All questions of Metal Casting for Mechanical Engineering Exam

Since given machining allowance is 3 mm on each side, add 3 mm on each side of the part shown in figure. The dimensions of pattern after machining will be

Shrinkage allowance is the difference in dimensions between the pattern and the final casting due to the shrinkage of the metal during solidification. The amount of shrinkage allowance required is dependent on the material being used for casting.

Explanation:

Steel has the highest shrinkage allowance among the given materials. This is due to the following reasons:

1. High melting point: Steel has a high melting point compared to other materials. This means that it takes longer for the metal to cool and solidify, resulting in more shrinkage.

2. High carbon content: Steel has a higher carbon content than other materials. Carbon is a strong carbide former, which can lead to a reduction in the solidification range of steel, resulting in more shrinkage.

3. Low thermal conductivity: Steel has a lower thermal conductivity than other materials. This means that heat is retained for a longer time, resulting in a longer solidification time and more shrinkage.

4. High density: Steel has a higher density than other materials. This means that it will shrink more during solidification.

Therefore, steel requires the highest shrinkage allowance among the given materials.

For same volume the ratio of surface areas of sphere : cylinder : cube: is 4.84 : 5.54 : 6

Explanation:

When pouring molten metal into a mould, it is important to ensure that the metal fills the entire cavity without any defects or voids. The use of risers is a common technique to prevent shrinkage defects in castings. A riser is an additional volume of molten metal that is connected to the casting through a channel, and it serves as a reservoir of molten metal that can compensate for the shrinkage during solidification.

If the molten metal does not appear in the riser during pouring, it indicates that a sound casting will be produced. This is because the molten metal has completely filled the cavity without any defects or voids, and there is no need for additional molten metal from the riser. This is a desirable situation, as it indicates that the casting will have good dimensional accuracy and mechanical properties.

If the molten metal does not appear in the riser, it rules out the possibility of the other three options:

- An obstruction between sprue and riser: If there was an obstruction, the molten metal would not have been able to flow freely into the cavity, and there would be defects or voids in the casting.
- Insufficient molten metal to fill the cavity: If there was insufficient molten metal, the cavity would not have been completely filled, and there would be defects or voids in the casting.
- Either insufficient molten metal to fill the cavity or an obstruction between sprue and riser: This is a combination of the previous two options, and it is also ruled out by the absence of molten metal in the riser.

Thus, the absence of molten metal in the riser during pouring is a good sign that a sound casting will be produced.

Riser in Casting Processes

Riser is an integral part of casting processes. It is a reservoir of molten metal that compensates for the shrinkage that occurs during solidification. This shrinkage can lead to various casting defects, such as porosity, shrinkage cavities, and cracks.

Purpose of Riser

The primary purpose of the riser is to feed the molten metal to the casting in order to compensate for the shrinkage. When the molten metal is poured into the mould cavity, it begins to solidify from the surface towards the centre. As the metal solidifies, it shrinks and creates a void or a cavity in the casting.

The riser compensates for this shrinkage by providing additional molten metal to the casting. The molten metal in the riser solidifies after the metal in the casting has solidified. This solidification of the riser provides an additional source of metal to compensate for the shrinkage in the casting.

Other Purposes of Riser

Apart from compensating for the shrinkage, the riser also serves other purposes in the casting processes. These include:

1. Acting as a reservoir for the molten metal: The riser acts as a reservoir for the molten metal and ensures that there is enough metal available to fill the mould cavity.

2. Providing a source of feeding: The riser provides a source of feeding to the casting by supplying additional molten metal.

3. Reducing the risk of defects: The riser reduces the risk of defects in the casting by compensating for the shrinkage and ensuring that the casting is filled with metal.

Conclusion

In summary, the primary purpose of the riser in casting processes is to compensate for the shrinkage that occurs during solidification. It provides additional molten metal to the casting to ensure that the casting is filled with metal and reduce the risk of defects.

The impurities in true centrifugal casting get collected in the middle portion in between the inner and outer surface. This is because of the unique nature of the centrifugal casting process, which involves the use of centrifugal force to distribute the molten metal and impurities.

Explanation:
1. The Centrifugal Casting Process:
- Centrifugal casting is a casting process that uses the principle of centrifugal force to distribute the molten metal and impurities.
- In this process, a mold is rotated at high speeds while the molten metal is poured into it.
- The centrifugal force generated by the rotation of the mold pushes the molten metal towards the outer surface of the mold, while the impurities tend to move towards the inner surface.

