Diecasting is used to produce small- to medium-sized castings at high-production rates. The metal molds are coated with a surface coating and preheated before being filled with molten metal. Premeasured amounts of molten metal are forced from a shot chamber into the permanent mold or die under extreme pressure (greater than 15,000 psi).
Castings of varying weights and sizes can be produced. Nearly all die castings are produced in nonferrous alloys with limited amounts of cast iron and steel castings produced in special applications.
Die castings and the diecasting process are suitable for a wide variety of applications in which high part volumes are needed. Benefits include:
- Excellent mechanical properties and surface finish.
- Dimensional tolerances of 0.005-0.01 in.
- Recommended machining allowances of 0.01-0.03 in.
- Thin-section castings.
Typical diecasting part size ranges from a few ounces to more than 100 lbs., but most parts fall on the lighter side of the range. Diecasting’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.
Diecast 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.
Diecast parts also have strong dimensional accuracy and excellent surface finishes. Aluminum alloys can be diecast 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 high pressure diecasting, metal molds are preheated and coated with a die release agent prior to each shot of metal.
Premeasured 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 diecasting), larger in-gates with slower hot velocities (squeeze casting) or semi-solid metalcasting (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, diecast parts cost less in the end, despite the high start-up costs.
However, because the molds used in diecasting must be so strong, they can be costly. The number of castings required to justify the use of diecasting rather than another process, such as permanent molding, is high. For high-volume jobs, the diecasting process, which is highly automated, often produces parts with the lowest per-unit price. Production runs above 10,000 pieces are possible 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.
Because of the shot chamber method of introducing metal into the mold, metal loss in diecasting is usually low.
When to Consider Diecasting
Diecasting and permanent molding are often compared because they both use metal molds to form a casting and can produce similar properties. Designers should consider the following characteristics during the decision-making process:
- Die castings can be made to closer dimensional limits with thinner sections.
- Permanent mold castings generally are sounder, can be produced at lower tooling costs and made with sand cores to yield shapes not available via diecasting.
- Die castings can be produced at higher rates with less manual labor and commonly cost less per casting when the production run is high.
- Diecasting produces smoother surface finishes (between 32-90 RMS compared to 150-250 RMS in permanent mold) and smaller cored holes.
- The tooling used in diecasting must be stronger to withstand higher pressures and is usually more expensive than permanent molds.
- Permanent mold castings are less porous than die castings.
- Diecasting is the least tolerant of varying alloys. Only highly castable alloys are used.
- The diecasting process is used to produce aluminum, magnesium and zinc parts. The permanent mold process can cast aluminum, magnesium and zinc, as well as copper alloys.
- Multislide diecasting can produce microsized castings as small as your fingernail.
- In high production, permanent mold casting typically can produce parts up to 100 lbs., although castings up to 400 lbs. are produced commercially. Diecasting is more limited in size due to the quick chilling of the metal. Typical diecast components range from ounces to 30 lbs.
Rules of thumb provide a base understanding of the strengths and weaknesses of diecasting and permanent mold, but if doubts remain on a suitable choice, contact a metalcaster from each process. Ultimately, metalcasters will have the best knowledge of a process’ capabilities. Plus, potential suppliers will have the best knowledge of their own process capabilities. Often, they also will be able to show you additional design measures to achieve your goals in cost-effective ways. CS
Click here to see this story as it appears in the July/August 2020 issue of Casting Source.