Gate In Casting
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| Gate In Casting |
Gate In Casting Process
What is a gate in the casting process and its function?
The gate in casting is the opening through which molten metal enters the mould cavity in the casting process.
Gate is mostly located in the drag section of the mould and should be designed with a trapezoidal cross-section for molten metal to pass smoothly into the mould cavity.
The gate is the entry point of the mould cavity followed by the runner. There can be single or multiple gate channels connected to the mould cavity depending upon the requirement of the casting.
The gate should be placed at the heavy, thicker and isolated sections, non-critical parts of the casting reducing bending, distortion and uneven shrinkage of the final casting.
The main function of the gate is to fill the mould cavity completely maintaining a steady speed of molten metal entering the mould cavity as shown below in diagram (a).
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| (a) Gate |
The gate is an inlet to mould cavity connected to the common runner.
The main purpose of the gate in the casting process is to fill and deliver the molten metal in the mould cavity fully without any turbulence.
The best cross-section area for the gate is a trapezoid and rectangular cross-section.
The gate in the casting process is designed in such a way that it should solidify slowly and can be broken while taking the final casting out after the final casting has solidified.
The position of the gate in the mould depends upon the application, types of material used, the requirement of the casting process and the final casting product.
If the diameter of the gate is too small it will cause slag, oxides, segregation, inclusions and impurities to trap at the gate.
The gate in casting is a part of the gating system in casting process.
Types of gates in the casting process according to the position of the gate in the mould cavity have been discussed below with diagrams.
In Gate
Types Of Gate In Casting Process
What are the 4 types of gates in the casting process?
- Top Gate
- Bottom Gate
- Step Gate
- Parting Gate
Top Gate
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| (b) Top Gate |
Molten metal poured during the process enters the mould cavity directly from a particular height from the top as shown above in diagram (b).
Molten metal enters perpendicular to the parting line in the mould cavity.
Opening of the top gate starts from the cope side of the mould.
There are impurities, dirt and dross in molten metal that can directly enter the mould cavity.
In order to avoid this strainer made of dry sand or ceramic is placed at the start of the sprue to trap any unwanted substance from entering the mould cavity.
Metal poured using the top gate causes mould erosion and turbulence in the mould cavity.
This is why the top gate is not the preferred gate in the casting process as it can result in the dross entering the mould cavity causing casting defects.
Strainer cores and ceramic filters are used to trap dross and other substances from being part of the final sand casting.
The top gate is suggested only for ferrous metal compared to non-ferrous metal.
Oxides forming metals such as aluminium and magnesium produce a lot of dross and are not preferred with top gates.
Top gate finds its application in casting which is small but shallow in nature having a defined height.
Simple and easily designed casting such as thin flat plates is easily made from the top gate.
Mould when using the top gate must be strong enough to retain its strength after molten metal has entered the mould cavity.
The advantage of using a top gate is that mould fills very quickly maintaining the temperature gradient during the pouring operation.
To maintain the temperature gradient top itself serves as a riser but is not as effective as a conventional riser system.
In the bottom gate, molten metal enters the mould cavity directly without passing through runner in the gating system.
Pencil Gate
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| (c) Pencil Gate |
Pencil gates are used for iron casting.
Pencil gates are a series of gates that control the flow of molten metal and pressure head during the pouring operation in the casting process with multiple streams.
Shown above pencil gate in diagram (c).
Finger Gate
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| (d) Finger Gate |
Modification of wedge gate used to fill the mould cavity with controlled flow rate with multiple streams.
Shown above the finger gate in diagram (d).
Wedge Gate
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| (e) Wedge Gate |
Wedge gates are used for lightweight casting. The name wedge get comes from the wedge shape.
Shown above wedge gate in diagram (e).
Bottom Gate
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| (f) Bottom Gate |
The bottom gate in the casting process is exactly the opposite of the top gate.
Molten metal enters the mould cavity from the bottom of the mould when the bottom gate is used.
