Extrusion International 5-2019

46 Extrusion International 5/2019 MEASURING TECHNOLOGY just the screws at the crosshead is fixed. When the product turns on its way to the measuring point, a rear projection of the wall thicknesses to the crosshead and, therefore, an al- location of the eccentricity to the ad- justing screws is required in order to make the appropriate adjustments. This is only possible when a 360 de- gree model is available. This is also the reason why well-known plant manufacturers for self-centering crossheads refer to X-ray measuring technology from SIKORA. Conclusion In summary, the application of X-ray measuring devices has many advan- tages for the production of hoses and tubes that other measuring methods cannot offer. The indepen- dency of coupling media, the object temperature and the insensitivity to optical interferences of the measur- ing environment by steam or smoke is a unique selling point for this tech- nology. Another point is the instant and constant generation of measur- ing values that are not based on sec- ondary measurements. In addition, a recalibration is not required as there are no movable parts. By using a physical model for the absorption and the knowledge of precise and in- dividual device geometry, statements about the entire circumference of the product can be made. This allows for a determination of the minimal wall thickness over 360 degrees of the circumference. All these points do not only ensure the highest prod- uct quality, but also the optimization of product processes and, therefore, a significant increase in efficiency. SIKORA AG Bruchweide 2, 28307 Bremen, Germany www.sikora.net K 2019: Hall 10, Booth H21 and the X-ray camera are facing each other from the opposite sides of the product (picture 1). Depending on material and thickness of the prod- uct, a part of the X-rays is absorbed by the material and the other part is transmitted. Intensity profiles are vis- ible on the camera (picture 2), which show the attenuation of X-rays by the material. This attenuation is occurring expo- nentially as it traverses material, thus, the intensity captured by the camera also decreases exponentially. The attenuation coefficient is depen- dent on the material and the density of the X-rays. Thereby, a “bend” in the intensity curve occurs at X-ray transitions from one material to the next one. By using a multi-layer mod- el based on the underlying physics, the exact position of the transition- ing from one material to another can be determined. Hence, with this approach and a geometric model of the product to be measured, the di- ameter as well as the wall thicknesses can be determined by optimizing the underlying model parameters. Not only individual information is being processed – as is the case for traditional measuring methods (such as level detection or impulse dura- tion, etc.) – but the camera image as a whole is being analyzed. Hoses or tubes are transilluminated entirely by X-rays due to the absence of a metal- lic core – contrary to measurements in the cable sector – which would ab- sorb the X-rays. The key advantage is that, regard- ing their parameters, all imaging ele- ments in a SIKORA X-ray measuring system are determined precisely and in context – for example, the posi- tions. For this reason, the measur- ing points stand in relation to each other, which allows for statements about the whole 360 degrees of the measured product. For instance, the minimal wall thickness can be deter- mined precisely and freely over the full circumference (picture 3). This also allows for conclusions about the positioning of a twisted product at the extruder. The direction to ad- significant advantage in comparison to other measuring methods. The stability of the measuring value and, therefore, the repeatability are significantly better compared to other methods and do not degener- ate over the lifetime because there are no moveable parts in use. There are no moving sensors, which have to be adapted to the product and, thus, changeover times do not oc- cur. In general, these characteristics sug- gest the application of X-ray mea- suring technology for online control of the extrusion of hoses and tubes. Not only the resolution of single lay- ers is possible, but also the presence of electric conductive additives in the materials (for example soot or metal- lic layers) or of ply do not represent an exclusion criterion. Functional principle The X-ray measuring devices from SIKORA work with X-ray transmis- sion, meaning that the X-ray source Picture 4: The system basedonX-ray technology measures precisely the inner andouter diameter, wall thickness, ovality and concentricity of hoses and tubes