Quality Assurance in Parts Cleaning

The key to manufacturing high quality parts consistently is eliminating variability. Thus, process monitoring systems are integrated in turning, drilling and milling machines. They catch poor quality production parts in process, minimize scrap and protect tools and machines. This results in high productivity and efficiency, which is a source of higher revenues. Now, look at parts cleaning—process management and control is often based on the experience of the machine operator and focuses on cleanliness inspection at the end of the process.

But, in aqueous cleaning processes, there is also variability. Cleaning results are influenced by temperature, concentrations of builders and surfactants in the cleaning agent and contamination in the baths. Cleaning agent performance is decreased by carried-over particulate and oily contamination. Builders and surfactants are depleted to different extents during the cleaning process, which also changes the quality of the cleaning results. The consequences vary, too: On the one hand, intervention in the process does not take place until quality problems become apparent, for example, insufficient adhesion of a coating, resulting in laborious rework, decreased productivity and high costs. On the other hand, the experience-based process leads to a costly, resource-intensive, overcautious operation: to be on the safe side, cleaning agent concentrations are significantly higher than required for adequate cleaning in many cases. 

As a result, the cleaning agent is carried over into the rinsing zones, where it’s removed by a bath. Because the concentration levels of the cleaning agent’s ingredients and contamination in the baths are often unknown, baths are changed earlier than necessary, either partially or entirely. In addition to increased consumption of water, chemicals and energy, this results in considerable wastewater reconditioning and disposal costs, as well as reduced availability of the cleaning system.

In order to achieve stable cleaning results in an economical and ecological way, all parameters that influence process and quality have to be monitored and managed. The basis, therefore, is to determine set points and limit values for bath variables such as concentration of the cleaning agent, respectively its component’s surfactant and builder, the bath contamination and temperature, as well as for other process parameters, for example, ultrasound properties. Often, optimizing runs for determining these parameters reveal that specified residual contamination limit values can also be reliably achieved with lower concentrations of cleaning agent ingredients and/or reduced temperatures.

For the monitoring of cleaning agent and contamination concentration in the baths, different measuring systems are available. An easy and quick way to measure the surfactant concentration is the bubble pressure method, which is based on determining the dynamic surface tension of liquids. There are also measuring systems based on sensors, which have been specially developed for cleaning processes. They allow for fully automated monitoring of cleaning agent concentration as well as bath contamination. Other systems detect and document the extent to which the bath is contaminated with particulates and oils, thereby reliably indicating when the bath needs to be changed. Additional parameters, which are relevant for the process, for example, pressure, temperature, pH value and specific conductance, can also be integrated in order to generate a complete process image. As an alternative for fully automated processes, monitoring systems are offered that not only measure bath contamination, but rather individual concentration levels of builders and surfactants as well. They can document the results and automatically add ingredients based on consumption.

Inspection of cleaned parts for film-like residual contamination such as residual oil and grease, as well as fingerprints and preservatives, can be performed by various test procedures. Test links are easy and work quickly. Beyond this, portable and inline test methods are available that also make it possible to document the ascertained values. These are based on various principles such as contact angle measurement and fluorescence measurement. 

When it comes to tracking down particulate residual contaminations on cleaned parts, the extraction and analysis methods of the VDA volume 19 and its international counterpart are usually applied. When manufacturing high quality parts, cleaning is a vital process, so why not eliminate variability here, too?