Design of Gating System MCQ Quiz - Objective Question with Answer for Design of Gating System - Download Free PDF

Concept:

The volume flow rate of metal in a sand casting process can be determined using Torricelli’s theorem, which states that the velocity of fluid flow under gravity is:

$v=\sqrt{2gh}$

The volume flow rate Q is given by:

$Q=Av$

where:

• A = Cross-sectional area of the sprue
• v = Velocity of the molten metal

Given:

• Diameter of sprue base: d = 10 mm
• Height of sprue: h = 200 mm
• Acceleration due to gravity: g = 10 m/s²
• π = 3.14

Calculation:

Step 1: Compute velocity using Torricelli’s theorem

$v=\sqrt{2gh}=\sqrt{2\times 10\times 0.2}$

$v=\sqrt{4}=2m/s$

Step 2: Compute cross-sectional area of the sprue

$A=\frac{\pi d^2}{4}=\frac{3.14\times (10\times {10}^{-3}{)}^2}{4}$

$A=\frac{3.14\times 100\times {10}^{-6}}{4}=7.85\times {10}^{-6}{m}^2$

Step 3: Compute volume flow rate

$Q=Av=(7.85\times {10}^{-6})\times 2$

$Q=15.7\times {10}^{-6}{m}^3/s$

Converting to mm³/s:

$Q=15700m{m}^3/s$

Explanation:

Gating System Components in Metal Casting

• In metal casting, the gating system is a crucial part of the mold design that controls how molten metal flows into the mold cavity. The gating system includes several components, such as the pouring basin, sprue, runner, and gate-on, each serving a specific function to ensure the proper filling of the mold.

Working Principle:

• The gating system guides the molten metal from the ladle to the mold cavity.
• The design of the gating system significantly affects the quality of the final cast product by controlling the flow rate, minimizing turbulence, and preventing defects such as air entrapment, slag inclusions, and cold shuts.

Components:

• Pouring Basin: This is the initial reservoir where molten metal is poured. It allows for a smooth and controlled flow of metal into the sprue, reducing turbulence and preventing air entrapment.
• Sprue: The sprue is a vertical channel that directs the molten metal from the pouring basin to the runner. It prevents the formation of vortexes that can trap air and inclusions.
• Runner: The runner is a horizontal channel that distributes the molten metal from the sprue to various gates. It helps in maintaining a consistent flow rate and minimizing turbulence.
• Gate-on: The gate-on (or simply gate) is the entry point through which the molten metal enters the mold cavity. Proper design of the gate ensures even filling of the mold and reduces the chances of defects. It controls the rate of entry of metal into the mold.

Explanation:

Casting Pattern Procedure:

• The pattern is a replica of the object to be cast, used to form the cavity in the sand.
• It is a crucial element in the casting process as it determines the shape and size of the final cast product.

Let us understand the role of a pattern in the casting process:

Purpose of a Pattern:

• The primary purpose of a pattern is to create the mould cavity where the molten metal will be poured.
• The pattern is placed in the sand to create an impression, which forms the mould cavity.
• Once the pattern is removed, the cavity retains the shape of the pattern, ready to be filled with molten metal.

Additional Information

There are different types of patterns used in casting, such as:

• Single-piece pattern: Used for simple shapes.
• Split pattern: Used for complex shapes, split into two or more parts.
• Match plate pattern: Both halves of the pattern are mounted on a plate.
• Cope and drag pattern: Similar to split pattern but specifically for cope (top half) and drag (bottom half) parts of the mould.

Concept:

For calculating Time (t) to fill the mould cavity we must know the discharge (Q) through the gate, as the amount of molten metal discharged through gate falls into the mould cavity.

Qgate = Agate × Vgate

where, 

Qgate = Discharge from the gate.

Agate = Area of gate

$V_{gate}=Velocityatthegate=\sqrt{2g{h}_{Sprue}}$

Qgate × t = Volume of Mould

$\therefore t=\frac{Volumeofmouldcavity}{A_g\sqrt{2g{h}_{sprue}}}$

Calculation:

Given:

hsprue = 45 cm,  Asprue = 5 cm2, Volume of mould = 3000 cm3, g = 10 m/s2 ⇒ 1000 cm/s2

$\therefore t=\frac{Volumeofmouldcavity}{A_g\sqrt{2g{h}_{sprue}}}$

$\therefore t={\displaystyle \frac{3000}{5\times \sqrt{2\times 1000\times 45}}}$

Explanation:

Skim bob: 

• It is an enlarged section at some place along the runner. This helps in trapping impurities such as dross of eroded sand and thus prevents them from going into the mould cavity. Both light and heavy impurities are trapped by the skim bob.

Sprue: 

• The passage through which the molten metal from the pouring basin reaches the mould cavity. In many cases, it controls the flow of metal into the mould.

