Wednesday, July 25, 2018

A Guide to Sand Casting

Sand casting is the process of casting metal via non-reusable sand molds. It is commonly used for
metal components ranging in size from just a few ounces to many tons. Sand casting can create
tremendously detailed castings, and it works for virtually any metal alloy. In fact, it is one of the few
processes that can be used for metals with very high melting temperatures such as nickel, titanium,
and steel. It is also relatively low-cost. Here is what you should know.
Molding Sand
Molding sand is inexpensive and easy to recycle, and it can withstand extremely high heat. While pure
sand breaks apart easily, molding sand contains bonding materials that allow it to hold its shape until
the metal inside has cooled and hardened.
Traditional sand casting used green sand, a mixture of sand, bentonite clay, pulverized coal, and water.
Today, modern chemically bonded mixes are becoming more popular. The most commonly used type
of sand, however, is still silica (SiO2).
Whether green sand or a modern chemical blend is used, molding sand must have certain properties
to be used for sand casting. These include:
Strength: The mold must be able to hold its geometric shape under mechanical stress.
Permeability: The mold must allow gases and steam to escape during casting.
Moisture: Too little moisture can make the mold brittle, while too much moisture can trap steam
bubbles inside the casting.
Flowability: Detailed castings need sand with a high flowability, or the capacity to even fill small
spaces in the pattern.
Grain size: The optimal size of each individual sand grain will vary according to the casting.
Grain shape: Molding sand comes in three different shapes. Rounded grains have high flowability
and permeability but poor bonding strength. Angular grains have high bonding strength but poor
flowability and permeability. For most applications, middle of the road sub-angular grains are ideal.
Collapsibility: A high level of collapsibility allows the sand mold to collapse under force. This lets the
metal casting shrink freely during hardening, reducing the risk of tearing or cracking.
Refractory strength: This is the molding sand’s ability to withstand extremely high heat.
Reusability: This refers to the molding sand’s ability to be recycled for new sand castings.
Sand Casting Mold
Sand casting molds have numerous parts that work together to develop the finished casting:
Pattern: The pattern is a full sized model of the finished piece that is used to create an impression in
the mold.
Core: A core is a separate piece of sand inserted into the mold to shape the interior of the pieces,
including such pieces as holes or passages. A core print and small metal pieces known as chaplets
may be added to support one or more cores.
Riser: A riser is a void in the mold that holds excess metal. It prevents voids from forming in the
casting by feeding liquid metal to the mold cavity as the casting hardens and shrinks.
Flask: The flask is a box that contains the entire sand mold. It is typically in two parts, with the
upper half known as the cope and the lower half known as the drag. The parting line separates the
two halves.
How Sand Castings Are Made
There are four basic steps to sand casting:
Mold assembly: The drag is partially filled with sand, and the pattern, core print, and cores are
inserted near the parting line. Then the cope is attached, and additional sand is poured until all pieces
are covered. The sand is compacted, and excess sand is removed with a strike off bar. The cope is
then removed so that the pattern can be extracted.
Metal pouring: The mold is prepared, a complex process that involves lubrication, positioning of the
cores, clamping, and possibly other steps to ensure that the mold is secure. Molten metal is then
poured into the mold via a pouring cup and gating system.
Cooling: During the cooling process, built up gases and displaced air escape through a series of
vents. The metal casting naturally shrinks as it hardens.
Removal: When the casting is completely cool, the sand mold is broken for removal in a manual or
automated process known as shakeout. The sand is then conditioned and recycled into a new mold.
Naturally, there are numerous different sand casting methods. Each is ideally suited for specific metal
items. At Atlas Bronze, we can create custom castings using the methods that, in our professional
opinion, are the best for your particular needs.
Ready to Start?
Atlas Bronze is a leading U.S. distributor of bronze, copper, brass, iron, and more. Contact us today at 1-800-478-0887 to place an order or learn about our custom products.


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