| Nanoporosity Requires Innovative Resin Impregnatin Processes to Stop Casting Leaks
Gary P. Stevens, Director of Sales&Marketing IMPCO, Inc. East Providence, Rhode Is/and
Eliminating porosity has long been the holy grail of casting engineers. From material refinements and design changes to modern process improvements, advances over the years have brought us even closer to that goal. To meet the quality demands of customers, casting engineers improved die design to create better flow of molten metal. They improved alloys to increase part strength. Their tighter process controls reduced fall-out. Advanced process controls enabled them to maintain uniform heat distribution throughout the casting process. With these and other improvements, engineers have succeeded in recent years in reducing porosity from macro- and micro-size holes to the nano level - often with the unintended consequences of making the porosity more difficult to seal. Today, nanoporosity is seen in both ferrous and nonferrous applications, but is most widely found in aluminum and magnesium castings that dominate the automotive and other industries. As the quality of the casting improves, unavoidable porosity becomes smaller and tighter, and it becomes progressively more abundant as the part geometry becomes more complex. While more expensive, highly engineered ferrous and nonferrous castings succeed in reducing porosity size, post-casting impregnation processes are still required to stop many smaller leaks from occurring in applications such as transmission housings, valve bodies, engine blocks, pumps and compressors, where the casting is required to contain gasses or liquids under pressure and failures can carry extensive risks. Before addressing exactlyhow the trend toward nanoporosity has affected the post-casting impregnation process, it is useful to review the nature of porosity.
Porosity Classification System
Some years ago, IMPCO developed a classification system to help people understand the various impregnation solutions to porosity problems and the expected rates of success (see "Porosity Classification System Helps Determine Solutions to Casting Leaks," Die Casting Engineer, July 2003). An updated chart illustrating three porosity types and five size classifications is printed in Table 1. The chart clearly shows that not all porosity is the same. The nature of the porosity in a given casting, the nominal size of the gas or liquid molecules that the casting contains, and the force applied will determine if a leak path occurs. Some porosity will not cause leakage under low pressure, but will cause leakage under high pressure. At a specified low pressure, for example, the nominal molecular size of the gas or liquid may be too large to penetrate the porosity of a given casting. With greater pressure or with greater heat, the larger molecules can be forced through the smaller casting porosity cavities. In fact, with enough force applied to any given material contained within a casting, most castings will leak to some degree. It should be noted that while leaks may not be present in castings with blind and totally enclosed porosity, machining processes may open previously undetected cavities, creating leak paths. Leak tests, therefore, should be performed after the machining process. After sealing of any leaks, testing to given pressure specifications provides assurance that leak problems are solved for particular applications.
Nanoporosity
Resin Impregnation Processes
Dynamically modified impregnation processes may be used to solvedifferent porosity problems. Process variations utilize the controllable parameters that impregnation technologies provide: impregnant engineering, pressure, time, and post impregnation actions such as parts washing and impregnant cure. As illustrated in Table 1, the resulting modifications are generally simplified to include standard vacuum/pressure, enhanced vacuum/pressure, and enhanced vacuum/pressure methods with further operations. Additionally, the vacuum/pressure operation may be accomplished through a wet or a dry process, both of which meet MIL-STD-276A. However, prior to any vacuum resin impregnation, cleaning and drying of castings is necessary. If cutting fluids, moisture or other contaminants are embedded in porosity, filling the void to seal a leak path is impossible. As casting engineers produce parts with smaller pores, the cleaning and drying process becomes progressively more important. It is critical for successfully sealing nanoporosity. To ensure proper cleaning, services that feature vacuum vapor degreasing and drying are available for die casters. Vacuum vapor degreasing prior to resin impregnation can elevate a company's sealing process so effectively that when pressure tested, the parts are as tight or better than the equipment generally available to test them. With parts that test within the leak band of the test equipment, any perceived leakage must be distinguished from "false positive" leaks that could actually be a result of conservative part rejection settings on the test equipment itself.
Wet Vacuum Process
The wet vacuum process involves placing the casting to be sealed directly into the sealant bath. The sealant liquid must completely cover the part. The air above the bath is evacuated and subsequently air from the porosity in the product is drawn through the surrounding liquid. Pressure, either atmospheric or compressed air, is then applied to help the sealant penetrate the pores. This process is most suitable for sealing castings with large pores such as powdered metal or sintered metal parts. Typically, 12 to 15 percent of each sintered metal part is made up of pores containing air. These pores must be sealed to avoid spotting, corrosion or other decorative imperfections on plated, painted, or coated surfaces and to prevent leaks - beforeor after machining- in parts that require pressure tightness.
Dry Vacuum Process
Over the years, impregnation technology companies have raised the bar of the process to resolve the problems of progressively smaller porosity. As engineers improved castings, modifications and improvements have been pioneered to enhance the natural ability of sealing resins to penetrate microporosity using a dry vacuum process. The dry vacuum process utilizes an autoclave in which a vacuum is drawn. After the air in the pores is exhausted, the liquid sealant is introduced while the product is still under vacuum. Next, a pressure cycle, generally compressed air up to 90 psi (approximately six atmospheres), forces the liquid sealant deep into the porous cavities located in the product. After the autoclave cycle, the product is washed in water to remove excess liquid sealant, which is water-washable, but not water-soluble, from all surfaces. The critical procedure here is to balance the requirements for the impregnation resin's removal from the part surfaces without removing it from the porosity. The sealants are engineered thermoset resins that have specific physical properties which enhance their ability to penetrate small porosity. The last step in the cycle is to turn the liquid resin into a solid by applying heat and/or chemical activators to initiate a chemical cross-linking process that forms a permanent seal in the cavities of the product. Once cross-linked, thermoset resins will not liquefy at elevated temperatures. Most thermoset sealants are suitable for operating at 350°F under continuous duty, and many meet MIL-I-17563C for processing military castings. Typically, cast products found in pressure-tight applications require the dry vacuum process with pressure cycle to ensure complete sealing of the product. Many times a double impregnation cycle is required to seal castings with macroporosity because sealants may wash out during the "washing" and/or "curing" cycles described above. Only pressure testing the product after the impregnation process will confirm that the product is sealed.
