Q.4 Explain the different types of pattern.
Related Questions -
Q. Describe the various kinds of patters in use. (AKTU - 2011 - 12)
Ans. Types of Pattern: -
1. Solid Or Single Piece Pattern: -
A solid pattern is made in one piece without any joints. These are generally used for simpler shapes and low quantity production. Solid patterns are generally made of wood and are inexpensive. Fig. 1.
2. Split Pattern: -
Many patterns cannot be made of a single piece because of the difficulty in moulding. To eliminate the difficulty the patterns are made split, half rests in lower part of the mould and half in upper part fig.2.
Related Questions -
Q. Describe the various kinds of patters in use. (AKTU - 2011 - 12)
Ans. Types of Pattern: -
1. Solid Or Single Piece Pattern: -
A solid pattern is made in one piece without any joints. These are generally used for simpler shapes and low quantity production. Solid patterns are generally made of wood and are inexpensive. Fig. 1.
2. Split Pattern: -
Many patterns cannot be made of a single piece because of the difficulty in moulding. To eliminate the difficulty the patterns are made split, half rests in lower part of the mould and half in upper part fig.2.
3. Gated Patterns: -
These are used in production work where many castings are required. Patterns are made of metal to give them strengths and to eliminate any warping tendency. The gates or runners for the molten metal are formed by connecting parts between the individual patterns (fig. 3).
These are used in production work where many castings are required. Patterns are made of metal to give them strengths and to eliminate any warping tendency. The gates or runners for the molten metal are formed by connecting parts between the individual patterns (fig. 3).
4. Loose Piece Pattern: -
It consists of loose pieces, which are necessary to facilitate withdrawing it from the mould (fig. 4).
It consists of loose pieces, which are necessary to facilitate withdrawing it from the mould (fig. 4).
5. Match Plates: -
These provide a substantial mounting for patterns. It consists of a flat metal or wooden plate to which the patterns and gate are permanently fastened. These are generally used for small castings with higher dimesional accuracy and mass production (fig. 5).
6. Sweep Pattern: -
These are used where the shape to be moulded can be formed by the rotation of a curved line element about an axis. This type of pattern is generally suitable for very large castings such as bells for ornamental purposes (fig. 6).
7. Skeleton Pattern: -
These consist of a simple wood frame outlining the shape of the casting. This type of pattern is used for large castings, required in small numbers (fig. 7).
8. Follow Board Pattern: -
These are used for those castings where there are some portions which are structurally weak and if not supported properly are likely to break under the force of ramming. Thus the bottcm board is modified as a follow board to closely fit the contour of the weak pattern and provide support to it during ramming of the drag (fig. 8).
9. Segmental Pattern: -
These are also known as part patterns. This type of pattern revolve about a centre and after ramming one section, it moves to another section to complete the mould. Such type of patterns are used for making large circular castings e.g., rings, wheel rims and gears (fig.9).
10. Cope and Drag Pattern: -
It is a type of split pattern. The pattern is made up of two halves, which are mounted on different plates. The cope and drag parts of the mould are made separately and then assembled.
Q.5 What are the main constituents of the moulding sand define its properties.
Related Questions -
Q. What are the important properties which are essential in a good moulding sand? and What are the ingredients of moulding sand ? (AKTU - 2010-11)
Ans. Moulding Sand: -
Moulding sand is one of the most important materials in production of sand casting because of the following advantages:
(1) It can withstand high temperatures without decomposition.
(2) In moistened state it can be easily packed to any shape.
(3) It permits the gases, air and steam formed in the mould to escape.
(4) It does not chemically react with the molten metal.
Sand is formed by breaking up of rocks due to natural forces such as, wind, rain and the action of heat. In nature it is found on the bottom and banks of rivers and lakes. Sand obtained from different localities vary in colour and composition.
Principle Constituents of Moulding Sand: -
In natural condition moulding sand is a mixture of silica sand (SiO2), clay i.e., Alumina (A12O3), and water.,In addition to these, other materials such as oxides of iron, Ti, Mn, and Ca, and some alkaline compounds are also present in the sand.
(1) Silica (SiO2): -
Grannular particles of silica comprise of 86 to 90% of total material of moulding sand. The shape and size of the sand grains has a remarkable effect on the physical properties of the moulding sand. The commonly formed shapes are round, sub-angular, angular and compound. Silica also gives higher thermal stability.
