Understanding the nature and composition of particles present both on and within automotive components is a critical analytical testing service PTL provides the automotive industry. Particles released from mechanical parts not only can shorten the life of the entire system but can also pose severe safety concerns. These particles may originate from the component itself, from wear and tear, or from contaminants during the manufacturing and/or packaging process.
Performing a thorough analysis and assessment of particle detection within automotive components involves extraction of particles from the critical components and then determination of the mass of loading recovered, the size of the particles, and in some cases, their composition. With this knowledge, the engineer can better control the quality of their final product and troubleshoot the source of contaminants, and the customer can be confident in the longevity of their purchase. In this article, we will cover briefly PTL’s approach when determining the cleanliness of a mechanical part.
ISO 16232 Cleanliness of Automotive Components and the Association of German Automotive Industry VDA 19 are similar in scope and are commonly cited standards for determining cleanliness in the automotive industry. VDA 19 is often cited by European automotive manufacturers. PTL also often receives company-specific procedures for determining cleanliness, but in general, all testing methodologies are similar.
They consist of three main steps –
1) Extraction of the particles from the surface of the part
2) Collecting the extracted particles on a filter
3) Determining the mass of the collected particles
In some specific cases, a fourth step is included during which more detailed characterization (e.g. particle size, shape, and/or identification) is performed.
Extracting the Particles
Several forms of extraction can be used. Selecting which is dependent on the component size, shape, loading level, geometry complexity, surface area, etc. PTL has the capability to perform only small-scale extraction processes. Our facilities allow for “small parts” and components such as fasteners, small manifolds, valves, gears, bearing housings, etc. Larger mechanical components such as engine blocks, transmission housing, etc. cannot be handled by our testing environment.
PTL can perform the following types of extraction. The maximum sizes for each extraction technique are also provided:
- Pressure Rinsing = maximum 16” (400 mm) (L) x 16” (400 mm) (W) x 7” (178 mm) (H)
- Ultrasonication = maximum 10” (254 mm) (L) x 8” (203 mm) (W) x 5” (127 mm) (H)
- Internal Rinsing = maximum 16” (400 mm) (L) x 16” (400 mm) (W) x 7” (178 mm) (H)
- Agitation = Varies depending on agitation method and duration. Contact us to discuss.
Pressure rinsing and ultrasonication are common approaches in many automotive extraction procedures. However, any of the four listed above can be employed depending on how the part will be used, the complexity of the part’s design/geometry, and other factors.
Each of the extraction techniques has parameters that must be considered and documented to ensure repeatability and reproducibility of the results. For example –
- Rinsing factors such as spray nozzle shape (fan, cone, or round)
- Pressure or flow rate
- Distance between the nozzle and part, and
- Duration of pressure rinsing
Ultrasonication duration, fluid temperature, and the power density are factors considered when utilizing the ultrasonic removal technique.
The power density is determined by the wattage from the ultrasonic bath provided by the manufacturer divided by the volume of liquid required to fully submerge the part(s). PTL has two ultrasonic baths (2 and 10 liters) available for ultrasonic removal of particles. The 2 L bath has a power density of 120 watts/liter and the 10 L bath is 24 watts/liter. All of PTL’s particle removal techniques use only aqueous miscible fluids due to environmental and safety considerations. Surfactants and detergents can be added to the aqueous rinse fluid to aid in liberating the particles from the part’s wetted surface.
If ISO 16232 is specifically requested and no previous information is available on how particles are extracted, a declining test should be performed. A declining test is a form of method development that investigates the proposed extraction process. This study evaluates the efficiency and repeatability of the extraction step. Six separate extractions are performed on the same part or batch of parts using the proposed extraction parameters. The extracted particles are pulled down onto a tared filter, dried to a constant mass, and then weighed. The recovered mass of particles from the six individual extractions is evaluated to determine if an acceptable endpoint is reached per the ISO standard. The cumulative time in the extraction process can then be used for routine testing of a new batch of current parts and future batches. The declining test does not need to be performed again for future batches unless a change in the manufacturing process occurs.
