Parts cleaning

Parts cleaning is essential to many industrial processes, as a prelude to surface finishing or to protect sensitive components. Electroplating is particularly a sensitive to part cleanliness, since molecular layers of oil can prevent the adhesion of the coating. ASTM B322 is a standard guide for cleaning metals prior to electroplating. Cleaning processes include solvent cleaning, hot alkaline detergent cleaning, electro-cleaning, and acid etch. The most common industrial test for cleanliness is the water-break test, in which the surface is thoroughly rinsed and held vertical. A quantitative measurement for this parameter is contact angle. Hydrophobic contaminants such as oils cause the water to bead and break up, allowing the water to drain rapidly. Perfectly clean metal surfaces are hydrophilic and will retain an unbroken sheet of water that does not bead up or drain off. ASTM F22 describes a version of this test. This test does not detect hydrophilic contaminants, but the electroplating process can displace these easily since the solutions are water-based. Surfactants such as soap reduce the sensitivity of the test, so these must be thoroughly rinsed off.

Definitions and classifications

For the activities described here, the following terms are often found: metal cleaning, metal surface cleaning, component cleaning, degreasing, parts washing, parts cleaning. These are well established in technical language usage but they have their shortcomings. Metal cleaning can easily be mixed up with the refinement of unpurified metals. Metal surface cleaning and metal cleaning do not consider the increasing usage of plastics and composite materials in this sector. The term component cleaning leaves out the cleaning of steel sections and sheets and finally, degreasing only describes a part of the topic as in most cases also chips, fines, particles, salts etc. have to be removed.

The terms 'commercial and industrial parts cleaning', 'parts cleaning in craft and industry' or 'commercial parts cleaning' probably best describe this field of activity. There are some specialists who prefer the term 'industrial parts cleaning', because they want to exclude maintenance of buildings, rooms, areas, windows, floors, tanks, machinery, hygiene, hands washing, showers etc.

Elements and their interactions

Cleaning activities in this sector can only be characterized sufficiently by a description of a number of different factors. These are outlined in illustration 1.

Parts and materials to be cleaned

First, consider the parts to be cleaned. They may consist of non-processed or hardly processed sections, sheets and wires, but also machined parts or assembled components needing cleaning. Therefore, they may be composed of different metals or different combinations of metals. Plastics and composite materials can frequently be found and indeed are on the increase because e.g. the automobile industry, as well as others, are using more and more lighter materials.

Mass and size can be very important for the selection of cleaning methods. For example, big shafts for ships are usually cleaned manually, whereas tiny shafts for electrical appliances are often cleaned in bulk in highly automated plants.

Similarly important is the geometry of the parts. Long, thin, branching, threaded holes, which could contain jammed chips, feature among the greatest challenges in this technical field. High pressure and the power wash process are one way to remove these chips, as well as robots, which are programmed to exactly flush the drilled holes under high pressure.

Contaminations

The parts are usually covered by unwanted substances, contaminations or soiling. The definition used is quite varied. In certain cases these coverings may be desired: e.g. one may not wish to remove a paint layer but only the material on top. In another case, where crack proofing is necessary, one has to remove the paint layer and it is regarded as an unwanted substance.

The classification of soiling follows the layer structure starting from the base material:

Deformed boundary layer, > 1 µm
Reaction layer, 1–10 nm
Sorption layer, 1–10 nm
Contamination layer, > 1 µm

See illustration 2: Structure of a metallic surface ^[1]

The nearer the layers are to the substrate surface, the more energy is needed to remove them. Correspondingly the cleaning itself can be structured according to the type of energy input:^[1]

Mechanical – abrasive: blasting, grinding
Mechanical – non-abrasive: stirring, mixing, ultrasound, spraying
Thermal – reactive: heat treatment much above 100 °C in reactive gases
Thermal – non-reactive: temperature below 100 °C, increased bath temperature, Vapor degreasing
Chemical – abrasive/reactive: pickling in liquids, plasma-assisted, sputter-cleaning, electropolishing
Chemical – non-reactive: organic solvents, aqueous solutions, supercritical CO₂

The contamination layer may then be further classified according to:

Origin
Composition: e.g. cooling lubricants may be composed very differently, thereby single components may account for big problems especially for job shop cleaners, who have no control over prior processes and thus don't know the contaminants. For example, silicates may obstruct nitriding.
State of aggregation
Chemical and physical properties

