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Selective impregnation of castings opens up clear benefits and new opportunities

The need to seal castings by impregnation is a well-understood and widely used practice across the castings industry. The elimination of the risks arising from leaks in casting products has a direct bearing on quality, cost and, ultimately, reputation in markets that are becoming increasingly competitive, so optimising the process is paramount. Now, with a new system development being introduced by Midland Impregnations Ltd (MIL), a highly effective alternative approach is set to gain widespread interest across the industry as it offers clear-cut benefits in terms of both performance and cost.    

“The most significant challenge facing casting manufacturers in this context relates to ‘through’ porosity which can see a major part of a casting’s integrity affected,” says MIL spokesman, Paul Young. “The fact that both ‘blind’ and ‘enclosed’ porosity can themselves transform into a ‘through’ issue as a result of machining operations, means impregnation is best carried out after the machining process has taken place. Our new selective impregnation system directly addresses this requirement.”

Conventionally, impregnation has been undertaken by total immersion of large baskets of castings in a vacuum and pressure vessel followed by washing off the excess sealant in cold water, before curing the casting in hot water. However, this is not only labour intensive but also treats the part in its entirety, rather than simply targeting the specific area of porosity.

Identification of the specific area of porosity in a casting that causes a leak is vital information for the foundry in order to make improvements to their casting process while, at the same time, allowing impregnation to be directed at the leak, resulting in both greater efficiencies and lower costs. 

“Moreover, manufacturing companies to date have had to invest in large, purpose-designed impregnation facilities and/or send leaking components to a dedicated sub-contractor,” said Young. “This can result in production delays, unnecessary stockholding, transport costs and potential handling damage. All of these issues can now be addressed by the selective impregnation system with clearly definable savings in terms of time, manpower and costs. As a result, production, budgeting and operational efficiencies are all significantly enhanced.”

A new concept

The selective impregnation system cleans the porosity, breaks the surface tension and accurately applies the impregnation sealant – MIL is making manual, bench-mounted and automated options available. The concept features a range of rubber washers from which the most appropriate is selected and positioned on the casting to match the precise point of porosity. A pressurised gun then applies the impregnation sealant with the operation completed by rapid curing – typically the process takes one to two minutes and the casting can be re-tested in as little as 30 minutes. Significantly, because the sealant cures and hardens inside the casting and not on its surface, there is no surface residue to be removed.

“The immediate gains result directly from the cost of the investment,” adds Young. “The low cost portable selective impregnation kit removes the need for capital investment which, typically, can amount to more than £100,000 for a ‘conventional’ arrangement. Following installation, the system goes on to avoid chemical wastage and saves processing costs by only treating areas of the casting that need sealing – in some cases, cost efficiency in this regard can be as high as 99 per cent. These are savings that are further enhanced by the fact that the management and disposal of wastewater and effluent are also eliminated.”

Possible 90 per cent reduction in sealant use

Young points out that the wide range of washers and nozzle adaptors supplied with the kit enables a long list of porosity issues to be addressed – from fine to gross leakers – and, overall, MIL believes that a 90 per cent reduction in sealant use can result. Additionally, only minimal training is required for the system, while further production efficiency can arise from its location alongside pressure testing equipment – a clear consequence of its markedly smaller footprint compared to alternatives. 

The company points out that the low viscosity and surface tension that are central to the selective impregnation system also result in excellent penetration of micro-porosity. When cured, the sealant resists a heat range of -50°C to 200°C, temperature cycling, vibration and a wide range of acids, caustics, solvents and hydrocarbons.

“The risk of sealant contamination in threads and of corrosion and discoloration of castings – common problems experienced with impregnation equipment to date – are also removed,” continues Young. “Additionally, the problems associated with cured sealant being left in oil ways and other internal galleries, which can result from ineffective washing during a traditional impregnation process and which, alarmingly, could potentially lead to product failure in service, are also eliminated.” 

The MIL development also means that there is no need for sealant recovery drain stations, cold water wash tanks and hot water curing tanks – or even for overhead gantries and hoists for handling large process baskets. The company points out that this represents additional significant cost savings for manufacturers and can also add up to major areas of a factory floor space being released.

Selective impregnation also enables a higher, and therefore more efficient, recovery rate of leaking castings to be achieved compared to a ‘conventional’ system alternative,” adds Young. “A recovery rate as high as 100 per cent is common as a result of a number of factors including the ability with selective impregnation to tailor the sealant to the type of porosity on a part-by-part basis. Significantly, there is no wash-out of sealant from the porosity during washing and curing resulting in a better quality seal.”

Young explains that, to date, typically up to 95 per cent of sealant used to impregnate a casting by conventional methods is washed down the drain as waste unless expensive sealant recovery facilities are installed – a consideration that is completely avoided with this new system. Moreover, potential damage, which can occur during transport off-site and processing through batch equipment, is avoided.

The launch of this new system from MIL represents a significant departure from methods that have long been considered conventional in this highly specialist area of casting repair. The new process opens up efficiencies, levels of quality consistency and tangible cost benefits that generate excellent results and which translate into new application opportunities. As the commitment to greater quality and efficiency comes into ever-sharper focus, these are factors that Midland Impregnations Ltd believes will be recognised by everyone involved in the casting production sector.

Contact: Paul Young, Midland Impregnations Ltd, Tel: +44 (0) 1384 891198, email: paul@midlandimpregnations.co.uk web: www.midlandimpregnations.co.uk