News Article
From November/December 2003 issue of MicroTec
There is a general impression among design engineers that die casting is old tech, that parts produced by this process are typically large, geometrically limited, and often require machining before they can be used. As with many impressions, however, this picture is inaccurate.
Zinc die-casting technology has evolved significantly in recent years, especially in the area of miniaturization. Very small, geometrically complex parts can now be die cast with great precision, including numerous features such as contoured shapes, very thin walls, flash-free sculpted shut-offs, and multiple cored throughholes. Design engineers charged with the development of miniature parts would do well to consider this process for a wide range of applications.
Of all methods for fabricating metal, die casting offers the quickest path from raw material to finished part, and one of the most economical. Die casting with zinc alloys has been around for well over half a century, and today's hot chamber process is highly automated and very fast. Using pneumatically operated, fourslide machines and single cavity tools, cycle times of 1000 to 2000 shots per hour are not uncommon for miniature parts with simple geometry. Accuracy and repeatability at these rates is also very high. Using a single cavity tool generates very little part-to-part variation, often as low as ± .0008 inch. This becomes increasingly important as parts become smaller, especially when it comes to assembly.
Zinc miniature and subminiature parts have been made so small that in one application, a pound of zinc yielded 17,000 finished castings. Modern zinc alloys themselves have a lot to do with this capability. Zinc parts are stronger and tougher than aluminum or magnesium, and offer better ductility as well. Zinc parts can be made smaller, with finer detail and greater complexity. Zinc is readily paintable and is easily plated with a wide variety of metals to provide a range of feature and finish options.
The melting point of zinc is also low (approximately 730°F) compared to other metals. This translates into low energy requirements. Melting a pound of zinc only requires one-third the energy required to melt a pound of aluminum. For design engineers, this also translates into two other very important benefits. Less energy in means less energy out, i.e., less time for parts to solidify in the mold makes for faster, more economical production. Second, since process temperatures never approach the tempering temperature of the die steel (steel hardness is not reduced like it is in aluminum die-cast tools), tool life with this process is very long. We've had tools producing a million or more parts per year for over ten years with little sign of wear.
Table 1 shows a comparison of the physical and mechanical properties of a typical zinc alloy with aluminum, magnesium, and iron. While zinc alloys are generally not suited for continuous heavy-duty or high-temperature applications, they do perform well under the moderate loads and high, but short-term, impulse loading typically applied to miniature parts. Impact values for zinc are comparable to gray cast iron at room temperature and even aluminum and magnesium at 40°F. These properties, combined with its excellent thermal and electrical conductivity make zinc favorable for a wide range of miniature part applications.
Another interesting property of zinc from a design perspective, is that it shrinks isotropically. This means shrinkage is uniform in all directions, unlike plastic, which may shrink at one rate axially and a different rate radially. With plastic, shrinkage is very hard to predict until you actually fill the tool, and with multiple cavity tools, shrinkage rates can differ from cavity to cavity depending on how they fill and how they are cooled. This can result in greater part to part variation than with similar parts die cast in zinc.
Zinc is available in a number of alloys, including Zamak alloys 3, 5, and 7, and a range of engineering alloys: ZA-8, ZA-12, and ZA-27. For our purposes, we have found that Zamak alloys provides the best combination of characteristics for miniature part production. It can be used with the hot chamber process, is very ductile and machinable, and provides the best finishing characteristics for fine detail.
Design Flexibility
The first thing designers notice when they begin working with the die-cast process is freedom of shape. This not only means that there is a broader choice of geometry for the part itself. It also means that, as the designer, you are not so much constrained by the manufacturing process. You can do more with the part, add functionality, design in features for orientation or design to facilitate assembly, consolidate several parts into a single casting or integrate fastening elements into a part, even incorporate information such as part numbers or corporate logos.
This flexibility allows you to approach the overall conceptual task more organically. You can look, not only at the individual component, but at the assembly to which it belongs and to the process of manufacturing, finishing and assembling it. This becomes increasingly important as parts become smaller in size. A good case in point is the simple switch assembly shown in Figure 1. One end of the barrel includes castin, fine-pitch external threads for mounting onto a housing. The other end incorporates two small lips which, because of the ductility of the ZA-8 alloy, can be roll-formed to attach a mating stamped component. Thus this single component not only fulfills its intended function, it eliminates the need for additional fastening elements as well.
Other examples include the fiber optic housing base component pictured in Figure 2, and the tiny, gold plated housing for an aerospace application pictured in Figure 3.
In the first example, the part contains a contoured lip to ease assembly and provide line-to-line fit when mated. It also incorporates thinwalled flanges on the bottom to facilitate mounting and a tapped 000-80 hole for connectivity. The gold plated housing features a very thin-walled base (.014") and mating cap which fit precisely. In the application, a circuit board fits into the housing, with an epoxied screen and wires exiting from the back. The whole assembly is hermetically sealed for mil-spec, severe weather capable applications.
Internal geometry is another consideration. Programmable controls and pneumatic four-slide technology provide tremendous flexibility to produce flash-free internal shapes by using core pulls and side actions, and controlling their sequencing. The board mount RF connector pictured in Figure 4 is a good, though fairly simple, case in point. It features a flashfree, sculpted shut-off with a full diameter core meeting a half-diameter. The part is tin lead plated for solderability, and also includes tapered crush ribs on the mounting lugs to keep it from moving around during wave soldering, an element which would have been extremely costly to machine. It also includes an embossed company logo on the underside for branding.
