Zinc Die Casting: A Cost-Saving Alternative for Multiple Processes

Zinc Die Casting

Complex, Thin-Wall Parts Are Produced in Net Shapes without Secondary Operations

By David Gaines and Mark Shortt
Design-2-Part Magazine

Complicated net-shape parts don't need to be produced using two or more separate processes. Instead of using multiple operations to produce a complex, multiple-piece component or assembly, engineers have a cost-effective alternative in zinc die casting. The process is known to be an efficient process for manufacturing high-quality, net-shape parts that don't require secondary processes and assembly. A complicated part that has traditionally been produced using multiple processes--such as screw machining, secondary machining, and assembly--can, instead, be manufactured as a one-piece, zinc die-cast part in a single operation that saves costs. Even better, the part can be produced with thin walls and tight tolerances on a fast-cycling production machine.

The zinc die casting process can be used to manufacture a one-piece, net shape zinc part, for example, with a round gear on one end and a threaded shaft stretching several inches outward. Carteret Die Casting, an ISO 9001:2000 certified, zinc die casting specialist headquartered in Somerset, New Jersey, has produced such a part. The company manufactured a one-piece, zinc die-cast part to replace a multi-piece component that required assembly of a plastic injection-molded gear to a screw-machined steel shaft. Carteret produced the part in one quick procedure instead of three time-consuming operations.

"A net-shape zinc casting is an excellent replacement for a screw machined part that doesn't drop off the screw machine completely finished," says John Mudrak, vice president of sales at Carteret Die Casting. "In that case, we can always be more competitive than the screw machined part. If something is produced on a screw machine that's made from a stainless steel, or some other very strong steel, obviously the zinc will not compare with that. However, if the part is made of mild steel or brass, the zinc die casting is generally very similar in properties."

If the part drops off the screw machine completely finished, the die cast part won't be as competitive, according to Mudrak. But if the part requires additional secondary operations, such as external threading, zinc die casting will generally be more competitive. Internal threading on the casting would require a secondary machining operation, but external threads come out of the die in a finished form, and the process is capable of holding pre-plating and after-plating dimensions.

Carteret Die Casting (www.carteretdiecasting.com) specializes in large and small zinc die castings with tight tolerances, complex geometry, and thin walls. Typically, on a single-cavity mold, the company can hold +/- 0.001 inch across the parting line. Multiple-cavity molds need a little more tolerance space, but the company can hold +/-0.0025 inch on a four-cavity mold. Zinc die castings are often produced with dimensions in the tenths of an inch, according to Mudrak, and the process is known to create parts with walls as thin as 0.007 inch. By producing parts with integral fastening elements, such as bosses and studs, zinc die casting can save costs by eliminating the need to add hardware or other fasteners during assembly.

In addition to serving customers in the medical, automotive, firearms, and lighting industries, Carteret performs work for OEMs that manufacture electronics, connectors, locks, architectural hardware, and fluid and liquid sensors. The company manufactures a variety of functional and cosmetic die-cast parts for the lock and hardware market; two-piece connectors; thin-wall die castings for fan housings; highly esthetic die castings for lighting, and thin-wall castings for electronic assemblies.

One of Carteret Die Casting's biggest strengths, according to Mudrak, is its readiness to assist customers with design and engineering challenges. Although the designs are supplied by its customers, the company can help its customers to make the designs more manufacturable. "The global marketplace requires domestic die casters to constantly search for ways to reduce costs, operate more efficiently, and provide maximum value and service," says Mudrak. "Our company focuses on helping our customers realize the full potential of the die casting process by participating fully in engineering and part design."

In addition to collaborative part design, Carteret offers its customers value analysis for cost reduction; review for process optimization; tool and die design; conventional and four-slide die casting; and secondary and finishing work that includes tapping, CNC machining, plating, and assembly.

Higher Strength, Lower Cost Are Advantages of Zinc Castings

Advantages of zinc die castings that OEM engineers should be aware of, according to Mudrak, are their strength, their lower manufacturing costs, and the fact that they are produced by a net shape process. Zinc die-casting alloys are reportedly stronger and more ductile than aluminum die casting alloys, and, when cast into complex shapes with sufficient radii, can even compete with welded steel components in strength.

"Zinc is very strong," says Mudrak. "A lot of people are under the impression that aluminum is stronger than zinc, but it's really not. Zinc has also better plating and paint capabilities than aluminum and longer tool life. You can cast smaller, more intricate details with zinc than you can cast in aluminum, and zinc has wonderful flow properties."

Mudrak says that zinc die castings have lower manufacturing costs than other alloys and processes, including aluminum die casting. A number of factors contribute to lower cost. The casting temperatures of zinc alloys are approximately one-third lower than aluminum; zinc die casting dies typically last 10 times longer than aluminum die casting dies; and the cycle time of zinc is significantly faster than other processes, he says.

Also, because zinc die casting is a net shape process, designers can design very complex components without any secondary machining processes. This allows for huge cost savings when converting multiple assemblies into one net-shape zinc die-casting component. And because zinc is more fluid than other casting alloys, engineers can reduce weight by casting thinner walls, which can meet a number of application requirements in the aerospace and automotive industries, for example.

