Udo Gommel, Andreas Schuele, Fraunhofer Institute for Manufacturing Engineering and Automation (IPA)
The processes resulting in the emission of particles from production utilities are those of friction and vibration. Frictional processes induced by relative movements between the surfaces of production utilities are the principle cause. To date, there are no scientifically sound representation models that generate particles from the frictional pairings of technical surfaces. Such a model would enable appropriate materials to be selected for clean manufacturing purposes right from the start of the development phase of components.
Approach to a solution
A standardized test method has been developed at the Fraunhofer IPA to clarify the particulate contamination behavior of different material pairings. A material test bench permits materials to be investigated in a contamination-free environment to assess their suitability for use in cleanrooms. The development of a scientifically sound test method ensures reproducibility, comparability, accuracy and the correct interpretation of the measurement results.
Specific assessment methods have been developed and applied in order to assess and classify the measurement data obtained. Analysis algorithms utilized for the measurement data clearly show the suitability of different material pairings for use in clean production areas.
Development of a material test bench
Different methods can be applied to achieve friction between two bodies (see Fig. 1). With the “ball-on-disk” test, a ball is pressed against the face of a disk. The area of contact is punctiform and the ball is fixed. With the “disk-on-disk” test, a rotatable disk run on bearings is advanced laterally towards the driven disk and then rolls against it. The area of contact is in the form of a line and both surfaces are curved at the site of contact. With the “roller-on-disk” test, a roller (e.g., stainless-steel or PA6 roller) is pressed onto the face of a disk that has been treated with a test coating. This method is used for testing coverings and coatings over which rollers actually move. For example, the roller-on-disk test is implemented to test a cleanroom floor covering where carts on rollers are used.
 Figure 1. “Roller-on-disk” test (top), “disk-on-disk” test (center) and “ball-on-disk” test (bottom). |
As the ball-on-disk test is often implemented in literature and in practice, comprehensive data records are available for correlation with particle generation. Due to a further advantage, i.e., the presence of a punctiform contact area between the materials, it is also possible to essentially standardize this method. As a result, the ball-on-disk test can be utilized for the considerations described below. With this test, a test disk rotates with a frequency (f) beneath a ball with a diameter (d). The latter is pressed onto the disk and a normal force FN is applied and a radius (r) used. At the site of contact between the ball and the disk, material abrasion occurs, which is accompanied by the emission of particles. Abrasion marks are formed. The recordable stress values on real components (area of applied force, distance traveled, multiple stresses on partial distances, etc.) can be correlated with the characteristics of the abrasion marks and the contact site both for analysis purposes and for fine differentiation.
Based on pretests, an appropriate test bench has been developed at the Fraunhofer IPA using the ball-on-disk principle (see Fig. 2).
 Figure 2. Design and realization of a test bench at the Fraunhofer IPA for assessing the cleanroom suitability and cleanliness suitability of materials. |
Parallel to the particle emission tests, it is also possible to determine tribological values. Particle emissions are used as a basis for comparing the results. From the measurement values obtained from the various particle size channels (>0.2 μm, >0.3 μm, >0.5 μm and >5.0 μm), it is possible to determine “particle volume” values. Tribological values are then related individually and in combined form (as derived values such as the volume of abrasion) to this value. In this way the correlation between tribological values and particle emission values can be assessed.
Classification model for material pairings
A major objective is to observe and characterize the development of particle emission during tribological stress. Optical particle counters are used, which supply particle measurement values differentially, i.e., in the form of counting events per measurement period or per measurement volume. In the process, a slight rise in particle measurement values is usually observed as the number of rotations increases. However, the increase in particle emissions cannot be assessed because the pattern is too inconstant (see Fig. 3, top). As the number of particles generated is directly related to the number of rotations and only indirectly related to the time elapsed, it is the number of rotations rather than the measuring time that is used as a new control variable.
If all the particles generated from a discreet number of rotations are added together, this equates to an integral representation of the particle measurement values. Using this representation, the resulting curves show a steady increase (see Fig. 3, bottom). After having approximated the cumulated particle emission graphs using nonlinear regression, the curves can be further considered accurately from a mathematical point of view. A measure of the cleanroom suitability of a material pairing is the rate of increase of this particle emission curve. Therefore, a shallow curve equates to a good degree of cleanroom suitability. As curves increase more quickly or more steeply, this indicates that material pairings are less and less suitable for use in high-level cleanroom environments.
One method of assessing the cleanroom suitability of material pairings is to compare the number of airborne particles generated during the tribological stress with the limiting values for determining air cleanliness. On converting the limiting values for the maximum number of particles permitted for each of the air cleanliness classes as specified in DIN EN ISO 14644-1 in dependence with the number of rotations, a material-class diagram results, which is adapted to the problem. In order to obtain a sound statement concerning the cleanroom suitability of the material pairing considered, a reference number of rotations needs to be fixed. Thus material pairings can be directly classified into material cleanroom suitability classes known as “Material Classes.”
Roadmap of material classifications
Preliminary groupings of material pairings, which were carried out in a pretest to check the material assessment method, already show sound results. For the purpose of the pre-tests where repeat measurements were only carried out twice, eight different material pairings were tested (see Fig. 4). The top diagram shows how material pairings were grouped into material classes.
The material pairing subjected to a typical stress on the material test bench generates particulate abrasion. Due to the measuring time interval set during the test, in which certain particle emission values are theoretically expected during the measuring time interval of one minute and with a test volume of one cubic meter, these concentrations of particles can be correlated with the air cleanliness classes as specified in DIN EN ISO 14644-1. As a result, it is possible to assess the cleanroom suitability of the material pairing based on these classes. In order to make it clear to the user that this classification is based on subjecting a material to systematic stress levels on a test bench and on a mathematical correlation calculation, the material classification of material class is used. Using the previously described test conditions, the conclusion obtained here indicates that a material pairing classified in Material Class 1 is suitable for use in a cleanroom fulfilling ISO Class 1 specifications; a material pairing classified in Material Class 2 is suitable for use in a cleanroom fulfilling ISO Class 2, etc.
Figure 4 (bottom) shows an overview of the cleanroom suitability classification of material pairings obtained from the pretests. The classifications show a good match; this is because of “real” emission values measured and the classifications calculated using regression.
In order to obtain preliminary values for the classification of the cleanroom suitability of various material pairings with different material modifications and stresses, the intention is to test as many material pairings as possible. Less of a focus will be placed on single, highly specific material stresses with a considerable amount of detail. This concept is also being pursued by the industrial alliance Cleanroom Suitable Materials (CSM). At the moment, approximately 40 to 50 different material pairings selected by participating industrial partners are being systematically tested and assessed with regard to their particulate emissions. The aim of this is to assist designers of cleanroom manufacturing equipment in choosing materials with low particle emission levels and to make the particle emission characteristics of the materials implemented clear to the equipment buyer.
Summary
The advantages of the classification model for assessing the cleanroom suitability of any material pairings are summarized as follows:
- Pre-assessment of the cleanroom suitability of a material pairing can be carried out using tribological pretests.
- Correlations can be made with a tribological database.
- Materials can be tested without having to construct costly molds for the actual manufacturing equipment.
- Process-independent assessment of the cleanroom suitability of material pairings for use in discreet air cleanliness classes is possible.
- Empirical tests are no longer required when selecting materials.
Udo Gommel is group manager for Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) in Stuttgart, Germany. He can be reached via e-mail at [email protected].
Andreas Schuele specializes in sterile manufacturing and measurement techniques for Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) in Stuttgart, Germany. He can be reached via e-mail at [email protected].