The American Society for Testing and Materials (ASTM) presents six groups of contaminations in their manual "Choosing a cleaning process" and relates them to the most common cleaning methods, thereby the suitability of cleaning methods for the removal of a given contaminate is discussed in detail.^[2] In addition they list exemplary cleaning processes for different typical applications. Since one has to consider very many different aspects when choosing a process, this can only serve as a first orientation. The groups of contaminants are stated as follows:

Pigmented drawing compounds
Unpigmented oil and grease
Chips and cutting fluids
Polishing and buffing compounds
Rust and scale
Others

Charging

In order to select suitable equipment and media it should be known also which amount and which throughput has to be handled. Small amounts can hardly be cleaned economically in larger plants. Also, the type of charging has to be ascertained. Sensitive parts sometimes need to be fixed in boxes. Very economically when dealing with large amounts is bulk charging, but it is quite difficult to achieve a sufficient level of cleanliness with flat pieces clinging together. Also drying can be difficult in these cases.

Place of cleaning

Another consideration is the place of cleaning. E.g., the cleaning is to be done on site, which can be the case with repair and maintenance work.

Usually, the cleaning takes place in a workshop. Several common methods are solvent degreasing, vapor degreasing and using an aqueous parts washer. Companies often want the charging, loading and unloading to be integrated into the production line, which is much more demanding as regards size and throughput ability of the cleaning system.

Such cleaning systems often exactly match the requirements regarding parts, contaminants and charging methods (special production). Nonetheless, central cleaning equipment, often built as multi task systems, are commonly used. These systems can suit different cleaning requirements. Typical examples are the wash stands or the small cleaning machines which are found in many industrial plants.

Cleaning equipment and procedure

First, one can differentiate among the following techniques (ordered from most to least technologically advanced):

Manual
Mechanical
Automatic
Robot supported

The process may be performed in one step, which is especially true for the manual cleaning, but typically it requires several steps. Therefore, it is not uncommon to find 10 to 20 steps in large plants e.g., for the medical and optical industry. This can be especially complex because non-cleaning steps may be integrated in such plants like application of corrosion protection layers or phosphating. Cleaning can also be simple, the cleaning processes are integrated into other processes as it is the case with electroplating or galvanising, where it usually serves as a pre-treatment step.

The following procedure is quite common:

Pre-cleaning
Main cleaning
Rinsing
Rinsing with deionised water
Rinsing with corrosion protection
Drying

Each of these steps may take place in its own bath or chamber or in case of spray cleaning in its own zone (line or multi-chamber equipment). But quite often these steps may have a single chamber into which the respective media are pumped in (single chamber plant).

Besides equipment and procedure, cleaning media plays an important role as it removes the contaminants from the substrate.

For liquid media the following cleaners are in use: aqueous agents, semi-aqueous agents (an emulsion of solvents and water), hydrocarbon-based solvents and halogenated solvents. Usually, the latter are referred to as chlorinated agents, but there are also brominated and fluorated substances in (limited) use, that is why we have chosen the higher-level classification. The hazardous traditionally used chlorinated agents TCE and PCE are nowadays only applied in airtight plants and the modern volume shift systems limits any emissions. In the group of hydrocarbon-based solvents, there are some newly developed agents like fatty acid esters made of natural fats and oils, modified alcohols and dibasic esters.

Aqueous cleaners are mostly a combination of various substances like alkaline builders, surfactants, sequestering agents, etc. In the case of ferrous metal cleaning, rust inhibitors are built into the aqueous cleaner to prevent flash rusting after washing. Their use is on the rise as their results have proven to be in many cases as good or better than hydrocarbon cleaners. Additionally, the wastes generated are less hazardous resulting in less costly disposal.

Aqueous cleaners have advantages as regards to particle and polar contaminants and only require higher inputs of mechanical and thermal energy to be effective, whereas solvents more easily remove oils and greases but have health and environmental risks. In addition, most solvents are flammable and create fire and explosion hazards. Nowadays, with the proper industrial parts washer equipment, it is generally accepted that aqueous cleaners remove oil and grease as easily as solvents.

Another approach is with solid cleaning media (blasting) which consists of the CO₂ dry ice process: For tougher requirements pellets are used while for more sensitive materials or components CO₂ in form of snow is applied. One drawback is the high energy consumption required to make dry ice.

