All metal casting processes have their own unique characteristics. When designing a new part for casting, the most appropriate process should be determined by a number of factors, including:
The goal of the manufacturer and the supplier should be to find the optimal balance that produces the lowest per-part cost at a given quantity, while meeting or exceeding all quality requirements.
Here, we compare investment casting to three metal casting processes: shell mold casting, greensand casting and permanent mold casting.
Investment casting is known for its ability to hold to tight tolerances and produce parts with intricate inner cavities and excellent surface finishes. Shell mold casting, which involves heat-bonded sand patterns and cores, also offers comparatively high quality in the same categories. However, investment casting is able to meet much tighter tolerances than shell mold casting, and can offer even smoother surface finish.
Tooling costs for investment casting are generally lower than for shell mold casting, but the process requires highly skilled and sometimes tedious labor. As production volumes increase, per-part costs for investment cast products often overtake those of shell mold casting.
Both processes can accommodate nearly any alloy, including ferrous and non ferrous metals.
Greensand casting is another sand-based casting method, but this time no heat is applied. Instead, clay-like substances in the sand bind the particles together around patterns to create molds. In terms of tolerances and surface finish, greensand casting falls behind both shell molding and investment casting.
When it comes to cost, however, greensand is often the winner. Tooling costs for greensand are lower than for investment casting, and the greensand process is not as labor intensive, resulting in fewer man-hours per part. Still, since greensand casting is a destructive mold process, per-pound costs quickly escalate as quantities increase.
Like investment casting and shell mold casting, greensand casting can also accommodate a wide variety of alloys.
Permanent mold casting is the only non-destructive casting method on this list. Unlike investment casting, shell molding or greensand casting, molds in permanent mold casting are reused again and again. Instead of using sand or refractory material to build up mold walls, permanent molds are generally made of steel or cast iron.
Tolerances and surface finish for permanent mold casting lie somewhere between shell molding and greensand casting. None of these methods approach the dimensional accuracy and surface finish produced by investment casting.
For high volume runs, permanent mold casting is often the most economically viable option, but for low or medium volume runs, tooling costs are often prohibitive. Despite high initial tooling costs, labor costs for permanent mold casting are lower than other casting methods.
Since permanent molds, themselves, are made from metals, their melting points create limitations. Metals cast in permanent molds must have significantly lower melting points than the metals making up the mold. Therefore, permanent mold casting is often employed using aluminum or other non-ferrous alloys.