by Peter T. Gebhard, Principal, IMPCO, Inc.

Reprinted from Die Casting Engineers, November, 2007

A window of opportunity has opened for American die casters who had the acumen and fortitude to survive the offshore outsourcing boom of recent years. Thanks to several factors, it is now possible for U.S. casting houses to win business away from their Asian and European counterparts by using the time-honored strategies of concentrating on price, quality, and delivery. However, if they don’t continue to improve their quality control, especially in the reduction and sealing of porosity, they could once again fall behind.

International outsourcing for die-cast parts is not as attractive as it once was, due in part to the increasing domestic demand for light metals in China and India. Add to the mix the increases in the cost of energy and raw materials, and now these traditionally low-cost countries face substantially increased production costs. Fact: China has to import a preponderance of its aluminum, and in a country that now is perennially short of electric power the 22,000kW-per-ton requirement for production puts a real strain on the system. Fact: India is close to running out of excess die-casting capacity, mostly because of increases in local demand for die-cast production. While labor costs in both of these countries are still a fraction of those in the U.S., the scarcity of skilled personnel has contributed to a dramatic increase in the local base labor cost. As important is the tradition to overstaff, with the assumption that “labor here is cheap.”

For the time being, the United States has a higher overall production capacity plus better recycling programs that keep metal (especially aluminum) supplies relatively high and metal processing and refining prices correspondingly low. Surprisingly, the U.S. is becoming a more economical place to produce several types of die-cast parts, especially those that require attention to manufacturing details. Leading the way are our continual emphasis on labor cost control and heavy emphasis on quality. Couple these with our location at the doorsteps of our customers, and the elements of a manufacturing resurgence are in place.

Other factors in our favor are the fresh yet painful memories some customers have of the hidden costs and darker side of some offshore contracting, such as delays, design errors, disappointing parts quality, and extra shipping costs. More automation has helped U.S. shops level the labor-cost playing field. Additionally, there is more business for U.S. casting shops due to the bittersweet fact that there are fewer shops than there were 10 years ago. Only the fittest remain, and they’ll have to grow fitter still to prosper in the next decade.

As I see it, the wave of the future (or one of them, anyway) is process control, and since my area of expertise is how to best seal porosity in cast parts, I must narrow the scope of this article. For many die-cast parts, advanced casting techniques can now reduce pore sizes all the way down to what is known as nanoporosity. Although they may have smaller pores, these parts are still “leakers." Parts with nanoporosity usually require greater attention to sealing techniques — the smaller the pores, the more difficult they are to fill or seal, with standard processing.

Nanoporosity is seen in both ferrous and nonferrous parts but is most widely found in the aluminum and magnesium castings that dominate the automotive and aerospace industries. As a general rule, we have found that nanoporosity is best sealed utilizing the dry vacuum resin impregnation process that removes all air from a part, introduces the appropriate resin to fill the pores, and then applies pressure to maximize penetration and sealing.

Because my business is selling resins and equipment for vacuum resin impregnation, I’ve noticed that the equipment we sell in the U.S. costs less, and has fewer controls and data measuring points than in growing foreign markets. And yes, this is probably another reason domestic casting houses can sell parts for less, but hold on. Why are European and other foreign companies willing to pay more for equipment? The answer carries a warning for the future. More and more foreign die casters and metal finishers are ordering high-end vacuum-resin-impregnation machines with improved process controls for consistently better parts. While this trend is predominantly European, Asian casters and metal finishers will soon follow suit. You read it here first!

Figure 1 - While casting technology continues to improve, most castings still exhibit porosity. When used to retain fluid or a gas, especially under pressure, impregnation methods that properly seal and prevent leaks are usually the best course of action.

For a casting (either die casting, permanent mold, or sand) designed to retain a fluid or gas, proper sealing is essential to prevent leaks. Porosity takes on additional importance in light of the trend toward smaller, lighter castings and higher operating pressures. The smaller the part, the more space, proportionally, is occupied by pores that could render it a defective “leaker." In casting larger parts, there are tricks of the trade that can push porosity to the least important sector of a part before it solidifies, but there is usually nowhere to hide porosity in a small part.

Because applications for many of today’s parts require them to withstand higher pressures than ever before, parts that were perfectly acceptable for standard “old” applications now leak and are therefore defective and unacceptable. Good examples of this trend are the increased hydraulic pressures utilized in aircraft controls and hydraulic transmissions for mechanical equipment. Aircraft parts are now routinely required to operate in the ±3000 psi range, 2-3 times that of past designs, and peak pressures in ordinary garden equipment transmissions can approach 4000 psi. In these cases, acceptable past performance for a casting in which there might have been no visible leak is simply not good enough anymore.

