Effect of impurities on Cast Iron

Cast Iron contains small percentages of silicon, sulphur, manganese and phosphorous. The effect of these impurities on the cast iron are as follows:

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  1. Silicon. It may be present in cast iron upto 4%. It provides the formation of free graphite which makes the iron soft and easily machinable. It also produces sound castings free from blow-holes, because of its high affinity for oxygen.
  2. Sulphur. It makes the cast iron hard and brittle. Since too much sulphur gives unsound casting, therefore, it should be kept well below 0.1% for most foundry purposes.
  3. Manganese. It makes the cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur.
  4. Phosphorus. It aids fusibility and fluidity in cast iron, but induces brittleness. It is rarely allowed to exceed 1%. Phosphoric irons are useful for casting of intricate design and for many light engineering castings when cheapness is essential.

INGREDIENTS OF MOULDING SANDS

PRINCIPAL INGREDIENTS OF  MOULDING SANDS :
          The principal ingredients of moulding sands are given below :
(1)    Silica sand grains
(2)    Clay
(3)    Moisture
(4)    Miscellaneous materials
>> SILICA  SAND GRAINS :
          Silica in the form of granular quartz, itself a sand, is the chief constituent of moulding sands.silica sand contains from 80% to 90% silicon dioxide and is characterized by a high softening temperature and thermal stability.it is a product of the breaking up of quartz rocks or the decomposition of granite, which is composed of quartz and feldspar.
>> CLAY :
          Clay is defined as those particles of sand ( under 20 microns in diameter ) that fails to settle at a rate of 25mm per minute, when suspended in water. Clay consists of two ingredients : FINE SILT and TRUE CLAY. Fine silt is the sort of foreign matter or mineral deposit and has no bonding power. It is the true clay which imparts the necessary bonding strength to the mould sand, so that the mould does not lose its shape after rimming. Most moulding sands for different grades of work contain 5% to 20% clay.
>> MOISTURE :
          Moisture in requisite amount, furnished the bonding action of clay. When water is added to clay, it penetrates the mixture and forms a microfilm which coats the surface of flake-shaped clay particles. The bonding quality of clay depends of the maximum thickness of water film it can maintain. The bonding action is considered best if the water added is the exact quantity required to form the film. The water should be between 2% to 8% .
>> MISCELLANEOUS MATERIALS :
          Miscellaneous materials that are found, in addition to silica and clay in moulding sand are oxide of iron, limestone, magnesia, soda, and potash. The impurities should be below 2% .

Types of Moulding Sands

Types of Moulding Sands :

1: Green sand :
The sand in its natural or moist state is called green sand. It is also called tempered sand. It is a mixture of sand with 20 to 30 percent clay, having total amount of water from 6 to 10 percent. The mould prepared with this sand is called green sand mould, which is used for small size casting of ferrous and non-ferrous metals.
2: Dry Sand :
The green sand moulds when baked or dried before pouring the molten metal are called dry sand moulds. The sand of this condition is called dry sand. The dry sand moulds have greater strength, rigidity and thermal stability. These moulds used for large and heavy casting.
3: Loam Sand :
A mixture of 50 percent sand grains and 50 percent clay is called loam sand. It is used for loam moulds of large Grey iron casting.
4: Facing Sand :
A sand which is used before pouring the molten metal, on the surface is called facing sand. It is specially prepared sand from silica sand and clay.
5: Backing or Floor Sand :
A sand used to back up the facing sand and not used next to the pattern is called backing sand. The sand which have been repeatedly used may be employed for this purpose. It is also known as black sand due to its color.
6: System Sand :
A sand employed in mechanical sand preparation and handling system is called system sand. This sand has high strength, permeability and refractoriness.
7: Parting Sand :
A sand employed on the faces of the pattern before the moulding is called parting sand. The parting sand consists of dried silica sand, sea sand or burnt sand.
8: Core Sand :

The cores are defined as sand bodies used to form the hollow portions or cavities of desired shape and size in the casting. Thus the sand used for making these cores is called core sand. It is sometimes called oil sand. It is the silica sand mixed with linseed oil or any other oil as binder.

Properties of Moulding Sands

Properties of Moulding Sands :

1: porosity or permeability :
It is the property of sand which permits the steam and other gases to pass through the sand mould. The porosity of sand depends upon its grain size, grain shape, moisture and clay components are the moulding sand. If the sand is too fine, the porosity will be low.
2: Plasticity :
It is that property of sand due to which it flows to all portions of the moulding box or flask. The sand must have sufficient plasticity to produce a good mould.
3: Adhesiveness :
It is that properties of sand due to it adheres or cling to the sides of the moulding box.
4: Cohesiveness :
It is the property of sand due to which the sand grains stick together during ramming. It is defined as the strength of the moulding sand.
5: Refractoriness :

The property which enables it to resist high temperature of the molten metal without breaking down o r fusing.

6: Chemical Stability :

The property of sand to resist chemical reaction with molten metal is termed as chemical stability.

Dry Sand Moulding

Dry Sand Moulding :

Introduction:

     Dry sand casting is a sophisticated form of green sand process, in which the sand mold is baked at a given temperature to make it stronger. This process in mostly used in large foundries to produce big ferrous and non-ferrous castings like engine blocks, construction parts, etc. Dry sand casting ensures precise size and perfect dimensions.
      The factors affecting the casting process is the proper baking of the mold and the accurate consideration of size, weight and mass of the casting. The key to this process is the proper baking time in relation to the binder and the moisture content.

Process:

     A chemical or adhesive binder is mixed with unbounded sand and then it is shaped in the form of the the mold or pattern. It is then baked at a specific temperature. Very intricate designs that are required for housing constructions and automotive parts can be molded perfectly by this method. When the mold is ready, the pattern made of wood, rubber or other material is set in the mold. Intricate pattern, spures and cores can be used in this method. The patterns is a exact replica of the shape that the metal is to casted in. Molten metal in poured into the mold where it solidifies. When the casting is complete, it is removed from the mold. For the best results in dry sand casting baking of the mold should be properly baked at a given temperature and the proportion between the binder and the moisture content should be maintained. Though the process is quite expensive this process produces exact result when casting large and heavy operation casting.

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Applications:

     Both ferrous and non-metals can be cast using thedry sand cast method. This process is largely used for producing intricate designs for heavy applications. Some of the casting that are produced are engine blocks, large gears, big housings, construction parts, big gear boxes, transmission housings, agricultural casting, automotive parts, etc.

Advantages: 

  • Complicate designs required for engines and automobiles can be designed with relative ease
  • Accuracy is terms of dimensions, size, designs, is the main benefit
  • Though an expensive process, here accuracy is maintained in every respect
  • A process is favored by large foundries.