Added Value with Optimised Cleaning Processes IMPROVE QUALITY – REDUCE COSTS

March 11, 2011 9:35 am

It is the task of industrial parts cleaning, both during and after manufacturing, to fulfil the surface cleanliness specifications which are required for further processing or the assembly of workpieces. Stricter requirements for component cleanliness, coupled with rising cost pressure, necessitate cleaning processes which are matched to actual demands. Doris Schulz writes.
 
Quality awareness for parts cleaning is frequently not as well developed as is the case with other manufacturing steps such as CNC machining, forming, casting and coating. However, requirements-oriented cleaning makes a significant contribution to increasing product quality and economy. In many cases, inefficient scrap and costly rework can thus be avoided, as well as image impairing product failures and recalls. The industry offers various processes to this end, which are also used in combination to an ever greater extent.
 
Matching the Cleaning Process to the Parts and the Contamination
With a cleaning process which has been matched to the actual requirements, optimisation potential can frequently be exploited throughout the entire production sequence, and manufacturing can be made more efficient. Essential factors include part geometry, material, type and degree of contamination, cleaning agent, process (e.g. wet chemical processes such as spray, immersion and ultrasonic cleaning, or dry methods like CO2 and plasma cleaning), treatment duration, number of process steps, throughput and cleanliness requirements. The process can be ideally adjusted to these parameters with the help of cleaning tests conducted in the laboratory, or the test facilities of the system manufacturer, using original contaminated parts. Depending upon the task at hand, it might also make good economical sense to combine various processes.
 
Wet Chemical Cleaning
The effectiveness of wet chemical cleaning is above all determined by the dissolving performance of the utilised cleaning agent. Common cleaning media include aqueous cleaners and solvents, of which the latter can be subdivided into halogenated hydrocarbons (HC), chlorinated hydrocarbons (CHC) and polar solvents. Aqueous media, available as alkaline, neutral and acidic cleaners, are used with preference where very large volumes of parts have to be cleaned, and/or where fine cleaning and micro-cleaning are required.
 
Interaction Amongst Cleaning Agents and Process Technology
In order to be able to achieve the desired cleaning results within short periods of time, the effectiveness of the cleaning medium is enhanced by means of various physical processes which demonstrate effects of varying magnitude. In the case of spray cleaning, which is primarily used for large, flat-shaped workpieces, contamination is partially dissolved or emulsified by the cleaning agent (usually an aqueous cleaner), and partially washed away by the kinetic energy of the spray jet. Additional motion of the goods to be washed and/or the spray nozzles assures uniform cleaning results.
 
Immersion cleaning processes are generally preferred for parts with complex geometries, for example with blind holes and undercuts. When the workpiece is immersed into the cleaning bath, contamination which adheres to its surfaces is dissolved by the chemical action of the cleaning agent. Rotating or swivelling the parts within the bath enhances the cleaning effect. Ultrasonic cleaning is also based upon immersion, and is capable of achieving high levels of cleanliness. The cleaning effect results from cavitation: The bath fluid is acoustically irradiated by means of an ultrasonic generator and a matching vibration system. Resulting vibration causes extremely small hollow spaces within the fluid, which then immediately collapse. Strong currents and turbulence develop which ‘blast’ the contamination away from the goods to be cleaned. In the case of pressurised flow cleaning, pumps draw fluid out of the cleaning bath, and subsequently inject it back into the bath at high pressure levels through nozzles located underneath the fill-level. This results in strong currents, thus causing turbulence at the edges of the components to be cleaned, which removes the contamination. When the fluid flows past blind holes and recesses, a suction effect is generated which ‘draws out’ contamination.
 
