Definitions

Metal surface cleaning

Parts cleaning

Parts cleaning is essential to many industrial processes, as a prelude to surface finishing or to protect sensitive components. Electroplating is particularly sensitive to part cleanliness, since molecular layers of oil can prevent 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, electrocleaning, and acid etch. The most common industrial test for cleanliness is the waterbreak test, in which the surface is thoroughly rinsed and held vertical. 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 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 here described activities you will often find the following terms: metal cleaning, metal surface cleaning, component cleaning, degreasing. These are well established in technical language usage but they have their shortcomings. Metal cleaning can easily be mixed up with 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 characterised sufficiently by a description of a number of different factors. These are outlined in illustration 1.

Parts and materials to be cleaned

Firstly to be considered are the parts to be cleaned. They may consist of non- or hardly processed sections, sheets and wires. But also machined parts or assembled components need cleaning. Thereby 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 uses 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 proves the geometry of the parts. Long, thin, branching, threaded holes, in which could be jammed chips, feature among the greatest challenges in this technical field. Among others robots are used, which are programmed to exactly flush the drilled holes under high pressure.

Contaminations

The parts are usually covered by unwanted substances, the contaminations or soiling. The definition used is quite flexible. 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 first of all 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 according to Brigitte Haase: Reinigen oder Vorbehandeln? Oberflächenzustand und Nitrierergebnis, Bauteilreinigung, Prozesskontrolle und –analytik. University of Applied Sciences Bremerhaven

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 energy aspects (see Brigitte Haase, l.c.); thereby the application of the energy can take place mechanically, thermally or chemically according to the type of cleaning.

Especially 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. 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

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

But usually the cleaning takes place in the workshop. Two common versions of this are Solvent Degreasing and Vapor Degreasing. Thereby 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 plant.

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

Cleaning equipment and procedure

Firstly one can differentiate between the following techniques:

  • Manual
  • Mechanical
  • Automatic
  • Robot supported

The process may be performed in one single step, which is especially true for the manual cleaning, but typically it needs several steps. Thereby 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. It can also be the other way round, that 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:

1. Pre cleaning
2. Main cleaning
3. Rinsing
4. Rinsing with deionised water
5. Rinsing with corrosion protection
6. 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 plants). But quite often these steps may have a single chamber only into which the respective media are pumped in (single chamber plant).

Besides equipment and procedure, cleaning media play an important role as they eventually remove the contaminants from the substrate.

As 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 traditionally used chlorinated agents TCI and PCE are nowadays only applied in airtight plants and the modern volume shift systems do hardly permit any emissions.

Aqueous cleaners are mostly a combination of various substances like builder, surfactants, sequestering agents, etc. In the group of hydrocarbon based solvents we find some newly developed agents like fatty acid esters made of natural fats and oils, modified alcohols and dibasic esters.

Aqueous cleaners have advantages as regards particle and polar contaminants but generally need a higher energy input, whereas solvents score in removal of oils and greases but have their health and environmental risks. In addition most of them are flammable and create fire and explosion hazards.

A fairly new approach in the field of solid media (blasting) constitutes the CO2 dry ice process: For tougher requirements pellets are in use whereas in case of more sensitive materials or components CO2 in form of snow is applied.

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

All cleaning steps are not only characterised by media and related temperatures but also by their individual agitation/application (mechanical impact). Also here we find a wide range of different methods and combinations of these methods:

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

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

Every 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 respectively 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 sharpening of the bath. With the help of oil separators, demulsifying agents and evaporators aqueous processes can be conducted 'waste water 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 cut 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 this turns out to be 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.

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 not possible. However it is possible to use general classifications. In Germany it was attempted to define cleaning as a sub category of metal treatment (DIN 8592: Cleaning as sub category of cutting processes), but this does not at all cope with the complexity of cleaning.

The rather general rules include 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 - acc.to Kurt Hertlein, Dt. Shell Chemie, 1989 Soils removed - acc. to John Durkee in A2C2, 2003 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 (among other things water break test, wipe test, measurement of contact angle, test inks, tape test) to complex analysis methods (among others gravimetric test, particle counting, infrared spectroscopy, glow discharge spectroscopy, energy dispersive X-Ray analysis, scanning electron microscopy and electrochemical methods). 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 (see e.g. German professional journal JOT 6, 2006 pg. 50-53)

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 also 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 for this area, describing methods which can be used to 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 minimised health and environmental risks. 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 Programme 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 university of Dortmund, assisting the users to analyse 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).