Last but not least there are processes without any media like vibration, laser, brushing and blow/exhaust systems.

All cleaning steps are characterized by media and applied temperatures and their individual agitation/application (mechanical impact). There is a wide range of different methods and combinations of these methods:

Blasting
Boiling under pressure
Carbon dioxide cleaning
Circulation of bath
Flooding
Gas or air injection into bath
Hydroson
Injection flooding
Megasonic, see megasonic cleaning
Movement of parts (turning, oscillating, pivoting)
Power wash process
Pressure flooding
Spraying
Sprinkling
Ultrasonic, see ultrasonic cleaning

Finally, every cleaning step is described by the time which the parts to be cleaned spends in the respective zone, bath, or chamber, and thus medium, temperature, and agitation can impact on the contamination.

Every item of cleaning equipment needs a so-called periphery. This term describes measures and equipment on the one hand side to maintain and control baths and on the other hand side to protect human beings and the environment.

In most plants the cleaning agents are circulated until their cleaning power has eventually decreased and reached the maximum tolerable contaminant level. In order to delay the necessary bath exchange as much as possible there are sophisticated treatment attachments in use, removing contaminants and the used up agents from the system. At the same time fresh cleaning agents or parts thereof have to be supplemented, which requires a bath control. The latter is more and more facilitated online and thus allows a computer aided adjustment of the bath. With the help of oil separators, demulsifying agents and evaporators aqueous processes can be conducted 'wastewater free'. Complete exchange of baths becomes only necessary every 3 to 12 months.

When using organic solvents, the preferred method to achieve a long operating bath life is distillation, an especially effective method to separate contaminants and agents.

The periphery also includes measures to protect the workers like encapsulation, automatic shut off of power supply, automatic refill and sharpening of media (e.g., gas shuttle technique), explosion prevention measures, exhaust ventilation etc., and also measures to protect the environment, e.g. capturing of volatile solvents, impounding basins, extraction, treatment and disposal of resulting wastes. Solvents based cleaning processes have the advantage that the dirt and the cleaning agent can be more easily separated, whereas in aqueous processes is more complex.

In processes without cleaning media, like laser ablation and vibration cleaning, only the removed dirt has to be disposed of as there is no cleaning agent. Quite little waste is generated in processes like CO₂ blasting and automatic brush cleaning at the expense of higher energy costs.

Quality requirements

A standardisation of the quality requirements for cleaned surfaces regarding the following process (e.g. coating, heat treatment) or from the point of view of technical functionality is difficult. However, it is possible to use general classifications. In Germany it was attempted to define cleaning as a subcategory of metal treatment (DIN 8592: Cleaning as sub category of cutting processes), but this does not cope with all the complexities of cleaning.

The rather general rules includes the classification in intermediate cleaning, final cleaning, precision cleaning and critical cleaning (s. table), in practice seen only as a general guideline.

Terms	Max. allowed dirt ^[3]	Soils removed ^[4]	Explanations
Intermediate cleaning			E.g. in metal cutting manufacturing
Final cleaning	≤ 500 mg / m² (1)	Mil-sized particles and residues thicker than a monolayer	E.g. before assembling or coating
Parts for phosphating, painting, enamelling	500 - ≤ 5 mg C / m² (2)
Parts for case-hardening, nitriding, nitro carburising resp. vacuum treatment	500 - ≤ 5 mg C / m² (2)
Parts for electroplating, electronic parts	20 - ≤ 5 mg C / m² (2)
Precision cleaning	≤ 50 mg / m² (1)	Supermicrometre particles and residues thinner than a monolayer	Controlled environment (Durkee)
Critical cleaning	≤ 5 mg / m² (1)	Sub-micrometre particles and non-volatile residue measured in Angstroms	cleanroom (Durkee)

(1) Related to the total dirt; (2) Only related to Carbon

Thus, in practice, the rule of thumb is still followed, stating that the quality requirements are met, if the subsequent process (see below) does not cause any problems, for example, a paint coating does not flake off before the guarantee period ends.

Where this is not sufficient, especially in case of external orders, because of missing standards there are often specific customer requirements regarding remaining contamination, corrosion protection, spots and gloss level, etc.

