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 solidifi cation of the casting material, which results in highly desirable fi ne-grained structures that have high strength and soundness. While diecasting can produce castings with closer dimensional limits and thinner sec- tions, permanent mold casting can produce parts with higher sound- ness (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 tight- ness and soundness. Permanent mold casting gen-
erally 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. T e 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 ma- chined 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 metalcast- ers to achieve higher complexity in the design (Fig. 3). T e use of metal cores is more economical, but when a casting has cavities that do not al- low a core to be pulled straight out,
Fig. 3. This 2.1-lb. electronic housing component originally was designed as a die cast- ing, but the conversion to permanent mold produced signifi cant savings in initial tooling costs. The component’s thick and thin sections would have made it hard to produce via the diecasting process, so it was converted to a highly cored permanent mold casting with complex parting.
If doubts remain on a suitable choice, contact a metalcaster from each process.
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 char-
acteristics that are benefi cial for diff erent applications. Here are a few guidelines when considering diecast-
ing and permanent mold: • Die castings can be made to closer dimensional limits with thinner sections.
• Permanent mold castings
• Diecasting produces smoother surface fi nishes (between 32-90 RMS compared to 150-250 RMS in permanent mold) and smaller cored holes.
• The tooling used in diecast- ing must be stronger to with- stand 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.
• T e diecasting process is used Nov/Dec 2014 | METAL CASTING DESIGN & PURCHASING | 43
• Die castings can be produced at higher rates with less manual labor and commonly cost less per casting when the produc- tion run is high.
generally are sounder, can be produced at lower tool- ing costs and made with sand cores to yield shapes not available via diecasting.
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