Q.10 Explain in brief the types of gates.
Ans. Metal can be made to flow into the mould cavity in several ways. Design of a gating system depends on its primary objectives and requirements. Therefore a gate is designed for simplicity and ease of moulding, to avoid turbulence, and to prevent washing out of sand from mould walls. A gate should be designed to avoid inclusion of dross or slag with the metal entering the mould.
Following are the various types of gate :
(i) Parting Gate: -
This gate enters the mould cavity along the parting line separating cope and drag components of the mould. This gate contains certain devices to collect slag and dross and to reduce pouring pressures.
(ii) Top Gate: -
In top gate, molten metal directly flows from pouring basin into the mould cavity. Since metal falls directly into mould cavity, mould should be very hard and strong to resist erosion caused by incoming molten metal. The advantage of top gates lies in the fact that all metal enter the casting at the top and hence hottest metal comes to rest in this region of casting. Consequently, proper temperature gradient favourable for directional solidification towards riser located at the top of casting is attained. The disadvantage of this gating system is the erosion of mould caused by falling metal. It leads to more oxidation. In top gating system, no riser is needed.
(iii) Bottom Gate: -
This gating system is used for very large castings. The liquid metal flows down the bottom of the mould cavity in the drag and enters the basement of the casting. Bottom gates keep the turbulence to a minimum during pouring operation of the metal and prevents mould from being eroded.
Advantage of thin gating system include less turbulence and mould erosion. It gives good casting surfaces. Disadvantage of this system is that the metal continues to lose its heat as it rises in mould cavity. Thus directional solidification is very difficult to attain. Moreover, the riser can not be located near the entrance of gate where metal is in hottest state.
(iv) Horn Gate: -
It resembles the horn of a cow. A horn gate enables the mould to be made in cope and drag only. It tends to produce a fountain effect in the casting.
Q.11 What are various casting defects and remedies? Describe in brief.
Ans. Common defects occurring in castings, their causes and remedies are explained below :
(i) Shifts : It is an external defect in a casting due to misplacing or mismatching of top and bottom components of casting at parting line. Shift is also caused due to misalignment of flasks.
Remedy : Shifts can be prevented by :
(a) proper alignment of die, pattern, moulding boxes.
(b) proper mounting of patterns on pattern plates, checking of flasks, locating pin, etc before using them.
(ii) Blow Holes : These are smooth, round holes appearing in the form of a cluster of a good number of small holes below the casting surface. The blow holes are entrapped bubbles of gas with smooth boundaries. Blow holes are developed due to excess moisture in the sand, low permeability of sand, too hard ramming of sand, fine grain sands, etc.
Remedy :
(a) Moisture in sand should be minimized.
(b) Proper grain sized sand should be used.
(c) Ramming should not be too hard.
(d) Adequate venting should be ensured.
(iii) War Page : It takes place during or after solidification. It is very common in large and flat sections or intersecting sections. Because of different solidification rates in different sections in a casting, thermal stresses are induced in adjoining surfaces resulting in war page in those areas.
Remedy : Uniform cooling rate should be maintained in all areas.
(iv) Swell : It is an enlargement of mould cavity due to metal pressure, resulting in local or overall enlargement of casting. It is caused by improper ramming of mould.
Remedy : Even and proper ramming of the sand.
(v) Fin : Fin is a thin and small projection of metal which is not a part of the casting. It generally occurs at parting line of mould or core sections. It is also because of insufficient weighting of molds or improper clamping of flasks.
Remedy : (a) Sufficient weight should be placed on top portion of mould to ensure tight fitting of the two parts.
(b) Moulds and cores used for casting should be properly and correctly assembled.
(vi) Hot Tears : Hot tears are also known as pulls. These are internal or external cracks having spoiled edges occurring after metal solidification. High residual stress cause hot tears in a casting.
Remedy : Improved design, uniform cooling rate, correct pouring temperature, proper directional solidification etc can eliminate hot tears.
(vi) Sponginess (or Honey Combing) : It is an external defect which consist of a large number of small cavities in a very close proximity.
Remedy : (a) Sand wash should be prevented
(b) Slay particles present in liquid metal should be removed by proper skimming.
(viii) Cold Shut and Misruns : It is an external defect developed because of imperfect fusion of two steams of metal in the mould cavity or because of unequal sections of pattern assembled together. Misruns appears like a crack or seam with round edges.
Remedy : (a) Patterns and core boxes should be frequently inspected and replaced.
(b) Proper designing of casting
(c) Hotter metal should be used
(ix) Metal Penetration : This defect is characterized as an uneven and rough external surface of the casting. It is due to low strength, larger grain size, high permeability and soft ramming of the sand.
Remedy : If these causes are removed, metal penetration can be prevented.
(x) Dirt : In some cases, sand particles coming out of cope get clogged on the top surface of casting. It is caused by crusting of mould due to improper handling, sand washout and presence of slag particles in liquid metal.
Ans. Metal can be made to flow into the mould cavity in several ways. Design of a gating system depends on its primary objectives and requirements. Therefore a gate is designed for simplicity and ease of moulding, to avoid turbulence, and to prevent washing out of sand from mould walls. A gate should be designed to avoid inclusion of dross or slag with the metal entering the mould.
Following are the various types of gate :
(i) Parting Gate: -
This gate enters the mould cavity along the parting line separating cope and drag components of the mould. This gate contains certain devices to collect slag and dross and to reduce pouring pressures.
(ii) Top Gate: -
In top gate, molten metal directly flows from pouring basin into the mould cavity. Since metal falls directly into mould cavity, mould should be very hard and strong to resist erosion caused by incoming molten metal. The advantage of top gates lies in the fact that all metal enter the casting at the top and hence hottest metal comes to rest in this region of casting. Consequently, proper temperature gradient favourable for directional solidification towards riser located at the top of casting is attained. The disadvantage of this gating system is the erosion of mould caused by falling metal. It leads to more oxidation. In top gating system, no riser is needed.
