Castings are not exempt from the trend towards increased component cleanliness. This frequently presents foundries with new challenges where cleaning their parts is concerned. In accordance with various requirements, the cleaning industry offers a broad spectrum of processes and blasting techniques, by means of which moulds and workpieces can be cleaned economically and ecologically in a time-saving fashion – even after chip-removing machining processes. Blasting with solid media and barrel finishing are amongst the well established process used by the casting industry for cleaning and deburring parts after the casting process. Blasting with dry ice consisting of carbon dioxide is also used.
Ice-Cold Cleaning of Castings and Moulds
This compact deburring system was
This dry cleaning process can be used to remove sand from cast steel and aluminium workpieces, as well as for cleaning moulds and core boxes – even while they’re still hot. The utilised carbon dioxide is recovered from other processes and is not generated from fossil fuels, and is thus environmentally neutral.
Dry ice is less abrasive than conventional blasting media. Its cleaning effectiveness is based upon three different modes of action:
The low temperature of the blasting medium, namely -78.5° Celsius, causes thermal stress due to the different coefficients of thermal expansion of the contamination and the metal, resulting in embrittlement and separation of the contaminants. The separation process is accelerated when castings and moulds are cleaned when hot. At the same time, the kinetic energy of the blasting medium results in mechanical separation. This is further promoted by the third mode of action, i.e. the pressure surge caused by sudden sublimation of the CO2 particles. As a consequence of sublimation of the blasting medium, only the washed away sand and the removed burrs or flitter remain. And only those burrs and flitter which would otherwise drop off during later use of the workpiece are removed by blasting with dry ice due to its lower levels of abrasiveness in comparison with conventional blasting processes. A completely deburred surface with smooth edges cannot be achieved by means of blasting with dry ice.
Cleanliness by means of Sound Waves
Particulate contamination such as release agents, sand and oxides, as well as film-like contaminants, for example oil and grease, can be removed quickly, reliably and gently from castings and moulds with ultrasound. Ultrasonic waves are produced with a generator which converts normal line frequencies into high frequency electrical signals. These signals are then transformed into mechanical vibration of the same frequency by means of so called sonic transducers or oscillators. Ultrasonic waves develop their full cleaning effectiveness in a liquid bath based on the physical principal of cavitation: The great intensity of the alternating sound pressure levels results in alternating phases of underpressure and overpressure within the liquid. Microscopically small cavities are formed during the underpressure phases. These bubbles implode during the subsequent overpressure phase, and generate shock waves with significant energy densities. Micro-currents are caused in the liquid as a result, which virtually “blast” particulates and film-like contamination away from the surfaces and out of undercuts, blind holes, gaps and hollow spaces in the parts to be cleaned, and rinse them away. Generally speaking, the lower the frequency, the larger the cavitation bubbles and thus the more energy is released.
When using ultrasound to clean moulds for aluminium, magnesium and zinc castings, as well as ingot moulds and core boxes, residues from production and deposits on the moulds including welded-on metals, release agents, facings, lubricants, oil, grease, lime and rust can be removed quickly and gently, and nevertheless thoroughly, without mechanical wear or impairment of surfaces and edges. In addition to this, otherwise customary dismantling of the moulds becomes unnecessary, because contamination can even be removed from difficult to access areas and complex geometries including, for example, vents, ejector guides, drill holes, scoring and hollow spaces thanks to the enormous cleaning effect of cavitation. The cleaning process is executed in systems which are laid out to fulfil actual requirements, usually with a sound frequency within a range of 20 to 30 kHz. Aqueous cleaning agents are used primarily, whose composition is matched to the respective types of contamination and materials to be cleaned.
Individual part cleaning allows for targeted
Image source: MAFAC
Ultrasound is used to clean castings before surface finishing processes such as galvanising, painting, burnishing and anodising, as well as in engine manufacturing in the automotive industry after etching aluminium die-castings in stripping systems, by means of which a homogenous surface is produced on the inside cylinder walls. The utilised lye reacts with the material and forms an aluminate on the surface which, like hydroxides, is very difficult to remove. The currents caused by cavitation make it possible to quickly and thoroughly remove this contamination.
