Metal Casting MCQ Quiz in తెలుగు - Objective Question with Answer for Metal Casting - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Concept:

Solidification time(τ):

Solidification time for molten metal in mold is given by 

τ = k₁M² , k₁ = mold constant or solidification factor, M = Modulus 

M= \(\frac{{Volume\;of\;casting}}{{Surface\;area}}\;\)

For sphere, M = \(\frac{D}{6}\)

\(∴ τ = {k_1}{\left( {\frac{D}{6}} \right)^2}\)

Calculation:

Given:

τ₁ = 10 min, V₂ = 8V₁, τ₂ =?

Now, we know that

 V2 = 8V1 ⇒ (D₂)³ = 8 (D₁)³⇒ D₂= 2D₁

From the above equation,

 \( τ = {k_1}{\left( {\frac{D}{6}\;} \right)^2}\)

\(​\frac{{{τ _2}}}{{{τ _1}}} = {\left( {\frac{{{D_2}}}{{{D_1}}}} \right)^2}\)

\(\frac{{{τ _2}}}{{10}} = {\left( 2 \right)^2}\)

∴ τ₂ = 40 min

Concept:

Solidification time:

It is the time required for a casting to solidify after pouring. This time is dependent on the size and shape of the casting.

There is an empirical relationship known as Chvorinov’s rule –

\({{\rm{t}}_{\rm{s}}} = {\rm{K }}{\left( {\frac{{\rm{V}}}{{{{\rm{A}}_{\rm{s}}}}}} \right)^2}\)

where ts = solidification time of casting (min), V = volume of casting (mm3), As = surface area of casting (mm2) and K = solidification factor (min/mm2).

i.e it indicates that casting with a higher volume-to-surface area ratio will cool and solidify more slowly than one with a lower ratio.

Calculation:

Given:

Volume of cube = l³, Volume of sphere = \(\frac{4}{3}\pi r^3\),

Given that the volume is equal

\(\therefore l^3=\frac{4}{3}\pi r^3\)

\(\therefore \frac{l}{r}=\left(\frac{4\pi}{3}\right)^{\frac{1}{3}}\)

\(({{{t}}_{{s}}}) = {{K }}{\left( {\frac{{{V}}}{{{{{A}}_{{s}}}}}} \right)^2}\)

\(\frac{({{{t}}_{{s}}})_{cube} }{({{{t}}_{{s}}})_{sphere} }= \left(\frac{A_{sphere}}{A_{cube}}\right)^2\)

\(\frac{({{{t}}_{{s}}})_{cube} }{({{{t}}_{{s}}})_{sphere} }= \left(\frac{4\pi r^2}{6l^2}\right)^2\)

\(\frac{({{{t}}_{{s}}})_{cube} }{({{{t}}_{{s}}})_{sphere} }= \left(\frac{4\pi }{6}\right)^2\times\left(\frac{ r}{l}\right)^4\)

Explanation:

Solidification of casting:

• In casting the starting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the material.
• Solidification is the transformation of the molten metal back into the solid-state (differs depending on composition and purity). pure metals freeze at a constant temperature while alloys, except for the eutectic compositions freeze over a temperature range.
• Whether the casting is pure metal or alloy, solidification takes time.
• The total solidification time is the time required for casting to solidify after pouring.
• Solidification time depends upon the size and shape of the casting.
• Cooling characteristic of the casting is the ratio of surface area to volume. 
• The volume of the casting represents the heat contained within the casting and surface area represents the heat loss from the casting. Thus, Higher the value of cooling characteristic faster will be the cooling of the casting.

Chvorinov's rule states that the solidification time is inversely proportional to the cooling characteristic of the casting.

Solidification time according to Chvorinov’s rule:

\({T_s} = K{\left( {\frac{V}{SA}} \right)^2}\)

where K = Solidification factor, Ts = total solidification time, (minute), V = volume of the casting (cm³) A = surface area of the casting, (cm²)

Explanation:

Casting defects are those characteristics that generate imperfection in a casting exceeding the quality limits imposed by the design or service condition of the casting. Some of them are mentioned below along with their cause.