Shown above the bottom gate in casting in diagram (f).
The flow of molten metal in the mould cavity is smooth without turbulence eliminating erosion of moulding sand in the mould cavity.
Core gating systems are employed for the bottom gate.
The bottom gate has an unfavourable temperature gradient as compared to the top gate as molten metal is fed into the mould cavity from the bottom.
Molten metal entering the mould cavity will be of higher temperature as compared to molten metal in the riser which will lose its temperature.
In order to solve the temperature gradient problem in the bottom gate following methods are used along the riser:
- Chills are provided to improve directional solidification.
- Exothermic material is used at the top of the riser.
- Insulation of the riser maintains the temperature of molten metal.
- Use of radiation shield to reduce heat loss through radiation.
- Filling the riser last with hot metal to avoid a large temperature difference between the metal in the mould and the riser.
Opening of the bottom gate starts from the drag side of the mould.
The molten metal fills slowly and gently as compared to the top gate reducing erosion of moulding sand.
Control over temperature gradient is difficult to achieve without using proper padding, riser and chills during the sand casting process.
Molten metal enters the mould cavity from the bottom through the pouring basin, runner and sprue.
When a bottom gate is used riser cannot be placed near the entrance of the entry of molten metal.
Riser in the top gate is placed at the side of the mould for uniform solidification and should be of a bigger size to compensate for uneven temperature distribution.
Directional solidification is difficult to achieve using the bottom gate.
Horn Gate
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| (g) Horn Gate |
A Horn gate is a type of bottom gate. The name horn gate is used as the gate looks like a horn.
The Horn gate is designed in such a way that the gate opening is from the cope but the entry of molten metal is from drag.
Horn gate covers cope and drag pouring molten metal into the mould cavity giving a curve to the sprue and feature to trap dirt, slag and dross.
Shown above the horn gate in the diagram (g).
Ring Gate
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| (h) Ring Gate |
Ring gate uses core to control the flow of molten metal into the mould cavity retaining the slag.
Step Gate
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| (i) Step Gate In Casting |
A step gate is used in the sand casting process for producing heavy casting of large size.
A step gate is used when the molten metal requirement is large and metal needs to fill the mould cavity in steps using the gate.
Shown above step gate in the casting process in diagram (i).
The molten metal fills the mould cavity steadily without any erosion like in top gate.
Step gates used in the casting process do not have exact openings in the mould cavity.
Gates in casting are in steps. The gate closest to the parting line has a smaller opening while the gate far away from the parting line has a wider progressive opening.
The step gate is placed from the parting line to the drag side of the mould.
In order to have an equal flow of molten metal in the mould cavity multiple in-gates cross-sections of the runner are decreased after each passing in-gate.
After each passing in-gates velocity and pressure of the molten metal in the channel is equalized. In order to achieve this runner is made trapezoidal in shape with passing length.
Step gate in sand casting is selected based on the following considerations:
- Casing size.
- Casting shape.
- Casting material.
- The complexity of the casting.
- Pressure and velocity of molten metal at the gate.
Parting Gate
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| (j) Parting Gate In Casting |
After learning the top gate, bottom gate and step let us learn the parting gate in detail.
This is the most common, simple in construction, economical gate in the casting process.
The position of the parting gate is horizontal in the mould along the parting line.
This allows the parting gate to easily accommodate the trapping of dirt, slag, sand and dross.
The parting gate is best suited for casting that is divided along the parting line in the cope and drag section of the mould.
As shown below parting gate at the entry of the mould cavity in diagram (j) above.
Dross, slag, dirt, oxides and unwanted substances which are light in weight float on molten metal.
Parting gates can easily accommodate slag, dross and dirt-trapping systems as molten metal travels through a runner.
The advantage of using a parting gate is hottest metal reaches the mould cavity and then the riser provides continuous feeding of molten metal to solidifying casting.
