Diecasting '96: a status report.
All the traditional reasons to use die castings-high-speed production, excellent dimensional repeatability, tight tolerances, the ability to cast thin walls and exceptional surface finish - are still in place. However, the traditional reasons more components aren't designed for diecasting are disappearing. Though there are fewer than 500 diecasters in the U.S. (down from nearly 700 in 1980), their products are found in almost every industry from sporting goods and toys to household appliances and automotive - accounting for more than one third of all metal castings, and with more than 60% of all aluminum casting shipments.
Because of the wide variety of industries it serves, diecasting's competitors are many. According to Brian Cochran, Wabash Alloys, "In terms of materials or processes, its competitors vary, depending on the market. In automotive, it's plastics; for industrial machinery applications, it's permanent mold and sand castings; in marine applications, it's lost foam casting." Despite this diversity of competition, diecasting is making new inroads into various markets, driven by quality improvements, technological advancements and the trend toward lighter materials with lower machining costs that the process delivers.
This article discusses the present and future place of diecasting among other casting processes, and provides an overview of the technology that is securing that position.
Diecasting, a process in use for about 100 years, is the pressurized (5000 psi or more) introduction of a precise "shot" of molten metal into a steel mold comprised of two or more die parts. The pressure is maintained until the metal solidifies. It is used in the casting of nonferrous metals, such as aluminum, magnesium and zinc. Traditionally, diecasting's niche has been smaller, nonstructural yet highly dimensionally accurate parts needed in high volumes (dies are typically built with an intended life of about 150,000 shots).
Compared to sand casting, many of the process steps and ancillary production equipment is unnecessary; most notably the entire sand system. Also, without sand and the attendant moisture and binder systems, there are fewer variables to casting. But diecasting has its own unique variables, including die and plunger lubrication, die cooling systems, hydraulics, and the entire set of mechanical parameters that must be adhered to in producing quality die castings.
There are two basic varieties of diecasting machines: hot chamber machines [ILLUSTRATION FOR FIGURE 1 OMITTED] and cold chamber [ILLUSTRATION FOR FIGURE 2 OMITTED]. In both units, the pressure for the shot is supplied by hydraulics, which also supplies the pressure for the "locking force" that clamps the dies together and keeps the cavity pressurized until solidification is complete. The size of the machine and that of the casting decide how much pressure is required. The pressure, along with the metal dies and their built-in cooling systems, causes the quick solidification of the part.
In diecasting, cores are incorporated as part of the mold. Known as "slides," they are moveable metal parts that slide into place when the mold is closed. In this way, many of the holes and flanges that would require machining on sand or permanent mold castings can be cast right in, saving on machining costs. On the other hand, this feature can be limiting, as core geometries must be such that they can be pulled straight out of the casting before the mold opens. Also, total part geometry is restricted compared to sand casting, and special machines with movable die parts are necessary to cast in undercuts that would present no problem in sand molding.
The other standard piece of equipment is the trim press. Located near the casting machine, this unit is set up to the casting dimensions and automatically removes the gating and flash from the die casting, thus saving numerous postcasting steps in the finishing room. Use of the trim press is facilitated by diecasting's high dimensional repeatability, which also lends itself to easy automation.
Production speed is diecasting's best attribute, with mold filling generally occurring in well under 1 sec. According to Cochran, "While a 70-lb casting might have a cycle time of 100-120 sec, small parts can be set up automatically to 30 shots per min, or one every 2 sec."
"Die castings can no longer be described as two dense skins separated by a sponge," Cochran stated, referring to the method's dubious reputation in some quarters when it comes to mechanical properties. "In the past," said Daniel Twarog, North American Die Casting Association (NADCA), "most die castings weren't designed with mechanical properties in mind. The engineer wanted the fastest method to produce a given shape, and diecasting fit the bill. As designers have become more sophisticated, so have die castings."
Porosity is an inherent problem in diecasting, forming because air and gases are entrained in the molten metal during the shot and trapped by the rapid solidification of the outer casting. Heat treating makes porosity particularly evident, causing the subsurface holes to form blisters in the casting face. This is not a problem in many cases, since the rapid chilling and solidification of the process forms a fine-grain structure that lends high fatigue strength and other good mechanical properties, precluding the need for heat treating. But it does impose a serious limitation on diecasting applications.
