11.16.06
Casting: What’s Old Is New Again
With material prices on the rise, these ancient skills are gaining renewed interest
Frank Celia
You might have to refer back to the discovery of the wheel or man’s dominion over fire to find an innovation more venerable than the casting of metal parts. This is a process that has existed pretty much since the dawn of civilization and, indeed, probably played a large part in the development of the human race. We know it was used to fashion jewelry more than 5,000 years ago; the ancient Romans built their casting molds in ditches; and there is evidence of rudimentary tools being cast some 7,000 years ago, or about a millennia before the outset of the Bronze Age.
And like the wheel and the mastery of fire, the casting of metal parts has undergone many improvements over the years, but the basic principles generally have remained the same. Advancements mainly involve technical improvements on the same basic themes.
The Birmingham Hip Resurfacing system is one example of the advances casting has made possible in orthopedic implants, given the metallurgy used. Photo courtesy of Smith & Nephew. |
Hence, there has been a push in the industry to streamline the investment casting process even more, as well as efforts to improve our understanding of metallurgy and the chemical reactions that occur when implants are inside the human body. Again, some companies are looking to a previous age. Metal-on-metal moving parts—once considered an anachronism in the world of implant technology—are now one of the most exciting areas of innovation in the industry.
Investment Casting
The metal casting industry plays a significant role in the US economy, representing the nation’s sixth largest manufacturing sector. Unlike many other manufacturing sectors, overseas competition is not seen as serious threat to the industry. Roughly 60% of investment casting worldwide occurs in the United States, and that market share is driven largely by our globally dominant aerospace industry. Of the $1.6 billion worth of investment castings produced in the United States, about half occur on aerospace- and military-related parts. The remaining half are used in a wide range of fields, including automotive components, hand tools, industrial valves, sports equipment, surgical instruments and, of course, orthopedic implants.
Many large orthopedic OEMs maintain their own in-house investment casting facilities, but do occasionally outsource for these services in cases of specialty jobs or when volumes become too high. Job shops that provide investment or other types of casting tend to be niche oriented—and extremely tightlipped about their proprietary trade secrets. It is common knowledge that all manufacturers closely guard their proprietary interests, but those in the casting industry seem to add an extra level of zeal to this secrecy, bringing to mind the mysterious trade guilds of the Middle Ages. Those in the industry often refer to casting as an “art” more often than a science or an industrial process.
Certainly, there are a quite a few steps in investment casting a metal part—and, thus, plenty of places to make mistakes…but also plenty of room for innovation and expertise.
The process begins with the creation of an expendable mold, usually wax, that has the same shape as the finished cast part. This is made by injecting liquid into a metal die cavity. This wax pattern is connected to other wax components—called the gate and running system—which will be used to deliver the molten metal into the mold and also serve as place for the wax to drain. The whole structure is then “invested” or dipped into a vat of liquid ceramic slurry, sometimes numerous times, after which the wax is burned away and drained. (Investment casting is sometimes known as “lost-wax casting” for this reason.) Finally, molten metal is poured into the remaining ceramic mold, and when it dries, the mold is removed and the cast metal part is created.
Net- or near-net shapes can be investment cast from almost any alloy, including aluminum, magnesium, titanium, stainless steel and copper. Because investment casting, also known as precision casting, offers a great deal of design flexibility and close dimensional tolerances, for most of human history it was employed to create jewelry and small ornamental objects. Not until World War II created a demand for aircraft parts that outstripped the capacity of the country’s machine shops did investment casting become a part of modern industry. Legend has it the first part created by investment casting was a stainless steel, three-inch turbine blade.
Materials
Obviously, materials—their properties, reliability, cost, availability, etc.—play a large part in this process, and casting shops in the orthopedic field are pushing the inherent cost effectiveness of their profession. “The materials most of the orthopedic implants are made from, usually cobalt chrome or even titanium, are kind of difficult to machine and polish,” said Wesley M. Shimizu, senior product engineer at Dameron Alloy Foundries in Compton, CA. “If you can reduce the amount of machine stock or polish stock on the part, that reduces cost.” Almost any part that can be cast can be machined with today’s advanced CNC machining, but machining whittles away much more material, which goes to waste, while the little material that is wasted in casting can be recycled and re-used, he added.
Tim Hall, vice president of sales and marketing at Cencast, an investment casting firm based in Portland, OR, agreed. “If you are machining from a round bar or a plate, there is a lot of material that gets lost there,” he noted. “We are hoping that the pendulum starts to swing more in our favor with these rising material costs. We can cast something—in some cases net, but in almost all cases near net—and that saves a tremendous amount of material.”
