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Sand casting is one of the oldest casting processes, introduced back at least 2000 years. Sand casting is also known as sand moulding. Sand casting is a very versatile process and can produce castings of extreme complexity in a wide range of aluminium alloys. It is the most widely used casting process.
Nearly all alloys can be sand cast, including metals with a high melting temperature, such as steel, nickel and titanium. Any aluminium alloy can be cast in sand, including alloys that may exhibit hot shortness in metal mould processes.
Sand casting process comes under the category of expandable mould process. Castings range in size from small to very large. The sand mould which is used in this casting process is made of green sand. Here, the green word is referring to the fact that this sand consists of moisture.
This sand is basically synthetic sand because its composition can be controlled more accurately. This sand is the mixture of silica, clay and water where clay is used as the cohesive agent to bond the sand particles, giving the sand better strength and water provides moisture to the sand.
Principle of Sand Casting
The sand casting processes consist of the following steps which are illustrated with above figure also
- Placing a pattern, having the shape of the desired casting, in the sand to make an imprint.
- Incorporating a gating system.
- Filling the resulting cavity with molten metal.
- Allow the metal to cool until it solidifies.
- Breaking away the sand mould.
- Removing the casting and finishing it.
Examples: -Parts made by sand casting are engine blocks, cylinder heads, machine tool bases and housings for pumps & motors.
Gating system is used to serve the passage to the molten metal being poured. Gating system consists the following terms-
- Flask or molding box : A frame made of metal or wood or plastic, in which the mold is formed. Lower molding flask is known as drag, upper molding flask as cope and intermediate molding flask, used in three piece molding, is known as cheek.
- Pattern: The replica of the object to be cast is known as pattern. The cavity in the mould is created with the help of the pattern.
- Parting line: The dividing line between the two molding boxes that makes up the mold.
- Molding sand: Sand, which is used for making the mould is called as molding sand. It is a mixture of silica sand, clay, and moisture in appropriate proportions. The molding sand must possess various properties such as permeability, flow ability, cohesive strength, etc.
- Facing sand: In order to give a better surface finish to the casting, a small amount of fine carbonaceous material, known as facing sand, is usually sprinkled on the parting surfaces of the molding boxes.
- Core: The part of mould, made of sand, used to create openings and various shaped cavities in the castings.
- Pouring basin: A funnel shaped cavity at the top of the mold into which the molten metal is poured.
- Sprue: The passage through which the molten metal flows from the pouring basin, and reaches to the mould cavity. It controls the flow of liquid metal into the mould.
- Runner: The channel through which the molten/liquid metal is carried from the sprue to the gate.
- Gate: A passageway through which the molten metal enters the mould cavity.
- Chaplets: Chaplets are used to support the cores inside the mould cavity. The chaplets are used to prevent the core against buckling or metallostatic pressure.
- Riser: The shapes of the Risers are like a sprue, which are placed at that part of the casting which is solidified in the last. the risers takes care of the shrinkage of the solidifying metal.
- Vent: Small opening in the mold to facilitate escape of air and gases.
- Drag: The bottom half of a horizontally parted mold.
- Cope: The top half of a horizontally parted mold.
- Draft: Slight taper given to a pattern to allow drawing from the sand.
Steps in making Sand Casting
There are five basic steps in producing a sand castings:
1. Pattern making
2. Core making
3. Mold preparation
4. Melting of metal and its pouring
5. Finishing of casting
The pattern is a physical model of the end product to be produced by metal casting process. It is an replica of the cast product. Several features depending upon the requirement of the end product and the type of metal are added in the pattern with an aim to produce a right casting. The pattern produced is used in the development of the mould to make the cavity in which the molten metal is filled through sprue and ingates.
Hollow castings can be produced with the help of introducing the core in the cavity produced in the mold. Cores are usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings.
Mold preparation is initiated by placing the pattern in the mould box and then the pattern is packed in sand aggregate. The sand aggregate consists of sand, binder and moisture. The sand aggregate is then rammed to make the mould dense and vents are produced, on the top surface of the mould, to let the gases generated after pouring the metal to escape.
Melting of metal and its pouring
The preparation of molten metal for casting is referred to simply as melting and once the metal is melted its is poured in the molding box to produce the casting. however, depending upon the requirement, molten metal may or may not be treated or refined before pouring.
Finishing of casting
After the solidification of casting in the mould, the molding boxes are opened and casting is knocked out from the sand aggregate. The solidified casting needs cleaning as the burned sand aggregate sticks on the casting surface. Also, vent wires, scales etc are need to be removed from the casting.