Parts cleaning

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Commercial and industrial parts cleaning is used to remove unwanted layers or particles from the parts to be cleaned in order to ensure the quality of either subsequent processes (such as paint coating, electroplating , hardening) or the purity of the end products.

Delimitations

Very often one finds for the activities described here, the following designations: Metal Cleaning (English: metal cleaning ), metal -Oberflächenreinigung (English: metal surface cleaning ), cleaning of components (English: parts or component cleaning ), degreasing (English: degreasing ). These have become established in linguistic usage, but suffer from a certain inaccuracy. Metal cleaning can easily be confused with the refining of contaminated metals and, like metal surface cleaning, it disregards the fact that other materials such as plastics and composite materials are increasingly being cleaned. The term component cleaning ignores the fact that raw materials such as steel profiles and sheets are also cleaned and degreasing only describes a sub-area, because in the vast majority of cases, chips, abrasion, particles, salts, etc. must also be cleaned off.

The terms “commercial and industrial parts cleaning”, “parts cleaning in trade and industry” or “commercial parts cleaning” are therefore probably the most appropriate descriptions of the specialist area. There are experts who prefer the term “industrial parts cleaning” in order to exclude maintenance of buildings, rooms, grounds, windows , floors , mixing tanks, machines , hygiene , hand washing, showers, etc.

Components and their interaction

Factors

Cleaning activities in this sector can only be adequately represented by describing the interactions of a number of factors.

Items to be cleaned

The starting point is the items to be cleaned. This can consist of unprocessed or hardly processed steels, sheets and wires. However, machined workpieces and assembled components also count as items to be cleaned. Very different materials can be used, and combinations of different metals and even plastics or composites often occur. These are even taking up more and more space because, for example, lightweight materials are increasingly being used in the automotive industry. Mass and size can be very important for the cleaning methods to be selected. For example, long ship shafts are usually cleaned manually (by hand), while small shafts for electrical devices, e.g. B. be cleaned as bulk material in highly automated systems. The geometry of the parts is just as important. For example, long, thin angled holes in which there may still be chips trapped are among the greatest challenges in this field. Here u. a. Robots are used that are programmed to precisely flush out the holes under high pressure.

Impurities

The items to be cleaned are covered with undesirable substances, the impurities or contaminants. These are defined flexibly. In certain cases, some coverings may well be desirable and you want z. B. Do not attack a paint coating, just remove the material lying on it. However, if you want to test for cracks, for example, the layers of paint must be removed and in this case they are among the undesirable substances.

The contamination is initially classified according to the layer structure:

Structure of a metallic surface
  • Deformed boundary layer,> 1 µm
  • Reaction layer, 1 to 10 nm
  • Sorption layer, 1 to 10 nm
  • Contamination layer,> 1 µm

See illustration 2: Structure of a metallic surface according to:

The closer the layers are to the substrate surface, the more energy has to be expended to remove them. Accordingly, there is a possible classification of cleaning according to the type of energy input:

  • mechanical - abrasive: blasting, grinding
  • mechanical - non-abrasive: stirring, mixing, ultrasound, spraying
  • thermal - reactive: heat treatment well above 100 ° C in reactive gases
  • thermal - non-reactive: temp. below 100 ° C, increased bath temperature, steam degreasing , depending on the cleaning agent
  • chemical - abrasive / reactive: pickling in liquid solutions, plasma assisted, sputter cleaning, electropolishing
  • chemical - non-reactive: organic solvents, aqueous solutions, supercritical CO 2

The contamination layer can then be further divided into:

  • origin
  • Composition: For example, cooling lubricants (KSS) can have very different compositions, individual components can lead to major problems, especially for contract cleaners who have no control over the processing steps. For example, silicates interfere with nitriding.
  • Physical state
  • Chem./physical properties

The American Society for Testing and Materials (ASTM) presents in its manual “Choosing a cleaning process” six groups of soiling and compares them with the most common cleaning methods, with detailed information on the suitability of the cleaning processes for removing the given soiling and sample cleaning processes for different ones typical applications are listed. However, since there are many different aspects to consider when selecting the procedure, this cannot be more than a first rough guide. The individual pollution groups are:

feed

In order to be able to select cleaning technology and media appropriately, it should also be known which number and with which throughput is to be cleaned. Small numbers of items can hardly be cleaned economically in larger systems. The form of the loading must also be determined. More sensitive parts sometimes need to be fixed in the batch containers. The bulk material is particularly advantageous in terms of loading, but it is very difficult here to achieve adequate cleaning and to ensure drying in the case of flat parts lying on top of one another.