Effects of the Cleaning Tank
Just how clean and spotless the workpieces are upon removal from the wet chemical cleaning system depends not only on the utilised process, chemical and duration of treatment: The cleaning tank plays a role as well. The tank often provides the necessary potential for a results, time and cost-optimised washing process: The layout of the cleaning tank and the parts basket influences the effectiveness of the utilised system technology, treatment time, the cleaning temperature and the medium. As a prerequisite for quick, reliable removal of contamination, the parts in the basket must be readily accessible to the cleaning process. Only in this way are the workpieces uniformly exposed to the cleaning agent, so that the mechanical washing process can develop its full effectiveness and wash out film-like contamination and particulates as efficiently as possible. Accessibility is also absolutely indispensable for drying with compressed or hot air. Ideal parts accessibility can be achieved by means of baskets which don’t have any large, contiguous surfaces. This is made possible through the consistent use of round wire. As opposed to closed containers or baskets made of perforated sheet metal, cleaning baskets made of round wire are also distinguished by significantly better draining characteristics. And this means that considerably less contamination and cleaning agent is carried over to downstream processes. This results in a longer service life for the cleaning bath, and thus improved cleaning system availability and efficiency. The utilised basket material also plays a role. Alkaline cleaners may cause peeling of the surface finish in baskets made of zinc plated steel, which contaminates the bath and the parts to be cleaned.
 
CO2 Cleaning – a Dry Alternative
CO2 snow jet cleaning makes use of liquid carbon dioxide as a cleaning medium, which is expanded as it passes through a nozzle and is accelerated with compressed air to ultrasonic speeds. Thanks to a combination of mechanical, thermal and chemical effects, the CO2 snow jet removes film-like contamination and particulates when it strikes the surface of almost any material – in a dry and residue-free fashion. This process can also be used to treat specific functional areas, for example sealing and bonding surfaces, without subjecting the entire component to the complex processing which is necessary in order to achieve the levels of cleanliness which are only required for the functional surfaces.
 
Removing Non-Polar Contamination with Supercritical CO2
Due to increasing demands for component cleanliness, as well as workpiece miniaturisation to an ever greater extent, cleaning with supercritical carbon dioxide is becoming more and more significant. ‘Supercritical’ because the carbon dioxide is used in an aggregation state in which its physical characteristics lie between the liquid and the gaseous state. While in this state, the CO2 demonstrates only minimal viscosity and surface tension. In this way, non-polar contamination such as oils and greases can be removed from the finest cracks and pores. In order to be able to remove polar contamination as well (chips, salts), suitable system concepts are utilised by means of which cleaning with supercritical and liquid carbon dioxide is combined.
 
Plasma – Cleaning with Additional Effects
Through the use of various reactive gases, plasma technology covers a broad spectrum of applications for cleaning individual parts and bulk goods of all types made of steel, non-ferrous metals, plastics, glass and ceramics. These processes are most effective when thin layers of organic contamination need to be removed. And with this method, achievable cleanliness is independent of the structure and geometry of the workpiece surface. An additional effect offered by this process is optimised preparation for subsequent surface finishing processes, for example improved adhesion of glues and coatings. Beyond this, coating by means of plasma technology – plasma polymerisation – makes it possible to create so-called easy-to-clean surfaces, or surfaces which protect the component from contamination during use. Plasma cleaning is employed, for example, in the fields of metalworking and plastics processing, before coating, in electronics and microsystems technology, as well as for optics and analytical chemistry.
 
For parts cleaning as well, quality has its price
As is also the case for milling machines and coating systems, quality has its price where industrial parts cleaning equipment and systems are concerned as well. In a supposedly less expensive quotation, it’s easy to overlook the fact that certain components are offered in differing qualities and types, for example zinc plated as opposed to stainless steel piping. And this usually has a negative effect on cleaning quality and/or the service life, and thus the overall economy, of the cleaning system.
 
parts2clean 2011
How can the cleaning task and the cleaning process for metal parts be ideally matched to each other? By means of which processes can cleaning be made more economical? How does the parts cleaning basket effect part cleanliness? Answers to these and other questions covering all aspects of industrial parts cleaning are provided by parts2clean. The leading international trade fair for industrial parts and surface cleaning will take place at the new exhibition centre directly adjacent to Stuttgart International Airport (Germany) from the 25th through the 27th of October, 2011. www.parts2clean.com