New Machinery Expands Large-Part Capabilities

Carteret's production capacity ranges from very small to very large parts. A number of recent casting equipment purchases have greatly helped the company enlarge its capacity. Carteret now has 19 die casting machines operating continuously on the shop floor. "At times in the past, we were asked to produce larger castings," Mudrak says. "So to expand our capabilities, the 200- and the 250-ton machines were purchased to produce these larger castings. We can now produce a shot up to five pounds, and we can advertise the production of a four-pound platable casting."

The casting machines at Carteret are automated, meaning they have programmable controls (PCs) attached to them. Several of the company's casting machines have robotic extractors that pull out the finished parts. The extractors are mainly used to protect parts that have a delicate surface finish or details. After extraction, the part is then die- trimmed or put into a bin to be taken to a secondary operation.  

"We have quite a few 40-ton machines, so the new 60-ton machine complements this size, along with a new 130-ton machine," says Mudrak. "We use the smaller-tonnage machines for the smaller parts that we produce, which is also one of our specialties."

Zinc's Properties Reduce Costs for Customers

Zinc's favorable flow properties and its lower operating temperature than aluminum can reduce costs for OEM customers by enabling faster cycle times and the production of net shapes, which eliminate subsequent machining. Good flow properties and lower operating temperatures lead to faster cycle times for zinc and enable Carteret to cast net shapes and thin walls that can't be cast using other processes. By casting a part in a net shape, the company can eliminate the secondary operation of machining external threads.

"Let's say we're running a two-ounce casting," says Mudrak. "Our running temperature would be somewhere in the 775- to 800-degree Fahrenheit range for zinc. With aluminum, you would have to run it at 1200 degrees Fahrenheit or higher." Mudrak also says that the silicon content in aluminum takes a higher toll on die steel than zinc. "The silicon is part of the mix for most aluminum alloys to give it strength," he notes.

"If you have something running in a common aluminum alloy--like maybe 380 aluminum--as opposed to running it with # 3 zinc, zinc will typically run twice as fast," he continues. This, he says, is due to the lower process temperature and the fluidity of zinc, which will save a client time and money. "There is a tremendous difference in the life of hard steel tooling with zinc," Mudrak maintains. "I can guarantee that the average tool for a zinc part will last for a million shots, whereas with tooling for an aluminum part, you're doing repairs or replacing the entire die at about 100,000 shots. So this is also a major cost and time savings during production."

Carteret can also cast very tiny cored holes with zinc. Aluminum, however, would typically require much larger core pins because of its higher process temperature compared to that of zinc. The lower operating temperature of zinc, Mudrak says, doesn't have the same erosive effect on the die steel or the core pins.

Multi-Slide Casting Machines Offers Speedy Cycling Times for Small Parts

Carteret has six multi-slide machines, which are designed for doing the small, highly-complex parts. Four of the multi-slide machines have a one-ounce shot capacity, which is for very small parts. With conventional die casting, molten material is shot into the sprue post, and then it goes into a runner system that fills the die cavity. With multi-slide machines, the molten metal is forced directly into the runner system, so there is no wait for the sprue post to solidify.

In conventional casting, if the sprue post is bigger than the part, cycle time will be affected. "The multi-slide machines are able to increase cycling times because there is no waiting for the sprue post to solidify," Mudrak points out. "With the really small parts, there isn't any sprue post, so the only limitation is waiting for the part to solidify."

Another benefit of multi-slide machines is that cores can be pulled in different sequences, which isn't typical with conventional machines. Instead of using ejector pins to push the part off of the die, it is stripped off the core pin with a stripper mechanism, which increases cycle times. Integral fastening elements, such as bosses and studs, are a design feature of zinc die casting that can save money and time during the assembly process. "We can cast something with a standing boss, put a mating part over the top of it, and then swage the standing boss," says Mudrak. "This is pretty much impossible with other materials because most of them are so brittle, but zinc is malleable enough to make this procedure possible."

To illustrate, Mudrak suggests that you picture a nail standing up on a part. Another piece of hardware is placed on top of the nail and then pressed down on the nail to collapse it, pushing the two pieces together without the means of a screw or other fastener. The same process could be used for a coaxial connector. "You have the outer shell and the inner insulator with a copper wire in the center," he explains. "First, you put the insulator inside of the other part, and then you can compress the ends of the zinc casting and peen it over it to retain the other part so it doesn't fall out."

Carteret performs in-house secondary processes that include die trimming, tumbled de-gating, vibratory finishing, machining (conventional drilling, tapping, and milling), and light assembly work that usually involves three- or four-piece components. Customers are often supplied with totally finished components. The painting, powder coating, chromating, and plating, however, are outsourced.

With manufacturing and design engineers on staff to assist its customers, Carteret is an engineering resource, especially during the initial design stages of a project. "We always like to be involved at the beginning of a project, especially the design stage," Mudrak continues. "We can offer additional value by giving customers engineering input--for example, to reduce the weight of the part by putting metal savers in the tooling. We can also analyze what can or cannot be done as far as the draft angles with intricate die steel, or features that may not be castable."

The Job Shop Company, Inc.--Design-2-Part Magazine. Reprinted with permission.

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