Areas of Improvement
Companies are scouring the investment casting process for unique ways to improve efficiency or to provide specialized services. Cencast, for example, specializes in casting very small and detailed endoscopic and laparoscopic surgical devices. With the use of the company’s proprietary High Acceleration Induction techniques, which employ centrifugal force, details as small as 0.004 inches can be achieved, along with thin sections as small as 0.01 inches, according to Hall. “It really gives us the unique ability to cast very complex and very highly detailed geometry,” he said. The company can produce high-quality metal parts in as little as six days, he added.
Automation and robotics are playing a bigger role in casting than ever before. But, for the most part, these processes occur in the grinding, polishing and cleaning steps of the casting, which are essentially machining techniques (all castings require at least some polishing or grinding, even if it is a very small amount). For example, the gate and running systems need to be grinded away, and robot cells can be far more efficient than humans in this regard.
The big challenge facing the industry is to develop ways to automate the casting process itself. One company, VA Technology, Ltd., of the United Kingdom, provides robot cells that can be programmed to dip the mold into the ceramic slurry, which adds greatly to the consistency of the parts.
The ceramic slurry dipping and drying stages are seen as areas where potential improvements are possible. Temperature, humidity, air speed and the like need to be carefully monitored and controlled to avoid shell cracking during this process. The techniques are so subtle and precise it may take two to 24 hours for each ceramic coat to dry. Hence, there is great incentive to speed up this process, but such efficiencies cannot come at the price of weakening the ceramic shell. In an attempt to solve this problem, a Chinese company called Dalian Guanghui Chemical Co. has developed a new shell-building technique that employs a super-absorbent polymer additive that it says dramatically reduces shell-drying time.
Another area of advancement is computer modeling, or cast solidification modeling, according to Nipendra (Nip) Singh, consulting partner/CEO for S&A Consulting Group in Cleveland, OH. These computer simulators can predict how castings will solidify in the mold, helping to determine things such as the grain size of the alloy. This gives engineers better forecasting ability and, thus, better control over the final product.
As design and material needs become more complex, companies are pushing hard to be involved in the design process at an earlier stage. “I think one of our biggest challenges is managing our forecasting with our customers,” said Andy Miclot, senior vice president of marketing at Symmetry Medical, Inc., based in Warsaw, IN. “When they scale back on their launches or have regional launches [instead of national ones], that impacts our business. The planning for that affects us. The sooner we know from our customers what they are doing, the better we know how much to invest in equipment and people.”
Metal-on-Metal Implants
Some of the most significant and far- reaching advances in the field of casting have occurred in the chemical compositions of the alloys themselves. Doncasters Medical Technologies—part of The Doncasters Group, which is headquartered in the United Kingdom with manufacturing facilities worldwide—worked with surgeon Dr. Derek McMinn to develop the specialized metallurgy used in the Birmingham Hip Resurfacing (BHR) system. This system gained FDA approval in May 2006.
Hip resurfacing is an alternative to total hip replacement and has been popular in Europe and other countries worldwide for many years. The main advantage of hip resurfacing is that it does not require the insertion of a large stem inside the femur. Instead, the surface of the head of the femur and the inner surface of the acetabular socket are prepared and lined with metal components. The procedure has been shown to be more stable than total hip replacement. Furthermore, patients can resume an active lifestyle and the device allows them to participate in strenuous physical activities without the fear of increasing device wear.
About 2,000 hip resurfacings have been performed in the United States so far, most on an experimental basis. This is a small number compared to the 220,000 total hip replacements that had been performed as of 2003. But many in the field expect hip resurfacing to become more popular now that it has gained FDA approval. (Several other orthopedic OEMs have hip resurfacing systems in the works, utilizing Doncasters’ technology.)
Hip resurfacing was first introduced as early as in the 1950s when Sir John Charnley tried Teflon-on-Teflon resurfacings. They showed very high early failure rates due to excessive wear and osteolysis (bone damage). In the 1970s, following the success of metal on polyethylene hip replacements, metal-on-polyethylene resurfacings were used. They too failed due to excess wear.
Metal-on-metal devices had been abandoned much earlier because it was expected that high friction between the components would result in early failures. Dr. McMinn, however, found that some patients who had metal-on-metal total hip replacements in the 1960s and 1970s showed no effects of wear or osteolysis when they were followed up for as long as 20 years or more.
Hip resurfacing was revisited again in the late 1980s, by which time it had been given up as a bad concept altogether. Dr. McMinn chose to use metal-on-metal bearings for his resurfacings and, along with a team of engineers from Doncasters, developed the modern resurfacings.
“The technology we have developed in casting the products to the required specification enhancing the low-wear characteristics is quite unique," according to Ken Birdsong, president and managing director of Doncasters Medical Technologies. Studies from Europe, Australia and around the world are continuing to provide evidence that the BHR device is proving successful in young and active patients with severe hip arthritis.
As opportunities continue to expand in the orthopedic market, the casting industry likely will serve as a sustained source of innovation and technological advancement. The elemental nature of these processes ensures their place at the core of the orthopedic implant trade.