Improving investment casting through innovations.
Technological and process innovations were on display for the attendees at ICI's 49th Annual Technical Meeting & Equipment Exhibition, held in Orlando, Florida, October 7-10. In addition to those displayed by suppliers showcasing at the Conference's exhibition, several of the 26 speakers also discussed advances for metalcasting. This article highlights a few of these advances by working its way through the investment casting process-from prototypes to cleaning and finishing.
Step 1: Making a Prototype-Rapid Prototyping Aids Design Validation
Rapid manufacturing tools add value to both customers' and casters' bottom lines by speeding up the prototype process. In his presentation, "Rapid Prototyping and Rapid Manufacturing for the Modem Investment Caster," Michael Hascher, 3D Systems, Inc., presented 3D Systems' Thermojet printer that can reduce the time required to introduce a new product via investment casting (Fig. I).
Patterns produced by this method can be substituted for waxes and used for the production of prototype and limited-run investment castings. They also can be used as models to prove out the casting process long before investing in production tooling.
According to Hascher, the printer uses multi-jet modeling (MJM) technology to create refined, thermoplastic wax patterns from electronic CAD designs. Three hundred fifty-two jets deliver the wax by building 0.0016-in, thick layers. Using such a model from the beginning of a project helps ensure that the project's full value is realized and development costs are reduced, he said.
In a case history presented by Hascher, a company contacted its casting supplier with CAD models for two castings. The foundry used the MJM printer to create initial patterns. The patterns cost $500 each, and three of each design were built in three days. The customer made changes to the design twice before final approval. Each literation took 1.5 wk, saving 28.5 days and $14,360.
Step 2: Making the Patern-Solid Wax Injection Speeds Up Patternmaking
Investment casting wax patterns can suffer from inconsistencies due to the way wax flows into the pattern and solidifies. Faulty patterns can create castings with defects or flaws. John Kight, Worldsource, Inc., and Earl Powers, C-Power Industries, presented their paper, "Significant Productivity Improvements from Solid Wax Injection Technology and Accompanying Automation," describing high-pressure injection of solid wax as a way to alleviate this problem. Solid wax technology produces patterns for small and large parts without shrink, dish, flow lines or other discontinuities, said Kight and Powers.
In this process, a wax preparation system delivers solid wax directly to the tool. The system has no accumulator or holding tank, and the wax temperature does not have the variation often seen with conventional paste machines. Solid wax injection uses pressures between 1000-2000 psi. While the solidification temperature depends upon the actual wax formula, conventional pattern waxes have been injected at or lower than 122F (50C).
According to Kight and Powers, the greatest advantage to the system is the speed of pattern production. In a test conducted in the U.S., the cycle time for injecting a typical pattern with solid wax was 10-15 sec. The same pattern made with liquid wax needed to cool in the tool for 2-3 min to achieve a face without sink.
In addition, solid wax injection produces patterns without shrinkage or any dish on the face. The patterns precisely replicate the dimensions of the tool, so an estimation of wax shrinkage is no longer a factor in the design of the tool. This eliminates dimensional miscalculations. Significant cost reductions also are achieved through the use of less expensive wax for the actual pattern--reclaimed wax works as well as virgin wax, Kight and Powers said.
Step 3: Coating the Pattern--Fiber-Modified Binder Thickens Shell Edges, Increases Permeability by 300%
In his presentation, "The Wexcoat Binder System," Dan Duffey, Wex Chemicals, presented the benefits of the binder system, which uses water-insoluble Wexperm fibers that disperse in the binder. Rather than dissolve, like water-soluble polymer additives, the organic fibers disperse in the binder and are designed to remain in the shell during dewaxing (Fig. 2). The fibers do not create foam, and the presence of bubbles in the slurry is low or nonexistent.
Advantages to the fiber-modified system include enhanced shell permeability caused when the fibers evacuate the shell during firing at a temperature of 1200F (650C). Despite the increased shell thickness of the organic fiber system, the permeability is 300% more than that of a water-soluble, polymer-modified system because of the passageways left behind from the fibers, Duffey explained. The fibers also change the characteristics of the shell build. They improve and even out the coating, which increases the thickness of the shell around the edges where cracking often occurs.
Duffey added that the system also reduces the number of necessary overall coats for a shell, makes shell removal easier, reduces the possibility of hot tearing and increases heat retention.
Step 4: Melting the Metal--Liquid Argon Atmosphere Addition Produces Cleaner Metal in Nickel Alloy Melting
A common melting problem with nickel boron and nickel-aluminide alloys is the increase of soluble gases such as oxygen, nitrogen and hydrogen in the melt. Robert E. Barber, Franklin Bronze and Alloy Co., Inc.; Terence La Sorda, Air Liquide America Corp.; and Kenneth Till, applications consultant, described a solution to this problem during their presentation, "The Application of SPAL on Nickel-Boron and Nickel-Aluminide Alloy Systems." Surface Protection Air Liquide (SPAL) is a liquid argon inert blanketing used on nickel-boron and nickel-aluminide alloy systems during melting. The process is designed to produce "cleaner" metal than all non-vacuum melting methods by reducing dissolved oxygen and hydrogen pickup.
