Expendable pattern casting: state of the process.
In the demand for improved quality while casting with an environmental conscience, expendable pattern casting (EPC) has intrigued progressive foundrymen since its birth 30-some years ago. Although several concerns have hindered the widespread use of this emerging technology, certain elements make it an attractive process for casting new parts.
In EPC (also known as lost foam casting), a low-density expendable foam replica is made of the part to be cast. Placed in a flask and compacted with dry granular sand, metal is then poured through a sprue. As it enters the mold, the metal melts and replaces the polystyrene pattern, creating a near-net shape casting.
Considered for years as a metalcasting renegade, EPC is believed by some to be tomorrow's long-term casting future. Others, however, see it as an unproven, headache-ridden process that makes the simple art of metalcasting unnecessarily complex.
This article, which is based on information presented at last September's AFS EPC Conference in Birmingham, Alabama, provides insight on where EPC stands today.
EPC's roots began in 1958 when H.F. Shroyer took a block of expanded polystyrene, machined it into a casting mold and supported it by bonded sand before pouring. Called the full mold process, as metal is poured into the foam cavity (surrounded by sand), the casting is created by replacing the foam.
Several years later, from the suggestion of a local artist looking to apply the technique to art casting, Merton Flemings, Massachusetts Institute of Technology, explored using a bead-formed foam pattern with unbonded sand. Developed for commercial use, this is EPC as it's known today. EPC differs from full mold by the use of unbonded sand.
Today, according to a market study headed by Jennifer Ballachino, University of Northern Iowa, and AFS, there are 26 EPC foundries (seven captive, 17 jobbing, two both) in North America. There also are an additional 12 full mold foundries (two captive, nine jobbing and one both). The Midwest has the greatest number of both in the U.S.
EPC foundries produce 63,000 tons of ferrous castings per year and more than 104 million lb of nonferrous castings annually. Full mold foundries produce 18,717 tons of ferrous castings and 60,000 lb of nonferrous castings a year.
Iron is the primary metal poured in both EPC and full mold foundries. The biggest end-user for both types of foundries is the auto/truck market with 33%, while plumbing is the next largest at 18%.
Role in Metalcasting
EPC addresses a number of current concerns for foundries, according to Tom Piwonka, University of Alabama. He said EPC's advantages over traditional sand casting include:
* the elimination of sand binders and sand preparation;
* the elimination of cores, core tooling and core sand recovery;
* the production of casting designs that would be very difficult to make using conventional sand casting techniques.
"EPC offers substantially reduced tooling costs over die castings, and the ability to make heavier wall castings with higher integrity than conventional pressure die castings," he said. "It eliminates investment casting problems of mold firing and dewaxing, and allows the casting of heavier section parts than normally possible for investment casting."
Others note EPC's improvements in dimensional accuracy, part consolidation, elimination of parting lines and as-cast detail.
"All these advantages, however," Piwonka said, "too often are offset by a number of problems." These include:
* "trial and error" gating and risering design;
* setting individual sand compaction cycles for each casting;
* unique casting defects arising from pattern decomposition;
* dimensional reproducibility, which at best is often only equal to that of competing casting processes.
Finding a Niche
"Obviously, EPC hasn't overtaken green sand as the casting process of choice," Piwonka said. "Five years ago, many foundries thought it had the potential to solve a lot of casting production problems. They were intrigued that the problems they had with green sand and cores could be banished merely by switching to EPC. Today, we know that EPC is a complex technology with problems that are uniquely its own."
Nevertheless, he said, it is clear EPC has unique possibilities for the proper class of castings. These are castings that:
* are complex, that would require many cores in sand casting;
* have wall thicknesses between 10-25 mm (too big for investment castings);
* require good properties (better than can be produced by die castings);
* require dimensional control as good as high-quality sand castings;
* are needed in quantities sufficient to justify tooling costs.
Current auto applications, such as cylinder blocks and heads, are excellent examples of the best uses of EPC, Piwonka said. Some components that aren't made by EPC but could include stationary gas turbine components (they are currently investment cast, but are actually too heavy to make successfully by this method) and die castings, where mechanical properties are ignored in favor of dimensional reproducibility. Concurrent engineering may be necessary to convince customers to go to EPC.
As far as the environment, the benefits are clear. "The EPC process is undoubtedly the most environmentally friendly foundry operation of our time," said Jim Deppler, Saturn Corp. "Solid, liquid and gaseous wastes all approach the zero emission category."
Working from actual foundry data, Barry Kornegay, Vulcan Engineering Co., Inc., a manufacturer of EPC equipment, said the most common source of EPC pollutants is airborne particulate, and EPC air contaminants must be addressed at pouring, cooling, shakeout, sand handling and finishing.
While gases such as styrene, benzene, ethyl benzene and toluene are given off at pouring, cooling and shakeout, Kornegay said the quantities are relatively low due to small amounts of organic materials in EPC molds. In a typical EPC mold for iron, total organic content is about 0.3% of total weight of iron poured. This is compared to about 5% for chemically bonded sand and 12% for green sand. "It's true that virtually all the organics in foam are volatilized while only a portion are in other molds," he said. He noted total emissions are much lower and occur for only a few seconds at pouring and at dump, making them easy to capture and control.
Several other factors make EPC even more valuable to a foundry, Kornegay said. There is an important decrease in worker exposure to noise, heat, carbon monoxide and silica. Also, no normal shakeout is required, eliminating one of the noisiest, dustiest and hottest processes in a foundry.
"Because grinding can expose workers to respirable silica in the foundry, the elimination of parting lines and core fins, in producing near-net shape castings, improves this problem," he said. With the almost complete recycling of dry sand and no additives or cores, solid waste disposal is made easier.
