Component cleanliness requires effort throughout manufacturing process

Specifications for particulate and film-residue cleanliness are a matter of course for many machined components. In addition to the parts cleaning procedure, manufacturers must consider the entire process chain when seeking to efficiently achieve and maintain the required level of cleanliness.

Even the tiniest particle or a microns-thick film residue can negatively impact later processes, part quality and the functionality of the finished product. For example, cutting tools are often coated with hard materials, such as diamond-like carbon, to improve their performance and service life. Depositing these coatings requires a high level of surface cleanliness, and any particulate or trace of film-forming residue will degrade the adhesion of the coating to the tool substrate, which can cause premature tool failure.

Solvent cleaning systems that are commonly used to remove nonpolar contaminants are equipped with an integrated distillation unit for continuous solvent conditioning. Image courtesy Höckh Metall-Reinigungsanlagen.

“Technical cleanliness” is, therefore, a quality consideration for many components, and design drawings often specify a cleanliness level. These specifications generally can only be met with considerable capital investments in industrial parts cleaning equipment. This includes units for maintaining the quality of the cleaning bath, filtration systems to remove particulates brought into the cleaning system, and rinsing fluids and systems for removing film-like contaminants, such as oil and grease. Among the systems available to maintain bath quality are vapor distillation, bag filters, multiple-tube filters, magnet separators and coalescence separators. The system selected is determined by part-cleanliness requirements, type and amount of impurities and the cleaning agent, whether a solvent or aqueous cleaner.

However, equipment alone is no guarantee of reliable and cost-effective cleaning.

Let the Scrubbing Begin

Cleanliness actually starts with the component-design phase. This is when part geometry and the individual steps in the manufacturing chain, such as turning, milling and grinding, are defined. This also determines the effort that’s necessary to clean parts. For example, a through-hole is easier to clean than a blind-hole, and designing a part with rounded inner contours instead of corners and edges reduces the amount of dirt that collects inside the part. This design feature also promotes laminar flow through the workpiece during cleaning, which prevents the formation of eddies and the resulting particle deposits. Surface finish influences a manufacturer’s ability to clean a part, too, because a smooth surface retains less dirt than a rough or porous one.

A change in contaminant input or modification of the coolant formulation can lead to insufficient conditioning of the cleaning solution and insufficient cleaning results. Image courtesy Karl Roll.

It’s also important to control the amount of contaminant build-up. The less oil, grease and other contaminants that adhere to parts, the quicker and easier it is to achieve the required cleanliness.

In multiple-step machining operations, intermediate cleaning processes prevent residue from accumulating and lubricants and coolants from mixing or drying, all of which can lead to problems when cleaning. Intermediate cleaning also prevents machining oil or coolant residue from acting like a magnet and attracting dirt from the shop environment.

Finally, proper preparation and filtering of the coolant or lubricant prevents contaminants already removed from parts from being redeposited on clean parts.

Fine-Tuning the Process

In metalworking, almost all parts cleaning tasks can be performed with wet chemicals. The effectiveness of the process used, and, therefore, the outcome in terms of quality, efficiency and stability, are primarily determined by the dissolving capacity of the chosen cleaning medium.

Aqueous cleaners and solvents are customarily used. Water-based media are available in pH-neutral, alkaline and acidic formulations.

For solvents, a distinction is generally made between chlorinated hydrocarbons, nonhalogenated hydrocarbons and modified alcohols. The latter exhibit hydrophilic and lipophilic properties, which are advantageous because they remove polar and nonpolar contaminants. Modified alcohols are an alternative to chlorinated hydrocarbons, hydrocarbons and aqueous cleaners. However, cleaning trials are recommended to ensure they provide the required level of cleanliness.

Cleaning containers made of round stainless steel wire provide good and complete access to the parts by the cleaning medium and cleaning mechanism, such as ultrasound waves. Image courtesy Metallform Wächter.

When choosing a cleaning medium, the basic chemical principle “like dissolves like” applies. In other words, for a mineral oil-based (nonpolar) contaminant, such as cutting oil, grease or wax, a solvent is generally the correct choice. Chips and particles lose their grip on the surface once the oil is removed.

Solvents are employed in closed-loop cleaning systems that comply with directives for volatile organic compounds. The cleaning process can be divided into several phases, such as cleaning, vapor degreasing, rinsing and drying. (For more about vapor degreasing, see the sidebar on this page.) Parts cleaning machines with multiple fluid tanks make it possible to integrate and use different solvent tanks. For example, a machine can be set up for preliminary cleaning, fine cleaning and corrosion protection.

Modern cleaning systems are equipped with an integrated distillation unit, which continuously removes oils and greases from the solvent. Additionally, these machines offer a filtration unit equipped with bag or multiple-tube filters for removing particulates like swarf, chips and burrs. The unit is usually integrated into fluid loops. When the solvent removes large quantities of metal particles from the parts, a magnetic separator can be integrated into the filtration system. These features enable the solvent to be used in a closed-loop system and assure consistent cleaning quality and low solvent consumption.

Polar Plunge

For water-based (polar) contaminants, such as coolant and lubricant emulsions, polishing pastes, salts, wear particles and other solids, water-based cleaning agents are generally employed. Cleaning tests with the aqueous cleaning agent are recommended to verify material compatibility and that the required results can be achieved.

To ensure consistent cleaning quality for aqueous processes, it is necessary to regularly monitor various parameters, such as the cleaner’s concentration level, temperature, rinse water quality and filter status.