TPT May 2012

T echnology N ews

Control in composite pipes production lines

A MEASURING device can be very progressive and precise: with just its integration in the production line it does not provide hose and tube manufacturers with decisive advantages. Only when it directly controls the process can the production be managed. The following example shows the different control methods in composite pipe extrusion lines. Composite pipes are made of an inner PE-tube, on which a thin aluminium layer is applied, covered by an additional outer PE- layer. The plastic layers are each connected with the aluminium by an adhesive layer. With an X-ray measuring system like the X-Ray 6000, the inner and outer diameter as well as the wall thickness of the single layers of the composite pipe can be measured and controlled. Moreover, faults in the inner tube can be detected. The improvement of the production process is nowadays possible with an automatic control of the line without intervention of the operator. From the setpoint-actual value difference of the diameter respectively the wall thickness the correcting variable is continuously calculated by a control module and directly transferred to the line control, for example via a Profibus interface. The correcting variable considers the product specific minimum values, the eccentricity and statistical data. This method is the most modern type of control because control parameters are directly transmitted to the line control via the Profibus interface.

Not every line can be controlled in this advanced manner. There are many lines with direct current drives. These usually work with a 0 to 10 volt signal. 0 means standstill of the line, 10 volt maximum line speed or extruder rpm. Both variables can be controlled alternatively continuously with highest precision to the nominal value. The specification of tolerance limits is not required. Rarely, we find contact controls (motor potentiometer). For this type of control the operator has two buttons to run the line faster or slower. Via the buttons motor potentiometers are controlled. Later, the buttons were replaced by contacts of a control module. This, in the 80s modern control, worked discontinuously and required that a tolerance band was accepted with no intervention in the control. There was always the question of a hot or cold value control. If the hot diameter measured and controlled, the material shrinkage is an unknown variable. The difficulty is that not the complete diameter is shrinking, but only the outer PE-layer. That is why the specification of a fixed percentage shrinking value is not a solution. Depending on the product type a separately calculated shrinking value has to be considered. Alternatively to the hot value control a control from the cold end is possible

with limitations. For processes that run with low line speeds this method is less suitable as the feedback about product parameters is only late as a result of the time delay. During the early 1990s Sikora launched a Hot-Cold-Control. This at that time patented method that combines the advantages of the fast measurement and control of the hot value control with the product dimensions, which are relevant for the end customer, namely the cold measuring values. From the hot/cold value difference the material shrinkage is continuously calculated and is automatically considered for the control of the diameter. Due to specific line configurations or product structure it is not always possible to install a measuring system directly after the extruder. For these applications a software concept with a virtual hot gauge head is used, which learns the control behavior of the line and conducts the hot/cold control by means of calculations without a hot measuring head. With regard to the diameter and wall thickness the line speed or the extruder rpm are controlled. As there are often reservations that, with an extruder control, the thermal balance is disturbed, hose and tube manufacturers prefer line speed control if the line is equipped with only one extruder. However, for tandem lines the control of the extruder rpm is typical. Integrated in the extrusion line the X-ray measuring system X-RAY 6000 measures the wall thickness, eccentricity, the inner and outer diameter and the ovality of hoses and tubes

Sikora AG – Germany Email: sales@sikora.net Website: www.sikora.net

In combination with the processor system ECOCONTROL 6000 the production data are visualised and controlled

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M ay 2012