Extrusion International 1-2017

29 Extrusion International 1/2017 materials and no coupling medium is needed. The tech- nology is independent of the temperature of the mate- rial, what makes it possible to integrate an X-ray mea- suring device directly into an extrusion line without any additional efforts. A calibration is not necessary. The system is either installed directly after the extruder (hot measurement) or at the end of the line (final quality con- trol). With a 4-point online measurement, the measuring values for the wall thickness, the eccentricity, the inner and outer diameter and the ovality are determined by one device. The system measures up to three different material layers. These measuring values are visualized numerically and graphically in the form of the tube/pipe cross section in real-time and enable the user to perfectly center the extrusion tool. Important for highest efficiency is the automatic control of the line speed or extruder rpm while considering the minimal values. Thus, the quality of the hose is ensured. On the other hand, the control to the minimal values ensures that only the needed material is used. X-ray technology is available for products with a diameter from 0.65 to 270 mm. Concerns on the safety of X-ray devices are arbitrary, as the radiation is because of the low energy of no relevance. Practically, a human is exposed to a much higher radiation on a flight from New York to Frankfurt. Technologies for the measurement of large plastic pipes For the dimension measurement of large plastic pipes starting at a diameter from 110 mm, as they are found in the building and service area, the above-described tech- nologies can be used. Nevertheless, these technologies reach their limits either functionally (ultrasound) as well as regarding the costs, the limited measuring range and number of measuring points on the circumference (X-ray) or the limitation in the measurement of the diameter only (laser). A further technology for the quality control is cur- rently tested. It uses terahertz pulses, which activate a powerful fiber laser that is aimed at the material. The wall thickness is determined by means of the reflected echoes from the inner and outer boundary layers. The usage of this technology for the measurement of larger wall thick- nesses and materials with a high damping, as for example PVC, are however limited. Furthermore, the durability of the laser is limited and the costs are very high. Millimeter waves technology An innovative, significantly less expensive technology for the dimension measurement and recording of the sag- ging is the radar technology FMCW (Frequency Modu- lated Continuous Waves). These systems work within the sub-terahertz range and are already used for some time in the automotive technology for distance measure- ment. They are based on semiconductor technology, are inexpensive and practically not limited regarding their lifespan. Within the chosen area from 80 to 300 GHz all plastic materials are penetrated with low absorption and thus, the wall thickness is measured. One or two continu- ously rotating transceiver continuously send and receive frequency modulated millimeter waves while moving around the tube. As an alternative, a static system mea- sures selectively the wall thickness and outer and inner di- ameter of a tube with two transceiver at 4 points. A rotat- ing gauge head is used when the complete measurement of the wall thickness around the whole circumference of the tube is required. In this version, also the sagging is measured and displayed precisely. The measurement uses the time difference of the signals that are reflected by the boundary layers of the front and back site of the plastic material. The measurement is realized with an accuracy of few micrometers and a measuring rate of 500 single measurements per second. The millimeter waves tech- nology measures products with a diameter from 110 to 3,000 mm precisely, around the complete circumference, with no need for coupling medium and is not influenced by the temperature or plastic material. Furthermore, the measuring system adapts the properties of the extruded plastics by itself – a calibration by the user is redundant. Furthermore, the technology provides information for centering the extrusion tool and thermal control of the line. Thereby, the measuring values are used to ensure an optimal concentricity and minimal wall thickness. Summary With the increasing quality requirements at the produc- tion of hoses and tubes, the precise and reliable quality control for plastic pipes at the extrusion by a Non-De- structive Testing (NDT) becomes significantly important. Furthermore, an efficient usage of materials for costs sav- ings is in focus of the plant management. Measuring and control systems monitor and control important product parameters continuously. Thereby, hose and tube manu- facturers may choose from various technologies with dif- ferent functions and diverse applications. The laser technology offers a reliable online measure- ment of the diameter from 0.05 to 500 mm. Additionally, X-ray measuring systems measure the wall thickness and eccentricity of products with a diameter up to 270 mm. A further innovative technology, based on millimeter waves, is used for extrusion lines where large plastic pipes up to 3,000 mm are produced. The technology is appli- cable for different material types and measures common tube dimensions as well as the sagging precisely. Which measuring technology should be used in an extrusion line depends, therefore, on the application area and the re- quirements of the user regarding measuring and control technology for quality assurance, process optimization and cost savings. www.sikora.net