Metal Casting

Metal Casting

While the word casting … “to make something out of metal by melting it and then cooling it into a more usable shape”… seems like a clear description of a process, there are actually many different casting methods.  So many, that each casting method deserves its own description.

First, here is a simple overview of what the casting process actually can be:

Ingots

An ingot is melted metal formed into an intermediate shape that is convenient for further shaping, remelting, or refining.  Ingots can then be extruded or forged directly into a finished product, or even rolled to a thinner dimension such as for sheet or wire products.

Traditionally, molten metal is poured into a mold which, when cooled, produces an ingot, or finished casting which then is machined to smooth any imperfections on the surface.

Casting

Casting is a general term often used to describe a specially shaped mold that is a finished shape in itself, often just requiring machined smoothing of the surface.

Continuous Casting

Continuous casting is a faster and more efficient way to produce an intermediate form before forming into a finished product.  Continuous casting forms metal into a continuous rectangular or round shape, which cuts time and labor costs.  Continuous castings are usually water cooled as the molten metal is poured into one end of the processer, with a solidified round or rectangular form coming out the other.  The time it takes for this process varies from producer to producer depending on what works best for a particular finished product.

Lost Wax Casting

The French name for this process, which is sometimes used, is cire perdue.  This process is mainly used in the formation of copies of an artist’s original sculpture, but is also used for industrial purposes where it is often called “investment” casting.  Lost wax casting can be used with metals such as silver, gold, brass or bronze.  The benefit of this type of casting is that more detailed and intricate originals can be duplicated.

This type of casting has been around for centuries and methods may vary slightly from foundry to foundry, but most follow a very standard routine.

First, the artist creates the original from something malleable such as wax or clay.  (Materials that retain their softness are often preferred.)  Then a mold is made of the original.  This outer mold can be made from plaster, fiberglass or a similar material.  Most molds are formed in at least two pieces to make removing the finished product easier.  Often, if there is a piece sticking out of the original model, these are molded and formed separately and then attached later.

Next, molten wax is poured into the mold in such a way to evenly coat the inner surface.  This step is sometimes repeated to achieve the desired thickness of wax, usually about 1/8” thick.  This thin wax copy of the original model is carefully removed from the mold and after some smoothing of the surface, looks like the original sculpture.

Then, through a complicated process, this wax mold is covered with a slurry and grit combination that becomes what is called a “ceramic shell” although it is not literally made of ceramic.  The finished shell is typically 1/2 inch thick.  The wax is melted out and there is now a sturdy mold to hold the molten metal.  In a carefully prepared setting, the molten metal is then poured into the shell.  Once the metal is cool the shell is hammered or sand-blasted off and the casting is released.

This sculpture is then “chased” or smoothed to remove any rough edges or impurities.  We now have a finished product.

Sand Casting

Sand casting, or sand molded casting, is a metal casting method where sand is used as the mold material.  A bonding agent, like clay either occurs naturally or is mixed with the sand.  The mixture is then moistened to form the mold.  The sand casting process accounts for over 70% of all metal castings.

Casting Aluminum

Aluminum casting is most used in automobile production.  Aluminum casting can be completed as “ingot” casting or what is called “mold” casting.

During ingot casting, the molten aluminum is cast into a slab or rolling ingot, a billet (a relatively narrow, usually square, bar of steel), or wire bar ingot, which is then formed into the finished product.

Mold casting can be described as sand casting, die casting, lost foam casting or wax pattern casting.  (Since wax pattern casting is seldom used in industry, we won’t discuss that here.)

Sand casting uses two types of molds that are re-usable and permanent.  In the first type, the “green sand” method, the mold is made of sand, clay and moisture.  In the second type, the “dry sand” method, the mold is made of sand and synthetic binders which are then cured.

Die casting aluminum uses also a mold that is permanent which is made of either iron or steel.  50% of all light alloy casting is done by high pressure die casting.  20% is completed by low pressure die casting, but its use is increasing.

Lost Foam Casting

Lost-foam casting was invented in 1964 by M.C. Flemmings.  Lost foam casting is much like lost wax or investment casting, using foam instead of wax.  The advantage of foam is its low boiling point, doing away with the need to melt the wax out of the mold.

Commonly cast metals cast with the lost foam method of casting are iron, aluminum alloys, steel, nickel alloys, stainless steel and copper alloys.