Place of cleaning

Another condition in the selection is the question of whether cleaning has to be carried out on site, which can be the case with repairs and maintenance.

However, cleaning usually takes place in the workshop. The loading should often be integrated into the production process and cleaning z. B. be part of the assembly line, which places increased demands on the system technology in terms of size and throughput.

Such systems are often precisely adapted to the requirements in terms of the items to be cleaned, contamination and loading processes (special systems). Central cleaning facilities are still standard, however, which are then mostly made as multitask systems, i.e. H. they can meet various cleaning needs. A simple, typical example are the washstands or simple cleaning machines that can be found in many workshops.

Cleaning technology

First of all, a distinction can be made between:

  • Manual
  • Machine
  • Automatic and
  • Robot-assisted

techniques

The cleaning process can be carried out in a single step, this is especially true for manual cleaning, but it typically involves several steps. In complex systems (e.g. in medical technology or the optical industry), up to 10 to 20 different steps can be run through. This can also become particularly confusing because non-cleaning steps are often integrated into the systems: e.g. B. the application of corrosion protection or phosphating . On the other hand, cleaning itself can also be integrated into other processes, such as electroplating or hot-dip galvanizing as a pretreatment step.

The following sequence is often encountered:

  1. Pre-cleaning
  2. Main cleaning
  3. do the washing up
  4. Rinse with deionized water
  5. Rinsing with corrosion protection
  6. dry

Each of these steps can take place in its own bath or container or, in the case of spray cleaning, have its own area (row or multi-chamber systems). Very often, however, the various steps also take place in a single chamber, into which the respective cleaning media are fed alternately (single-chamber systems).

In addition to technology and processes, the cleaning media play a very central role as they ultimately remove the dirt from the substrate.

The liquid media used are aqueous cleaners, emulsion cleaners (they consist of an emulsion of solvents in water), hydrocarbon solvents (KWL) and halogenated hydrocarbons. Usually we speak of chlorinated substances here. However, brominated and fluorinated agents are also used to a lesser extent, which is why the overarching term is chosen here. The traditional chlorinated cleaners (CHC) Tri and Per are only used in closed systems under vacuum today, with the modern volume shifting process virtually eliminating emissions. Aqueous cleaners are usually made from a combination of ingredients (builders, surfactants, complexing agents). There have been a number of recent developments in hydrocarbon solvents such as: B. the vegetable oil esters, the modified alcohols and the dibasic esters.

For certain applications such as For example, molten salts are used to remove molding sand and scale from cast parts or to remove layers of enamel .

The aqueous cleaners have advantages for particulate and polar soiling, but usually require a relatively large amount of energy. The solvents score points for greasy and oily soiling, but they have their health and environmental risks. Many are also flammable, creating fire and explosion hazards.

A relatively new approach to solid media / beam method is the use of CO 2 - dry ice in the form of pellets for coarser requirements or in the form of snow for sensitive materials or components.

There are also processes that do not require cleaning media: vibration, laser, brush and blow-suction processes.

In addition to the cleaning media and the associated temperatures, all cleaning steps are also characterized by their respective special agitation / application (mechanical effect). Here, too, there is a wide range of different methods and combinations of these methods:

  • Spray
  • Syringes
  • Rays
  • Floods
  • Vapor degreasing
  • Moving the items to be cleaned (rotating, oscillating, swiveling)
  • Bath circulation
  • Injection of gases
  • Vacuum cooking
  • Injection floods
  • Pressure floods
  • Hydroson
  • Ultrasound of different frequencies (high-frequency US from 1 MHz, used on sensitive components, is called megasonic in English-speaking countries), see also ultrasonic cleaning device

Finally, each cleaning step is also characterized by the time in which the items to be cleaned are in the relevant zone / bath / container and thus medium, temperature and agitation can act on the contamination.

Every cleaning technique requires a so-called periphery. This includes, on the one hand, measures and techniques for bathroom maintenance and monitoring (extension of service life) and, on the other hand, measures to protect people and the environment.

