Cleaning methods encompass solvent cleaning, hot alkalinedetergent cleaning, electro-cleaning, and acid etch. In industrial settings, the water-break test is a common practice to assess machinery cleanliness. This test involves thoroughly rinsing and vertically holding the surface. Hydrophobic contaminants, like oils, cause water to bead and break, leading to rapid drainage. In contrast, perfectly clean metal surfaces are hydrophilic and retain an unbroken sheet of water without beading or draining off. It is important to note that this test may not detect hydrophilic contaminants, but they can be displaced during the water-based electroplating process. Surfactants like soap can reduce the test's sensitivity and should 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, and 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 un-purified 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, chips, fines, particles, salts, etc. also 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, and other non-commercial objects.
Elements and their interactions
Factors
Cleaning activities in this sector can only be characterized sufficiently by a description of several factors. These are outlined in the first image above.
Parts and materials to be cleaned
First, consider the parts to be cleaned. They may comprise 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 lighter materials.
Mass 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, contaminants, or soiling. The definition used is quite different. 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 cases, where crack proofing is necessary, one has to remove the paint layer, as it is regarded as an unwanted substance.
The classification of soiling follows the layer structure, starting from the base material:
Structure of a metallic surface
Deformed boundary layer, > 1µm
Reaction layer, 1–10nm
Sorption layer, 1–10nm
Contamination layer, > 1µm
See illustration 2: Structure of a metallic surface [1]
The closer a layer is to the substrate surface, the more energy is needed to remove it. Correspondingly, the cleaning itself can be structured according to the type of energy input:[1]
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 differently. 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, the suitability of cleaning methods for the removal of a given contaminate is discussed.[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:
In order to select suitable equipment and media, it should be known also which amount and which throughput have to be handled. In larger factories, little amounts are virtually ever cleaned economically [clarification needed]. Additionally, the pricing method needs to be determined. Sensitive parts sometimes need to be fixed in boxes. When dealing with large amounts, bulk charging can be used, but it's difficult to achieve a sufficient level of cleanliness with flat pieces clinging together. Drying can also be difficult in these cases.
Place of cleaning
Another consideration is the place of cleaning. Cleaning in a workshop calls for different methods as compared to cleaning that is to be done on site, which can be the case with maintenance and repair work.
Usually, the cleaning takes place in a workshop. Several common methods include solvent degreasing, vapor degreasing, and the use of 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 throughout the ability of the cleaning system.
Such cleaning systems often exactly match the requirements regarding parts, contaminants and charging methods (special production). Central cleaning equipment, often built as multi task systems, is 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.
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.
Each of these steps may take place in its own bath, chamber, or, in case of spray cleaning, in its own zone (line or multi-chamber equipment). But often these steps may have a single chamber into which the respective media are pumped in (single chamber plant).
Cleaning media plays an important role as it removes the contaminants from the substrate.
For liquid media, the following cleaners can be used: 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 brominated and fluorinated substances can be used. The traditionally used chlorinated agents, TCE and PCE, which are hazardous, are now only applied in airtight plants and the modern volume shift systems limit 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, and sequestering agents. With ferrous metal cleaning, rust inhibitors are added into the aqueous cleaner to prevent flash rusting after washing. Their use is on the rise as their results have proven to be most times as good or better than hydrocarbon cleaners. The waste generated is less hazardous, which reduces disposal costs.
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, creates fire and explosion hazards. Nowadays, with proper industrial parts washer equipment, it is accepted that aqueous cleaners remove oil and grease as easily as solvents.
Another approach is with solid cleaning media (blasting) which comprises 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 with no 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:
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 affect 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 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 bath exchange as much as possible, there are sophisticated treatment attachments in use, removing contaminants and the used up agents from the system. 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 change 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 shutoff 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 standardization 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 include the classification in intermediate cleaning, final cleaning, precision cleaning and critical cleaning (s. table), in practice seen only as a general guideline.
