Where die casting makes "cents"

Stuart Burke
Application Engineer
FisherCast
Peterborough, Ont.
Canada
(www.fishercast.com)

Designers of a new automotive seat were in a quandary over the most economical way to build the seat's geometrically complicated, horizontal gear drive. It was proving tough to find a process that met the gear drive's precise tolerancing requirements and do so on a budget. The gear drive incorporated not only an Acme screw thread at one end of a bearing journal but also a cross-axis helical gear on the other.

One potential option was machining. But this was costly and time consuming. It would have entailed making two components in two separate operations, and then press fitting the assembly together with a spline engagement. Additionally, there were concerns about inconsistent runout and whether gear teeth could be positioned with enough relative accuracy. Powdered-metal processes were another possibility for the gears. But tooling constraints and tolerance-control issues precluded their use. Plastics also were ruled out due to tolerance and strength limitations.

That left die casting. One plus was that it combined the individual components -- the screw thread, helical gear, internal bearing journal, and two thrust faces -- into a single part. Zinc alloy also provides strength and dimensional stability. Moreover, die-casting production costs were 40% below those for machined steel.

Tooling to form the Acme-screw thread cavity incorporated four side cores. Gating through the center bore ensured the alloy would consistently fill in the tooth forms. The resulting gear drive is cast ready to use and needs no finishing or deburring operations.

The option to convert multiple components and operations into a single die-casting operation is a major reason designers consider this process. But, die casting also offers many other cost-cutting opportunities and can improve part quality as well.

Why die casting?
The potential for piece price reduction is the usual motivation for high-volume die casting. Economies of scale start at 50,000 pieces annually. Numerous factors can affect production economics. These include component complexity, alloy properties, die-casting technology used, precision of the die-cast tool, and cycle rate. One reason die casting can be thrifty is that a single cast part often replaces multiple components. And it is frequently possible to incorporate features in the casting that eliminate secondary milling, boring, reaming, and grinding operations.

Flash-free die-cast tooling also eliminates the need for finishing operations. And additional savings come from material reduction, use of less-expensive metals, improved tolerances, and good part-to-part consistency.

As a rule of thumb, designs incorporating complex configurations are well suited for die casting. Good candidates include gears, shafts, cams, ratchets, levers and pinions, and others performing mechanical functions. Containments such as end bells, plates, motor and gear housings, spacers and seats also are frequent choices for die casting.

Die casting is well known for reducing manufacturing costs in external, internal, face, helical, spur, and worm gears -- casting them to AGMA 6 to 8 specification. Most tooth forms can be cast, including teeth with helix angles as great as 20*. Up to 50 external threads/inch are cast flash-free to Class 2A tolerance without cleaning or chasing, as are multistart threads.

Choosing an alloy
Zinc, magnesium, and aluminum are the most widely used die-casting alloys. Alloy mechanical and physical properties dictate material selection based on design specifications. Most typical of these are component function, intended loads, and operating environment.

Zinc alloys, such as those in the Zamak, ZA, and ACuZn families, offer a wide range of casting properties. Zinc is the strongest die-cast alloy at room temperature. The bulk of zinc alloy components are die cast in Zamak 3, as it offers the best combination of mechanical properties and economics. Hot chamber die-cast zinc alloys can be cast flash-free to tight tolerances with complex detail. Dimensional stability ensures part-to-part consistency over long production runs. Net-shape manufacturing is one of the main advantages of hot-chamber die-cast zinc alloys.

Magnesium (AZ91D, designated by ASTM as die-casting grade) is the lightest of all structural metals but has the highest strength-to-weight ratio. It is two-thirds the weight of aluminum and about one-quarter that of steel and zinc. Both magnesium and zinc alloys are dimensionally stable. They also exhibit consistent, predictable shrinkage rates that ensure part-to-part consistency. High fluidity of the alloys also makes possible thin-wall sections (a minimum 0.020 in.) for realizing complex, intricate details. And both alloys have inherent EMI/RFI shielding properties.

Aluminum alloys are lightweight with good physical properties, dimensional stability, and high electrical conductivity. They resist corrosion and cast easily. With strength comparable to zinc alloys, aluminum alloys are frequently chosen when weight is at a premium. Al 380 is the most common aluminum alloy in use, but there are others tailored for corrosion resistance, improved ductility, or superior strength at elevated temperatures.

The diverse properties of metal alloys help simplify production. For example, in complex, multipart assemblies designers often use alloys of varying strength depending on how strong a particular component must be to function properly. Tailoring the alloy for individual components while using an unvarying production process helps reduce overall costs.

Choosing a casting process
The choice of alloy and component size dictate which die-casting process to use. Pressure die casting is the most common method of producing small zinc-alloy and magnesium components -- using either hot or cold-chamber processes. Aluminum and those zinc alloys with high aluminum contents (ZA-12, ZA-27, and ACuZn-10) aren't suitable for hot-chamber die-casting and must be cast using the cold-chamber process.

Other options include sand and investment casting of zinc, magnesium, and aluminum alloys, permanent mold casting for aluminum and zinc alloys, and semisolid-metal (SSM) processes for both magnesium and cold-chamber zinc alloys. Powdered metal is yet another option for producing aluminum and magnesium components. And spin casting makes small zinc-alloy components.

