During the manufacture of plastic tubes with large diameters and wall thicknesses the product quality as well as the reduction of material costs have highest priority. Norms and standards precisely define the minimum and maximum permissible diameter and wall thicknesses of a tube dimension. In addition, the determination of sagging plays an important role. Due to standards and growing demands in the tube extrusion, manufacturers use measuring and control devices for quality assurance in production lines.
 
In cooperation with the Fraunhofer Research Institute for High-frequency Physics and Radar Technology (FHR) and the South German Institute for Plastics (SKZ) the company SIKORA has developed a new technology on the basis of millimeter wave technology for non-contact, precise online measurement of inner and outer diameter, ovality, wall thicknesses, and sagging of large plastic tubes with a diameter from 120 mm. Thanks to the innovative concept of the measuring system it adapts the characteristics of the extruded plastics and does not require any calibration by the operator. The technology allows to increase product quality and ensures significant material and cost saving during extrusion.
 

Picture 1: System on the basis of millimeter wave technology for measuring the diameter, ovality, wall thicknesses and sagging of large tubes
 
Technologies for dimension measurement of plastic tubes during extrusion
 
Today, there are diverse technologies used for quality assurance at the production of plastic tubes, such as optical methods e.g. laser for determination of the diameter or X-ray for the additional measurement of concentricity and wall thicknesses. Conventional technologies such as ultrasonic also measure tube dimensions, however they often reach their functional limits.
 
An additional technology that is used for quality control works with terahertz impulse technique. For this method a powerful fiber laser generates terahertz impulses directed to the measuring object. From the reflected echoes at the inner and outer boundary layers, the wall thicknesses are determined.
 
Millimeter wave technology for the measurement of large tubes
 
A precise measurement of big tubes independent from environmental or material influences can be assured by the use of the new system on the basis of millimeter wave technology, introduced in this paper.
 
In the last years, enormous successes regarding measuring accuracy have been achieved by researching metrological applications with frequencies in the millimeter waves range. Nevertheless, the results could not yet be used for the coating thickness measurement of cylindrical products. The newly developed millimeter wave technology creates the prerequisite for reliable measurement of the nominal size as well as the outer diameter, ovality and wall thickness of all kinds of extruded tubes.
 
Without any knowledge of the properties of the extruded materials and its temperatures, the system measures the outer contour as well as the wall thicknesses simultaneously at several positions of the circumference. Also layer thicknesses of multi-layer tubes are measured precisely. Thus, the system represents a key technology for future-oriented quality assurance at the production of large tubes.
 
The measurement via millimeter wave technology is based on the FMCW runtime method. Several static or rotating transceiver, arranged around the circumference of a tube, continuously send and receive frequency modulated millimeter waves. From the runtime difference the product dimensions are defined.
 

Picture 2: Scheme: Measuring system with rotating (left) and static (right) sensors
 
Boundary layers, as for example each front and back site of a plastic, reflect these radio waves, which are detected and demodulated by the receive unit of the regarding transceiver. The receive signals contain information regarding the distance between boundary layers of different materials, that means the inner and outer diameter, ovality, wall thicknesses and sagging. After an algorithmic processing of the receive signals of each sensor, the requested measuring results are ready for visualization and control of the diverse tube dimensions in real time. A connected processor system provides in addition to a numeric display of the measuring values also their graphic representation as well as comprehensive trend and statistical information.
 

Picture 3: Video image: Evaluation of the receive signals and determination of the tube dimensions
 

Picture 4: Absorption coefficient for PVC depending on frequency: With increasing frequency the absorption of radio waves for PVC increases. The millimeter wave measuring method works in a low frequency range (80 GHz). Therefore, even large PVC wall thicknesses can be measured precisely.
 
Millimeter wave technology for optimization of tube quality as well as time and cost saving
 
As product temperatures have no influence on the measuring result when using millimeter wave technology, the system is installed for hot measurement as well as at the cold end of the line for final quality control. Directly after the first cooling, the CENTERWAVE 6000 provides precise information about inner and outer diameter, ovality, wall thickness and in particular sagging. The millimeter wave technology selected for the measurement covers the entire range of plastics such as PE, HDPE, PP, PA6 etc., as well as PVC (see also picture 4). If we assume that a line, where tubes are produced with an outer diameter of 400 mm and a wall thickness of 27.5 mm, at a line speed of 0.5 m/min, the machine operator receives accurate measuring results already after ca. 10 to 30 min.
 
In contrast, the measurement of plastic wall thicknesses with high temperatures via ultrasonic technology, represents a special challenge. As a result of the temperature dependence of ultrasonic a temperature compensation is required. But, the goal is to achieve as early as possible in the production process reliable and most precise information about tube dimensions in order to take actions if necessary and to avoid failure badges. Moreover, it is necessary to approach early in the production process the minimum permissible tube dimensions to produce a minimum meter weight. Cost savings resulting from the low meter weight are often decisive in competition. At the extrusion of plastic tubes material costs account for up to 90 per cent of the total manufacturing costs. Depending on throughput of the line, used material type and underlying norm there is between the minimum and maximum permissible tube dimensions an annual saving potential in the single-digit millions range. In addition, the production of standardized plastic tubes assures a flawless processing of the tubes. For example, quality tubes can be welded easily. Thus, the use of a millimeter wave measuring system leads to significant time and material savings as well as to a high qualitative end product.
 
Areas of application of millimeter wave technology
 
The millimeter wave technology is suitable for the measurement of any kinds of plastic tubes with a diameter from 120mm to 2,500mm that are for example used for conducting water, gas, chemicals and oil. In particular interesting is the application of tubes made of all common plastics such as PE, HDPE, PP, PA6, PVC etc. Here the system provides precise measuring values, even for tubes with wall thicknesses of 200mm.
 
Another area of application is the measurement of single and multi-layer tubes. During the production there is the risk that the melt that leaves the tube tool, flows down as a result of gravity, and thus negatively influences the tube wall thickness distribution. This so called sagging is identified by the millimeter wave measuring method. Via a display and control device the machine operator immediately receives information on the production process to take actions.
 
Conclusion and outlook
 
Quality demands at the manufacture of large plastic tubes are continuously increasing. Norms precisely define the dimensions of the products to be produced. The precise and reliable quality assurance of plastic tubes during extrusion is increasingly gaining importance. By the use of a new system on the basis of millimeter wave technology for hot measurement and at the cold end for final quality control the product parameters inner and outer diameter, ovality, wall thickness and sagging are continuously online monitored.
 
The method is applicable to different material types such as for example PVC. Curved product surfaces as well as wall thicknesses of multi-layer tubes are determined and precisely measured. Consequently, the introduced millimeter wave technology in combination with processor systems contributes to process optimization, increase of tube quality, minimization of the material consumption as well as time and cost saving.
 
 
 
1Frequency Modulated Continuous Waves
2http://www.kraussmaffeiberstorff.com/media/files/kmdownloadlocal/de/EXT_BR_Grossrohr_de.pdf, S. 7, 2014.
3Regel, K.: Sagging nicht erwünscht. K-Profi, Ausgabe 3-4, S. 14, 2014.