Literature and further information

Literature

  • John B. Durkee: "Management of Industrial Cleaning Technology and Processes," 2006, Elsevier, Oxford, United Kingdom, ISBN 0080448887.
  • Carole A. LeBlanc: The search for safer and greener chemical solvents in surface cleaning : a proposed tool to support environmental decision-making. 2001, Erasmus University Centre for Environmental Studies, Rotterdam, the Netherlands.
  • David S. Peterson: Practical guide to industrial metal cleaning. 1997, Hanser Gardner Publications, Cincinnati, Ohio, USA. ISBN 1-56990-216-X
  • Barbara Kanegsberg ed.: Handbook for critical cleaning. 2001, CRC Press, Boca Raton, Florida, USA. ISBN 0-8493-1655-3
  • Malcolm C. McLaughlin et al.: The aqueous cleaning handbook : a guide to critical-cleaning procedures, techniques, and validation. 2000, The Morris-Lee Publishing Group, Rosemont, New Jersey, USA. ISBN 0-9645356-7-X
  • Karen Thomas, John Laplante, Alan Buckley: Guidebook of part cleaning alternatives : making cleaning greener in Massachusetts. 1997, Toxics Use Reduction Institute, University of Massachusetts, Lowell, Massachusetts, USA
  • ASM International: Choosing a cleaning process. 1996, ASM International, Materials Park, Ohio, USA. ISBN 0-87170-572-9
  • ASM International: Guide to acid, alkaline, emulsion, and ultrasonic cleaning. 1997, ASM International, Materials Park, Ohio, USA. ISBN 0-87170-577-X
  • ASM International: Guide to vapour degreasing and solvent cold cleaning. 1996, ASM International, Materials Park, Ohio, USA. ISBN 0-87170-573-7
  • ASM International: Guide to mechanical cleaning systems. 1996, ASM International, Materials Park, Ohio, USA. ISBN 0-87170-574-5
  • ASM International: Guide to pickling and descaling, and molten salt bath cleaning. 1996, ASM International, Materials Park, Ohio, USA. ISBN 0-87170-576-1
  • Klaus-Peter Müller: Praktische Oberflächentechnik. Edition 2003.XII, vieweg, Braunschweig/Wiesbaden, ISBN 978-3-528-36562-2
  • Thomas W. Jelinek: Reinigen und Entfetten in der Metallindustrie. 1. Edition 1999, Leuze Verlag, Saulgau, ISBN 3-87480-155-1
  • Brigitte Haase: Wie sauber muß eine Oberfläche sein? in: Journal Oberflächentechnik. Nr. 4, 1997
  • Brigitte Haase: Reinigen oder Vorbehandeln? Oberflächenzustand und Nitrierergebnis, Bauteilreinigung, Prozesskontrolle und –analytik. Hochschule Bremerhaven
  • Bernd Künne: Online Fachbuch für industrielle Reinigung. in: bauteilreinigung.de. Universität Dortmund, Fachgebiet Maschinenelemente
  • Reiner Grün: Reinigen und Vorbehandeln - Stand und Perspektiven. in: Galvanotechnik. 90, 1999, Nr. 7, S. 1836-1844
  • Günter Kreisel et al.: Ganzheitliche Bilanzierung/Bewertung von Reinigungs-/Vorbehandlungstechnologien in der Oberflächenbehandlung. 1998, Jena, Institut für Technische Chemie der FSU

Professional journals

  • Controlled Environments, USA (formerly A2C2??)
  • JOT, Journal Oberflächentechnik, vieweg Verlag, Germany
  • mo, metalloberfläche, I.G.T. Informationsgesellschaft Technik mbH, Germany
  • Galvanotechnik, Eugen G. Leuze Verlag, Germany
  • MM, Maschinenmarkt, Vogel Industrie Medien GmbH & Co.KG, Germany

Fora, List mail

  • NMFRC.org, USA
  • JOT-Oberflaeche.de, Germany
  • PIUS-Info.de, Germany (also English pages)

Research

  • TURI, Toxic Use Reduction Institute of the Lowell University in Massachusetts, USA
  • RTI, Research Triangle Institute, Raleigh, N.C., USA
  • IWT, Stiftung Institut für Werkstofftechnik, Germany
  • AWT, Arbeitsgemeinschaft Wärmebehandlung und Werkstofftechnik e. V., Germany
  • Fraunhofer Allianz Reinigungstechnik, Germany
  • Conference proceedings, published by PIUS, Germany, partly available in English

Work groups

  • AWT Fachausschuss 14, Germany
  • Fachausschuss Reinigen der Metall Berufsgenossenschaften, Germany

Federations

  • Fachverband Industrielle Teilereinigung e.V., FiT e.V., fit-online.de, Germany
  • Industrieverband Hygiene und Oberflächenschutz für industrielle und institutionelle Anwendung e.V., IHO, iho.de, Germany

See also

Trade fairs

  • parts2clean, the leading trade fair for cleaning in production, Germany

External links

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