Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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continuous measurement in a bypass have to be homogeneous and must be kept on a defined temperature level as well. A special measuring vessel with slow-flow- ing characteristics in the capillary area minimizes negative influences on the bubble and avoids particle accumulation. The flow rate of 2 l/min ensures a suf- ficiently homogeneous and non-demulsifying sample near the bath tempera- ture level. A crucial problem is the lifetime of the capillary. Accumulations of cleaning bath ingredients can change the capillary characteristics during the time of usage. Common glass capillaries used in the bubble pressure tensiometry were substituted with a PEEK-capillary consisting of a more robust and hydropho- bic material. The PEEK material and the increasing diameter behind the new capillary's tip minimize the capillary effect, which means that the sample hardly gets into the measuring capillary. Under the same conditions, a con- ventional laboratory capillary has a lifetime of approximately 300 hours in com- parison to an optimized PEEK capillary with a lifetime more than 10 times high- er. This high insensitivity against accumulations ensures long-lasting and reproducible bubble generation.2 The process measuring technology SITA DynoLine is used for continuous bath monitoring. The fully automatic device works as a plant component and is adapted to the specific requirements of the process, the chemistry, and the plant. It differs from other sensors and conventional surface tension measuring technologies in that the SITA DynoLine automatically controls the supply of the sample, the calibration water, and the self-cleaning procedure. Other essential char- acteristics include a self-monitoring function, error detection, and alarm signaling for trouble-free operation. Any changes in the capillary characteristics that occur (e.g., through an accumulation of contamination) can be identified by the measuring instrument itself due to the regular self-monitoring function. For controlling the liquids, the instrument has a valve for both the sample and the water. Furthermore, it has a pressure sensor for monitoring the input pres- sure. Together with calibration water, the sample is led back into the bath with- out pressure (in general, the evaporation of bath liquid is higher than the supply of calibration water). All measuring values can be saved in the instrument for one year in order to record the measurements. Interfaces allow communication with the superior control system of the cleaning plant. The upgrading of a plant for automatic dosing can be easily achieved in order to keep the surfactant con- centration of a process within an optimal range (see Fig. 8). In many applications of surface treatment processes, from car bodies to pre- cision parts for pumps and bearings, the cleaning processes were successfully opti- mized by monitoring and dosing the cleaning agent components (surfactant and builder automatically). Furthermore, control of the process achieves sufficient cleanliness quality with a minimum use of water, cleaning agent, and energy. Finally, a new dimension in the process reliability as well as in the economic and ecological process management may be achieved. NOTES 1. Brunn, K. Besser und weiter. Metalloberfläche (2003) H.11, S15–16. 2. Haberland, R. Dimensionierung und Gestaltung von Oberflächenspannungsmessgeräten für den Prozess-Einsatz. Dissertation, Technische Universität Dresden, 2004. 63

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