The Sand Casting Process
Metal casting is the process in which molten metal is poured into a mold and allowed to solidify into an object. The object that results from this process is also called a casting. In sand casting, sand is used to define the cavity inside a mold. Sand casting is one of the most basic and at the same time one of the most versatile method of forming metals that include processes like forging, punching, stamping, extrusion and others. Sand casting affords the designer a great degree of any forming method and an almost unlimited choice of metals and alloys. Sand casting foundries are basic to industrial processing and responsible for a great part of the industrial growth of the world. In addition, sand is used to make any cores that are contained in the mold. The molten metal solidifies in the cavity between the interior of the mold and the exterior of the core. There are the five basic steps to creating a sand casting. Sand casting can be mass manufactured in foundries ranging from a few cm to many feet, and from less than 50 gm to weighing in tons. Sand casting is utilized in industries like automotive, manufacturing, etc. to make parts that comprise of iron, bronze, brass and even aluminum. In fact, sand casting is one of the most popular of casting process in India and elsewhere, with a share of almost 70% of all metal casting processes. The sand casting process is considered an economical option because it requires less technology and cost.
Sand casting uses natural or synthetic sand (lake sand) which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cut-off and re-used also.
The Sand Casting Process
A material in a liquid or semisolid form is poured or forced to flow into a die cavity and allowed to solidify, thus taking the solid shape of the cavity. The process can be applied on metals and plastics. The term casting is commonly used for metals and the term moulding is used for plastics.
Patternmaking
The first step in sand casting is patternmaking. The pattern is a replica of the exterior of the casting with dimensional allocation for shrinkage and finishing. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is (almost) identical to the shape of the part we need to make. There are two reasons why patterns are not made to match the exact size of the casing:
Core making
The next step in the process is core making. Cores are forms which are placed into the mould to create the interior contours of the casting. They are typically made of sand mixture- sand combined with water and organic adhesives called binders- which is baked to form the core. This allows the cores to be strong yet collapsible, so they can be easily removed from the finished casting. The finished cores, which can be solid or hollow, are inserted into the mould tin order to provide the internal cavities of the casting before the mould halves are joined. Sand cores are also widely used in die-casting, where permanent metal moulds are employed. Since cores are made in moulds, they require a pattern and mould, called a core box. The core pattern is made in the same fashion as the casting pattern, but the core box is created from a durable material like metal or wood. Core making processes normally use a special kind of sand known as green sand and mixing it with other components to make the sand stronger when used to produce a core.
Moulding
In the sand moulding process, the molten metal flows into the gap between the two parts of the mould. The top half of the mould is called the cope, and bottom part is the drag. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is very close to the part to be casted. The liquid metal is poured into the pouring cup (the top of the funnel), and flows down the sprue (the pipe-shaped neck of the funnel is the sprue). Runners, which are horizontal conduits at the bottom of the spuresto carry the molten metal, carry the molten metal to the mould cavity. The region where the runner joins with the cavity is called the gate. To take care of the excess metal that may flow into the mould cavity, risers (extra cavities) are made connecting to the highest / top surface of the mould.
Cores are also placed in the drag portion of the mould if they are needed.
Melting & Pouring
Melting is the preparation of the metal for casting, and its conversion from a solid to a liquid state in a furnace. It is then transferred in a ladle to the moulding area of the foundry where it is poured into the moulds. The mould is put together by inserting the core into the drag’s cavity, then placing the cope over top and securing/locking the mould. After the metal has solidified, the moulds are vibrated to remove the sand from the casting, and the casting removed for further processing like heat treatment. Sometimes, further grinding or shot blasting is undertaken as well.
Inspection
Sand casting uses natural or synthetic sand (lake sand) which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cut-off and re-used also.
The Sand Casting Process
A material in a liquid or semisolid form is poured or forced to flow into a die cavity and allowed to solidify, thus taking the solid shape of the cavity. The process can be applied on metals and plastics. The term casting is commonly used for metals and the term moulding is used for plastics.
Patternmaking
The first step in sand casting is patternmaking. The pattern is a replica of the exterior of the casting with dimensional allocation for shrinkage and finishing. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is (almost) identical to the shape of the part we need to make. There are two reasons why patterns are not made to match the exact size of the casing:
- the pattern is only needed to make the outer surfaces with the ; the inner surfaces are made by the core
- you need to allow for the shrinkage of the casting after the metal solidifies
Core making
The next step in the process is core making. Cores are forms which are placed into the mould to create the interior contours of the casting. They are typically made of sand mixture- sand combined with water and organic adhesives called binders- which is baked to form the core. This allows the cores to be strong yet collapsible, so they can be easily removed from the finished casting. The finished cores, which can be solid or hollow, are inserted into the mould tin order to provide the internal cavities of the casting before the mould halves are joined. Sand cores are also widely used in die-casting, where permanent metal moulds are employed. Since cores are made in moulds, they require a pattern and mould, called a core box. The core pattern is made in the same fashion as the casting pattern, but the core box is created from a durable material like metal or wood. Core making processes normally use a special kind of sand known as green sand and mixing it with other components to make the sand stronger when used to produce a core.
Moulding
In the sand moulding process, the molten metal flows into the gap between the two parts of the mould. The top half of the mould is called the cope, and bottom part is the drag. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is very close to the part to be casted. The liquid metal is poured into the pouring cup (the top of the funnel), and flows down the sprue (the pipe-shaped neck of the funnel is the sprue). Runners, which are horizontal conduits at the bottom of the spuresto carry the molten metal, carry the molten metal to the mould cavity. The region where the runner joins with the cavity is called the gate. To take care of the excess metal that may flow into the mould cavity, risers (extra cavities) are made connecting to the highest / top surface of the mould.
Cores are also placed in the drag portion of the mould if they are needed.
Melting & Pouring
Melting is the preparation of the metal for casting, and its conversion from a solid to a liquid state in a furnace. It is then transferred in a ladle to the moulding area of the foundry where it is poured into the moulds. The mould is put together by inserting the core into the drag’s cavity, then placing the cope over top and securing/locking the mould. After the metal has solidified, the moulds are vibrated to remove the sand from the casting, and the casting removed for further processing like heat treatment. Sometimes, further grinding or shot blasting is undertaken as well.
Inspection
It is important to check the quality of the casting before it is dispatched to the client in order to inspect it for defects and in order to satisfy decided quality standards. This inspection may include nondestructive testing to determine whether the part will adequately perform for its intended use. All good foundries in India inspect the casting quality before shipping it.