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|NDT.net Issue - 2019-02 - NEWS |
|SIKORA develops key technology for quality assurance during the extrusion of big tubes
IntroductionDuring the manufacturing of plastic pipes with large diameters and wall thicknesses the product quality as well as the reduction of material costs have the highest priority.
Norms and standards precisely define the minimum and maximum permissible diameter and wall thicknesses of a pipe dimension. In addition, the determination of sagging plays an important role. Due to standards and growing demands in the pipe extrusion, manufacturers use measuring and control devices for quality assurance in production lines.
SIKORA, in cooperation with the Fraunhofer Research Institute for High-frequency Physics and Radar Technology (FHR) and the South German Institute for Plastics (SKZ), has developed a new system based on millimeter wave technology for non-contact, precise, online measurement of inner and outer diameter, ovality, wall thicknesses, and sagging (“sagging of the melt during solidification at a too low viscosity”) of large plastic pipes with a diameter from 90 to 3,200 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. This new millimeter wave technology allows for an increase of product quality and ensures significant material and cost savings during extrusion.
Figure 1: System on the basis of millimeter wave technology for measuring the diameter, ovality, wall thicknesses, and sagging of large pipes
Technologies for dimension measurement of plastic pipes during extrusionToday, there are diverse technologies used for quality assurance during the production of plastic pipes such as optical methods e.g. lasers for determination of the diameter or X-ray for the additional measurement of the concentricity and wall thicknesses. Conventional technologies such as ultrasonic also measure pipe dimensions, however they often reach their functional limits.
An additional technology for quality control is presently undergoing practical trials. It works with terahertz impulses generated by a femto-second fiber laser, whereby the terahertz beam is directed onto the measured object. The wall thicknesses are determined from the reflected echoes striking the inner and outer boundary layers.
Millimeter wave technology for the measurement of large pipesAlthough frequency modulated radar technology FMCW has been discussed for years1 the system introduced in this article is the first that works in the sub terahertz range and is applied for measuring pipe dimensions during production. The FMCW method has already been used for some time in the automotive technology for distance measurement. It is based on semiconductor technology that is economically priced and its lifespan is practically unlimited. However, it was necessary to increase the bandwidth of the frequency modulation by a multiple to increase the resolution. In the selected range from 80 to 300 GHz all plastics with a low absorption are penetrated and can be measured regarding their wall thickness. This applies for all plastics such as PE, cross-linked PE, HDPE, PP, PA6, PVC, and many others (see also picture 4). In recent years, enormous success, regarding measuring accuracy, has been achieved by researching metrological applications with frequencies in the millimeter wave 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 pipes.
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. Thus, the system represents a key technology for future-oriented quality assurance at the production of large pipes.
Measurement by millimeter wave technology is based on the runtime method. Several static or rotating transceivers, arranged around the circumference of a pipe, continuously send and receive frequency modulated millimeter waves. A static system measures selectively the wall thickness and the outer as well as inner diameter of the pipe. If there is a complete recording of the wall thickness around the entire circumference of the pipe required, a rotating gauge head is used.
This design also allows to precisely measure and represent the sagging. From the runtime difference the product dimensions are defined.
Figure 2: Scheme: Measuring system with rotating sensor
Boundary layers, as for example each front and back site of a plastic pipe, reflect these radio waves. The signals are detected and demodulated by the receiver of the transceiver. The signals contain information regarding the distance between boundary layers of different materials that means the inner and outer diameter, ovality, wall thickness, and sagging. Measurements are made with an accuracy of a few micrometers and with a measuring rate of 500 single measuring values per second. After an algorithmic processing of the received signals of each sensor, the requested measuring results are ready for visualization and control of the diverse pipe dimensions in real time. A connected processor system takes the measured values and displays them numerically and graphically. It also includes comprehensive trending and statistical information.
Figure 3: Runtime method: From the runtime of the (at the boundary layers) reflected millimeter waves, which are continuously frequency modulated, pipe dimensions are defined.
Figure 4: Absorption of radio waves in PVC: The measuring method operates within a spectrum from 80 to 300 Hz, in which the absorption coefficient is small, so that even large PVC wall thicknesses can be measured precisely.
Millimeter wave technology for optimization of pipe quality as well as time and cost savingsAs 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. If we assume that a line, where pipes 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 after ca. 10 to 30 min.
In contrast, the measurement of plastic wall thicknesses with high temperatures via ultrasonic technology, represents a special challenge, because the absorption of sound waves is enormous, in particular at high temperatures. Thus, the measurement of larger wall thicknesses in the hot area is limited. Moreover, the accuracy of the measuring result – in the hot as well as cold area – is largely limited as a result of the temperature-dependent runtime of the sound. Though, the goal is to achieve as early as possible in the production process, reliable and precise information about pipe dimensions in order to take actions if necessary and to avoid failure deliveries.
Moreover, it is necessary to approach early in the production process the minimum permissible pipe dimensions to produce a minimum meter weight. Cost savings resulting from the low meter weight are often decisive in competition. Material costs of extruded plastic pipes account for up to 90 % of the total manufacturing costs.2An example: An extruder has an output capacity of 1,000 kg/h. Assuming an operating time of 5,000 h/year this line produces 5 million kilogram material. Presumed that the material costs are €2/kg, the material consumption is €10 million/year. A lower wall thickness of 2 % results to €200,000 savings per year. In addition, the production of standardized plastic pipes assures a flawless processing of the pipes. For example, quality pipes 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 quality end product.
Areas of application of millimeter wave technologyThe millimeter wave technology is suitable for the measurement of any kind of plastic pipes with a diameter from 90 to 3,200 mm that are for example used for conducting water, gas, chemicals, and oil. Particularly interesting is the application of pipes made of all common plastics such as PE, HDPE, PP, PA6, PVC etc. Here the system provides precise measuring values, even for pipes with larger wall thicknesses. During production, there is the risk that the melt that leaves the pipe tool flows down as a result of gravity, and thus, negatively influences the pipe wall thickness distribution.3 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 action.
Conclusion and outlookQuality demands when manufacturing large plastic pipes are continuously increasing. Norms precisely define the dimensions of the products to be produced. The precise and reliable quality assurance of plastic pipes 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 measuring end for final quality control the product parameters, inner and outer diameter, ovality, wall thickness and sagging are continuously monitored online.
The method is applicable to different material types such as PVC. Curved product surfaces are also determined and precisely measured. Consequently, the introduced millimeter wave technology in combination with processor systems contributes to process optimization, increase of pipe quality, minimization of the material consumption as well as time and cost savings.
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