Choosing Between Permanent Mold and Diecasting

The metal molds used in diecasting and permanent mold casting help drive cost, material choice and casting soundness, but each process offers unique advantages.

Shannon Wetzel, Senior Editor

(Click here to see the story as it appears in the Nov./Dec. issue of Metal Casting Design & Purchasing.)

The permanent mold and diecasting processes share a common ground. While the molds used in sand molding, investment casting and lost foam casting are expendable, metal diecasting 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 diecasting 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 Diecasting?

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. Still, unusually large parts cannot be diecast.

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 the diecasting process, also called high pressure diecasting, metal molds, or dies, 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 ingates with slower shot 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 stronger than those used in permanent molding, they can be more costly, and the number of castings required to justify the use of diecasting is higher than permanent mold. 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 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 diecasting is usually low.

Why Permanent Mold?

Casting size for permanent mold ranges from less than a pound to more than several hundred pounds. Surface finish varies between 150 to 400 RMS, basic linear tolerances are about +/-0.01 sq. in. and minimum wall thickness is 0.1 in.

In the permanent mold process, molten metal is poured into a mold made in two halves either directly by gravity, by using low pressure, or by tilt-pour, where the metal is poured into a cup attached to the mold that is then tilted from a horizontal to a vertical position.

Like diecasting, the metal mold aids in quicker solidification of the casting material, which results in highly desirable fine-grained structures that have high strength and soundness. While diecasting can produce castings with closer dimensional limits and thinner sections, permanent mold casting can produce parts with higher soundness (Fig. 2).

Porosity that often occurs in diecasting lowers the mechanical properties of the part and may cause blistering during thermal treatment. Permanent mold castings typically contain lower levels of entrapped gas, resulting in superior pressure tightness and soundness.

Permanent mold casting generally is used in high production volumes that will compensate for the high tooling costs, although these costs are generally not as high as with diecasting.

The wear life of a permanent mold can range from 10,000 to 120,000 castings. A general number of castings needed to be produced annually for permanent mold to be economical is 3,000, although this varies by metalcasting facility and casting size. Permanent mold jobs with production runs as few as 100 a year are possible.

When designing for permanent mold castings, be aware that the process should not be expected to cast key ways, exterior screws or threaded designs or holes. Because all casting features must be machined into the metal mold, the permanent mold process cannot produce the complexity capable with sand molds. However, permanent molding can be paired with sand cores for semi-permanent molding, and this method allows metalcasters to achieve higher complexity in the design (Fig. 3). The use of metal cores is more economical, but when a casting has cavities that do not allow a core to be pulled straight out, an expendable sand core often will do the trick. Too many sand cores in a semi-permanent mold casting can result in the deterioration of its strength advantages, so highly complex castings may be better cast in a full sand mold process.

Head to Head

Each casting process has characteristics that are beneficial for different applications. Here are a few guidelines when considering diecasting and permanent mold:

  • 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.
  • Multi-slide diecasting can produce micro-sized castings as small as your fingernail (Fig. 4).
  • 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., although it can produce parts up to 100 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.

 

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