There are many causes for porosity, including some shared with other casting processes. "A lot depends on how well you gate and vent," said NADCA's Stephen Midson. But the problem is exacerbated by the physics of diecasting. The "injection profile" - the mechanics and route of the molten metal shot from the shot sleeve through the gating and into the mold cavity - is the largest source of variables affecting porosity. In conventional cold chamber machine design in which the shot sleeve is horizontal, "you have approximately half metal and half air in the cylinder," Midson said. As the plunger moves forward it creates a wave of metal, some of which can break away and roll forward, entrapping air.
It has been found that manipulating shot acceleration and velocity by either slowing the entire shot or abruptly dropping injection speed just before shot injection allows air and gases a chance to escape prior to solidification. The application of a vacuum to the cavity and gating just prior to the shot can remove much of the air that causes porosity.
Vertical diecasting is an important development in the elimination of porosity. In this process, the metal is introduced from below, rather than from the side of the die [ILLUSTRATION FOR FIGURE 3 OMITTED]. The dies are set up either in the conventional horizontal way or can be inverted to lie flat, depending on machine design. In vertical injection, the metal shot rests on the plunger face, which moves the metal straight upward and into the cavity. No wave is created and all the air is pushed ahead of the metal and purged through venting. The method also requires less extensive gating and holds metal temperature well.
The delivery of the shot is not the only cause of mechanical property problems in diecasting. As Twarog said, "The processing of metal has always been the key to better properties." Metallurgically, diecasters face the same challenges as other nonferrous foundrymen, and clean metal is paramount in producing good castings. In many diecasting shops, metal is delivered from the melting furnace to smaller holding furnaces near or attached to the diecasting machines via covered, heated launders [ILLUSTRATION FOR FIGURE 4 OMITTED].
Cochran cites "dosing" furnaces as an effective means of protecting the alloy from oxidation. The liquid metal enters one end through the launder and an insulated tube on the other side connects the furnace to the shot cylinder of the machine. Pneumatic pressure or an electromagnetic pump takes the metal from 6 in. below the bath surface up the refractory pipe and into the shot sleeve. "This guarantees not only clean metal delivery but the benefit of better temperature control," he said.
Compared to its other metalcasting counterparts, diecasting is somewhat limited in its ability to cast complex parts with intricate coring. The more complicated a part's geometry, the more expensive the die. Multiple slides represent huge diemaking costs and even then are restricted in what can be cast.
The last several years have seen the development of technology making it possible to use disposable cores made from sand or salt, allowing the casting of intricate internal passages, which was unheard of a few years ago. However, disposable core use is not yet widespread in the industry.
Generally, die casting quality has been improved most through the use of computers and other high-tech process controls. "The faster computers become," said Twarog, "the more they can provide instantaneous feedback and adjustments." Closed loop monitoring systems for shot control, metal temperature, die cooling and hydraulic pressure allow operators unprecedented control over the mechanics of the process, while solidification and flow modeling are optimizing part castability.
On the technological horizon, diecasters would seem to be in the best position of any metalcasting segment to take advantage of semisolid and squeeze casting, two processes currently headed from the developmental to the production stages. According to Kenneth Young, Buehler, Ltd., semisolid metalcasting "involves injecting aluminum alloy slugs the consistency of toothpaste into a die cavity. Since the alloy is already 60% solid at injection, there is substantially less solidification shrinkage to accommodate." There is also no porosity or gas-related defects, so the castings can be heat-treated.
In squeeze casting, a metered amount of liquid metal is introduced to the mold at a slower, less turbulent rate than in traditional diecasting (up to 3 sec). Once the metal is semisolid, pressure is applied and it is squeezed. The resultant casting is porosity-free and has high integrity. It provides exceptional surface finish and high yield, since there is no need for gating or risering.
Because of the equipment and process similarities, these two methods are very compatible with diecasting, and indeed, some diecasting machine manufacturers are producing units capable of combinations of traditional diecasting, semisolid and squeeze casting.
"More than any other casting process," said Cochran, "diecasting is a capital-intensive endeavor. A basic medium-sized machine of 650 tons locking force costs about $350,000. A single cavity die for that machine with no slides and simple geometry will run $50,000. If it has slides and complex geometry, it can easily cost three times that amount. Large diecasting machines can cost $1.5-2 million, with dies costing up to $1 million, depending on complexity."
