Permanent Mold Casting Quality

 "Permanent Mold Castings: Pathway to Quality"
Technology Tips as reprinted from MetlFax

The casting approach enables metal to be put where it is required by part function, in contrast to other processes that produce shapes more rigidly constrained by the limitations of the production process.

In selecting a method of producing parts, many factors must be taken into account. A major reason for the selection of castings is their close approximation to the shape desired by the designer. Also, the wide range of casting methods and castable metals has earned the process a key role in parts manufacture.

An outstanding performer in the medium production range is the permanent mold process. For quantities from a few hundred, up to hundreds of thousands, permanent mold aluminum castings often provide the maximum combination of economy and high quality. A look at the nature of the process and the requirements of mechanical equipment reveals the specific reasons that favor the selection of permanent mold aluminum castings.

Costs and Quality

Gray iron castings are usually thought of as being the cheapest castings available. However, when cost of providing a function is considered, aluminum castings often prove to be more economical or as economical as gray iron castings. When considering the economics of competitive aluminum casting processes, permanent mold casting occupies a unique place. Usually, the cost of a permanent mold casting is less than a sand casting (often substantially less), and usually higher production rates can be obtained. However, the cost of tooling for sand casting is less than that for permanent mold.

When compared to die casting, permanent mold tooling is dramatically lower than dies cast tooling, but piece prices are somewhat higher. It is the combination of these two cost factors, piece price and tooling costs, that enables permanent mold castings to be extremely competitive over a wide range of medium quantity orders.

Each of the casting processes has a particular combination of properties in addition to the cost factors just mentioned. When quality (strength and soundness), is the key consideration, permanent mold aluminum casting stands out as clearly superior to either sand or die casting. This is so because strength in castings (alloy and heat treating being held constant), depends on two considerations:

    * Fineness of metallurgical grain structure.
    * Absence of voids caused by shrinkage or the presence of gas or air.

A fine grain structure is obtained when castings are cooled rapidly from a molten to a solid state. The iron of permanent molds draws away heat rapidly from the aluminum, which creates a fine grain structure in the aluminum casting. In contrast, aluminum cast in sand requires much more time to solidify, which allows a coarser grain structure to form. Hence, sand castings are not as strong as those of the same alloy cast in permanent molds.

For example, 356T6 aluminum alloy sand cast will typically provide an ultimate tensile strength of 33,000 psi. The same alloy cast in permanent mold will have an ultimate tensile strength of 38,000 psi. Yield strength in sand will be 24,000 psi compared to 27,000 psi in permanent mold. Elongation of the alloy sand cast is 3.5%, while the permanent mold value is 5%. Compressive strength values are 25,000 psi and 27,000 psi, while shear strengths are 26,000 psi and 30,000 psi.

Although metal dies are used in the production of die castings, comparable strengths cannot be achieved by the process. The metal near the die cast surface does exhibit a fine grain structure. However, when molten metal is injected into the die cavity under pressure, air often becomes entrapped in the metal. Also, after the metal is injected into the die, additional metal cannot flow into the cavity as cooling and shrinkage take place.

As a result, die cast parts have good strength and soundness near the surface, but the more central portions of the castings often contain voids caused either by entrapped air or gas, or by metal shrinkage. This lowers the mechanical properties of the castings and may also cause blistering during heat treatment.

Reservoirs of molten metal (called risers), are used in both sand and permanent mold casting to supply additional metal as cooling and metal contraction take place. This enables both sand and permanent mold castings to be made without shrinkage voids of the type often present in die castings.

Entrapment of air is not a problem in pouring of either sand or permanent mold castings. However, sand molds can generate gasses that can be entrapped in the metal.

Additional Comparisons

Sand Casting, Aluminum Sand Casting, Gravity Die Casting, Aluminum Die Casting, Die Casting, Aluminum Casting Processes, Sand Casting and Die Casting Processes, Permanent mold casting, Die Casting manufacture, Green Sand Molds, Aluminium Casting Foundry, Zinc die casting