Project team develops a surface optimization method for use with pharmaceutical, food, and process technology
Surface finishes on stainless steel are significantly influenced by chemical and physical properties. Technical terms such as “hygienic design” are becoming increasingly important in sensitive areas, such as food and beverages, or the pharmaceutical industry. This is because firms want to do whatever they can to avoid particles being carried over to other areas and contaminating different batches. Surface cleaning properties play a crucial role in this, as does the ability of particles to gain adhesion. The question is: Which factors influence particle adhesion on hygienic surfaces, and how can these factors be optimized? This was the question posed by stainless steel specialist Bolz Intec from Eisenharz in Baden-Wuerttemberg. The firm has been producing tanks and customized vessels from chromium-nickel steels for many years, and for a number of those years it has been working with the University of Constance and Technology – Organization – Human Resources, the Steinbeis Transfer Center in Ravensburg, to investigate surface properties.
Currently, the main attribute used to characterize surface properties is surface roughness. Despite this, the research team decided to focus on other factors such as final surface energy.
Current technology revolves around the assessment of surfaces using non-destructive testing, such as roughness measurement in combination with visual observations. There are, however, further important criteria such as how and in what ways surfaces are finished. “We discovered that even if the Ra of different surfaces is the same, in the final surface assessment different grinding techniques result in different adhesion properties,” explains Steinbeis entrepreneur Professor Edmund Haupenthal. It became clear that it is important exactly how material is removed. An experiment was set up to grind a vessel over a longer period using automated grinding rather than grinding surfaces manually. Gradually removing material over a time resulted in less build-up and thus better cleaning properties. This is what the experts refer to as final surface energy.
OGF – optimized grind finishing produces optimized surfaces
Bolz Intec has made full use of these new insights to develop a process capable of removing small amounts of material on a continual basis over a long period of time. This process, which is semi-automated, is called optimized grind finishing, or OGF for short. To remove material, indeterminately shaped abrasive fragments are allowed to swill around inside vessels. The big advantage with this process is that the resulting surface is not only of an outstanding quality, it is also reproducible. The processes used by Bolz Intec do not depend on imprecise variables, such as manual pressure during conventional grinding carried out by an operative, or the quality of individual abrasive materials. The results are defined roughness depths with low influences on the depth of microstructures and visually appealing surfaces.
The differences versus conventional processes are also obvious to the eye. The project team compared a surface produced using conventional grinding processes with a surface finished using the OGF method. The different surfaces were then examined using a 3D optical measuring instrument, which also made it possible to render false-color images. The comparison showed that the OFG method does not produce linear abrasions and as a result, there are significantly lower peaks and troughs on the surface versus the standard sample. The surface does display some imperfections, but these are significantly reduced by the extremely fine and gentle removal process of the OGF method.
Visual and chemical improvements
During further investigation, the surfaces were refined even further and underwent electropolishing. This resulted in surface spikes being removed, which not only delivered visual benefits but also improved chemical properties, such as enhanced corrosion protection due to strengthening of the passive layer.
In its final assessment of the study, the project team also demonstrated improved cleaning properties using a test based on VDA19.1 (March 2015)/ISO16232 (December 2018). The OGF method reduces the build-up of residual dirt in vessels. For users, this significantly reduces the risk of contamination between different batches.
The project partners felt positive about the results of the project. The desired parameters were achieved and thanks to the research, it was possible to create surfaces that can be accurately reproduced in terms of both roughness and visual impact. In addition, the surfaces offer advantages when it comes to cleaning. This added value could be especially important for critical, extremely valuable, or very fine particles. In industries such as nanotechnology, biotechnology, and pharmaceuticals, batch purity and avoiding contamination are a crucial aspect of the production process.