(2) Clay (Alumina): -
Moulding sand may contain 2 to 5 percent of clay with suitable water content. The clay acts as a binder. The clay used for binding the sand grains should be refractory and should withstand high temperature. Clay imparts strength and plasticity to the moulding sand and reduces permeability.
(3) Water: -
Water present in amounts of about 1.5 to 1.8 percent activates the clay in the sand, causing the mixture to develop plasticity and higher dry strength. Water in excess of that, which can be absorbed by clay is called free water. Free water acts as lubricant and makes the sand more plastic and more mouldable but reduces the strength.
(4) Impurities: -
In addition to the above ingredients sand also contains iron oxide, alkali oxide, line, magnesia, soda, potash etc. The presence of excessive amounts of these substances can cause objectionable lowering of the fusion point in some sands.
(5) Special Additives: -
In addition to the three basic ingredients namely silica, clay and water, other materials added in the new as well as used sand to improve its qualities as a moulding material are as under:
Bentonite: -
Bentonite is a special clay which makes an excellen bond for the moulding sand.
(6) Cerel Binder: -
It is finely ground corn flour or gelatinized and ground starch from corn. It may be used in moulding sand to increase. green or dry strength or collapsibility. Generally it is used in the range of 0.25 to 2.00%.
(7) Ground Pitch: -
It is distilled from soft coal at about 175°C and above. It is used to improve hot strength and casting finish on ferrous castings.
(8) Sea Coal: -
It is finely ground soft coal used in gray and malleable moulding sands to improve surface finish and ease of cleaning the castings.
(9) Wood Flour: -
Ground wood flour may be added to moulding sands to increase their thermal stability. It functions as a cushion or control of expansion of sand by burning out at elevated temperatures.
(10) Silica Flour: -
Pulverised silica is sometimes used to increase hot strength of sand.
(11) Perlite: -
It is an expanded aluminium silicate mineral used in small quantities to obtain better thermal stability of sand and used as riser (insulator).
(12) Oils: -
“Drying” oils, such as linsed oil are often mixed with silica sand to serve as bonding agents. They develop maximum bond strength after baking.
Properties: - (AKTU - 2010 - 11, 11 - 12)
Refractoriness: -
(1) Refractoriness: -
It is the property of the moulding sand to withstand high temperature of the molten metal without breaking down or fusing. The moulding sand possesses this property largely to the presence of quartz content or SiO2 and the shape and size of the grains. Rough and larger grains lead to higher refractoriness. The presence of iron oxides, cakium, sodium and potassium reacts chemically, during the pouring of molten metal, to form compounds which have very low melting point. Thus, their presence in the sand reduces its refractoriness.
The extent to which this property is needed depends upon the metal which is to be cast. For example, steel is poured at a higher temperature than cast iron, therefore, the mould for steel casting should have higher degree of refractoriness than that used for iron casting. If sand lacks this property it will fuse on coming in contact with the molten metal, form slag on the surface of the casting and will result in defective casting.
(2) Cohesiveness: -
The ability of the sand particles to stick together is termed as cohesiveness or the strength of the moulding sand. The sand particles bind together firmly after ramming due to cohesiveness and the pattern is withdrawn from the mould without damaging the mould surfaces. Also due to this property the mould faces get sufficient strength to withstand the pressure of the flowing molten metal and do not get washed. This property of the sand in green state is called as green strength. When the molten metal is poured in the mould or the mould is baked in an oven, it gcts dried. The cohesiveness of the sand grains is still required to give sufficient strength to the mould faces to retain their shape and resist the flow of molten metal into the sand mass. This property in dry state of the sand mould is called as dry strength. The dry strength should not be allowed to exceed beyond requirements otherwise it will reduce collapsibility and restrict contraction of solidifying metal, resulting in cracks and unsound casting.
Permeability (Porosity): -
It is the property of the moulding sand which allows the gases and steam to escape through the sand mould. The sand used for casting must be porous enough, so as to allow the gases, water vapours to escape freely when the molten metal is poured in the mould. Molten metal contains some dissolved gases which are evolved on solidification. Further molten metal comes in contact with moisture, coal dust, oil and other similar materials present in the sand and froms steam and vapour which must find passage to escape completely. Insufficient porosity of moulding sand leads for casting defects such as gas holes and pores.