The various die-casting methods for each metal have merits and limitations. These depend on component size and complexity, tolerance specifications, production volume, and tooling cost.

Overall, hot-chamber pressure die casting is the process of choice for small zinc and magnesium-alloy components of up to 8 in.3 For larger components other casting processes may be a better fit. While initial tool costs are high for the hot-chamber process, large volume production and part-to-part consistency helps reduce piece price.

Additionally, zero parts-per-million scrap is common with pressure die casting. Tooling techniques let designers specify net-shape parts with intricate and complex external and internal features and tolerances of +/-0.001 in. Parts are flash-free with no secondary finishing or machining required. This helps offset tool costs when compared to other processes.

Getting started
It's a smart idea to have a die-casting supplier on board as part of the initial project-planning team. Most suppliers have technical specialists whose sole job is to review component designs for die casting. Having a supplier on board early can translate into major cost savings in both design and production by maximizing tooling and die-casting techniques.

Before the design can be finalized, the die-casting specialist must calculate process factors such as flow vectors, gate and runner design, fillets, radii, draft, metal velocity, and fill time. Die casters employ a number of techniques to maximize tool and component design. Even minor changes can improve performance and reduce costs. Elements such as ribs may be incorporated to boost component strength, stability, and density. At the same time, wall thickness can be cut to as little as 0.020 in. Cross sections can be reduced or recesses designed into the component to remove material if weight is a concern. Likewise, inside corners designed with fillets enhance creep resistance. And the addition of extra threads in a bolt connection reduces creep and helps retain the load over long periods.

Gating can also be critical with some component configurations. Generally, the molten alloy injects into the cavity along a parting line of a noncritical feature, although other options are possible. Where the component design has a through-hole 0.4-in. diameter or less, a center gating technique introduces the alloy into the cavity around the circumference of the through-hole. This ensures uniform alloy distribution from the center to the outside.

Design engineers need to think beyond die casting as just the forming of metal components. At its simplest, that is what it is. But the real benefits come when die casting serves as a manufacturing process to reduce production costs. Where an application consists of several parts, the die caster looks for ways to combine components into a single part -- consolidating gears with shafts, ratchets, and cams. It's often possible to devise one die-cast part to eliminate assemblies that incorporate swaging, riveting, screw machining, stamping, press fitting, and welding.

For example, a slotted cylindrical post and a stamped plate, formerly assembled by swaging, can be cast as a single unit. Or, rather than staking a gear and a pinion to a stainless-steel shaft, the two fabricated components can also be cast directly on the shaft. Manufacturing costs are cut by eliminating separate components and joining operations. They are also further reduced thanks to the inherent close tolerances and part-to-part consistency of hot-chamber die casting. In the case of the die-cast gear and pinion, concentricity is held to 0.002 in. TIR.

Where a component manufactured from another material must become an integral part of the final configuration, the molten alloy is cast around it. For example, a pivot arm to which a brass pin was press fit is now cast to precisely encapsulate part of the pin. The inside diameter of the pivot arm is held parallel to the outside diameter of the pin to within 0.002 in. over the 1.03-in. length. Another example is an alloy ring cast around a ceramic magnet to a consistent outside diameter of 0.0005 in. Replacing an adhesive bonding operation increases the production rate of the ready-to-use component.

Tooling techniques
Complex components with intricate features are commonly pressure die cast using sophisticated tools. Tool tolerance is critical. Flashing at tool faces can defeat the economics of die casting if it necessitates deburring or secondary finishing. In conventional die-casting tools, molten alloy is forced into the cavity until it flashes out between adjoining surfaces. For zinc-alloy die casting of small components, tools are assembled to tolerances of +/-0.0001 in. -- a tight seal around the cavity which eliminates flash.

A die-casting tool is basically a six-sided cube that opens and closes like a clamshell, with a parting line where the two halves meet. The cavity inside is the shape of the component to be formed. Any component feature parallel to that open/close motion is easily incorporated into the two halves with the use of cores. For example, a fixed core pin in the movable half of the tool forms a center hole. For features offset from the parting line, movable side cores are driven in a sideways motion to be retracted before the die-cast component is ejected from the tool. These cores can be at any angle. For a wheel that requires features on the outside diameter corresponding to each month of the year, 12 cores are used, one every 30*.

Cylindrical cores form holes with a 0.001 in. tolerance, which can be tapped to 60 to 75% full thread without drilling. Side cores enable the production of holes and undercut features that are parallel to the major parting line of the tool. A movable core can form a hole or slot of virtually any shape to tolerances of 0.002 in. External, internal, face, helical, spur and worm gears are cast to angles of 20° and can incorporate shafts, ratchets, and cams.

Consistently close tolerances are characteristic of the hot chamber die-casting process. Linear tolerances are typically ±[0.0008 in. + (0.001 X dimension)] with C_pk = 1.33. Straightness is 0.001 in./in., flatness is 0.0015 in., perpendicularity is 0.001 in./in., and concentricity is within 0.002-in. TIR. Wall thickness can be as thin as 0.020 in. Surface finish typically runs from 16 to 64 mm. Center bores can be cast to a dimensional tolerance of +/-0.0005 in. n