Nanoporosity Resin Impregnation Process
In the case of nanoporosity, the wet vacuum process, which relies solely on the forces of gravity, atmospheric pressure, and the natural ability of a standard resin to wet a metal surface and penetrate porous cavities, is not robust enough to force the resin into ultra small pores within a reasonable time frame. Instead, an enhanced dry vacuum/pressure process that is adjusted to suit the particular nanoporosity application must be employed. Adjustment of process parameters to force large molecules of sealant into small holes include: Pulling a lower vacuum to create less resistance to the sealant entering the nanopores; Holding the vacuum for a longer time to ensure that all the air and any molecular moisture is removed from the porosity; Extending the pressure cycle time; Increasing the pressure applied to the top of the resin in the chamber; Aggressively engineering resins for specific sealing applications.
The parameters for all these measures may have to be adjusted for a given casting and its particular nanoporosity characteristics. Overall, the impregnation processes required to solve the nanoporosity challenge are more complex than the standard impregnation processes required to resolve microporosity problems. Achieving nanoporosity with highly engineered and tightly controlled casting operations and the concomitant requirement of a more complex post-casting impregnation process has the unintended potential of making castings more expensive, while not eliminating leakage problems. In some applications, such extensive engineering of castings can be counterproductive since standard impregnation of micropores has been proven to reliably seal parts for applications as sensitive as high-volume automotive components. The counter productivity of achieving nanoporosity in castings is illustrated in the re-engineering of a safety restraint part that IMPCO seals for the automotive industry. Initially, a standard process cycle, requiring a one-hour process time, was used to seal the microporous part. This was extremely successful, resulting in sealed castings that met all specifications. In an effort to eliminate all porosity, engineers redesigned the casting. The immediate objective was to cut the cost and time required to impregnate the part. What actually happened was just the opposite. Instead of eliminating all porosity, the engineers succeeded in reducing the microporosity to the nano level. However, the part still leaked. To successfully seal the newly created nanoporosity, IMPCO engineered an enhanced impregnation process which included increasing the time of the impregnation pressure cycle up to 12 hours from the one hour previously used for sealing the microporosity in the old part design. In this case, the successful engineering effort to reduce the porosity had the unintended consequence of actually increasing costs. As part of this quality enhancement program, it was anticipated that the cost of new tooling for the redesigned part would be repaid by the improved quality. Instead, because the porosity had only been changed from micro to nano, the cost to seal the part, and the added time to seal it, increased overall production time and the savings were not achieved.
Conclusion For most applications, engineers should fully understand that the effort to eliminate microporosity often has the result of creating nanoporosity. Since nanoporosity still leaks, although at a slower rate, it must be sealed using enhanced and generally more complex and expensive techniques. The determination to spend scarce engineering resources on casting porosity can be based on the application and the nature of the gas or liquid that the part must hold. If very small molecules of gas must be contained by a critical part, and the part is small, then improving a casting to achieve a nanoporosity level may be justified. However, if the application requires containment of large liquid or large gaseous molecules, such as water, oil or air, and/or if the application is not highly safetysensitive over a prolonged operating life, investing resources to improve a casting with microporosity may not necessarily provide economic results. Still, some casting engineers cannot resist the temptation of trying to eliminate all porosity. While achieving nanoporosity may be a step in the right direction, there is no guarantee that perfection can be achieved in today's foundries. Porosity is the result of many dynamic factors. While materials may be improved, uniform heat distribution may be maintained throughout the casting process and other elements of the process may be consistently controlled, something as uncontrollable as a passing thunderstorm may quickly change the relative humidity within a foundry and, thereby, increase porosity in whatever castings may be in process at the time. Eliminating all porosity may be a worthy goal. Sometimes, the best and most economic solution is to accept naturally occurring microporosity - and take advantage of the proven and cost-effective sealing processes of reliable impregnation technology.
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Nanoporosity is a new classification created for porosity that is nearly the same size as the molecules of the sealants used in standard resin impregnation processes for castings. Like micro, micro/macro, funnel and macroporosity, nanoporosity can exist in many shapes and can be blind, continuous or enclosed in nature. Just as with microporosity, determination ofleak paths through nanoporosity is governed by the same principles of porosity type, pressure and the nominal size of gas or liquid molecules relative to the size of the nanoporosity. The figure below compares the size of micro- and nanoporosity relative to the size of standard sealant molecules. The top example shows that standard sealant molecules are small enough to penetrate micropores. The lower example depicts the same sealant molecules relative to the size of nanoporosity. Clearly, nanoporosity in castings creates a greater challenge for the impregnation processor trying to force the larger sealant molecules into the smaller porosity passages.
