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The advantage and disadvantage of investment casting
- May 22, 2018 -

Advantages of investment casting

  • Excellent surface finish

  • High dimensional accuracy

  • Extremely intricate parts are castable

  • Almost any metal can be cast

  • No flash or parting lines

Disadvantages of investment casting

The main disadvantage is the overall cost, especially for short-run productions. Some of the reasons for the high cost include specialized equipment, costly refractories and binders, many operations to make a mould, a lot of labor is needed and occasional minute defects. However, the cost is still less than producing the same part by machining from bar stock; for example, gun manufacturing has moved to investment casting to lower costs of producing pistols.

Additionally:

  • It can be difficult to cast objects requiring cores.

  • This process is expensive, is usually limited to small casting, and presents some difficulties where cores are involved.

  • Holes cannot be smaller than 1/16 in. (1.6 mm) and should be no deeper than about 1.5 times the diameter.

  • Investment castings require longer production cycles compared to other casting processes.

Counter-gravity casting

The variation on the gravity pouring technique is to fill the mold using a vacuum. A common form of this is called the Hitchiner process after the Hitchiner Manufacturing Company that invented the technique. In this technique, the mold has a downward fill pipe that is lowered into the melt. A vacuum draws the melt into the cavity; when the important parts have solidified, the vacuum is released, and the unused material leaves the mold. The technique can use substantially less material than gravity pouring because the sprue and some gating need not solidify.

This technique is more metal efficient than traditional pouring because less material solidifies in the gating system. Gravity pouring only has a 15 to 50% metal yield compared to 60 to 95% for counter-gravity pouring. There is also less turbulence, so the gating system can be simplified since it does not have to control turbulence. The metal is drawn from below the top of the pool, so the metal is free from dross and slag (which are lower density (lighter) and float to the top of the pool). The pressure differential helps the metal flow into every intricacy of the mold. Finally, lower temperatures can be used, which improves the grain structure.

This process is also used to cast refractory ceramics under the term vacuum casting.

Vacuum pressure casting

Vacuum pressure casting (VPC), properly referred to as vacuum assist direct pour, uses gas pressure and a vacuum to improve the quality of the casting and minimize porosity. Typically VPC machines consist of an upper and a lower chamber—the upper chamber, or melting chamber, housing the crucible, and the lower casting chamber housing the investment mould. Both chambers are connected via a small hole containing a stopper. A vacuum is pulled in the lower chamber, while pressure is applied in the upper, and then the stopper is removed. This creates the greatest pressure differential to fill the molds. The most common materials for vacuum casting process are the high nickel-based alloy and super alloys. Turbocharger products are a common applications for this casting process, though it is also regularly used in the manufacture of silver and gold jewellery.


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