Design away die-casting difficulties

Sept. 8, 2011
Three broad design strategies can drastically reduce die-casting costs and improve part quality.

Authored by:
Tim Sparkman

Engineering Manager Dynacast
Elgin, Ill.
Edited by Kenneth J. Korane
[email protected]

Key points:
• Part geometry and flow characteristics are keys to casting cost and quality.
• Net-shape parts can substantially reduce manufacturing costs.

North American Die Casting Assn.

Design for Manufacturing (DFM) is one of those “best practices” that engineers routinely claim to follow. The reality, though, is that manufacturing often takes a back seat to functionality during the design process. With die-cast parts, ignoring manufacturability can be a particularly expensive mistake.

Parts designed without regard for die-casting’s DFM guidelines are more likely to require expensive secondary operations, such as finish machining or extra assembly steps. They may suffer from lower yields and quality problems, driving up scrap rates. In the worst cases, parts may even be impossible to cast and require an expensive redesign.

All these factors add dramatically to the part and life-cycle costs of diecast components. Consider that secondary operations alone can represent as much as 80% of a cast-part’s cost. Low yields likewise drive up manufacturing expenses. Poor quality can trigger immediate and longterm cost increases, too, not just from lower yields but also as premature failures in the field. And those trips back to the drawing board to rework unsatisfactory designs can delay product introduction and drive up development costs.

Fortunately, designing with the die-casting process in mind helps avoid all these pitfalls. Dynacast engineers, for example, use a Design for Die Casting (DFDC) methodology that optimizes casting manufacturability while preserving functionality. The gains from DFDC vary with each job, depending on whether the initial design is casting-friendly or not. But our engineers routinely manage to cut total costs by 30% or more. Here’s a look at the three key strategies behind DFDC.

Improve the geometry
Engineers must follow part-design guidelines for most manufacturing processes, and die casting is no exception. Its design-for-manufacturing guidelines cover geometry, wall thickness, draft angles, and more.

Much of this information is readily available from qualified manufacturers. Dynacast’s Web site for example, includes interactive tools that can guide engineers through important part-geometry decisions. The North American Die Casting Assn.’s Web site is also a good source of information, including detailed die-casting specifications with design recommendations.

These guidelines go a long way toward improving the die-casting process as well as the quality of the parts themselves. It’s important to keep in mind, however, that published design guides are best regarded as the basics. Pushing the envelope with die casting sometimes requires that engineers bend or even break the basic design rules.

And that’s where the skill of the die caster comes into play. Those with advanced machines, process controls, tooling, and engineering know-how can offer more design freedom than the basic rules typically permit.

Consider the wall thickness of zinc die castings, for example. Traditional design rules specify uniform walls at least 1.1-mm thick, without any abrupt thickto- thin transitions. But engineers increasingly want thinner parts or localized thinwall sections to reduce mass or part size, or to save on materials. Dynacast regularly produces parts significantly thinner than traditional guidelines recommend. The thinnest of these parts have nominal walls down to 0.3 mm with localized transitions down to 0.2 mm.

Engineers may know intuitively that die casting, whether hot or cold-chamber, requires the unimpeded flow of molten metal to fill the entire die cavity. Still, much of the design-for-manufacturing work involves revisions that target potential flow and filling problems.

Left unchecked, these problems result in defective parts — anything from minor surface blemishes to unusable short shots, where the mold does not completely fill. Less-than-ideal flow can also present itself in more insidious ways not visible to the naked eye, including localized porosity that can weaken parts.

Engineers can easily resolve many simple flow problems if they are tackled early in the design process. Many times, simple tweaks to part geometry will do the trick. For example, changing sharp surface intersections to fillets promotes flow, in addition to reducing stress concentrations and die wear. Bosses, ribs, and other long, narrow features also need fillets at their bases to fill properly.

Tweaking part geometry, though, only gets you so far with complex parts that push the limit of what’s possible with die casting. The expertise to produce thin-wall parts, for example, rests in no small measure in the ability to precisely manage flow.

Identifying and solving the most troublesome flow and filling problems is typically not a do-it-yourself endeavor for most engineers because it requires a deep knowledge of die-casting process parameters and tooling, particularly gate and runner design. And like other design-for-manufacturing improvements, it pays to partner early in the design process when changes are easier to make.

Aim for net shape
The cost-saving benefits of casting parts as close as possible to their final or “net” shape cannot be overemphasized. The nearer a part is to net shape, the less likely it will need expensive finishing operations. Net-shape parts, for example, require little or no milling to meet dimensional tolerances. Milling can double the cost of a die-cast part.

Net-shape manufacturing also tends to make assembly processes simpler and faster. That’s because true net-shape parts often emerge from the die with dimensional accuracy suitable for mechanical or adhesive joining — so parts take less time to assemble. Another more-subtle reason is that components designed for net-shape die casting also tend to have a higher degree of functional integration. Put differently, the same design expertise that makes netshape manufacturing possible also lets designers consolidate several parts into one.

But net-shape casting is easier said than done. Some parts do not lend themselves to true net-shape die casting. They may have tolerances, either across the entire design or locally, that are too tight for even the most capable die-caster. Other parts may have functional requirements at odds with achieving a net shape. For example, a part’s end-use requirements may not allow tapered surfaces and would therefore require some touch-up machining to selectively remove draft.

Keep in mind, however, that net shape is not an allor- nothing proposition. Pushing designs as close as possible to net shape will drive down costs, even if there are a few secondary operations left on the table. It is worth the time and effort to aim for true net-shape on every job because even getting close will drastically reduce the time and money needed to produce superior die-cast components.

© 2011 Penton Media, Inc.

About the Author

Kenneth Korane

Ken Korane holds a B.S. Mechanical Engineering from The Ohio State University. In addition to serving as an editor at Machine Design until August 2015, his prior work experience includes product engineer at Parker Hannifin Corp. and mechanical design engineer at Euclid Inc. 

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