Pouring Basin:

• Pouring Basin acts as are reservoir from which molten metal moves smoothly into the sprue. The pouring basin stops the debris from going into the main casting by means of a skimmer or skim bob.

Pouring Basin →

→ Pouring Basin act as are reservoir from which molten metal moves smoothly into the sprue.

→ Pouring basin stop slag from entering the mould cavity by means of a skimmer or skim bob.

→ The depth of the pouring basin is 2.5 times the sprue entrance diameter for smooth metal flow and to prevent vortex formation:

Runner:

→ It is generally located in a horizontal plane (parting plane) which connects the sprue to its ingates, thus, allowing the metal to enter the mould cavity.

Ingate (Gate):

→ These are the opening through which molten metal enters the mould cavity.

→ The shape and cross-section of the ingate should be such that it can readily be broken off after casting solidification and also to allow the metal to enter quickly into other mould cavities.

Choke:

Also called sprue base well.

→ Choke is that part of the gating system which has the smallest cross-sectional area and helps in controlling the rate of metal flow, thus ensuring lower metal flow velocity in runners and minimizing sand erosion in runners.

Concept:

The mould filling time is given by:

$t=\frac{Volumeofcasting}{Volumeflowrate}=\frac{V}{A\times \sqrt{2gh}}$

Now, h is the pressure head and can be written as h = P/ρg 

Thus:

$t=\frac{V}{A\times \sqrt{2gh}}=\frac{V\sqrt{\rho }}{A\times \sqrt{2P}}\Rightarrow t\propto \sqrt{\frac{\rho }{P}}$

Calculation:

Given:

ρ₁ = 1000 kg/m3, ρ₂ = 2000 kg/m3, P₁ = 200 bar, P₂ = 400 bar, t₁ = 0.05 sec, t₂ = ?

$\frac{t_2}{t_1}=\sqrt{\frac{{\rho }_2}{P_2}\times \frac{P_1}{{\rho }_1}}=\sqrt{\frac{2000}{1000}\times \frac{200}{400}}=1$

Explanation:

Chills:

Chills and padding are used to provide uni-directional solidification.

Chills are metallic objects, which are placed in the mould to increase the cooling rate of castings. Tapering of thinner section towards thicker section is known as 'padding'. This will require extra material. If padding is not provided, centre line shrinkage or porosity will result in the thinner section.

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 prints are the projections on the pattern and recess inside the mould cavity to position the core properly.
• 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.

Chaplets: 

• Chaplets are used to support the core inside the mould cavity. 
• Chaplet also takes care of its own weight and overcomes the metallostatic forces.

Concept:

To avoid aspiration effect

$\frac{{\mathbf{A}}_1}{{\mathbf{A}}_2}=\frac{{\mathbf{V}}_2}{{\mathbf{V}}_1}=\mathbf{R}=\sqrt{\frac{{\mathbf{h}}_{\mathbf{t}}-{\mathbf{h}}_s}{{\mathbf{h}}_{\mathbf{t}}}}$

where ht = = Total height = hs + hb, hs = sprue height, hb = basin height

Calculation:

Given:

h2 = 1500 mm, hb = 100 mm

total height, h_t =  1500 + 100 = 1600 mm

we know that

$\frac{{\mathbf{A}}_1}{{\mathbf{A}}_2}=\frac{{\mathbf{V}}_2}{{\mathbf{V}}_1}=\mathbf{R}=\sqrt{\frac{{\mathbf{h}}_{\mathbf{t}}-{\mathbf{h}}_s}{{\mathbf{h}}_{\mathbf{t}}}}$

${\left(\frac{{\mathbf{d}}_1}{d_2}\right)}^2=\sqrt{\frac{1600-1500}{1600}}=\sqrt{\frac{100}{1600}}=\frac{10}{40}$

∴ d₁: d₂ = 1 : 2

In top gating system:

$t_f=\frac{Mouldvolume\left(V_m\right)}{Moltenmetalflowrate\left(A_c{V}_c\right)}$

As gating ratio is given:

A_choke : A_runner : A_ingate = 1 : 1.5 : 2

Given A_ingate = 400 sq-mm.

Thus, $A_{choke}=\frac{400}{2}=200sq-mm$

Choke area will be considered for molten metal flow rate because choke regulates the rate of pouring of molten metal.

$\begin{array}{l}{V}_c=\sqrt{2g{h}_t}\\ =\sqrt{2\times 9810\times 250}\end{array}$

= 2214.7234 mm/s

$\begin{array}{l}{t}_f=\frac{\frac{\pi }{4}{d}^{2}h}{200\times 2214.7234}\\ =\frac{\frac{\pi }{4}\times {150}^2\times 200}{200\times 2214.7234}=7.979sec\end{array}$

Explanation:

The volume of metal compensated by rises does not constitute cooling during the solid state. Riser can compensate for volume shrinkage only in liquid stage and transition stage.