The cleaning agents are used again and again in most systems until their cleaning performance is ultimately too low or the maximum tolerable dirt-carrying capacity has been reached. In order to delay this replacement time as long as possible, sophisticated processing systems are used that remove impurities and exhausted cleaners from the system. At the same time, cleaning agents have to be added, which also requires bath monitoring. The latter is happening more and more online and allows a computer-controlled automatic re-sharpening of the bathroom. With the help of oil separators, demulsifying cleaners and evaporators, aqueous processes can now be run "without wastewater". Complete bath changes are only required every 3 to 12 months on average.

In the case of organic solvents, distillation is usually used to extend the service life, which allows a particularly effective separation of cleaning media and contamination.

The periphery also includes measures to protect employees, such as equipment for system encapsulation, automatic shutdown, automatic filling with the cleaning media (e.g. gas displacement method), explosion protection systems, extraction systems, etc., as well as measures to protect the environment, e.g. B. the collection of volatile solvents, collecting trays, the separation, treatment, processing and disposal of the waste generated. Solvent-based cleaning processes have advantages here, as dirt and detergent can be separated more easily, whereas this is more complex with aqueous processes.

In the case of media-free processes such as laser treatment and vibration cleaning, i. d. Usually only the cleaned off dirt for disposal and not additionally used cleaner. In proportion, processes such as (CO 2 ) dry ice blasting / snow blasting and automated brush cleaning generate little waste .

Quality requirements

The standardization of the quality requirements for a cleaned component surface with regard to the subsequent process (coating, heat treatment) or from a technically functional point of view (intended use) is not possible. The division into relatively general areas is possible, however. An attempt was made to tie cleaning into the corset of metalworking (DIN 8592: cleaning as a sub-category of separation processes), which cannot do justice to the complexity of cleaning.

One of the more general rules is the classification into intermediate cleaning, final cleaning, fine and ultra-fine cleaning (see table), which in practice is only viewed as a general guideline.

designation Max. permissible dirt
(according to Hertlein) 		Soils removed
(after Durkee) 		Explanations
Additional cleaning for example in machining
Final cleaning ≤ 500 mg / m² (1) Mil-sized particles and residues thicker than a monolayer for example, before assembly or coating
  • Parts for phosphating, painting, enamelling
 500… 5 mg · C / m² (2)
  • Parts for case hardening, nitriding, nitrocarbonizing respectively. Vacuum treatment
 500… 5 mg · C / m² (2)
  • Electroplating parts, electronic parts
 20… 5 mg · C / m² (2)
Fine cleaning (ger .: Precision cleaning ) ≤ 50 mg / m² (1) Supermicron particles and residues thinner than a monolayer Controlled environment (Durkee)
Fine cleaning ( critical cleaning ) ≤ 5 mg / m² (1) Submicron particles and non-volatile residue measured in Angstroms cleanroom (Durkee)

Remarks:

(1) based on the total dirt
(2) only related to C.

In practice, it is still the case that the quality requirements are met if the subsequent process (see below) does not pose any problems, e.g. if the paint layer does not peel off before the required time.

Where this is not sufficient, especially in the case of external orders, there are customer-specific specifications for example on residual dirt, corrosion protection, stains, degree of gloss etc. in the absence of binding standards.

Measurement methods to ensure quality therefore do not play a particularly important role in workshops, although there is a wide range of different methods, ranging from visual testing to simple test methods (including water drainage test, wipe test, contact angle measurement, test inks, scotch tape test) to complex examination methods (including gravimetry, particle counter, infrared spectroscopy, UV fluorescence measurement, glow discharge spectroscopy, X-ray fluorescence analysis, electron spectroscopy and electrochemical measurement methods). However, there are hardly any processes that can be used directly in the line and offer reproducible and comparable results. Major development steps have only recently taken place here (see e.g.).

The general situation has only recently changed because the cleanliness requirements for certain components in the automotive industry are increasing dramatically. Brake and injection systems, for example, have to have ever smaller diameters and withstand ever higher pressures. Even minor particle contamination can therefore lead to major problems. With the increasing speed of innovation, one can no longer afford to recognize possible errors relatively late. That is why the standard VDA 19 / ISO 16232 “Road Vehicles - Cleanliness of Components of Fluid Circuits” was developed for this area, which describes procedures with which compliance with the cleanliness specifications can be checked.

