Excerpt from the September 2019 TheLibraryofManufacturing.com article.
Sand casting is the most widely used metal casting process in manufacturing. Almost all casting metals can be sand cast. Sand castings can range in size from very small to extremely large. Some examples of items manufactured in modern industry by sand casting processes are engine blocks, machine tool bases, cylinder heads, pump housings, and valves, just to name a few.
Most sand casting operations use silica sand (SiO2). A great advantage of sand in manufacturing applications is that sand is inexpensive. Another advantage of sand to manufacture products by metal casting processes, is that sand is very resistant to elevated temperatures. In fact, sand casting is one of the few processes that can be used for metals with high melting temperatures such as steels, nickel, and titanium. Usually sand used to manufacture a mold for the casting process is held together by a mixture of water and clay. A typical mixture by volume could be 89% sand, 4% water, 7% clay. Control of all aspects of the properties of sand is crucial when manufacturing parts by sand casting, therefore a sand laboratory is usually attached to the foundry.
Use Of Binder In Sand Casting
A mold must have the physical integrity to keep its shape throughout the casting operation. For this reason, in sand casting, the sand must contain some type of binder that acts to hold the sand particles together. Clay serves an essential purpose in the sand casting manufacturing process, as a binding agent to adhere the molding sand together. In manufacturing industry other agents may be used to bond the molding sand together in place of clay. Organic resins, (such as phenolic resins), and inorganic bonding agents, (such as phosphate and sodium silicate), may also be used to hold the sand together. In addition to sand and bonding agents, the sand mixture to create the metal casting mold will sometimes have other constituents added to it in order to improve mold properties.
Types Of Sand Used In Sand Casting
There are two general types of sand used in the manufacturing process of sand casting.
Naturally Bonded- Naturally bonded sand is less expensive but it includes organic impurities that reduce the fusion temperature of the sand mixture for the casting, lower the binding strength, and require a higher moisture content.
Synthetic Sand- Synthetic sand is mixed in a manufacturing lab starting with a pure (SiO2) sand base. In this case, the composition can be controlled more accurately, which imparts the casting sand mixture with higher green strength, more permeability, and greater refractory strength. For these reasons, synthetic sand is mostly preferred in sand casting manufacture.
Properties Of A Sand Casting Mixture
Type And Content Of Binder And Other Additives
As mentioned, controlling the type and content of the sand binder and other additives is the key to controlling the properties of the casting's mold sand mixture.
Moisture content affects the other properties of the mixture such as strength and permeability. Too much moisture can cause steam bubbles to be entrapped in the metal casting.
This property represents the size of the individual particles of sand.
Shape of Grains
This property evaluates the shape of the individual grains of sand based on how round they are. Less round grains are said to be more irregular.
The explanation of strength is, the ability of the sand casting mixture to hold its geometric shape under the conditions of mechanical stress imposed during the sand casting process.
The ability of the sand mold to permit the escape of air, gases, and steam during the sand casting process.
The ability of the sand mixture to collapse under force. Collapsibility is a very important property in this type of casting manufacture. Collapsibility of the mold will allow the metal casting to shrink freely during the solidification phase of the process. If the molding sand cannot collapse adequately for the casting's shrinkage, hot tearing or cracking will develop in the casting.
The ability of the sand mixture to flow over and fill the sand casting pattern during the impression making phase of the manufacturing process, more flowability is useful for a more detailed casting.
During the pouring of the molten metal in sand casting manufacture, the sand mixture in the mold must not melt, burn, crack, or sinter. The refractory strength is the ability of the mold sand mixture to withstand levels of extreme temperature.
The ability of the mold sand mixture to be reused to produce other sand castings in subsequent manufacturing operations.
When planning the manufacture of a particular casting, remember some properties of a sand casting mold mixture are contradictory to each other. Tradeoffs in different properties are often needed to achieve a compromise that provides a sand casting mold mixture with adequate properties for the specific part and casting application. There are some things to consider when selecting a sand mixture for a manufacturing process. Small grain size enhances mold strength, but large grain size is more permeable. Sand casting molds made from grains of irregular shape tend to be stronger because of grain interlocking, but rounder grains provide a better surface finish. A sand casting mold mixture with more collapsibility has less strength, and a sand casting mixture with more strength has less collapsibility.
Sand Conditioning For A Metal Casting Operation
If the sand is being reused from a previous sand casting manufacturing process, lumps should be crushed and then all particles and metal granules removed, (a magnetic field may be used to assist in this). All sand and constituents should be screened. In industrial practice shakers, rotary screens, or vibrating screens, are used in this process. Then continuous screw-mixers or mulling machines are used to mix the sand uniformly.
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