Comparing Aluminum Permanent Mold and Diecasting
Despite the similarities, each process may better suit a particular application, depending on the property requirements, casting size, production rate and design complexity.
To Die For
In the diecasting process, 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 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, whereas in other casting processes, adding material in a high stress area will help dissipate the loads.
The dies have a relatively long wear life and can be used for 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. Production runs above 10,000 pieces generally are considered for this method. For high volume jobs, the diecasting process, which is highly automated, often produces parts with the lowest per-unit price.
Diecast parts 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.
Because of the shot chamber method of introducing metal into the mold, metal loss in diecasting is usually low. Die casting part size ranges from a few ounces to more than 100 lbs., but most parts fall on the small 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.
“If a part can be diecast, then from a cost advantage standpoint, you’re better going that way,” said Bob Braun, vice president of engineering for Wisconsin Aluminum Foundry, Manitowoc, Wis. “But diecast cores are pretty limited.”
Make It Permanent
The permanent mold process works much like the sand casting process, where molten metal is poured into a mold that is made in two halves. In typical permanent mold casting, the metal is poured either directly by gravity or by pouring the metal into a cup attached to the mold and tilting it 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 can produce castings with higher soundness.
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.
“With permanent mold, you have the ability to machine the part and not open up porosity,” Braun said. “An unmachined diecast part is stronger than a permanent mold casting, but throughout the part, permanent mold has the advantage because of the lack of porosity.”
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. Permanent molds usually are made of a high-alloy iron or steel, and diecasting dies generally are made of hot-work tool steels, such as H-13. These steels, which were developed for the diecasting process in particular, are stronger and harder to machine.
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 in order for permanent mold to be economical is 3,000, although this varies by metalcasting facility and by casting size. Wisconsin Aluminum Foundry has some permanent mold jobs with production runs as few as 100 a year.
“Tooling costs have become pretty low in the last few years, as more CNC machining is being used to make the tools,” Braun said. “We’re quoting some jobs that normally would be sand castings because the tooling cost difference is not as much as it used to be, and you’ll get better dimensions and mechanical properties with 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.
When designing for aluminum 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. 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 core often will do the trick. Too many sand cores in a permanent mold casting can result in the deterioration of its strength advantages, so highly complex castings may be better cast in a full sand process.
Head to Head
Each aluminum casting process has characteristics that are beneficial for different applications. Following 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 are sounder, can be produced at lower tooling costs and be 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 and smaller cored holes;
- Dies used in diecasting must be stronger and are therefore 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.