Obviously, the only way to recoup these kinds of capital outlays is through high-volume work. Another reason diecasting must be done in high volumes is the lead time for initial part development and die manufacture, which can easily take six months or a year.
Therefore, perhaps more than any other process, diecasting stands to benefit from the use of computers in part design and prototyping. "The modeling of dies and solidification has helped substantially reduce lead time," Twarog said.
Diecasters are taking advantage of concurrent engineering to speed the development process. New designs are immediately translated into CAD/CAM and computer-aided engineering (CAE) data. "As the customer refines the design and conducts structural analysis," Cochran explained, "the caster is doing fluid and thermal analysis, and solidification simulations." Linked via computer, the caster and the customer can review all details immediately, before the prototype stage. "This approach saves months of time and at least one generation of dies," Cochran says.
The various types of rapid prototyping also aid the process. A new, high-speed option called precision strataform machining provides high-integrity prototype parts. It uses thin slices of aluminum plate fused together to form an exact model of the part with high structural integrity. In some cases, it can be made from the same alloy for even closer correlation to the intended use.
A Booming Market
All these improvements are timely indeed, because they will allow diecasting to take advantage of myriad market opportunities emerging in the next decade. Diecasting is already enjoying the effects of the current design preference for lighter, nonferrous metals replacing iron and steel in many applications - particularly automotive. According to Cochran, each one of the 14.6 million vehicles produced in North America in 1994 contained 72 lb of secondary aluminum, 14.6 lb of zinc and 3.5 lb of magnesium die castings, a total of 1.3 billion lb.
As auto manufacturers face the problem of the corporate average fuel economy (CAFE) requirement being ratcheted up to 37 miles per gal, and customers who want bigger cars loaded with options, the demand for lighter parts in extremely high volumes will soar - exactly the niche for diecasting.
With zinc being the notable exception (it is far too heavy to figure in the weight reduction trend), the future looks bright for diecast parts. Cochran predicts that by 2000, annual vehicle production will be 16.5 million units, each including 121.8 lb of aluminum and 7.27 lb of magnesium die castings. Expanded automotive applications already include steering wheels and columns, dashboard frames, brake rotors, suspension parts, frame members, engine blocks and seat frames, to name a few.
Magnesium comprises a healthy segment of die casting applications. According to Peter Caton and Kenneth Barnes, Diemakers, Inc., "The need for weight reduction has stimulated North American engineers to be a little more adventurous in their choice of materials." Magnesium die castings, with only one fourth of the density of steel, one third that of aluminum and a higher strength to weight ratio than either, have seen a North American production increase from fewer than 4000 tons in 1982 to 40,000 tons in 1995, they said.
Wider use of magnesium is hindered by its high cost and supply problems. However, with a low heat content and low reactivity to steel, it can provide faster production and result in longer die life. "The good fluidity of molten magnesium allows complex and finely detailed components to be easily cast," said Caton and Barnes, "and the low heat results in lower thermal distortion and close tolerances, often eliminating secondary machining operations."
It also has excellent damping properties for noise, vibration and vehicle handling improvements. Accordingly, said Kenneth Kirgin, Stratecasts, Inc., "Magnesium has been specified for clutch and brake pedal supports, steering gear assembly parts and clutch housings." Kirgin forecasts magnesium casting production to rise to 52,000 tons in 2005.
Zinc die casting shipments will continue to decline as engineers favor lighter materials. Zinc's 394,000-ton total of 1995, Kirgin said, will decrease at a rate of 1.7% a year to 332,000 tons in 2005. Cochran agreed, saying that in the automotive market, "At best, zinc may hold its current levels, but more likely will steadily decline as magnesium or aluminum take over those applications."
As for aluminum die castings, Kirgin predicts shipments of 858,000 tons in 1996, with 447,000 tons going to automotive applications. But as the diecasting process continues to improve, and market and design trends continue to favor its particular attributes, it appears diecasters will be enjoying a demand direction that, as Cochran puts it, "is UP, UP, UP!"
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|Author:||Philbin, Matthew L.|
|Date:||Feb 1, 1996|
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