Follow-up process

The processes that are to be carried out with the cleaned parts are of particular importance when selecting the cleaning technology, cleaning agents and procedures.

The classification essentially follows metal engineering:

  • To edit,
  • Separate
  • Put
  • Coating
  • Heat treatment
  • Assembly
  • Measure, test
  • Repair, maintenance

Over time, empirical values ​​have emerged as to how thorough cleaning has to be carried out in order to guarantee these processes for the respective guarantee period and beyond. When selecting the cleaning process, the subsequent process is often the starting point.

Challenges and tendencies

The above statements show how extraordinarily complex the subject is. Even small changes in the requirements can make completely different processes necessary. It thus eludes a strictly scientific-technical determination. On the other hand, it is becoming more and more important to achieve the required degree of purity as cost-effectively as possible with minimal risks to people and the environment, since cleaning has now become of central importance for the value chain in production. As a rule, users rely on a small group of well-known manufacturers who, based on their wealth of experience, can name suitable systems and processes, which are then adapted to the detailed requirements in pilot plant tests. However, these providers remain limited to the technology developed in their homes. In order to be able to offer the practitioners all the appropriate options according to their requirement profile, different tools have been developed by various institutes:

  • SAGE: A comprehensive expert system for parts cleaning and degreasing that made a tiered list of relatively general procedures available online according to user requirements (unfortunately taken offline in 2007). Developed by the Surface Cleaning Program at the Research Triangle Institute, Raleigh North Carolina USA, in cooperation with the US EPA.
  • Cleantool: A "best practice" database with comprehensive and specific documentation that is collected directly in the user companies. This database also contains an integrated assessment tool that covers technology, quality, occupational and environmental protection as well as costs; a comprehensive glossary is also included; (in 4 languages, [1] )
  • Component cleaning: A selection system for component cleaning from the Technical University of Dortmund with which users can have their cleaning task analyzed with regard to the cleaning methods and cleaning agents that can be used (only in German, [2] ).
  • TURI, Toxic Use Reduction Institute: A department at Lowell University in Massachusetts that has published the results of its metal surface cleaning tests for various companies here (in English only, [3] ).

Individual evidence

  1. ↑ a b Brigitte Haase, cleaning or pretreatment? Surface condition and nitriding result, component cleaning, process control and analysis. Bremerhaven University of Applied Sciences.
  2. ↑ Kurt Hertlein, Ger. Shell Chemie, 1989
  3. ↑ John Durkee in A2C2, 2003
  4. ↑ Doris Schulz: Increasing demands on cleaning quality - controlled cleanliness . In: JOT Journal for Surface Technology Vieweg Verlag / GWV Fachverlage GmbH . No. 6 , 2006, p. 50-53 .
  5. ↑ Fraunhofer Cleaning Technology Alliance, market and trend analysis in industrial parts cleaning , 2007.

Literature and further information

literature

  • Klaus-Peter Müller: Practical surface technology. Edition 2003. XII, vieweg, Braunschweig / Wiesbaden, ISBN 978-3-528-36562-2 .
  • Thomas W. Jelinek: Cleaning and degreasing in the metal industry. 1st edition, Leuze Verlag, Saulgau 1999, ISBN 3-87480-155-1 .
  • Brigitte Haase: How clean does a surface have to be? In: Journal surface technology. No. 4, 1997.
  • Brigitte Haase: cleaning or pre-treatment? Surface condition and nitriding result, component cleaning, process control and analysis. Bremerhaven University of Applied Sciences.
  • Bernd Künne: Online reference book for industrial cleaning. In: bauteilreinigung.de. University of Dortmund, Department of Machine Elements
  • Pure green: cleaning and pretreatment - status and perspectives. In: Electroplating. 90, No. 7, 1999, pp. 1836-1844.
  • Günter Kreisel et al .: Holistic accounting / evaluation of cleaning / pretreatment technologies in surface treatment. Jena 1998, Institute for Technical Chemistry at FSU.
  • Martin Bilz, Mark Krieg, Fraunhofer Institute for Production Systems and Design Technology: Methodical action in cleaning technology - efficient planning of cleanliness. In JOT industrial parts cleaning , Vieweg + Teubner Verlag / GWV Fachverlage GmbH Wiesbaden, 1/2009, pp. 7–9

See also