The metal molds used in die casting and permanent mold casting help drive cost, material choice and casting soundness, but each process offers unique advantages.
Excerpt from the November 2014 issue of Casting Source
The permanent mold and die casting processes share a common ground. While the molds used in sand molding, investment casting and lost foam casting are expendable, metal die casting and permanent molds are used repeatedly. The material and manufacture of the metal molds make initial tooling costs more expensive than the other processes, but if the volume of production is high enough, this cost difference can be overcome. Permanent mold and some types of die casting also offer superior mechanical properties because the metal mold acts as a chill.
Despite the similarities, each process may better suit a particular application, depending on the property requirements, casting size, production rate and design complexity.
Why Die Casting?
Die casting part size ranges from a few ounces to more than 100 lbs., but most parts fall on the lighter side of the range. Die casting’s minimum size is smaller than most other casting methods, so the process usually is associated with small parts with thin sections.
The demand for larger, more complex die castings with improved quality and lower cost has led to the development of high precision equipment and the extension of casting technologies to larger pieces with heavier wall thicknesses. Still, unusually large parts cannot be die cast.
Die cast parts trend toward the less complex, partly because the metal cores must be designed to be pulled straight out of the casting. This limits the shapes of the cores and passageways of the casting.
Die cast parts also have strong dimensional accuracy and excellent surface finishes. Aluminum alloys can be die cast to tolerances of +/-0.004 sq. in. and feature finishes as fine as 50 RMS. Walls can be cast as thin as 0.04 in.
In the die casting process, also called high-pressure die casting, metal molds, or dies, are preheated and coated with a die release agent prior to each shot of metal. Pre-measured amounts of molten metal then are metered into a shot sleeve and forced into the die under extreme pressure (usually from 10,000 to 15,000 psi).
Rapid filling of the mold and solidification under pressure can produce a dense, fine-grained and refined surface structure with excellent properties, including fatigue strength. But the typical injection speeds of the metal into the mold do not allow enough time for air to escape the die cavity. If turbulence occurs as the metal flows through the shape of the casting, porosity results. The use of a vacuum during die filling (vacuum die casting), larger ingates with slower shot velocities (squeeze casting) or semi-solid metal casting (in which metal somewhere between the liquid and solid phase is injected into the die) can overcome these problems and produce parts that can be heat treated and welded.
In designing for a die casting, thick sections may be less strong than thinner areas, because they can breed shrink porosity as the outer layer solidifies before the interior metal.
Dies have a relatively long wear life and can be used for up to 100,000 shots, depending on the application, so when large quantities are required, die cast parts cost less in the end, despite the high start-up costs. However, because the molds used in die casting must be stronger than those used in permanent molding, they can be more costly, and the number of castings required to justify the use of die casting is higher than permanent mold. For high volume jobs, the die casting process, which is highly automated, often produces parts with the lowest per-unit price. Production runs above 10,000 pieces are connected with this method most often, but rapid tooling technology advances have made shorter runs—between 500 and 2,000 pieces—more economical while also significantly reducing lead times to one to four weeks (Fig. 1).
Because of the shot chamber method of introducing metal into the mold, metal loss in die casting is usually low.