TPT January 2020

AR T I C L E

Sikora AG

Figure 4: The system based on X-ray technology measures precisely the inner and outer diameter, wall thickness, ovality and concentricity of hoses and tubes

Figure 2: X-ray image of a hand, and intensity profile of a single layer tube in comparison

Depending on material and thickness of the product, a part of the X-rays is absorbed by the material and the other part is transmitted. Intensity profiles are visible on the camera (figure 2), which show the attenuation of X-rays by the material. This attenuation is occurring exponentially as it traverses material, thus, the intensity captured by the camera also decreases exponentially. The attenuation coefficient is dependent 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. By using a multi- layer model based on the underlying physics, the exact position of the transitioning from one material to another can be determined. Hence, with this approach and a geometric model of the product to be measured, the diameter as well as the wall thicknesses can be determined by optimising 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 duration, etc) – but the camera image as a whole is being analysed. Hoses or tubes are transilluminated entirely by X-rays due to the absence of a metallic core – contrary to measurements in the cable sector – which would absorb the X-rays. The key advantage is that, regarding their parameters, all imaging elements in a Sikora X-ray measuring system are determined precisely and in context – for example, the positions. For this reason, the measuring points stand in relation to each other, which allows for statements about the whole 360° of the measured product. For instance, the minimal wall thickness can be determined precisely and freely over the full circumference (figure 3).

This also allows for conclusions about the positioning of a twisted product at the extruder. The direction to adjust 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 allocation of the eccentricity to the adjusting screws is required in order to make the appropriate adjustments. This is only possible when a 360° model is available. This is also the reason why well- known plant manufacturers for self-centring crossheads refer to X-ray measuring technology from Sikora. Conclusion In summary, the application of X-ray measuring devices has many advantages for the production of hoses and tubes that other measuring methods cannot offer. The independency of coupling media, the object temperature and the insensitivity to optical interferences of the measuring environment by steam or smoke is a unique selling point for this technology. Another point is the instant and constant generation of measuring values that are not based on secondary measurements. In addition, recalibration is not required as there are no movable parts. By using a physical model for the absorption and the knowledge of precise and individual 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° of the circumference. All these points do not only ensure the highest product quality, but also the optimisation of product processes and, therefore, a significant increase in efficiency.

Figure 3: Determination of the minimum wall thickness: the wall thickness under 45° from the outside (red dotted line) is larger than the actual minimum wall thickness (yellow solid line)

Sikora AG Bruchweide 2, 28307 Bremen, Germany

Tel: +49 421 48900 0 Fax: +49 421 48900 90 Email: sales@sikora.net Website: www.sikora.net

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JANUARY 2020