Measuring methods to ensure quality therefore do not play a bigger role in the workshops, although there exist a broad scale of different methods, from visual control over simple testing methods (water break test, wipe test, measurement of contact angle, test inks, tape test, among others) to complex analysis methods (gravimetric test, particle counting, infrared spectroscopy, glow discharge spectroscopy, energy dispersive X-ray analysis, scanning electron microscopy and electrochemical methods, among others). Nevertheless, there are only few methods, which can be applied directly in the line and which offer reproducible and comparable results. It was not until recently that bigger advancements in this area have been made ^[5]

The general situation has changed meanwhile, because of dramatically rising cleanliness requirements for certain components in the automotive industry. For example, brake systems and fuel-injection systems need to be fitted with increasingly smaller diameters and they have to withstand increasingly higher pressures. Therefore, a very minor particle contamination may lead to big problems. Due to the rising innovation speed, the industry cannot afford to identify possible failures at a relatively late stage. Therefore, the standard VDA 19/ISO 16232 'Road Vehicles – Cleanliness of Components of Fluid Circuits' was developed which describes methods that can control the compliance with the cleanliness requirements.

Subsequent process

When choosing cleaning techniques, cleaning agents and cleaning processes, the subsequent processes, i.e. the further processing of the cleaned parts is of special interest.

The classification follows basically the metal work theory:

Machining
Cutting
Joining
Coating
Heat treatment
Assembling
Measuring, testing
Repairing, maintenance

In the course of time empirical values were established, how efficient the cleaning has to be, to assure the processes for the particular guarantee period and beyond. Choosing the cleaning method often starts from here.

Challenges and trends

The details above illustrate how extremely complex this specific field is. Already small changes in the requirements can necessitate completely different processes. Thus, it defies scientific technical determination. On the other hand, it becomes more and more important to receive the required degree of cleanliness as cost-effective as possible and with continuously minimized health and environmental risks, because cleaning has become of central importance for the supply chain in manufacturing.^[6] Applying companies usually rely on their suppliers, who—due to a big experience base—suggest adequate equipment and processes, which are then adapted to the detailed requirements in tests stations at the supplier’s premises. However, they are limited to their scope of technology. To put practitioners in a position to consider all relevant possibilities meeting their requirements, some institutes have developed different tools:

SAGE: Unfortunately no longer in operation the comprehensive expert system for parts cleaning and degreasing provided a graded list with relatively general processes of possible solvent and process alternatives. Developed by the Surface Cleaning Program at the Research Triangle Institute, Raleigh, North Carolina, USA, in cooperation with the U.S. EPA (used to be available under: http://clean.rti.org/).

Cleantool: A ‘Best Practice’ database in seven languages with comprehensive and specific processes, directly recorded in companies. It contains furthermore an integrated evaluation tool, which covers the areas technology, quality, health and safety at work, environmental protection as well as costs. Also included is a comprehensive glossary (seven languages, link see below).

Bauteilreinigung: A selection system for component cleaning developed by the University of Dortmund, assisting the users to analyze their cleaning tasks with regard to the suitable cleaning processes and cleaning agents (German only, link see below).

TURI, Toxic Use Reduction Institute: A department of the University of Lowell, Massachusetts (USA). TURI's laboratory has been conducting evaluations on alternative cleaning products since 1993. A majority of these products were designed for metal surface cleaning. The results of these tests are available on-line through the Institute’s laboratory database (English only link, see below).

References

^ ^a ^b :Brigitte Haase: Reinigen oder Vorbehandeln? Oberflächenzustand und Nitrierergebnis, Bauteilreinigung, Prozesskontrolle und –analytik. University of Applied Sciences Bremerhaven.
^ ASM International: Choosing a cleaning process. 1996, ASM International, Materials Park, Ohio, USA. ISBN 0-87170-572-9
^ :Kurt Hertlein: Dt. Shell Chemie, 1989.
^ :John Durkee in A2C2, 2003.
^ :Doris Schulz: Steigende Anforderungen an die Reinigungsqualität - Kontrollierte Sauberkeit.JOT Journal für Oberflächentechnik Vieweg Verlag/GWV Fachverlage GmbH, No. 6, 2006 pg. 50-53.
^ :Fraunhofer Allianz Reinigungstechnik: market and trend analysis in the industrial parts cleaning, 2007.