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Vol. 9
November Issue
Year 2008
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Articles


in Vol. 9 - November Issue - Year 2008
Automatic Dosage of Cleaning Agents and Efficient Cleanliness Control for an Optimization of Cleaning Processes



Pic. 1: SITA DynoLine for an automatic cleaner dosage according to the consumption


Picture 2: Fluorescence measuring device for the parts


Picture 3: Economic process management

The industrial cleaning of metal parts offers an extensive optimization potential. An appropriate dosing of the cleaner according to the consumption as well as a cleanliness inspection of parts’ surfaces can lead to an ecological and economic process management. Furthermore, the process reliability can be increased in a sustainable way. The cleanliness of parts’ surfaces can be inspected due to a new fluorescence measuring technology in order to evaluate the cleaning process. The concentration of cleaning active surfactants can be analyzed by measuring the surface tension in the cleaning bath.
A high quality of surface treatment or surface coating of parts after shaping requires sufficiently cleaned parts’ surfaces. Insufficiently cleaned surfaces can cause defects of the surface quality of products which lead to high costs and consequential damages. The function of the cleaning process is to remove oils, greases and particles from parts’ surfaces. The aqueous cleaning requires stable conditions of the state variables in cleaning baths for ensuring a high process reliability. The concentrations of the used cleaning agent components, builder and surfactant (wetting agent) play an especially important role next to the temperature of the cleaning fluid and the contamination concentration.
At present, cleaning components get strongly overdosed in order to intensify the cleaning power in baths for ensuring reliable cleaning results. In contrast to overdosing, the cleaning bath often cannot be used anymore at an early stage due to a depletion of the cleaner caused by exhaustion and carry-over.
The basis for the optimization is a continuous monitoring of the cleaner agent concentration and therefore a reasonable dosage according to the consumption.
For monitoring and dosing of the builder-component, it is possible to control its concentration by measuring the conductivity or by acid-base-titration. Monitoring the surfactants concentration and dosing according to the consumption is possible due to the parameter surface tension. Therefore, the SITA bubble pressure tensiometers are used for measuring the dynamic surface tension of liquids by the newly developed bubble pressure principle based on the differential pressure method. In this process, air is pumped through a capillary in the liquid which has to be analyzed. A pressure sensor measures the internal pressure of the developed bubble on the tip of the capillary. The surface tension is calculated by the difference between the maximum and minimum internal bubble pressure and a calibration factor. The surfactants concentration can be determined by a reference chart developed in the laboratory.
The advantage of this measuring principle is, that only the free and cleaning active surfactants are measured. Those surfactants will attach to new developed bubble surfaces and have an influence on the measured dynamic surface tension.
The SITA DynoLine (pic. 1) is used for a continuous-monitoring and an automatic dosing of wetting agents worldwide. For realizing quick and simple measurements as well as analysis about bath monitoring at the process, the mobile tensiometer SITA pro line t 15 or the hand-held tensiometer SITA DynoTester are suitable.

A simple inspection of residual contamination on parts’ surfaces can be realized due to the new developed fluorescence measuring device SITA CleanoSpector. The SITA CleanoSpector detects the contamination layer by exciting its fluorescence by an UV light source. Organic materials show a strong fluorescence effect when being excited by UV light. Therefore, it is possible to prove oil and grease residuals as well as corrosion prevention agents. A photodiode in the sensor head of the SITA CleanoSpector measures the intensity of the emitted fluorescence radiation. The higher the intensity, the higher is the residual contamination on the parts’ surfaces.
The measuring device displays the cleanliness results of the part’s surface in percentages. The calibration of the device happens on parts free of any contamination (ideally clean) which correspond to a cleanliness of 100 %. The reference samples support the definition of a limit value for a sufficient cleanliness.

Due to the continuous monitoring of the bath state variables as well as the cleanliness inspection, it is possible to detect an insufficient parts’ cleanliness and its causes in order to introduce necessary actions. An insufficient cleanliness of parts’ surfaces in the cleaning process can be avoided for example by increasing the cleaner’s concentration or by realizing bath care actions in order to eliminate defects in following surface treatment processes.

The optimization of the cleaner concentration is the basis for an economic process management. With a continuous cleanliness control of the parts’ surfaces and monitoring the cleaner concentration it is possible to decrease the safety reserve of the cleaning agent in the bath until the defined limit value for a sufficient cleanliness is reached. Afterwards the limit values for the cleaning agent concentration have to be defined and maintained through an automatic dosing based on measured values (pic. 3).
Next to an increase in process reliability, the optimization of the cleaning process leads to:

- a decrease in costs of the cleaning agent’s usage
- an avoidance of the enrichment of the cleaner components in subsequent rinsing baths
- an extension of the bath lifetime
- a decrease in water and wastewater  costs

In many application examples of the surface treatment, from car bodies up to precision parts for pumps and bearings, the cleaning processes can be optimized successfully by monitoring and automatic dosing the cleaner components, builder and surfactant. Therefore, it is possible to control the processes in a way that a sufficient cleanliness quality can be reached by using a minimum of water, cleaner and energy. With the new developed SITA CleanoSpector, the bath monitoring process can be combined with the parts’ control in order to achieve a new dimension in the process reliability as well as in the economic and ecological process management.




Author: Dipl.-Ing. Daniel Schümann
For Information:
SITA Messtechnik GmbH
Gostritzer Straße 61-63
01217 Dresden, Germany
Tel.: +49 351 871 8047
E-Mail: info@sita-messtechnik.de
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