Adhesiveness: -
Adhesiveness is the property of the moulding sand due to which it is capable of adhering to the surface of other materials. It is due to this property that sand mass is held in the moulding box properly and does not fall down when the mould is moved.
Flowability or Plasticity: -
Flowability is the property of the moulding sand due to which it flows during ramming to all portions of moulding flask, packs properly around the pattern to acquire the desired shape and distributes the ramming pressure evenly to all parts of the mould. Generally fine grained sand has better plasticity. It increases with the addition of clay and water content and reduction of green strength and grain size.
Collapsibility: -
Sand mould should break (collapse) easily after the solidification of the molten metal for easy removal of casting. In absence of this property the contraction of the metal will be hindered by the mould and this will result in tears and cracks in the casting.
Other Requirements: -
In addition to the above main characteristics, the moulding sand should also possess the following properties
(i) It should be reusable for the sake of economy.
(ii) It should not react chemically with the molten metal.
(iii) Its coefficient of expansion should be sufficiently low.
(iv) It should be cheap and easily available.
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Refractoriness: -
(1) Refractoriness: -
It is the property of the moulding sand to withstand high temperature of the molten metal without breaking down or fusing. The moulding sand possesses this property largely to the presence of quartz content or SiO2 and the shape and size of the grains. Rough and larger grains lead to higher refractoriness. The presence of iron oxides, cakium, sodium and potassium reacts chemically, during the pouring of molten metal, to form compounds which have very low melting point. Thus, their presence in the sand reduces its refractoriness.
The extent to which this property is needed depends upon the metal which is to be cast. For example, steel is poured at a higher temperature than cast iron, therefore, the mould for steel casting should have higher degree of refractoriness than that used for iron casting. If sand lacks this property it will fuse on coming in contact with the molten metal, form slag on the surface of the casting and will result in defective casting.
(2) Cohesiveness: -
The ability of the sand particles to stick together is termed as cohesiveness or the strength of the moulding sand. The sand particles bind together firmly after ramming due to cohesiveness and the pattern is withdrawn from the mould without damaging the mould surfaces. Also due to this property the mould faces get sufficient strength to withstand the pressure of the flowing molten metal and do not get washed. This property of the sand in green state is called as green strength. When the molten metal is poured in the mould or the mould is baked in an oven, it gcts dried. The cohesiveness of the sand grains is still required to give sufficient strength to the mould faces to retain their shape and resist the flow of molten metal into the sand mass. This property in dry state of the sand mould is called as dry strength. The dry strength should not be allowed to exceed beyond requirements otherwise it will reduce collapsibility and restrict contraction of solidifying metal, resulting in cracks and unsound casting.
Permeability (Porosity): -
It is the property of the moulding sand which allows the gases and steam to escape through the sand mould. The sand used for casting must be porous enough, so as to allow the gases, water vapours to escape freely when the molten metal is poured in the mould. Molten metal contains some dissolved gases which are evolved on solidification. Further molten metal comes in contact with moisture, coal dust, oil and other similar materials present in the sand and froms steam and vapour which must find passage to escape completely. Insufficient porosity of moulding sand leads for casting defects such as gas holes and pores.
Adhesiveness: -
Adhesiveness is the property of the moulding sand due to which it is capable of adhering to the surface of other materials. It is due to this property that sand mass is held in the moulding box properly and does not fall down when the mould is moved.
Flowability or Plasticity: -
Flowability is the property of the moulding sand due to which it flows during ramming to all portions of moulding flask, packs properly around the pattern to acquire the desired shape and distributes the ramming pressure evenly to all parts of the mould. Generally fine grained sand has better plasticity. It increases with the addition of clay and water content and reduction of green strength and grain size.
Collapsibility: -
Sand mould should break (collapse) easily after the solidification of the molten metal for easy removal of casting. In absence of this property the contraction of the metal will be hindered by the mould and this will result in tears and cracks in the casting.
Other Requirements: -
In addition to the above main characteristics, the moulding sand should also possess the following properties
(i) It should be reusable for the sake of economy.
(ii) It should not react chemically with the molten metal.
(iii) Its coefficient of expansion should be sufficiently low.
(iv) It should be cheap and easily available.
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