2. Movement of Impurities:
- As the molten metal is poured into the rotating mold, the centrifugal force acts on it, causing it to move towards the outer surface of the mold.
- At the same time, the impurities present in the molten metal also experience the centrifugal force and tend to move towards the inner surface.
- However, due to their higher density compared to the molten metal, the impurities are not able to move as easily as the molten metal.
- As a result, the impurities get trapped in the middle portion of the casting, which is in between the inner and outer surface.

3. Collection of Impurities:
- The impurities that get collected in the middle portion of the casting are typically in the form of slag, oxides, or other non-metallic contaminants.
- These impurities are undesirable in the final casting as they can weaken the material and affect its mechanical properties.
- To minimize the presence of impurities, various techniques can be used, such as the use of proper gating systems, filtering the molten metal, and controlling the pouring temperature.

In conclusion, in true centrifugal casting, the impurities get collected in the middle portion in between the inner and outer surface. This is due to the movement of the molten metal towards the outer surface under the influence of centrifugal force, while the impurities, being denser, tend to remain in the middle portion. Proper control and techniques can be employed to minimize the impurities and ensure a high-quality casting.

To determine the time taken to fill the casting cavity, we need to consider the volume of the cavity and the flow rate of the metal.

1. Calculating the volume of the casting cavity:
The volume of a cylinder is given by the formula V = πr^2h, where r is the radius and h is the height. In this case, the diameter is given as 150 mm, so the radius is 75 mm (or 0.075 m). The height of the cylinder is 200 mm (or 0.2 m). Therefore, the volume of the casting cavity is:
V_cavity = π(0.075)^2(0.2) = 0.00707 m^3

2. Calculating the flow rate of the metal:
The flow rate of the metal depends on the gating ratio and the in-gate area. The gating ratio is not provided in the question, so let's assume it to be 2.

The in-gate area is given as 10 cm^2 (or 0.001 m^2). Therefore, the total gating area is:
A_gating = gating ratio * in-gate area = 2 * 0.001 = 0.002 m^2

3. Calculating the flow rate:
The flow rate of the metal can be calculated using the formula Q = A_gating * V_flow, where Q is the flow rate and V_flow is the velocity of the metal.

Since the height of the metal during pouring is 50 mm above the cope, the total height of the metal column is 200 mm + 50 mm = 250 mm (or 0.25 m).

Using Bernoulli's equation for incompressible flow, the velocity of the metal can be calculated as:
V_flow = √(2gH), where g is the acceleration due to gravity (9.81 m/s^2) and H is the height of the metal column.

V_flow = √(2 * 9.81 * 0.25) = 2.21 m/s

Now, we can calculate the flow rate:
Q = 0.002 * 2.21 = 0.00442 m^3/s

4. Calculating the time taken:
The time taken to fill the casting cavity can be calculated using the formula t = V_cavity / Q, where t is the time taken.

t = 0.00707 / 0.00442 ≈ 1.6 seconds

Since the time needs to be rounded to the nearest whole number, the correct answer is option 'C' - 8 seconds.

Shrinkage Allowance in Materials

Introduction
Shrinkage allowance is the amount of additional material that needs to be added to a casting pattern to compensate for the shrinkage that occurs during the solidification and cooling process. The shrinkage allowance ensures that the final casting dimensions are accurate and match the desired specifications. Different materials have different shrinkage rates, which is why different shrinkage allowances are required for different materials.

Explanation
Among the materials mentioned in the options (Cl, Lead, Brass, and Aluminium alloy), the material with the highest shrinkage allowance is Brass.

Shrinkage Allowance Factors
Several factors influence the shrinkage allowance in a material, including:
1. Cooling rate: Faster cooling rates tend to increase shrinkage.
2. Solidification time: Longer solidification times lead to higher shrinkage.
3. Alloy composition: Different alloy compositions can result in varying shrinkage rates.

Shrinkage Allowance Comparison
Let's analyze the shrinkage allowances for each material:

1. Cl (Chlorine)
Chlorine is not a casting material but a chemical element. Therefore, it does not have a shrinkage allowance.

2. Lead
Lead is a soft and malleable material with a relatively low melting point. It exhibits minimal shrinkage during the cooling process. Therefore, the shrinkage allowance for lead is relatively low compared to other materials.

3. Brass
Brass is an alloy composed of copper and zinc. It has a higher melting point compared to lead. Due to the different thermal properties of copper and zinc, brass experiences a significant amount of shrinkage during cooling. Consequently, a larger shrinkage allowance is required for brass castings.

4. Aluminium Alloy
Aluminium alloys are widely used in various industries due to their lightweight and excellent mechanical properties. They have a relatively low shrinkage rate compared to materials like brass. Therefore, the shrinkage allowance for aluminium alloy castings is lower compared to brass.