Explanation:

Ladle:

• Ladle is used to feed molten metal to casting set-up.
• Types of ladles:

​Hand Ladle:

• It is a bucket with removable, lever arm and handle shank. It is used when the quantity of molten metal is small. It can be carried by a single person. Its carrying capacity varies from 10 to 20 kg.

Shank or Bull Ladle:

• A shank or bull ladle is carried by two persons and used for medium capacity of molten metal. Its carrying capacity varies from 30 to 150 kg.

Tea Pot Ladle:

• Tea pot ladle is used for small and medium-sized mould. Tea pot ladle allows the molten metal to be taken out from the bottom opening provided. The bottom opening is advantageous as it does not disturb the slag floats on top.

Bottom-Poured Ladle:

• Bottom poured ladle is used for top-run or direct-pour into the mould. The molten metal is poured through the bottom hole, which is operated by a graphite stopper and lever. Slag, being lighter, floats at the top of the molten metal and pure metal is poured into the mould. Therefore, it is also known as self-cleaning ladle.

Monorail or Trolley Ladle:

• The molten metal is carried in a trolley. The trolley is mounted on the mono­rail for easy movement to the pouring site. The molten metal is poured through a lever provided with crucible. A hand wheel is also provided for raising and lowering the crucible

​​

Concept:

To avoid aspiration in a gating system, the diameter of the downsprue at its bottom should be calculated based on Bernoulli’s equation and the principle of continuity. The flow of molten metal must be such that the pressure at the bottom of the sprue remains atmospheric to prevent air entrainment.

Calculation: 

Given:

Length of downsprue = 180 mm, Diameter at top = 20 mm, Height of liquid metal in pouring cup = 60 mm

Total head (H) = 180 + 60 = 240 mm = 0.24 m

Using Bernoulli’s equation:

$v=\sqrt{2gH}=\sqrt{2\times 9.81\times 0.24}=\sqrt{4.7088}\approx 2.17\text{m/s}$

Assuming atmospheric pressure at both ends and negligible velocity at the top due to larger cross-section area:

Apply continuity equation: $A_2{v}_2=A_3{v}_3$

Let D₂ = 20 mm, and assume v₂ negligible, then use ratio of areas:

$D_3=D_2\times \sqrt{\frac{v_2}{v_3}}=20\times \sqrt{\frac{1.085}{2.17}}=20\times \sqrt{0.5}=20\times 0.707\approx 14.14\text{mm}$

Explanation:

Gating ratio:

The term gating ratio is used to describe the relative cross-sectional areas of the components of gating system.

It is defined as the ratio of the sprue area (A_s) to the total runner area (A_r) to the total gate area (A_g).

i.e. Gating ratio a : b : c = Sprue area : Runner area : Gate area.

Gating ratio is grouped in two classes i.e. pressurised and unpressurised gating system.

Additional Information

Pressurised gating system:

• The proportion of sprue, runner and gate area are so arranged that back-pressure is maintained in the gating system.
• This requires that the total gate area is not greater than the area of the sprue. Eg. 1 : 0.75 : 0.5, 1 : 2 : 1,  2 : 1 : 1.
• A pressurised gating system keeps itself full of metal.
• Air aspiration is minimised.
• Smaller loss of metal and greater yield.
• High metal velocities may cause turbulence at the junctions and corners in mould cavity.
• Method suitable for ferrous material and brass. 

Un-pressurised gating system:

• The unpressurised gating system produces lower metal velocities and permits greater flow rates. 
• Requires careful design to ensure complete filling, and large-sized runners and gates, which reduce the yield and increase the wastage of metal.
• this system is generally adopted for metal such as Aluminium and Magnesium. The ratio used are 1 : 2 : 2, 1 : 3 : 3.

Explanation:

Gating System: 

• It refers to the channels through which the molten metal flows to the die cavity.
• Its key objective is to ensure its smooth and complete flow from the ladle to the mould cavity.
• To achieve perfect castings, it is important to have a gating system that is well designed.

Important elements related to the Gating system are:

Cope: Top half of the mould.

Drag: Bottom part of the mould

Mould Cavity: Gap created in mould using pattern

Pouring Cup: From here metal is poured

Sprue: Pipe shaped. From the pouring basin, the metal flows into the sprue.

Runner: The horizontal hollow channels that connect the bottom of the sprue to the mould cavity.

Ingate: The region where runner joins with the cavity

Riser: Excess metal poured into the mould flows into these cavities. They act as reservoirs, as the metal solidifies inside the cavity, it shrinks, and the extra metal from the riser flows back down to avoid holes.

Hence, A and B in question are Riser and Runner respectively.

Additional Information

Gating ratio =  a : b : c

Where, a = cross-sectional area of sprue; b = total cross-sectional area of runner;  c = cross-sectional area of ingates.