Sand casting is a process that utilizes non-reusable sand molds to form metal castings. This production method is versatile and can be used for metal components of all sizes and complexities from simple to very complex, detailed castings and with nearly any metal alloy.
How to Make a Sand Casting
There are four basic steps to sand casting:
1. Create the sand mold
2. Pour the liquid metal into the mold
3. Allow the metal to cool
4. Breakaway the sand mold and remove the casting
The mold needs to be prepared before it can be filled with liquid metal. Often, the mold cavity is lubricated with mold washes to make it easier to later remove the castings. Then the cores are positioned, additional mold materials such as cope rope are added to help prevent runouts, and the mold halves are closed and securely clamped together with the aid of pins and guides. The mold halves must remain secure so liquid metal can’t leak through the parting line.
Molten metal enters the mold cavity through a gating system. The molten metal is poured into a mold via the pouring cup, continues down the sprue (the vertical part of the gating system), and then through the runners (horizontal portions). There are vents that allow gases and displaced air to escape. through vents. The multiple points where metal is introduced into the mold cavity from the runners are called the gates.
Once the casting has fully cooled, it’s broken out of the sand mold by a process known as the shakeout. This can be done manually or by automated machinery.
What are the Parts of a Sand Casting Mold?
In order to make metal castings that are well-formed in the desired shape, you must first have a good mold. Molds have multiple parts that work together to properly create the final product.
You must have a pattern. The mold cavity is shaped by the pattern which is what makes an impression in the sand mold. Some internal surfaces are not included in the pattern and will instead be created by separate cores.
A core is a sand shape inserted into the mold to shape the casting interior to create holes or passages. Cores are usually made out of molding sand so they can be shaken out of the casting. A core print is a region added to the pattern, core, or mold to locate and support the core within the mold.
Molding sand has three important advantages over other mold materials:
– It’s inexpensive
– It’s easily recycled
– It can withstand extremely high temperatures.
Molding sands are prepared in mullers, which mix the sand, bonding agent, and water. Aerators are used in conjunction to loosen the sand to make it more amenable to molding. It can then be delivered to the molding floor where it is formed into molds and brought by conveyors to a pouring station. After pouring, the castings are removed from the adhering sand at a shakeout station. The used sand, in turn, is returned to the storage bins by belt conveyor or other means.
Sand casting is the most often used casting process because it works for metals with high melting temperatures such as steels, nickel, and titanium. It is inexpensive, heat resistant, and flexible.
The mold cavity must retain its shape until the metal has cooled and fully solidified or it will not produce the desired casting/part. Pure sand breaks apart easily, but molding sand contains bonding material that increases its ability to resist heat and hold shape.
Traditionally, green sand has often been used in sand casting, however, modern chemically bonded molding systems are becoming more popular. The most widely used casting sand is silica (SiO2).
Recycling Molding Sand
Once the casting is complete and the sand has been shaken off, the lumps are cooled and crushed. All particles and metal granules are removed. All sand and constituents are screened with shakers, rotary screens, or vibrating screens. Now that the sand has been cleaned it can be reintroduced to the molding sand production cycle to start over.
Casting sand is typically recycled and reused through many production cycles. Approximately 100 million tons of sand are used in production per year. Thanks to the recycling of the sand, only four to seven million tons are discarded annually. Happily, even that sand is often recycled by other industries making this a both economical and environmentally friendly process.
If you are interested in learning more about sand casting or have a project that might benefit from this process, call the professionals at Faircast Inc. One of our engineers will meet with you and to discuss your needs and how we might help.