Explanation:
Pattern:

• Pattern is the replica of final casting to be produced with some modification. 
• Modification are done in the form of allowances.

Pattern Colour Code:

The pattern are normally painted with contrasting colors such that the mould maker would be able to understand the functions clearly.

The colour code used is as follows:

Explanation:

Core: 

• Cores are used to get the internal cavity in the castings.
• Cores are placed into the mould after the removal of the casting. These are made up of sand are used in permanent moulds also.
• The cores are surrounded by molten metal and therefore subjected to severe thermal and mechanical conditions.
• Core sand should be of higher strength than the moulding sand.

Core prints: 

• Core print is an open space provided in the mould for locating, positioning and supporting the core.
• As we know the density of core (made of sand) is less than the density of metal being poured in the cavity.
• So there will be an upward buoyancy force on the core.
• To overcome this defect the core prints are used.
• This force and hence core print design will depend on the moulding sand characteristics.

Explanation:

Shrinkage Allowance: 

• During the cooling of the material in the casting process, the molten metal tries to shrink in size because of the increase in density. The reduction in the volume of cast metal occurs during the Cooling of liquid metal to solid-state.
• Liquid shrinkages are always compensated by providing a riser in the casting process. Solid shrinkage is compensated by the giving shrinkage allowance. It is expressed as mm/m.

​Additional Information

Shrinkage values for different metal

• Bismuth – Negligible 
• White metal (Tin) – 5 mm/m
• Cast Iron – 10 mm/m
• Aluminum – 13 mm/m
• Brass – 15 mm/m
• Copper – 17 mm/m
• Steel – 20 mm/m
• Zinc – 24 mm/m

Explanation:

Investment casting process:

• The investment casting process also called the lost-wax process begins with the production of wax replicas or patterns of the desired shape of the castings
• It is preferred where intricate shapes are required like sewing machines, firearms, surgical and dental devices, turbine blades, gear impeller, hand tools, cranks, and levers

The investment casting process or lost wax process has the following steps:

1. Produce expendable wax, plastic, or polystyrene patterns.
2. Assemble these patterns onto a gating system
3. Investing or covering the pattern assembly with a refractory slurry
4. Melting the pattern assembly to remove the pattern material
5. Firing the mould to remove the last traces of the pattern material
6. Pouring molten metal
7. Knockout, cutoff, and finishing

As in this casting, the pattern disappears so this is also known as lost pattern casting.

Explanation:

During solidification of pure molten metal, the grains in the casting near the mould wall are fine and randomly oriented because the rate of solidification is high at the surface of mould during solidification.

Explanation:

Casting: It is a manufacturing process in which liquid material is filled into the mould, which contains a hollow cavity of the desired shape and then allowed to solidify. The solidified part is known as a casting, which injected or broken out of the mould to complete the process.

Investment casting:

• The investment casting process also called the lost-wax process begins with the production of wax replicas or patterns of the desired shape of the castings
• It is preferred where intricate shapes are required like jewellery, sewing machines, firearms, surgical and dental devices, turbine blades, gear impeller, hand tools, cranks, and levers.

Additional Information

Sand casting:

Steps involved in sand casting are:

• Make the pattern. The material of the pattern can be wood metal or plastic.
• With the help of a pattern prepare the mould and necessary cores.
• Melt the metal or alloy to be cast.
• Pour the molten metal or alloy into the mould cavity.
• Allow the molten metal to cool and solidify.
• Remove the casting from the mould. This operation is called a shakeout.
• Remove the defects if any and if possible.

Product application includes metallic components, taps, tools, and many more.

Centrifugal casting:

• This is a process where the mold is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, continuous pressure will be acting on the metal as it solidifies.
• The slag, oxides, and other inclusions being lighter gets separated from the metal and segregates toward the center.
• There are three main types of centrifugal casting processes. They are,
  • True centrifugal casting,
  • Semi-centrifugal casting, and  
  • Centrifuging. 

Die casting:

• Die casting is a moulding process in which the molten metal is injected under high pressure and velocity into a split mould die.
• It is also called pressure die casting.
• The split mould used under this type of casting is reusable.

Some parts produced by die-casting even do not require machining after casting or may require only light machining to achieve the desired dimensions.