Defined standards for residual particulate contamination of functionally relevant components in the automotive industry are self-evident today. This is especially true in Europe, where the German Association of the Automotive Industry’s VDA Volume 19 is the first comprehensive standardization document to deal with the approaches and procedures for characterizing the cleanliness of products within the automotive industry’s quality chain.
As more manufacturers and finishers in the U.S. look to see if VDA 19 standards may come to the North American supply chain, attention should be paid not only to the cleaning process, but to the entire process chain. Even the tiniest particles left behind in the wrong place can cause damage and system failures. “Technical cleanliness” is therefore a quality criterion, particularly when it comes to parts for the automotive industry, precision engineering and hydraulics.
The progressive downsizing of motors is one trend driving the need for significantly cleaner parts in the automotive industry, in particular for the fuel intake, pressurized and unpressurized oil chambers, and the coolant chamber. Investments in technology for industrial component cleaning are necessary in order to meet and document these standards. However, modern cleaning technology alone is no guarantee of sufficiently clean parts.
Designing To Support Cleanliness
Cleanliness actually starts with component construction. This is where the geometry and individual steps in the manufacturing process are decided, including turning, milling, joining and coating. As a general rule, the question of how the design could support cleanliness is not raised, so components often have corners, edges or holes from which particulates and residue cannot be removed, or can only be removed with great difficulty. Surface texture also influences cleanability: A smooth surface simply retains less dirt than a rough or porous one.
The cleaning effort needed to meet existing contaminant guidelines is also determined by the manufacturing process. The less particulates, oil, grease, cooling lubricant and other contaminants adhere to the parts, the quicker and easier it is to achieve the required level of cleanliness. In multi-step machining operations, intermediate cleaning processes prevent residue from accumulating and lubricants/coolants from mixing or drying out, all of which can lead to problems in cleaning.
Tool maintenance and upkeep is also of relevance to cleanliness. For example, clusters of chippings that accumulate during machining—especially as the tool loses its sharp edge—and are then difficult to remove can be prevented by timely tool-changing.
Additional factors that affect cleaning include feed rates and tool geometry. Preparation or filtering of the coolant/lubricant also prevents contaminants already removed from components from redepositing on them. A separate rinsing step for the tool in the machine can also contribute to minimizing particulate volumes.
An Optimal Cleaning Process
Modern cleaning systems are able to meet very high requirements for component cleanliness, as long as the cleaning process is optimized for the impurities to be removed, the component geometry, the material and the required cleanliness specifications.
In addition to the process engineering and the cleaning medium, the container also significantly affects the results and affordability of the cleaning process. Two questions are of primary importance here: Are the parts in the container easily accessible from all sides by the cleaning agent and the cleaning system mechanics? Can the component be positioned in the container so that critical areas can be targeted?
Efficient cleaning also requires that impurities be removed from the cleaning bath to prevent them from redepositing on the components. A filtering system in good working condition and appropriate for the particulate size is needed to ensure continuous particle removal.
Inspection of Baths
Monitoring impurity levels in the cleaning and rinsing baths is also very important for meeting cleanliness requirements adequately and affordably. Measuring systems for water-based cleaning fluids are available that detect and document particulate and fluid contaminants, and reliably indicate when a change of solution is needed.
Additional separate measurement and documentation of concentrations of builders and surfactants in the cleaning agents is also possible. Other parameters that are relevant and can help form a comprehensive picture of the process include pressure, temperature, pH and conductance.
When cleaning with solvents, oils and emulsions accumulate in the cleaning agent and react, distilling over time to form free acids. These not only reduce the cleaning quality and lifetime of the cleaning agent, but can also lead to corrosion of the cleaned parts or the system. Test sets are available for regular inspections for chlorinated hydrocarbons (CHC) and some modified alcohols.
Inspecting And Documenting
Inspecting particulate cleanliness of automotive industry parts has been governed since 2005 by VDA Vol. 19, Part 1 (“Inspection of technical cleanliness – Particulate contamination of functionally relevant automotive components”) or the international equivalent, ISO 16232, Vol. 1 to 10 (“Road vehicles – Cleanliness of components of fluid circuits”). The purpose of the guideline is to objectively assess and compare the technical cleanliness of a component based on clear and precisely defined methods and procedures for extracting and analyzing particulate contaminants from manufacturing and the environment. This is the reason for the interest shown in this set of standards by other sectors such as medical and precision engineering, or the hydraulics industry.
A key criterion with VDA 19 is that the required cleanliness level is always linked to an inspection specification containing unequivocal information about the cleanliness inspection parameters and particulate measurement techniques. It also stipulates that parameters for cleanliness inspection for the given component type shall be tested and optimized using a so-called extraction curve to achieve the most complete removal of particulates possible without damaging the component substance.
Hindered By Geometry
Because verification of particulates is hindered by geometry for most workpieces and cannot take place directly on the product surface, a cleaning step in which the particulates are transferred to a fluid medium is necessary. This extraction or removal of particulates from the component can be achieved through a variety of different fluid methods, including spraying, ultrasound, rinsing or shaking, however the extraction procedure is not predefined. This freedom continues to present one of the biggest problems for comparing the results of component cleanliness analyses. The revised VDA 19 that is currently being drafted should, therefore, include a decision matrix that enables users to select the appropriate extraction techniques for their inspection task.
Different methods can be used to evaluate samples, with varying levels of validity: gravimetric microscopy, automated microscopy with image processing and scanning electron microscopy. The latter provides information about particulates’ origin and potential for damage, and can examine even the smallest particulates.
Even more precise information can be gained from microtomography, which can measure contaminant particulates in three dimensions. These are extremely precise, but also very tedious, expensive and time-consuming laboratory analyses.
Another topic addressed in the revision to VDA 19 is the strict definition of cleanliness requirements, in terms of both their formulation and how to respond when they are not met. Thinking has evolved in this area. Rather than ever-emptier discussion around the “final micron,” in the future processes and process chains will come into greater focus for understanding and inspecting cleanliness. This will require new, rapid and cost-effective particulate monitoring systems, and the necessary solutions are in part already available or under development.
Detecting Film Residue
Various inspection techniques are available to check cleaned components and surfaces for film residues such as grease and oil, fingerprints, or conservation agents that can detract from the quality of subsequent coating, painting, adhesive, hardening or welding processes. They range from test inks to mobile and inline-enabled testing methods that also permit documentation of the detected values.
Once workpieces meet the required cleanliness specifications after manufacture, this cleanliness must be maintained through later steps such as transport, inspection, storage and packing. To prevent particulate contamination from the environment, these subsequent steps must be carried out in a location removed from the manufacturing area, and personnel must be equipped with the necessary clothing and gloves. Volatile Corrosion Inhibitor (VCI) packaging is available to protect cleaned components from corrosion. These packaging materials create a corrosion-proof atmosphere inside the packaging, as well as providing protection from outside contamination.
Doris Schulz is with parts2clean, a trade show held every year in Germany. For more info, visit parts2clean.com.