Barber said that Franklin Bronze and Alloy Co., Inc., Franklin, Pennsylvania, a 33-employee steel and bronze foundry, tested the process on four of five induction melting units--two 500-lb capacity tilt furnaces for nickel alloys and two 300-lb capacity lift furnaces, one for copper, bronze, brass and aluminum alloys and the other for steel and stainless steel alloys. The foundry wanted to improve metal matrix integrity and process cost reductions during melting.
The process uses insulated flexible hoses that deliver low-pressure argon into a stationary lance and through a sintered diffuser. As the liquid approaches the melt surface, it begins to expand. As the expansion occurs (840 times its own unit volume for liquid argon), atmospheric gases are displaced through this purging action, explained La Sorda.
Liquid argon was applied prior to incipient melting for each heat cycle of the alloys, and the atmosphere was maintained throughout the heat cycle through pouring into the transfer ladles. This continual liquid-to-gas phase change provided the protective atmosphere necessary during the tap-to-tap cycle, including transfer ladle inserting.
The system implemented at the foundry resulted in an average 85% decrease of slag and dross on all heats. With the reduction of atmospheric gas pickup, the fluidity of the molten alloys increased due to reductions in dissolved oxygen. With this higher fluidity, a 100F drop in pouring temperature was achieved on nickel alloys and a 50F on ferrous alloys. The process also increased the amount of revert material from a 50:50 revert/virgin charge ratio to 100% revert charge composition, said Barber. Overall, the foundry realized 20% reduction in melt-deck generated scrap.
Step 5: Pouring, Lifting Molds-Manipulators Reduce Operator Fatigue through Help with Pouring, Lifting
Utilizing lifting devices such as manipulators can make improvements in productivity, safety, employee morale and product quality while reducing labor costs, said Scott Benda, Ergoflex, Inc., in his presentation, "Increase Profits Through Utilization of Mechanical Lifting Devices in the Investment Casting Industry."
Manipulators are manually controlled lifting machines that aid an operator in moving material from one location to another and also can be particularly effective in pouring metal, handling molds in the cut-off area, moving shells into and out of furnaces and dipping wax treesinto slurry.
Typically, several axis of movement are possible in a manipulating arm as the arm lifts the load while the operator manually pushes the work piece in a horizontal direction.
Benda detailed several benefits that manipulators offer, including:
* increased production through reduction of operator fatigue because operators are guiding, not carrying, the load (Fig. 3);
* increased safety because the machines do the lifting, relieving worker strain;
* reduced manpower because one employee can lift a heavy piece with a manipulator, a job that normally would require several workers
* increased workforce flexibility--no longer are only young, strong men needed for the job;
* improved worker morale, work quality and consistency. Employees don't experience stress and strain, so the work quality stays high.
Benda referred to InvestCast, a steel and bronze investment casting foundry in Minneapolis that uses a manipulating arm to pour metal. The company was able to reduce its pouring crew by one man and increase production. Prior to using the manipulator, it took 14 hr/day for the crew to pour molds. With the manipulator, this same work requires 11 hr. This production increase is due to the ability to pour larger trees and reduced operator fatigue. The foundry experienced payback in 1 yr on direct cost savings alone,
Step 6: Grinding the Finished Casting-Pressure-Assisted Grinding Eases Gate Removal, Reduces Abrasive Costs
Millions of dollars are spent annually by foundries on grinding operations. Manual grinding methods using coated abrasive belts on backstand grinders are most common, but these rely on the operator's effort to remove the gates. High-pressure rapid grinding operations use hydraulics or motorized feed methods to remove gates and can reduce abrasive costs while improving throughput of gate removal operations.
According to Nick Orf, 3M Abrasive Systems Div., in his presentation, "Maximizing Pressure-Assisted Gate Removal Operations," the application of high pressures at a constant feed rate with a hydraulic or motorized feed system allows fast material removal rates with the added benefits of increased belt life, cooler grinding temperatures and improved dimensional tolerance. A few examples of benefits include:
* a stainless steel gate (0.75 x 0.75 in. with a 0.5-in. projection) can be removed in 1.5 sec using a plunge grind that forces the part directly into the belt. Rapid grinding techniques typically reduce the degating time by 0.67 when compared to manual method;
* during manual grinding, the tips of the grains on belts are used, and when the mineral becomes rounded and dull, the operator replaces the belt. The high pressures of rapid grinding applications allow the mineral to fracture, revealing new, sharp grinding edges. This method uses the full width of the abrasive belt and can improve abrasive life by 150% vs. manual methods;
* rapid grinding techniques index the part into the coated abrasive belt to a fixed position. Since parts are held firmly in a fixture, a repeatable grind can be maintained to reduce scrap and the chance of out-of-tolerance parts. With proper machine controls and compensation for belt wear (0.0250-0.04 in.), parts can be dimensioned to a flatness of 0.005 in;
* gate removal through rapid grinding improves the safety of the grinding operation because the operator is not in direct contact with the abrasive. The operator's job consists of picking and placing parts in a fixture, which removes him from potential injury and reduces the chance for repetitive stress disorders.
In one case history provided by Orf, a Midwestern investment casting foundry recently converted from manual degating to pressure-assisted grinding. This change reduced labor by $60,000 annually as one rapid grinding operator is doing the work of 2-3 manual grinders. In addition, abrasive costs have been reduced by 300/0 annually based on a combination of increased belt life and the ability to consume the used belts from the manual operations.
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|Date:||Jan 1, 2002|
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