Mil-sized particles and residues thicker than a monolayer
E.g. before assembling or coating
Parts for phosphating, painting, enamelling
500 - ≤ 5mg C / m² (2)
Parts for case-hardening, nitriding, nitro carburising resp. vacuum treatment
500 - ≤ 5mg C / m² (2)
Parts for electroplating, electronic parts
20 - ≤ 5mg C / m² (2)
Precision cleaning
≤ 50mg / m² (1)
Supermicrometre particles and residues thinner than a monolayer
Controlled environment (Durkee)
Critical cleaning
≤ 5mg / 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, 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 are 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). There are only a 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. Because of 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:
In 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. Small variations in requirements can cause completely different processes. It becomes more and more important to receive the required 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—because of 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 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 of technology, quality, health and safety at work, environmental protection and 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 regarding 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 are available on-line through the Institute's laboratory database.
In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some electrochemical applications such as electrocoagulation for wastewater treatment, amperometric chemical sensing, and electrochemical synthesis.
Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning is often performed for aesthetic, hygienic, functional, safety, or environmental protection purposes. Cleaning occurs in many different contexts, and uses many different methods. Several occupations are devoted to cleaning.
In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.
Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object. A chrome plated part is called chrome, or is said to have been chromed. The chromium layer can be decorative, provide corrosion resistance, facilitate cleaning, and increase surface hardness. Sometimes, a less expensive substitute for chrome such as nickel may be used for aesthetic purposes.
Dye penetrant inspection (DP), also called liquid penetrate inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials. The penetrant may be applied to all non-ferrous materials and ferrous materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.
Ultrasonic cleaning is a process that uses ultrasound to agitate a fluid, with a cleaning effect. Ultrasonic cleaners come in a variety of sizes, from small desktop units with an internal volume of less than 0.5 litres (0.13 US gal), to large industrial units with volumes approaching 1,000 litres.
Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, and rust or scale from ferrous metals, copper, precious metals and aluminum alloys. A solution called pickle liquor, which usually contains acid, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes.
An air knife is a tool used to blow off liquid or debris from products as they travel on conveyors. Air knives are normally used in manufacturing or as the first step in a recursive recycling process to separate lighter or smaller particles from other components for use in later or subsequent steps, post manufacturing parts drying and conveyor cleaning, part of component cleaning. The knife consists of a high-intensity, uniform sheet of laminar airflow sometimes known as streamline flow.
Electronic packaging is the design and production of enclosures for electronic devices ranging from individual semiconductor devices up to complete systems such as a mainframe computer. Packaging of an electronic system must consider protection from mechanical damage, cooling, radio frequency noise emission and electrostatic discharge. Product safety standards may dictate particular features of a consumer product, for example, external case temperature or grounding of exposed metal parts. Prototypes and industrial equipment made in small quantities may use standardized commercially available enclosures such as card cages or prefabricated boxes. Mass-market consumer devices may have highly specialized packaging to increase consumer appeal. Electronic packaging is a major discipline within the field of mechanical engineering.
The RCA clean is a standard set of wafer cleaning steps which need to be performed before high-temperature processing steps of silicon wafers in semiconductor manufacturing.
Adhesive bonding describes a wafer bonding technique with applying an intermediate layer to connect substrates of different types of materials. Those connections produced can be soluble or insoluble. The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate surfaces. Adhesives, especially the well-established SU-8, and benzocyclobutene (BCB), are specialized for MEMS or electronic component production.
Cleaning agents or hard-surface cleaners are substances used to remove dirt, including dust, stains, foul odors, and clutter on surfaces. Purposes of cleaning agents include health, beauty, removing offensive odor, and avoiding the spread of dirt and contaminants to oneself and others. Some cleaning agents can kill bacteria and clean at the same time. Others, called degreasers, contain organic solvents to help dissolve oils and fats.
Vapor degreasing is a surface finishing process. It involves solvents in vapor form to cleanse the workpiece in preparation for further finishing operations.
Solvent degreasing is a process used to prepare a part for further operations such as electroplating or painting. Typically it uses petroleum, chlorine, dry ice or alcohol based solvents to dissolve the machining fluids and other contaminants that might be on the part.