Cleaning Machined Castings
In addition to casting itself, machining by means of chip-removing processes is included within the spectrum of services offered by many foundries. Not infrequently, precisely defined residual contamination specifications have to be adhered to when cleaning these workpieces.
The castings are often cleaned by means of a wet chemical batch process in bulk goods baskets or workpiece carriers which have been laid out specifically for the workpieces, allowing for high volume throughput within relatively short periods of time. Water-based media which are available as acidic, neutral and alkaline cleaning agents, as well as solvents, are used for cleaning. The latter are subdivided roughly into chlorinated hydrocarbons (CHC), non-halogenated hydrocarbons (HC) and modified alcohols, as well as polar solvents. The state-of-the-art for cleaning with solvents involves closed systems which fulfil requirements for reducing the emission of volatile compounds (e.g. the VOC directive). These are usually offered as single chamber systems, and are laid out for multistage cleaning processes such as cleaning, steam degreasing, rinsing and drying. Various physical processes which demonstrate effects of varying magnitude, such as ultrasound and flushing under pressure, are usually used in order to increase the effectiveness of the cleaning agent and to shorten the duration of the process.
Modern, combination cleaning and deburring solutions with robots offer high levels of flexibility and allow for quick adaptation to new or modified casting geometries.
Image source: Strama-mps
In addition to the utilised process technology, chemicals and treatment duration, the type of cleaning basket also influences cleaning results and costs. As a prerequisites for quick, reliable removal of contamination, the workpieces must be readily and uniformly accessible to the cleaning agent and the mechanical washing process must develop its full effectiveness, so that film-like contamination and particulates can be washed away as efficiently as possible. 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. This results in a longer service life for the cleaning bath, and thus improved cleaning system availability and efficiency.
Individual Part Cleaning – Targeted Treatment of Castings
Individual part cleaning is advisable for geometrically complex castings which have to meet demanding cleanliness requirements. This is due to the fact that channels, drill holes, undercuts and functional surfaces can be processed in a targeted fashion in this way. In addition to geometry, the avoidance of damage resulting from unnecessary parts handling also speaks in favour of individual parts cleaning.
|Cleaning baskets used in batch processes assure good results within short periods of time by making parts readily accessible from all sides.|
Image source: Dürr Ecoclean
Cleaning and Deburring in a Single Step
If the castings not only need to be cleaned, but rather deburred as well, flexible systems are available in which the workpieces are deburred by means of high pressure, brushing or rigid tools, and are cleaned at the same time. In this case, modern, automated cleaning solutions which are integrated into the production line and make use of robots offer high levels of flexibility and allow for quick adaptation to new or modified casting geometries. Integrated into the wet zone, the robot comprises the handling and processing unit of the cleaning system, which precisely positions the workpieces at various treatment stations such as spray cleaning, injection flood washing, high pressure deburring and drying. Control is implemented by means of a standard CNC system, and trouble-free programming is assured with an easy to use teach-in panel. These compact cleaning units are usually offered as standard modules with integrated media preparation.
Which time and cost optimising potential is provided by parts cleaning in the casting industry? With which processes can various components be cleaned both reliably and economically? Answers to these and other questions covering all aspects of cleaning processes for castings and moulds are provided at parts2clean. The Leading International Trade Fair for Cleaning within the Production Process and Maintenance will take place at the New Exhibition Centre directly adjacent to Stuttgart International Airport (Germany) from the 20th through the 22nd of October, 2009. The exhibition portfolio encompasses systems, processes and process media for degreasing, cleaning, deburring and pre-treatment of parts, parts baskets and workpiece carriers, handling and process automation, cleanroom technology, quality assurance, test methods and analysis procedures, media treatment and disposal, job-shop cleaning, research and technical literature. For the first time in 2009, an international congress will also be held in keeping with the motto, “Worldwide Requirements, Technologies and Markets”.