Manager, Engineering Research and Development
Fishercast div. of Fisher Gauge Limited
M. DAVID HANNA
General Motors Corp., Research and Development Center
EDITED BY DAVID S. HOTTER
Engineers at a valve-manufacturing plant never considered die casting while searching for ways to reduce production costs. They thought the process wouldn’t help. This illustrates a common misconception about metal part forming, particularly when using zinc alloys. In reality, designers can slash costs substantially by changing over to die-casting methods.
The valves, used to prevent blow-back in oxyacetylene torches, were originally machined from brass bar stock. Measuring only 0.875 in. long and 0.400 in. in diameter, the miniature valves required milling, deburring, and critical process controls to hold close tolerances. But by switching to zinc-alloy die casting, multiple manufacturing steps were replaced with a single casting step, and the manufacturer now produces flash-free check valves at half the cost.
Before making the switch, workers machined the valves, milled six side windows, and deburred the final component. Now, movable slides in the die form the windows, sealing against a pin that forms the inside bore. A simple center gate in a die lets the molten alloy enter the cavity while ensuring a homogeneous flow to all sections of the tool.
Consequently, die casting slashes costs by eliminating secondary operations such as boring, reaming, and grinding. It uses less material to produce complex net shapes with tight tolerances and thin-wall sections, and replaces multistep manufacturing processes with a single component while improving quality.
Hot and cold-chamber die casting are two widely used methods of manufacturing small components at volumes greater than 50,000 pieces/yr. But because the casting equipment can’t withstand long periods at high temperatures, the process is usually limited to alloys with low melting points, such as zinc, aluminum, and magnesium — parts weighing between 4 oz and 55 lb.
In hot-chamber casting, an alloy reaches its melting point in a temperature-controlled furnace. The furnace contains an injection plunger and gooseneck continuously immersed in the molten material. When a cycle begins, the two half-shells of the die close and the molten alloy enters through the gooseneck under high pressure (2,000 to 5,000 psi). Finished parts eject from automated presses at rates as high as several thousand/hr, depending on size and the number of cavities.
Cold-chamber casting is suitable for zinc alloys with high aluminum content and higher melting points, including ZA-12, ZA-27, and ACuZinc-10. Molten alloy is ladled, poured, or pumped into an injection cylinder during each cycle. The cylinder then pumps the metal into the die at pressures from 3,000 to 10,000 psi. The die-casting tools squeeze molten alloy out between adjoining surfaces, much like excess glue oozes out from two pieces of wood forming a joint. This flash must then be trimmed. This last operation makes production rates generally slower than the hot-chamber process.
Die casters have developed tooling techniques to form complex shapes to close tolerances. An example is fixed cores within tooling that form holes with 0.001-in. tolerance. In addition, tooling can produce up to 50 external threads/in. to Class 2A tolerance — comparable to those produced by screw machining — without cleaning or chasing.
Manufacturers often tap cylindrical cores to 60 to 75% of full threads. In some applications internal threads are formed to Class 2B tolerance using equipment that rotates cores within the tools. Additional cores mounted on the sides of tools produce holes and undercuts parallel to major parting lines. A moving core can form almost any shape hole or slot to within 0.002 in.
Die casting also reduces the costs of manufacturing external, internal, face, helical, spur, and worm gears, by casting them to AGMA 6 to 8 specifications. All tooth forms are possible, including teeth with helix angles as great as 20°, and die casting lets designers consolidate parts with gears, such as shafts, ratchets, and cams.
Engineers use insert die-casting techniques when parts made from other materials must be integrated into zinc casts. One example is a linkage that must incorporate a previously machined steel pin to meet performance requirements. Rather than assembling a die-cast linkage and steel pin, the pin is inserted into the die-casting tool and held in position as molten zinc alloy injects into the tool cavity. Part of the pin is captured in the die casting and mechanically locked solidly in place.
ASSESSING THE ALLOYS
Zinc, aluminum, and magnesium are the most common alloys used for die casting. The best alloy for an application is determined by examining its physical and mechanical properties. For instance, traditional Zamak and ZA (zinc-aluminum) alloys have strength comparable to aluminum and magnesium, with good mechanical and physical qualities. However, the biggest advantage of these alloys is they hold closer tolerances than aluminum. Zinc is also the most ductile material, which works well for thinwall sections and can be easily crimped or staked.