(iii) Bottom Gate: -
This gating system is used for very large castings. The liquid metal flows down the bottom of the mould cavity in the drag and enters the basement of the casting. Bottom gates keep the turbulence to a minimum during pouring operation of the metal and prevents mould from being eroded.
Advantage of thin gating system include less turbulence and mould erosion. It gives good casting surfaces. Disadvantage of this system is that the metal continues to lose its heat as it rises in mould cavity. Thus directional solidification is very difficult to attain. Moreover, the riser can not be located near the entrance of gate where metal is in hottest state.
(iv) Horn Gate: -
It resembles the horn of a cow. A horn gate enables the mould to be made in cope and drag only. It tends to produce a fountain effect in the casting.
Q.11 What are various casting defects and remedies? Describe in brief.
Ans. Common defects occurring in castings, their causes and remedies are explained below :
(i) Shifts : It is an external defect in a casting due to misplacing or mismatching of top and bottom components of casting at parting line. Shift is also caused due to misalignment of flasks.
Remedy : Shifts can be prevented by :
(a) proper alignment of die, pattern, moulding boxes.
(b) proper mounting of patterns on pattern plates, checking of flasks, locating pin, etc before using them.
(ii) Blow Holes : These are smooth, round holes appearing in the form of a cluster of a good number of small holes below the casting surface. The blow holes are entrapped bubbles of gas with smooth boundaries. Blow holes are developed due to excess moisture in the sand, low permeability of sand, too hard ramming of sand, fine grain sands, etc.
Remedy :
(a) Moisture in sand should be minimized.
(b) Proper grain sized sand should be used.
(c) Ramming should not be too hard.
(d) Adequate venting should be ensured.
(iii) War Page : It takes place during or after solidification. It is very common in large and flat sections or intersecting sections. Because of different solidification rates in different sections in a casting, thermal stresses are induced in adjoining surfaces resulting in war page in those areas.
Remedy : Uniform cooling rate should be maintained in all areas.
(iv) Swell : It is an enlargement of mould cavity due to metal pressure, resulting in local or overall enlargement of casting. It is caused by improper ramming of mould.
Remedy : Even and proper ramming of the sand.
(v) Fin : Fin is a thin and small projection of metal which is not a part of the casting. It generally occurs at parting line of mould or core sections. It is also because of insufficient weighting of molds or improper clamping of flasks.
Remedy : (a) Sufficient weight should be placed on top portion of mould to ensure tight fitting of the two parts.
(b) Moulds and cores used for casting should be properly and correctly assembled.
(vi) Hot Tears : Hot tears are also known as pulls. These are internal or external cracks having spoiled edges occurring after metal solidification. High residual stress cause hot tears in a casting.
Remedy : Improved design, uniform cooling rate, correct pouring temperature, proper directional solidification etc can eliminate hot tears.
(vi) Sponginess (or Honey Combing) : It is an external defect which consist of a large number of small cavities in a very close proximity.
Remedy : (a) Sand wash should be prevented
(b) Slay particles present in liquid metal should be removed by proper skimming.
(viii) Cold Shut and Misruns : It is an external defect developed because of imperfect fusion of two steams of metal in the mould cavity or because of unequal sections of pattern assembled together. Misruns appears like a crack or seam with round edges.
Remedy : (a) Patterns and core boxes should be frequently inspected and replaced.
(b) Proper designing of casting
(c) Hotter metal should be used
(ix) Metal Penetration : This defect is characterized as an uneven and rough external surface of the casting. It is due to low strength, larger grain size, high permeability and soft ramming of the sand.
Remedy : If these causes are removed, metal penetration can be prevented.
(x) Dirt : In some cases, sand particles coming out of cope get clogged on the top surface of casting. It is caused by crusting of mould due to improper handling, sand washout and presence of slag particles in liquid metal.
Remedy : By proper fluxing and use of dirt traps, this defect can be removed.
(xi) Scabs : Scab is a rough and thin layer of a metal on the surface of casting. Scab is developed when the upheaved sand is separated from mould and molten metal flows into the space between mould cavity and displaced sand.
(xii) Shrinkage Cavity : It is a void or depression in the casting due to uncontrolled and random solidification of the metal. A very high pouring temperature also results in the formation of shrinkage cavity.
Remedy : (a) It can be prevented by proper use of chills, padding, etc
(b) Uncontrolled and random solidification of metal should be controlled
(xiii) Drop : A drop occurs when the upper surface of a mould cracks and the sand particles fall into the liquid metal. It is caused due to low strength, soft ramming of sand, improper fluxing of liquid metal and inadequate reinforcement of sand projections in the cope.
Remedy : (a) Proper fluxing of molten metal.
(b) Sufficient reinforcement of sand projections in the cope.
(xiv) Pin Holes : They are micro blow holes, less then 2mm, entrapped in the liquid metal and visible at or just below the surface of a casting cleaned by shot blasting.
Pin holes are caused by sand with high moisture content, absorption of hydrogen or CO.
Remedy: -
It can be minimized by using good melting and fluxing practices, by reducing moisture content of moulding sand; increasing its permeability and by promoting a rapid solidification.
(xv) Buckle: -
It appears as a long, fairly shallow, broad and V-shaped depression on the surface of a flat casting. The high temperature of liquid metal causes an expansion in the thin layer of sand at the mould face. This expansion of thin layer of sand is restricted by the moulding body, as a result the mould face tends to buckle in V-Shape.
Remedy: -
Voltile additives, in proper quantity, should be mixed with sand to compensate for this expansion and avoid buckles.
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