Conclusion
Among the materials mentioned in the options, brass has the highest shrinkage allowance. This is because brass, being an alloy of copper and zinc, experiences significant shrinkage during the cooling process. It is important to consider the shrinkage allowance when designing casting patterns to ensure accurate final dimensions in the castings.

Metallic moulds are used in die casting process.

The correct answer is option C: 1, 2, and 4 only.

Shell mould casting, also known as shell molding or shell molding casting, is a casting process that uses a resin-coated sand shell to form the mold. This process offers several advantages over other casting methods. Let's discuss each advantage in detail:

1. Close dimensional tolerance: Shell mould casting provides close dimensional tolerances, meaning that the cast parts can be manufactured with high precision and accuracy. The use of a shell mold allows for greater control over the final dimensions of the cast part, resulting in minimal dimensional variations. This advantage is particularly important in industries where tight tolerances are required, such as aerospace or automotive.

2. Good surface finish: Shell mould casting produces castings with a smooth and consistent surface finish. The resin-coated sand shell used in this process helps to create a mold cavity with excellent surface quality. As a result, the cast parts have a clean and polished appearance, requiring minimal post-processing or finishing operations.

3. Low cost: Shell mould casting is a cost-effective casting method. The process uses readily available materials, such as sand and resin, which are relatively inexpensive compared to other mold materials. Additionally, the use of reusable shell molds reduces the overall production costs as compared to traditional sand casting, where a new mold is required for each casting. The low cost of shell mould casting makes it an attractive option for large-scale production runs.

4. Easier: Shell mould casting is relatively easier compared to other casting methods. The process involves fewer steps and requires less equipment and manpower. The use of pre-coated sand shells simplifies the mold-making process, reducing the time and effort required to produce molds. This advantage makes shell mould casting more efficient and less labor-intensive than some other casting techniques.

In conclusion, shell mould casting offers advantages such as close dimensional tolerance, good surface finish, and ease of use. These advantages make it a preferred choice for many applications where precision, aesthetics, and cost-effectiveness are important factors. Therefore, option C: 1, 2, and 4 only is the correct answer.

No core is used in centrifugal casting method.

Top gate causes turbulence due to high head and velocity

Shell mould casting uses silica sand and phenal formaldehyde.

Purpose of Inoculation in Casting
Inoculation is a crucial process in metal casting that primarily aims to enhance the structure and properties of the cast metal. Here’s a detailed overview of its significance:
1. Structural Modification
- Inoculation introduces specific alloying elements or inoculants during the solidification of molten metal.
- These inoculants promote the formation of a refined microstructure, which can significantly enhance the mechanical properties of the metal.
2. Improvement in Properties
- The inoculation process helps in controlling grain size and distribution in the cast metal.
- A finer grain structure leads to improved strength, ductility, and toughness of the final product.
3. Reduction of Defects
- By modifying the solidification process, inoculation can minimize casting defects such as porosity and segregation.
- This results in a more homogeneous material with consistent properties throughout.
4. Enhanced Performance
- The tailored microstructure achieved through inoculation often results in better performance characteristics under operational conditions.
- This is particularly important in applications where mechanical properties are critical, such as in automotive and aerospace components.
5. Cost-Effectiveness
- Inoculation can lead to a reduction in the amount of expensive alloying elements needed to achieve desired properties, making the casting process more economical.
In summary, the primary purpose of inoculation in casting is to modify the structure and properties of cast metals, ensuring that they meet specific performance standards and operational requirements. This process is essential for producing high-quality cast components in various industries.

Introduction:
The sand ramming method is used in foundry processes to pack sand around a pattern in order to create a mold cavity. There are various sand ramming methods, and each method produces different results in terms of hardness and density of the mold. The question asks which method results in the hardest layer at the parting plane and around the pattern, while being less dense in the top layers.

Jolting:
Jolting is a sand ramming method where the flask containing the pattern is subjected to a jolting action. This action helps in compacting the sand around the pattern. The jolting action causes the sand particles to rearrange and settle more tightly, resulting in a denser mold. However, this method does not provide uniform compaction throughout the entire mold.

Squeezing:
Squeezing is another sand ramming method where the flask containing the pattern is squeezed by applying pressure. This pressure helps in packing the sand around the pattern. Squeezing provides better uniform compaction compared to jolting and results in a denser mold. However, the hardness of the mold may not be as high as in the case of jolting.

Jolting and Squeezing:
Jolting and squeezing are both used in combination in some sand ramming methods. This combination helps in achieving better compaction and density of the mold. However, the hardness of the mold may still not be as high as in the case of jolting alone.

