Defects that form during casting or solidification include microscpic pores in surfaces and leak paths in casting walls. Without some remediation, the casting may be scrapped time and production cost. And, for automotive or aerospace castings, the effect of the defect may be even more consequential.
The need to seal castings by impregnation is well understood and the practice is widely applied across the metalcasting industry. Eliminating the risks arising from defects in finished casting has a direct bearing on quality, cost and, ultimately, the producer’s reputation in markets that are increasingly competitive.
Impregnation systems have become increasingly efficient too, in line with the producers’ need to improve product quality and increase productivity. Among the latest developments are in-house systems for high-volume impregnation programs.
Sealing porosity on a cast surface.
But, not every foundry or diecaster has the need for a high-volume impregnation process. The ‘Selective Impregnation’ development by Impregnation Solutions Ltd. is an effective alternative, and it’s drawing interest now for its simplicity, performance, and low cost.
Identifying the specific area of porosity in a casting that causes a leak is vital information for the metalcaster, in order for it to make improvements to the production process. By then targeting impregnation directly at the leak, greater efficiencies and lower costs can be realized when compared to impregnating large areas of the casting that do not require treatment.
The manual Selective Impregnation kit from ISL.
“To date, manufacturing companies have either invested in large, purpose-designed impregnation facilities or sent leaking components to a dedicated sub-contractor,” explained Paul Young, director at ISL. “This can result in production delays, unnecessary inventories, transport costs, and potential handling damage – all issues that can now be addressed by the Selective Impregnation system, with clearly definable savings in terms of time, manpower and costs,” he said. “As a result, production, budgeting, and operational efficiencies, alll are significantly enhanced.”
The Selective Impregnation process accurately applies the impregnation sealant only to the identified leak area, using a simple to operate, hand-held Spot-Seal pressure impregnation gun. Bench-mounted and automated options also are available. The method uses a range of rubber washers from
The ISL Selective Impregnation system is supplied with a choice of porosity
sealants, to address multiple types of porosity.
which the most appropriate is selected and positioned onto the casting to match the precise point of porosity.
Next, a pressurized gun is applied, both to clean the porosity and then carry out impregnation, 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,” Young added. “Significantly, because the sealant cures and hardens inside the casting and not on its surface, there is no surface residue to be removed.”
For the sealant, IPL offers three different formulations. Each one has a different viscosity so that the appropriate sealant can be selected in relation to fine, medium, or gross leakage rates. “This is not possible with vacuum impregnation where there is only one viscosity,” Young noted, “so often castings with a gross leakage rate are not recovered, even though the casting is sound.”
The range of sealants can be used to seal porosity in all types of metal.
As for the types of defects to be corrected, Young noted the difficulty of classifying porosity defects dimensionally, as they are variably shaped crevices or cavities. “Therefore, leakage rates tend to be used to categorize fine to gross leakers,” he explained. “For example, a leakage rate of 0-20 cc/min at 2 bar maybe classed as fine porosity; 20-50cc/min as medium porosity; and over 50cc/min as gross porosity. Because we have a range of viscosities, the appropriate sealant can be selected to seal a specific leak rate.”
The system developers claim a 90% reduction in sealant usage compared to a conventional impregnation process, which also results in reduced or eliminated management and disposal of wastewater and effluent. “Importantly, only minimal training is required, while further production efficiency can arise from its location alongside pressure testing equipment – a clear consequence of its markedly smaller footprint,” Paul Young continued.
IPL contends that the low viscosity and surface tension of the sealant is central to the Selective Impregnation process, and results in “excellent” penetration of microporosity. When cured, the sealant resists a heat range of -50°C to 200°C, temperature cycling, vibration, and a 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 –also are removed,” Young said. “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 ISL process uses a targeted method of impregnation, he reported, which means the sealant is applied directly to the area of porosity that leaks, versus vacuum impregnation, which applies the sealant to the complete casting regardless of where it leaks and is consequently wasteful of chemicals and energy.
“As a result, the larger the casting is the greater will be the savings,” according to Young. Typical applications to date have included automotive engine castings, namely cylinder blocks, ladders, sumps, cam carriers, cam covers, etc. Also, off-road and marine engine and heat exchanger castings, pumps, compressors, etc., have been treated.
ISL pointed out that the Selective Impregnation method, using the Spot-Seal gun, achieves a higher (therefore, more efficient) recovery rate of leaking castings, compared to a conventional system. “A recovery rate as high as 100% is common,” Young said, “a direct 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,” Young added.
“Significantly, there is no wash-out of sealant from the porosity during washing and curing, which results in a better-quality seal.”
The developers of the Selective Impregnation system say it opens up efficiencies, levels of quality consistency, and tangible cost benefits, which translate into new application opportunities.