Commercial Zamak alloys for the hot-chamber diecasting process, primarily Zamak 3 and Zamak 5, were developed in the 1930s to meet demands for strong, stable die-casting materials and are still used to produce a wide variety of industrial parts. These alloys contain approximately 4% aluminum, little magnesium, and 0.25% copper in the case of Zamak 3 and about 1% copper for Zamak 5. Zamak 3 is the most widely used alloy, offering the best combination of mechanical properties, castability, and price. Increased copper levels in Zamak 5 boost strength and improve wear resistance. In addition, approximately 20 years ago, another family of zinc alloys were developed, including ZA-8 which provides greater strength over traditional zinc alloys. ZA-8 contains a nominal 8% aluminum and between 1 and 2% copper.
The most significant advancement in die casting are the new ACuZinc alloys. Developed by engineers at the General Motors Corp. research and development center, ACuZinc is a ternary zinc-copper-aluminum alloy. It’s composed primarily of zinc, with 5 to 6% copper and 2.8 to 3.3% aluminum. The copper content is substantially higher than in Zamak 3, Zamak 5, and ZA- 8, while the aluminum content is lower. The ACuZinc-5 formulation used in hot-chamber die casting contains about 5% copper, while the formulation of ACuZinc-10 for cold-chamber casting contains about 10% copper.
Engineers die cast ACuZinc in the same way as traditional zinc alloys, yet components are much stronger, harder, and resist creep and wear better. Its physical and mechanical properties extend the range of applications where traditional zinc alloys failed and more expensive materials and processes were required. Consequently, engineers can now consider hot-chamber die-cast zinc as a replacement for powdered metals and brass, as well as steel stampings, machinings, and fabrications.
Higher copper content increases the tensile strength of ACuZinc. Creep resistance of the alloy is also greater than other zinc casting alloys. Under high stresses and loads, Zamak and ZA alloys deform or relax over time, reducing yield strength. Engineers can further boost tensile strength and creep resistance by modifying part designs to include uniform wall sections, and sufficiently toleranced fillets and radii.
With Brinell hardness of 118, ACuZinc is harder than other zinc alloys, as well as aluminum, magnesium, and brass. ACuZinc’s high copper content helps it resist wear as well as bearing bronzes and aluminum alloys, which are regarded as high-wear resistance materials. With a low friction coefficient of 0.06, ACuZinc provides a low-cost replacement for bearings and bushings.
In addition to zinc alloys, die casting works well for aluminum and magnesium. Aluminum alloys weigh little, resist corrosion, and cast easily. They are also dimensionally stable, and conduct electricity well. With strength comparable to zinc alloys, engineers specify aluminum when weight is critical. The most widely used alloys are Al 380, Al 384, Al 13, and Al 360. Because molten aluminum attacks steel components, it is cast using the cold-chamber process.
Magnesium can be cast using either hot or cold-chamber equipment. It is among the lightest die-casting material and easiest to machine during postprocessing. Although compared to aluminum, magnesium has slightly lower strength, but it has a higher strength-toweight ratio — it weighs two-thirds that of aluminum.
MAKING THE MOST OF THE DIE-CASTING PROCESS
Die casters employ a number of techniques to maximize designs and reduce costs. An advantage of zinc alloys is that their mechanical properties call for less material which, in turn, increases production rates.
Part weight may be slashed in several ways, including reducing cross sections to a thickness as little as 0.020 in. and adding recesses to parts. Structural ribs are added to reinforce components when necessary.
Making minor design changes is another way engineers increase strength and reduce creep. For example, an inside corner designed with a fillet rather than a sharp corner enhances creep resistance, improves metal flow, and prevents stress concentrations. Extra threads in a bolt connection also reduce creep and ensure that loads hold over long periods.
Porosity is often a concern to engineers familiar with die casting. Although small die-cast components always contain some porosity, uniform wall thickness, fillets, and radii minimize porosity or limit it to areas where it won’t affect performance.
One quality that attracts designers to zinc alloys is its dimensional stability and ability to consistently cast within small tolerances. Center bores can be cast parallel within 0.0005 in. without draft, with dimensional tolerances held to ±0.0005 in., and a surface finish of 4 to 8 μin. Linear tolerances are typically ±0.001 in., while straightness, flatness, roundness, and perpendicularity maintain 0.001-in. tolerance, and concentricity within 0.002 in. T.I.R. (total indicator reading).