiles makes the collection efficiency to change very much; emissivity changes with the alloy and the surface properties. For all those reason frequent calibrations should be made, in contrast with productivity needs. Multi wavelength instruments have been developed, which evaluate the temperature from the shape of the spectrum and not from the radiation intensity, makes it not of importance the change in collection efficiency. More complex instruments make real time calculations of the emissivity and can compensate also for the non gray body behavior of aluminum. These instruments are usually quite slow, and are suitable for the conventional direct extrusion of profiles of sizable cross section. In that case extrusion ratio is low, and therefore speed is not too high, in the range of tens of meters per minute, and fast signal are not expected.