The size of the molecules to be retained by a part is also important, and likely to become more so. Hydrogen, for example, under consideration as a clean fuel for the future, is the smallest molecule of all, and will have to be retained by the cast components of hydrogen fuel systems. No leaks welcome! So, while casting technology continues to improve, the only real assurance to meet production and quality requirements will be to use impregnation methods to seal nanoporosity.

Coated or plated castings are another problem: Without filling porosity by impregnation, acids and other contaminants trapped inside pores during plating or anodizing can bleed out, rising to the surface and spoiling the anti-corrosive or decorative finish. New, environmentally friendly coatings are now being formulated without heavy metals, and this trend is likely to continue, but the inherent problems caused by porosity will remain. Die casters should be aware that these new coatings often require different sealing resins, which are specifically engineered to withstand the higher process temperatures that these parts will see.

Porosity in casting is generally the unavoidable consequence of one or more of the following: gasification of contaminants at molten-metal temperatures; shrinkage that takes place as molten metal solidifies; and unexpected, uncontrolled changes in temperature or humidity. Porosity can be classified as continuous, blind, or totally enclosed. Continuous porosity is usually a fissure meandering from one side of the part to the other. Blind porosity comes to a dead end inside the part. Totally enclosed porosity is isolated within the part’s walls, like a bubble. If a dead end or bubble is close to the part’s surface, machining may open it up, creating an additional leak path.

There are two basic methods of vacuum resin impregnation, which require different equipment: “Dry Vacuum and Pressure” and “Wet Vacuum.” In the “Dry Vacuum and Pressure” method, parts are placed in an autoclave and a minimum vacuum of 25 torr (29 in. Hg) is “pulled” (to use our industry jargon). At this level, all air is removed from the pores along with moisture contamination. The vacuum is maintained while liquid sealing resin is introduced, to approximately 6 inches above the parts. The next step is to release the vacuum. Pressure is then applied (in the form of compressed air at 80-100 psi/ 5-7 atmospheres), forcing the sealant deep into all pores. After a period of time ranging from several minutes to many hours, the sealing resin is transferred back to a storage tank and the parts are removed from the vessel and washed in water to remove any resin on the surface. The parts are then placed in a hot (195ºF/95ºC) water bath where the resin solidifies. Other resins utilize a slightly different polymerization method that relies on the interaction of the base metal with a catalyst to produce the polymerization reaction. Once cross-linked and cured, the resin will not return to a liquid state.

Though not as effective for nanoporosity, the “Wet Vacuum” method is an alternative for impregnation. In this method the parts are submerged in resin, above which a vacuum is drawn. Bubbles of air come out of the pores, and that air is also removed, as the vacuum is held. When the vacuum is released, this method uses normal atmospheric pressure to force resin into the pores.

For both methods there are variations, such as “Dry Vacuum/No Pressure” and “Wet Vacuum/Pressure.” “Dry Vacuum/No Pressure” relies on a soak cycle of the parts in a bath of resin. “Wet Vacuum/Pressure” applies compressed air after the vacuum is released, to help the sealant penetrate the pores. In both variations, after a sufficient time the parts are removed from the resin bath, washed, and cured.

A third, though little used, type of vacuum resin impregnation, the internal-impregnation-or-pressure method, requires the least investment in equipment and provides excellent sealing results in even the finest porosity, but can process only one or two parts at a time. This method utilizes the part as the vessel. The part is first filled with sealing resin while venting any trapped air. The sealant within the part is then pressurized up to the test pressure of the part (often to much higher pressures than those of other impregnation methods). The pressure is typically held until sealant is seen weeping from the part. After impregnation the part is then washed and cured in a way similar to all other methods. An advantage to this method is that impregnation and leak testing are performed simultaneously.

It is important for metallurgists, casting engineers, and impregnation engineers to collaborate throughout the entire parts-production process, beginning with the early stages of casting design. Just as there are many variables in the casting process (mold design, alloy metal ratios, additives, temperature, heat distribution, pressure, etc.), there are many variables in the impregnation process as well. For any given part, the vacuum level and pressure can be adjusted. Temperature is important at several stages, and the best curing temperature is different for different resins and parts. Some resins require the choice of an appropriate chemical activator to properly solidify the impregnated resin. Time is another important parameter — vacuum cycles, soak cycles, pressure cycles, heating cycles, and rinse cycles can all be lengthened or shortened as needed. Very important, but often overlooked, are parts washing and other pre-impregnation cleaning techniques such as vacuum vapor degreasing and drying.