Slinging:
Slinging is not a sand ramming method. It involves the process of throwing or slinging sand particles onto the pattern to create a mold. This method does not provide proper compaction or density and is not suitable for creating molds with a hard layer at the parting plane and around the pattern.

Conclusion:
Based on the explanations above, it can be concluded that the sand ramming method that results in the hardest layer at the parting plane and around the pattern, while being less dense in the top layers, is jolting (option A). Jolting provides better compaction and hardness compared to squeezing, jolting and squeezing, and slinging.

Statement 1 is wrong because if the velocity is as high as possible the flow will become turbulent which results in sand erosion, respiration effects etc.

Mechanical properties of cast parts refer to the physical and mechanical characteristics of the materials used in the casting process. These properties determine the strength, durability, and performance of the cast parts. Different casting methods have varying effects on the mechanical properties of the cast parts. Among the given options, centrifugal casting is known to provide superior mechanical properties compared to the other casting methods.

1. Shell-Mould Casting:
- Shell-mould casting is a precision casting process that uses a shell made of a resin-bonded sand mixture to produce complex and accurate castings.
- The process involves creating a hollow shell by curing the sand mixture coated on a pattern, which is then removed to leave a cavity.
- Molten metal is poured into the shell, and after solidification, the shell is broken to retrieve the casting.
- While shell-mould casting allows for high dimensional accuracy and good surface finish, the mechanical properties of the cast parts may not be as superior as in centrifugal casting.

2. Investment Casting:
- Investment casting, also known as lost-wax casting, is a process that uses wax patterns to create molds for casting.
- The wax patterns are coated with a ceramic shell, which is then heated to remove the wax and harden the ceramic.
- Molten metal is poured into the ceramic shell, and after solidification, the shell is broken to retrieve the casting.
- Investment casting offers excellent dimensional accuracy and surface finish but may not provide the same level of mechanical properties as centrifugal casting.

3. Permanent Mould Casting:
- Permanent mould casting, also known as gravity die casting, is a process that uses a reusable metal mold to produce castings.
- Molten metal is poured into the metal mold, and after solidification, the mold is opened to retrieve the casting.
- Permanent mould casting can provide good dimensional accuracy and surface finish, but the mechanical properties may not be as superior as in centrifugal casting.

4. Centrifugal Casting:
- Centrifugal casting is a process that utilizes centrifugal force to distribute molten metal in a mold.
- The mold is rotated at high speed, causing the molten metal to be forced against the mold walls.
- This process ensures a dense and uniform structure, resulting in superior mechanical properties.
- The centrifugal force helps to eliminate gas and impurities, leading to improved strength and integrity of the cast parts.
- Centrifugal casting is particularly suitable for producing cylindrical and symmetrical parts, such as pipes, cylinders, and rings.

In conclusion, centrifugal casting is known to provide superior mechanical properties compared to shell-mould casting, investment casting, and permanent mould casting. The centrifugal force and uniform distribution of molten metal in the mold result in dense and strong cast parts with excellent strength, durability, and performance.

Metallastatic pressure on the mould = pgh

The correct reasons for the occurrence of hot tear in casting are:

i. Hindered contraction occurring immediately after the metal has solidified:
- When the metal solidifies, it undergoes thermal contraction. However, if there are restrictions in the mold or core, the contraction is hindered, leading to the development of internal stresses in the casting.
- These internal stresses can cause the casting to crack or tear during the cooling and solidification process.
- Hot tear defects are more likely to occur in areas where the metal is thickest, as the contraction is more pronounced in these regions.

ii. Poor collapsibility of the mold and core:
- Collapsibility refers to the ability of the mold and core to collapse or shrink away from the casting as it cools and solidifies.
- If the mold or core has poor collapsibility, it will not shrink away from the casting properly, leading to the development of internal stresses and potential hot tear defects.
- Poor collapsibility can be caused by factors such as improper design of the mold or core, inadequate venting, or insufficient mold or core material.

iii. Too high pouring temperature:
- Pouring temperature refers to the temperature at which the molten metal is poured into the mold.
- If the pouring temperature is too high, it can result in excessive thermal gradients and differential cooling within the casting, leading to the development of internal stresses and hot tear defects.
- High pouring temperatures can also cause the metal to solidify too quickly, reducing its ability to deform and accommodate the thermal contraction, further increasing the likelihood of hot tear formation.

In summary, the occurrence of hot tear defects in casting can be attributed to hindered contraction occurring immediately after metal solidification, poor collapsibility of the mold and core, and too high pouring temperature. These factors contribute to the development of internal stresses in the casting, which can lead to cracking or tearing during the cooling and solidification process.