Kornegay also challenges the common misconception that the earth's ozone layer is affected by the process. "The foam used in this process doesn't contain or give off the chlorofluorocarbons associated with other foam materials," he said. Designer Requirements
Like every casting process, educating customers on its benefits is essential. In a day where virtually every metalforming process also can be weighed against material substitution, EPC hinges on several factors affecting a design engineer's decision.
"Designer goals are form, fit, function and total lowest cost," said Duane Graham, a casting specialist at Cummins Engine. "A decision to use EPC for many parts is easy until you look at lowest total cost."
Answering the question, "Why might EPC capture a designer's interest?" Graham notes EPC is attractive because:
* manufacturers must deliver product on time at the lowest possible cost to the customer.
* they must be an effective user of new technology to maintain their competitive position.
* the process offers extended design freedom as component functional requirements continue to grow in the '90s. "A good part one or two years ago won't meet today's requirements," Graham said. "We're in a continuing state of rising expectations."
* lower cost components are possible. Less machining/equipment (and less capital) is required, and raw parts are cast in near-net shape.
* design flexibility. EPC allows integral designs, dimensional precision, as-cast features that are otherwise difficult to machine, and the cost-effective development of combination parts.
Even if it proved ideal for specific manufacturing needs, other factors extend beyond the designer's desk, Graham said. EPC must pass several other tests. These concerns are:
Existing machines, processes and tooling/fixtures--It's important to minimize capital investment.
Risk factor--If the design isn't mature, there is a heavy risk.
Capital tooling commitment--This must be made 18-24 months before production release. It's hard to have the final design and the EPC mold ready at that time. New processing procedures--These are required by manufacturing engineers. They don't believe you'll deliver. They're comfortable with sand, permanent mold and diecasting.
Issues with contract machiners, etc.--The value added by this process is a threat to their work. These workers aren't excited when the value added by their work is removed and put into the casting process.
Graham said four hurdles must be crossed in working with designers. These include: lack of published design standards; a long tooling lead time; understanding casting cost in terms of long-term savings; and the long casting development lead time to get EPC under way. "In starting with EPC, the just-in-time effort and EPC clash," Graham said.
Foundrymen and suppliers agree collective efforts are needed to allow the industry to take full advantage of this technology.
"EPC is now emerging as a capable process with unique capabilities, which will continue to fill a need for many casting applications," Piwonka said. "Its full potential, however, won't be reached unless it is included in R&D programs investigating improvements in foundry technology, particularly clean casting and simulation engineering."
Marketing is central to EPC's future casting niche, and current users are the best marketers.
"Exchanging technical knowledge is critical--both by foundry-to-foundry and foundry-to-designer sharing," notes Ballachino in her market study. "To remain competitive in the world market, foundries must be willing to help their colleagues and stand together on a united front to market the EPC process."
There's more at stake than the age-old contest of casting process versus casting process. With constant technological changes, designers have more options in different methods than ever before. If embraced by the foundry industry, EPC could be another ace to keep parts from being lost--to other metalforming processes and material substitutions.
For details on conference proceedings and other EPC publications, call AFS Publications at 800/537-4237 or 708/824-0181.
EPC Technology Part of Saturn Foundation
Saturn Corp.'s foundry in Spring Hill, Tennessee, is one example of a casting facility that made the commitment to go with EPC and seen profound results. Residing in Saturn's powertrain business unit, the foundry occupies 120,000 sq ft.
Saturn's EPC strategy began in 1986 when casting processes were finalized for the new auto. Aluminum EPC would be used for cylinder blocks and heads, and ductile iron EPC would be used for the crankshaft and differential cases.
"The manufacturing plan told us what we were going to make, how many and at what rate," said Jim Deppler, Saturn project engineer. "We didn't need to plan for added capacity or expensive equipment flexibility for major future product changes.
"This was a process designer's dream and led to a product driven process design for casting operations--design the process for individual products rather than force the product into a rigid generic process."
Seven years later, Saturn found EPC's documented advantages to be true. The company produced near-net shape castings with reproducible accuracy, as-cast detail resulting in reduced finish stock and greater design flexibility.
"To put the benefits into perspective, on the block and head machining lines alone, over 16 in. of drilled holes are avoided using as-cast passages not attainable in other processes," Deppler said. "This equals more than 16 in. of aluminum not generated into chips on each engine, yielding significant reduction in variable operating costs and aluminum material cost. We paid for the foundry in machining, component and assembly cost avoidance."
He also noted that tangible benefits include improved reliability with fewer parts, fewer operations and reduced dimensional variability in the long run. Investment and product costs are reduced through reductions in parts, material, capital equipment, and tooling, labor and floor space.
To maximize the process, Deppler said, simultaneous product/process engineering must occur. Product designers can achieve an optimum design that is castable and buildable only with the synergy and real-time input from casting, machining and assembly engineers and production technicians.
"You can't convert a green sand part to EPC and realize any major cost reduction unless the part has been redesigned to take advantage of EPC," Deppler said. "A new machine line designed to manufacture green sand parts won't save any money for the program when changed to EPC--it didn't eliminate any machining." The bottom line, he said, isn't individual casting costs, but the unit cost. "Value is engineered into the EPC castings so that it won't need to be added on later," he said. "The final product cost (engine) is what we focused on."
Deppler said when Saturn was conceived, successful EPC was one of the firm's biggest reaches. Suggesting a serious look at EPC before investing capital in other processes, he adds: "EPC is unquestionably one of the biggest, if not the biggest win, in our short history."