Manual on Vapor Degreasing (Astm Manual Series)

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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.

Wypych Handbook of Solvents - Стр 55

Correspondingly the cleaning itself can be structured according to the type of energy input: [1]. 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. 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:.

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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. Another consideration is the place of cleaning.

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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 insdustrial plants. First, one can differentiate among the following technique ordered from most to least technologically advanced :. 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. 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 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 inibitors 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 CO 2 dry ice process : For tougher requirements pellets are used while for more sensitive materials or components CO 2 in form of snow is applied.

One draw back is the high energy consumption required to make dry ice. 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 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 '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 shut off of power supply, automatic refill and sharpening of media e.

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 CO 2 blasting and automatic brush cleaning at the expense of higher energy costs. A standardisation of the quality requirements for cleaned surfaces regarding the following process e. However it is possible to use general classifications.

In Germany it was attempted to define cleaning as a sub category of metal treatment DIN Cleaning as sub category of cutting processes , but this does not cope with all the complexities of cleaning.

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The rather general rules includes the classification in intermediate cleaning, final cleaning, precision cleaning and critical cleaning s. 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.

Investigation should be made to determine the time limitation of the adhesive and the rate of change on the surface of the adherend. On the basis of the results of such an investigation, it may be possible to extend the length of time and establish standards for the interval between surface preparation and adhesive application to?

Contaminating operations in the area should be avoided. Especially detrimental are paint or other spraying operations, processes using powdered materials, oil vapors from pumps and other machinery and spraying of mold release agents. This can be accomplished by wiping with a cloth wetted with a variety of commercial solvents, such as acetone, methyl ethyl ketone, lacquer thinner and naphtha. Commercial vapor degreasing equipment is usually used. Parts are suspended in the vapor zone above the hot liquid and allowed to remain until there is condensation and run off approximately 5 min.

This step is repeated if necessary until all visible contamination is removed. The parts are raised above the tank and allowed to dry thoroughly. Previously bonded parts, such as honeycomb core materials, are usually not degreased due to difficulty in removing residue. This test depends on the observation that a clean surface one that is chemically active or polar will hold a continuous?

This is known as a water-break-free condition. A break in the water? Distilled water should be used in the test, and a drainage time of about 30 s should be allowed. Any trace of residual cleaning solution should be removed or a false conclusion may be made. If a water-break-free condition is not observed on the treated surface, it should not be used for bonding.