In addition to determining the particle loading from the parts themselves, a “blank” test is recommended as part of the testing scheme. The blank is usually collected from the testing environment, collection vessel, and filtration equipment to ensure the testing environment and equipment are properly prepared and cleaned prior to working with the samples. PTL performs part cleanliness testing in laminar flow hoods which are tested on a designated schedule for particle counts during operation using an externally calibrated portable particle counter. Our laminar flow hoods are designed by the manufacturer to meet and exceed ISO Class 5.
Characterizing the Particles
Now that the particles have been collected, it’s time to take some measurements. The mass of the particles collected is typically first determined by drawing down the extraction fluid onto a filter. The pore diameter of the filter is typically 5 µm, however, smaller or larger filter pores sizes are available depending on the client’s requirements and size range of interest. The mass of particles collected is reported as mass per part or mass per unit area (e.g. milligrams per part or milligrams per square centimeter). The surface area of the part must be provided by the client for this type of reporting. PTL uses a 5 place analytical balance to determine the mass collected on the filters. Our analytical balances meet the lower detection limit (LDL) of 1 mg and lower quantitation limit (LQL) of 10 mg stated in the ISO 16232 standard.
Several different technologies are available at PTL for characterizing the recovered particles. Microscopy counting is commonly used to provide a size distribution or sizes within specific intervals either required by international standards or requested by customers. The ISO and VDA standards refer to this microscopy evaluation of the filter as the Light Optical (LO) technique. Automated image analysis can be used to provide shape measurements of the collected particles. During the light optical assessment, the instrument can provide information on the size of metallic particles present by evaluating the light intensity differences between two polarization planes.
Particle sizing by Single Particle Optical Sizing (SPOS), also referred to at times as Automatic Particle Counters (APC), is another technique. PTL’s SPOS systems conform to ISO 21501-2 Determination of particle size distribution – Single particle light interaction methods – Part 2: Light scattering liquid-borne particle counter) and 21501-3 Determination of particle size distribution – Single Particle light interaction methods – Part 3: Light extinction liquid-borne particle counter.
PTL also has capabilities to identify the foreign particles in some cases using either Raman Spectroscopy or Energy Dispersive Spectroscopy (EDS) depending on the material, particle size, and atomic number of the elemental composition.
ISO 16232 outlines three analysis packages depending on the requirements agreed upon by the supplier and end customer. They are referred to as Standard, Extended, and Shortened analysis packages. A short explanation for each can be found below:
- Standard Analysis = consists of performing the particle extraction followed by determination of the mass collected, then characterizing the size of particles > 50 µm using the light optical technique. Particles can be classified as metallic, non-metallic, and fibers.
- Extended Analysis = consists of analyzing particles > 5 µm by the light optical technique in addition to further characterizing the particles collected to determine their elemental composition or identifying the material type using techniques such as Raman Spectroscopy or EDS.
- Shortened Analysis = consists of analyzing the particles in the extraction fluid directly using the SPOS/APC technique. The extracted particles are not pulled down onto a filter prior to analysis.
Reporting the Results
Once the analysis is completed, the results are reported typically in one of three formats depending on if the ISO standard was requested or if a standard PTL report is sufficient. A standard PTL report consists of a short description of any observations made by the chemist, result summary table, and the instrument output(s).
If ISO 16232 reporting is requested, the reporting format is dependent on the analyses required during the project. The two types of report generated using ISO 16232 are:
- Qualification Report = contains the results from the Declining Test, details on the extraction setup, and results from the batch of parts.
- Inspection Report = contains the results from the batch of parts and a brief summary of the extraction setup previously determined through a Declining Test. An Inspection Report does not include the Declining Test results as it assumes a Declining Test was performed at some point in the past.
How PTL Can Help
Parts cleanliness projects can vary depending on the industry, part, component design, end-use, client needs, etc. As a service laboratory, PTL works with our clients to discuss the goal of the testing and how the part is used, and PTL takes the time to review any established testing instructions as well as previous results (if available) to assist in creating customized studies to produce meaningful results. These results are used by process and design engineers to reduce defects, system failures, and production downtime. PTL looks to be a partner in our client’s testing needs and works to achieve cost-effective solutions to their manufacturing challenges.
Consider PTL and speak with one of our knowledgeable staff the next time you are faced with a small component contamination challenge or are asked by a customer or government agency to demonstrate if your product will meet cleanliness requirements!
By William Kopesky, Director of Analytical Services.