Last but not least, thermoset sealing resins, whether designed for elevated temperature cure or ambient temperature cure, are not ordinary resins. Many formulations have been engineered with various mono and poly functional methacrylate monomers, so it is important to use a formulation with specific properties that enhance its ability to effectively seal the part in question. To produce pressure-tight castings, the impregnant must fill the porosity and then solidify completely. It should be a polar, low-viscosity liquid containing no inert solvents and no filterable solid materials in suspension. It should produce no gaseous or liquid by-products on curing. It should have a long pot life and should be easy to handle without introducing unacceptable health and safety hazards in the work environment. Obviously, the molecules of the resin should be small enough to fit in the smallest pores of the casting to be sealed.

Resins should be formulated for stability, high strength, and non-brittle resilience. They also should be resistant to fuels, oils, transmission and power-steering fluids, antifreeze, alcohols, solvents, acids, alkalis, and refrigerants and certified to current military impregnation standards (MIL-STD 276A and MIL-STD-I-17563C), which form the basis of many aerospace, automotive, and general industry requirements. Parts impregnated with these resins should withstand operational temperatures from -80°F to 450°F (-62°C to 232°C).

Figure 2 - With the IMPCO Partially Automated Controls Impregnation System, the operator places a wire basket of castings into the vacuum pressure vessel. A touch of a button carries out the complete “Dry Vacuum and Pressure” cycle according to easily pre-programmed requirements. Manual override is available if needed.

Overall, the vacuum-resin-impregnation processes required to meet the nanoporosity challenge are more complex than those required to resolve regular porosity problems. Ironically, achieving nanoporosity with highly engineered and tightly controlled casting operations and the concomitant requirement of a more complex post-casting impregnation process sometimes has the unintended consequence of making castings more expensive. In certain applications, such as high-volume automotive components And Critical Aerospace Parts, Standard, Lower-Cost, “Wet Vacuum” impregnation cannot reliably seal the “new” porosity. For those castings, extensive engineering sometimes can be counter-productive.

Such counter-productivity is illustrated by the re-engineering of a safety-restraint part that my company, IMPCO, seals for the automotive industry. For the original microporous part, we used a standard 1-hour vacuum-resin-impregnation cycle. This was extremely successful, resulting in sealed castings that met all specifications. In an effort to eliminate all porosity, engineers redesigned the casting as well as the die-casting process. 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. The part still leaked. After the standard impregnation cycle, helium leak tests showed an increase in the number of rejected parts.

To successfully seal the newly created nanoporosity, IMPCO engineered an enhanced impregnation process that substantially changed the cycle. In this case, the successful engineering effort to reduce the porosity had the unintended consequence of creating the requirement to enhance the impregnation process. The die-casting engineers had anticipated that the cost of new tooling for the redesigned part would be repaid by the improved quality. Instead, because the porosity had only changed from micro to nano, the longer sealing time increased overall production time — the expected savings were not achieved.

The message here is that casting engineers should fully understand that the effort to eliminate microporosity can have the unintended result of creating nanoporosity. Since parts with nanoporosity still leak, although at a slower rate, they must be sealed using enhanced techniques that are usually more expensive. 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, or if the casting will not be highly safety-sensitive over a prolonged operating life, investing resources to improve such a casting may not be necessary.

Figure 3 - IMPCO offers several automated and partially automated vacuum resin impregnation systems and will design custom systems. The type and capacity of a system are determined by production requirements and part sizes. For the “Dry Vacuum and Pressure” method, standard IMPCO vacuum pressure vessels are available in six diameters from 24” to 54”.

For castings to be used in very-high-pressure applications (above 5000 psi/350 atmospheres) special processing should be considered. While resins have been successfully tested and used in applications of over 10,000 psi (700 atmospheres), combining high pressures and nanoporosity presents specific processing challenges for the impregnation process.

Still, some casting engineers cannot resist the tempting quest 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, but sometimes the most economical solution is to take advantage of the proven process of standard vacuum resin impregnation.

When properly planned and implemented, impregnation decreases manufacturing costs and increases productivity by ensuring part quality and reducing the number of rejected parts. Prevention of pressure leaks and “bleedouts” is the main benefit, but impregnation also can enhance the machinability of a part, minimize its corrosion and oxidation, and enable the part to better withstand heat and chemicals.

Quality counts. On that, I think, we can all agree. As I mentioned previously, the global marketplace can change quickly, and the quality of cast parts is likely to count even more as the curtain opens for the next act on the world stage. I’m no casting (pun intended) director, but I’ll bet the starring roles will go to those who match the latest casting techniques with the latest sealing techniques.