Another example of internal geometry is the shielded connector for a telecommunications application pictured in Figure 5. This electroless nickel-plated, 8- to 16-wire connector had very thin walls and needed to be flash-free to avoid short circuits. It was cast flash-free in a tool using four sliding cores with minimal draft.
One of the more interesting jobs we've done recently really tested our tooling and processing capabilities. This very tiny part Figure 6 for a fiber optics application measured only .030" .050" .220". Figure 7 shows this part inserted into a rectangular slot on its mating part. This slot contained crush ribs to facilitate assembly by gripping the part. The center through-hole was cast by pulling a core at a 4° angle with no machine modifications. Converting this application from machining to zinc die casting reduced costs by a factor of 20.
Tooling Considerations
One of the biggest challenges in zinc die casting miniature parts is dealing with scale. As parts get smaller, the degree of difficulty rises exponentially, starting with tooling. A blown-up part design looks easy, but when you start looking at in terms of die steel, you say, “How are we going to do that? Can we make steel pins that small? Will they be strong enough? How can we add corner radii to reduce stress concentrations?” On larger parts, you can add a .010" corner radius and strengthen the steel. But if your part has a .010" wall thickness to begin with, you can't add a .010" radius. However, even a .002" radius on a corner like that can make all the difference, and even a little draft on tiny core pins can be critical. The key is to focus on areas of stress concentration and do whatever possible to relieve them.
Also, as parts get smaller, dimensional tolerances tend to shrink as well. A zinc die-cast part that fits into a five-inch cube might have a tolerance range of ±.005" or ±.003" on most features. But when you get into micro parts, you might be looking at ±.002", or even ±.001". You've got to be that much tighter because there's less room for error in assembly. Some parts for fiber optic and electro-mechanical applications may have tolerances on critical dimensions that are down to ±.0005".
Other issues include material flow and gating. The part in Figure 1 for example, had experienced failures due to stress cracking in the roll-formed flanges. When we got the job, we redesigned the tooling and sequencing around the roll form and completely modified gating and parting lines to create overflows. That way, we got nice hot material to that spot first, and eliminated the problem.
Another difficulty with miniature parts is part ejection. Unlike plastic, inc is very unforgiving, and pin ejection can damage small parts. To get around this, very often we use stripper plates to strip off a core, then air eject. But it depends on part geometry.One of the keys is to have full machine control and flexibility in sequencing so you can design the tool to sequence in a way that the part has to be handled. For example, a typical sequence might be to open the die, then run an ejection knock-out system forward. A better solution might be to cast the part around core pins, then sequence the opposite way: pull those pins, then strip before opening the mold.
Die Casting Limitations
As far as ultimate limitations on part size, I'd like to think there aren't any. Of course, day to day we review part designs for quotation with walls that are too thin, or unable to eject due to geometry. But most of these things are solvable. It just comes down to volume. If you're making enough of something, then the engineering and tooling capital budget is great enough to build the tooling and sequence it the right way.
Ultimately, there certainly are limitations on how thin you can go with a wall, depending on the material and its viscosity. There are limitations having to do with any kind of shut-off features. If you've got a hole coming through a part and you've got to shut two core pins off, and you've got two very small core pins, you can't get too much butt pressure, or you'll actually deflect one of the pins and that may lead to premature failure. And how you fill will even affect things like flash. That can be a real challenge with very small parts.
Intersection of tolerance bands can also become a limiting factor in small parts. As the part becomes smaller, the variation from part to part is much more critical and must become smaller for assembly with other parts. Basically, your tolerance window just gets smaller. That's why the key is good engineering and good tooling. Make sure you've got a process and the tooling to make the part to the right sizes and control tolerancing so the parts will fit in assembly.
But there is not much geometry the process cannot produce, particularly external geometry, so long as the dies can be pulled in a straight line and there are no undercuts. The key in making small parts is attitude. You eed to work with a zinc die-caster who is willing to look at challenging applications and push the limits on what can be done.
As a designer, you have to think differently for real small parts. You've got to put all your disciplines together: part design, tooling, and processing. If you don't think about all the required downstream processes and design for them up front, you're going to build in some real headaches going forward. You need a much more integrated approach to design and engineering, to successfully make real small parts.
Forged Relations
On micro-miniature parts, involve your die-cast supplier/partner early on in the design process. This will help to develop parts that are castable and trouble free. Also, early designs can be rapid prototyped and tested to assure function, fit, and form—and help you get customer feedback and involvement early! And the die-caster can often offer suggestions that may eliminate parts, facilitate assembly, or eliminate potential failure modes.
Once properly tooled and into production, you can expect millions of trouble free parts with very little part to part variation, at prices that will keep you competitive and profitable!
About the Author:
Steven Fielding is the President at Fielding Manufacturing. He may be reached at 800-230-8690, x 214 or stevenf@fieldingmfg.com.
Reprinted by permission of the publisher from the November/December 2003 issue of MicroTec.
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(800) 230-8690
· (401) 461-0400
· Fax (401) 941-2222
E-mail: sales@fieldingmfg.com
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