Thursday, July 26, 2018

Understanding The Difference Between Cast Metal and Wrought Metal

Many people use the terms “cast metal” and “wrought metal” interchangeably, but the two actually
refer to very different processes. Cast metal has been melted, molded, and cooled. Wrought metal has
been heated and then worked with tools. Here is what you should know.
Cast Metal
Any metal that can be wrought can also be cast. However, both cast metal and wrought metal are
generally alloys rather than pure metals, and their compositions are different. This means that when
compared to wrought metal, cast metal is harder, more brittle, and less malleable.
Cast metal has a relatively low tensile strength, which means that it is likely to break rather than bend,
but it has a high compression strength, making it useful for constructions and other applications where
it needs to hold significant weight. Cast iron, for example, was a common building material from the
18th century until it was replaced by steel in the early 20th century.
Wrought Metal
Wrought metal is heated and then worked with a hammer and other tools while hot before being
cooled. It is softer and more ductile than cast metal. Since it is extremely malleable, wrought metal
can be reheated and reworked over and over again. This reworking actually strengthens wrought metal
rather than damaging it.
Wrought metal has a very high tensile strength, making it ideal for applications such as horizontal
construction beams that must deform and reform regularly. It is extremely resistant to fatigue, and it is
unlikely to break unless it is distorted from intense heat such as a fire or is severely overloaded.
Today, wrought metal is mostly used as a decorative material.
Some metals form a protective oxidative coating over time, while others are susceptible to corrosion.
Both cast iron and wrought iron are particularly at risk for corrosion, especially in areas of high
humidity and frequent precipitation. Consequently, both need to be protected in some way. Paint and
powder coatings are two of the most common ways to protect bare metal.
A powder coating consists of a polymer resin mixed with a variety of additives such as leveling agents,
curatives, flow modifiers, and perhaps pigments. The ingredients are melted, mixed, cooled, and
ground into a power that resembles baking flour. The powder is then loaded into a special electrostatic
spray gun that gives it a negative charge and applies it evenly to the metal item. The metal then enters
a curing oven where it is baked to create a chemical reaction that ensures even, long-term adhesion.
Powder coatings are highly durable and are not prone to flaking, fading, cracking, or chipping.
Both cast metal and wrought metal can be highly decorative as well as extremely functional. Yet the
two are not interchangeable. It is important to understand the differences in order to purchase the
product that best meets your needs and desires.
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.

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.