Key Factors for Efficient Cleanliness Analysis

February 26, 2014

In the automotive industry, even the smallest particles on single components can cause malfunctioning. Fuel, lubricant and urea filter systems are particularly vulnerable. Component cleanliness has therefore become a central quality characteristic in the modern manufacturing process, especially when fitting components of various suppliers.

Efficient cleanliness analysis has to begin at the supplier’s already. What are the most important factors for efficient particle analysis? What facilitates the workflow for quality control staff? What challenges does the process environment impose on the analysis system?

Prerequisite for analysis: dialog on specifications

"The basic requirement for efficient particle analysis is open communication between automobile manufacturers and their suppliers" says Dr. Nicol Ecke, Teamleader Sales Industry Application at Leica Microsystems. "The dialog on cleanliness and measurement protocol specifications is often neglected."

First of all, the supplier must know exactly how to clean the parts, whether they need cleaning separately or can be cleaned together. Customers and suppliers must also agree on the measurement parameters (length, breadth, height) the supplier is to measure, the classification parameters, and whether and how the supplier needs to distinguish between fibers and particles.

With respect to particle size, the parties must agree on the smallest size to be measured. The aim is to customize the analysis system to specific needs. The partners should also discuss how the results are to be documented: Are particles to be relocalized and recalculated? May particles be deleted and edited? Do the measurement protocols, images and diagrams have to be documented?

"The  customer and the supplier have to agree on these key issues before the right system can be chosen and efficient particle analysis can be performed," says Nicol Ecke. Leica Cleanliness Expert, a complete solution consisting of a microscope, digital camera and analysis software, offers user-defined configuration and classification options allowing any number of measurement protocols to be stored. 

Turbocharger, 3D rendering (© GP – Fotolia)
Shock absorbers for back wheels of motor vehicles (© withGod – Fotolia)

Authoritative regulations for component cleanliness analysis: VDA 19 and ISO 16232

A guide to measurement specifications is given by the VDA, volume 19 and ISO 16232. The standardization of inspection practice promoted by both these works leads to reproducible, reliable and comparable results. Even though large companies often apply factory standards for their own production and for their customers, they are usually based on the above-mentioned standards.

"In practice, however, it has emerged that the standards do not contain enough specifications," explains Nicol Ecke. "Leica Microsystems is playing a leading role in the current revision of the VDA 19 standard, particularly with regard to software implementation. After all, we have over 15 years of market success." The new VDA 19 will simplify the comparability of analysis and analysis systems in a defined measurement range. The new version will therefore contain a so-called "recipe" for setting image brightness and measurement thresholds, for instance, in order to ensure comparable results.

Exact choice of measurement parameters

Conclusive analysis of component cleanliness depends on the right choice of measurement parameters. In the past, the maximum particle length was usually taken as a measure of damage potential. For components such as jets, however, in which particles align themselves under pressure, maximum particle length cannot exactly represent the damage potential. Particle breadth (e.g. feret breadth) is a more accurate measurement parameter here.  In the case of bent particles, on the other hand, it is better to measure the inner circle diameter, as the feret breadth of a particle would result in too high a value. The same applies to particle height, which is a compulsory measurement criterion for moving components like turbochargers or cylinders. The height of a particle is determined with the low field depth of high-power objectives. "Depending on the product and function, the choice of the right measurement parameters can improve the accuracy of damage potential assessment," says Nicol Ecke.

Measurement of particle height

Approaching the lower focal plane (filter background)
Approaching the upper focal plane (top of the particle)
Adapted from Pall Europe Limited

Measurement of particle width

In this curved fiber the max. inscribed circle diameter shows the more realistic damage potential. The minimum Feretmin here result in a far too great width.

The right analysis system for every user group

In many companies, component cleanliness is measured and analyzed by staff without microscopy experience. Here it is particularly important that the microscope system automatically guides the operator through the analysis routine to a fail-safe result. If the measurement protocol is selected in such an analysis system, the operator only has to check the focus points. The brightness, threshold value and contrasting technique are adjusted by the system, the filter is automatically scanned and evaluated. If there is more than one operator using the same microscope, the system should be able to store several user settings and reconstruct them as required. If the particles to be analyzed are not smaller than 30–50 µm and high sample throughput is an important consideration, an analysis system with a macroscope Leica DMS1000 could be the right solution. The low magnification allows a quick overview of the filters and provides the right degree of resolution. Tiny particles as small as 5 µm require higher optical resolution – standard-compliant measurement of such particles is only possible using materials microscopes with fixed optics. These systems are also a sound investment in terms of future usability (measurement of extremely small particles, z height measurement) and versatility (they can be used as materials microscopes in metallography, for example). 

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