Sand casting is used to make large parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic).
In conventional sand casting, the mold is formed around a pattern by ramming sand, mixed with the proper bonding agent, onto the pattern. Then the pattern is removed, leaving a cavity in the shape of the casting to be made. If the casting is to have internal cavities or undercuts, sand cores are used to make them. Molten metal is poured into the mold, and after it has solidified the mold is broken to remove the casting. In making molds and cores, various agents can be used for bonding the sand. The agent most often used is a mixture of clay and water.
Sand casting imposes few restrictions on product shape. The only significant exceptions are the draft angles that are always needed on flat surfaces oriented perpendicular to the parting line. Dimensional control and consistency in sand castings ranges from about + 0.030 to + 0.125 in (+ 0.8 to 3.2 mm). Within this range, the more generous tolerances apply across the parting line. Surface finish ranges between approximately 300 and 500 gin (7.7 – 12.9 gm) rms.
SAND CASTING PROCESS
The pattern is housed in a box called the flask, and then packed with sand. A binder helps harden the sand into a semi-permanent shape. Once the sand mold is cured, the pattern is removed. This leaves a hollow space in the sand in the shape of the desired part. The pattern is made larger than the cast to allow for shrinkage during cooling. Sand cores can then be inserted in the mold to create holes and improve the casting`s overall shape. Simple patterns are usually open on top, allowing molten metal to be poured into them. Two-piece molds are clamped together. Molten metal is poured into a pouring cup where it will then travel down a sprue and into the gating system. Vent holes are created to allow hot gases to escape during the pour. Ideally, the pouring temperature of the molten metal is a few hundred degrees higher than the melting point, assuring good fluidity. The temperature difference also prevents premature cooling and resulting voids and porosity. After the metal cools, the sand mold is removed and the metal part is ready for additional operations, such as cutoff and grinding.
Properly locating the parting line reduces the number of cores and the amount of waste while increasing dimensional accuracy. Uniform casting thickness results in uniform cooling and solidification, as well as distortion-free castings. When uniform cross sections cannot be maintained, gradual changes in cross sections are required. This can be achieved by using a transition radius of 1/3 of the thicker section and blending in the radius with a 15-degree slope line. To minimize problems at intersecting points within the core, use staggered rather than continuous ribs and avoid large, unsupported areas. Maintaining minimum wall thickness will prevent voids and non-fill areas. With a solid understanding of the process and its advantages, you can confidently select sand casting as the low-cost method for creating your metal components.
PRECISION DRY CASTING
The mould is dried at a temperature just above 100°C (212°F) the majority of the free moisture will be removed. This is the principal of the dry sand moulding process. Removal of the free moisture is accompanied by a significant increase in the strength and rigidity of the mould. This enables the mould to withstand much greater pressures and so, traditionally, the dry sand process has been used in the manufacture of large, heavy castings.
GREEN SAND CASTING
The relative abundance of sand and clay minerals means that the material costs for the process are low. The mould material is reclaimable, with between 90 and 95% of the sand being recycled, although new sand and additions are required to make up for the deliberately discarded loss. From an environmental view the process is also acceptable, although problems may arise from the use of organic additions to the sand and from the generation of fine silica dust. These features, combined with the relative ease of mould production, have ensured that the green sand moulding process has remained as the principal method by which castings are produced.
The green sand process remains unchallenged as the process capable of the mass production of castings at low cost, especially in cast irons.
The sand used for green sand moulding must fulfil a number of requirements:
1. It must pack tightly around the pattern, which means that it must have flowability.
2. It should be capable of being deformed slightly without cracking, so that the pattern can be withdrawn. In other words, it must exhibit plastic deformation.
3. It must have sufficient strength to strip from the patter and support its own weight without deforming, and to withstand the pressure of molten metal when the mould is cast. It must therefore have green strength.
4. It must be permeable, so that gases and steam can escape from the mould during casting.
5. It must have dry strength, to prevent erosion of the mould surface by liquid metal during pouring as the surface of the mould cavity dries out.
6. It must have refractoriness, to withstand the high temperature involved in pouring without melting or fusing to the casting.
7. With the exception of refractoriness, all of these requirements are dependent on the amount of active clay present and on the water content of the mixture
Detailed process :
The five basic steps in making sand castings are,
(i) Pattern making,
(ii) Core making,
(iv) Melting and pouring,
(v) Cleaning Pattern making
i) Pattern: Replica of the part to be cast and is used to prepare the mould cavity. It is the physical model of the casting used to make the mould. Made of either wood or
-The mould is made by packing some readily formed aggregate material, such as moulding sand, surrounding the pattern. When the pattern is withdrawn, its imprint provides the mould cavity. This cavity is filled with metal to become the casting.
– If the casting is to be hollow, additional patterns called ‘cores’, are used to form these cavities.
ii) Core making: Cores are placed into a mould cavity to form the interior surfaces of castings. Thus the void space is filled with molten metal and eventually becomes the casting.
iii) Moulding: Moulding is nothing but the mould preparation activities for receiving molten metal. Moulding usually involves: (i) preparing the consolidated sand mould around a pattern held within a supporting metal frame, (ii) removing the pattern to leave the mould cavity with cores.
Mould cavity is the primary cavity.
The mould cavity contains the liquid metal and it acts as a negative of the desired product.
The mould also contains secondary cavities for pouring and channeling the liquid material in to the primary cavity and will act a reservoir, if required.
iv) Melting and Pouring: The preparation of molten metal for casting is referred to simply as melting. The molten metal is transferred to the pouring area where the moulds
Cleaning: Cleaning involves removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed.
Flask: A metal or wood frame, without fixed top or bottom, in which the mould is formed. Depending upon the position of the flask in the moulding structure, it is referred to by various names such as drag – lower moulding flask, cope – upper moulding flask, cheek – intermediate moulding flask used in three piece moulding.
Pattern: It is the replica of the final object to be made. The mould cavity is made with the help of pattern.
Parting line: This is the dividing line between the two moulding flasks that makes up the mould.
Moulding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.
Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mould cavity to give a better surface finish to the castings.
Core: A separate part of the mould, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.
Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten metal is poured.
Sprue: The passage through which the molten metal, from the pouring basin, reaches the mould cavity. In many cases it controls the flow of metal into the mould.
Runner: The channel through which the molten metal is carried from the sprue to the gate. Gate: A channel through which the molten metal enters the mould cavity.
Chaplets: Chaplets are used to support the cores inside the mould cavity to take care of its own weight and overcome the metallostatic force.
Riser: A column of molten metal placed in the mould to feed the castings as it shrinks and solidifies. Also known as “feed head”.
Vent: Small opening in the mould to facilitate escape of air
1. Molten material can flow into very small
sections so that intricate shapes can be made by this process.
As a result, many other operations, such as
machining, forging, and welding, can be minimized. 2.Possible to cast
practically any material: ferrous or non-ferrous.
3.The necessary tools required for casting
moulds are very simple and inexpensive. As a result, for production of a small
lot, it is the ideal process.
4.There are certain parts (like turbine
blades) made from metals and alloys that can only be processed this way.
Turbine blades: Fully casting + last machining.
5.Size and weight of the product is not a
limitation for the casting process.
1.Dimensional accuracy and surface finish of
the castings made by sand casting processes are a limitation to this technique.
2.Metal casting is a labour intensive process
3.Automation: a question
used in blowers or impellers, electronic equipment, engine blocks, gas or oil tanks, automobile parts, agricultural machines, pistons, valves, medical equipment, mining equipments etc.