A parts washer is a piece of equipment used to remove contaminants or debris, such as dirt, grime, carbon, oil, grease, metal chips, cutting fluids, mold release agents, ink, paint, and corrosion from workpieces. Parts washers are used in new manufacturing and remanufacturing processes; they are designed to clean, degrease and dry bulk loads of small or large parts in preparation for assembly, inspection, surface treatment, packaging and distribution. Parts washers may be as simple as the manual "sink-on-a-drum" common to many auto repair shops, or they may be very complex, multi-stage units with pass-through parts handling systems. Parts washers are essential in maintenance, repair and remanufacturing operations as well, from cleaning fasteners, nuts, bolts and screws to diesel engine blocks and related parts, rail bearings, wind turbine gears boxes and automotive assemblies.
Ultrapure water (UPW), high-purity water or highly purified water (HPW) is water that has been purified to uncommonly stringent specifications. Ultrapure water is a term commonly used in manufacturing to emphasize the fact that the water is treated to the highest levels of purity for all contaminant types, including: organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile; reactive, and inert; hydrophilic and hydrophobic; and dissolved gases.
Manganese(II) phosphate is an inorganic compound with the chemical formula Mn3(PO4)2. It has industrial importance as a constituent of manganese based phosphate conversion coatings.
Chemical milling or industrial etching is the subtractive manufacturing process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape. Other names for chemical etching include photo etching, chemical etching, photo chemical etching and photochemical machining. It is mostly used on metals, though other materials are increasingly important. It was developed from armor-decorating and printing etching processes developed during the Renaissance as alternatives to engraving on metal. The process essentially involves bathing the cutting areas in a corrosive chemical known as an etchant, which reacts with the material in the area to be cut and causes the solid material to be dissolved; inert substances known as maskants are used to protect specific areas of the material as resists.
Oxygen compatibility is the issue of compatibility of materials for service in high concentrations of oxygen. It is a critical issue in space, aircraft, medical, underwater diving and industrial applications. Aspects include effects of increased oxygen concentration on the ignition and burning of materials and components exposed to these concentrations in service.
Diffusion bonding or diffusion welding is a solid-state welding technique used in metalworking, capable of joining similar and dissimilar metals. It operates on the principle of solid-state diffusion, wherein the atoms of two solid, metallic surfaces intersperse themselves over time. This is typically accomplished at an elevated temperature, approximately 50-75% of the absolute melting temperature of the materials. A weak bond can also be achieved at room temperature. Diffusion bonding is usually implemented by applying high pressure, in conjunction with necessarily high temperature, to the materials to be welded; the technique is most commonly used to weld "sandwiches" of alternating layers of thin metal foil, and metal wires or filaments. Currently, the diffusion bonding method is widely used in the joining of high-strength and refractory metals within the aerospace and nuclear industries.
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. ISBN0-87170-572-9
Fraunhofer Allianz Reinigungstechnik: market and trend analysis in the industrial parts cleaning, 2007.
Further reading
John B. Durkee: "Management of Industrial Cleaning Technology and Processes," 2006, Elsevier, Oxford, United Kingdom, ISBN0-08-044888-7.
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. ISBN1-56990-216-X
Barbara Kanegsberg ed.: Handbook for critical cleaning. 2001, CRC Press, Boca Raton, Florida, USA. ISBN0-8493-1655-3
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. ISBN0-9645356-7-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. ISBN0-87170-572-9
ASM International: Guide to acid, alkaline, emulsion, and ultrasonic cleaning. 1997, ASM International, Materials Park, Ohio, USA. ISBN0-87170-577-X
ASM International: Guide to vapour degreasing and solvent cold cleaning. 1996, ASM International, Materials Park, Ohio, USA. ISBN0-87170-573-7
ASM International: Guide to mechanical cleaning systems. 1996, ASM International, Materials Park, Ohio, USA. ISBN0-87170-574-5
ASM International: Guide to pickling and descaling, and molten salt bath cleaning. 1996, ASM International, Materials Park, Ohio, USA. ISBN0-87170-576-1
Thomas W. Jelinek: Reinigen und Entfetten in der Metallindustrie. 1. Edition 1999, Leuze Verlag, Saulgau, ISBN3-87480-155-1
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
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