If the drag side of the mould is larger parting gate will cause air entrapment, sand erosion and turbulence in the sand-casting process for non-ferrous alloys.
Parting gate benefits both advantages of the top and bottom gate in the sand casting process.
There will be an erosion of sand due to turbulence if the depth of the mould cavity is higher after using a bottom gate.
The parting gate is only suitable for casting which is not shallow.
Skim Gate
A skimming gate is a parting gate that rests in the mould along the parting line.
Molten metal flowing through the skimming gate and whirlpool gate traps molten meal and removes slag and dross.
Gate with shrink bob serves as additional slag and dirt-trapping element in the casting process.
Swirl Gate
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| (k) Swirl Gate |
Molten metal is swirled using a runner and feeder to remove impurities from molten metal. The swirl gate is a part of the parting gate.
Whirl Gate
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| (l) Whirl Gate |
Molten metal is whirled centrifugally with centrifugal force forcing dirt and slag to the centre of the riser while letting molten metal pass to the mould cavity.
The whirl gate prevents dirt, molten metal and slag from entering the mould cavity.
Whirl gate is an effective slag-trapping system in the casting process.
Branch Gate
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| (m) Branch Gate |
Branch gates are multiple gates connected to a common runner used to feed mould cavities. The name branch gate is given to the gate as it looks like a branch as shown above in the diagram (m).
Gating Ratio
What is the gating ratio in the casting process and why it is provided?
The gating ratio is the relation between the area of the choke (sprue) to the total area of the runner and gate.
Molten metal flowing through the gate is the function of the cross-section area of the gate, runner and sprue.
The gating ratio is a ratio between gating elements such as sprue, runner and gate.
The gating ratio reveals if the total cross section increases or decreases as the molten metal moves through the mould cavity. This helps to design gating systems into pressurised and non-pressurized gating systems.
According to the gating ratio, gates are divided into two ways.
- Pressurized gating system.
- Non-pressurized gating system.
The ideal gating ratio in casting is 1:3:3. 1:3:3 represents the following:
Runner cross-section area = 3㎠
Gate cross-section area = 3㎠
Types Of Gating Systems
How many types of gating systems there are in the casting process?
There are two types of gating systems in the casting process.
- Pressurized gating system.
- Non-pressurized gating system.
I have explained both types of gating systems below.
Pressurized Gating System
In the pressurized gating system the system becomes pressurized and the gate is served as a choke. The entire gating system is pressurized here having a restriction in the flow of the molten metal in the mould cavity.
The pressurized gating system is also called a chocked system or gate control system and is suitable for brass, magnesium, aluminium, aluminium alloy and steel.
In a pressurized gating system cross-section of the sprue and runner is larger than the gate. This will cause a positive pressurised head towards the gate.
This gating system has a higher casting yield as molten metal consumed is less.
Oxidation and aspiration are reduced in a pressurized gating system as the sprue is always full but turbulence remains high at the entry of the molten metal.
Slag, dross, inclusions and unwanted substances in molten metal are difficult to trap as the pressure in the system is high.
Pressurized gating systems are suitable for ferrous metals mostly.
Gating Ratio For Pressurized Gating System
The gating ratio is: Sprue area: Runner area: Ingate area
Typical gating ratios for pressurised gating systems are 1:2:1 and 2:1:1.
1:2:1 means the cross-section area of a runner is larger than the ingate.
2:1:1 means the cross-section area of the in-gate is smaller than the runner and sprue.
The typical gating ratio for a pressurized gating system is 1:0.75:0.50.
More examples of a pressurized gating system are 3:9:1, 2:2:1 or 4:4:2, 2:8:1, 1.6:1:3:1 and 1:2:1.
A pressurized gating system is best suitable for ferrous metal, ductile iron, copper and brass.
Metal flowing in the mould cavity with her velocity will cause mould to erode the moulding sand and cause casting defects in the final casting.
Non-Pressurized Gating System
The non-pressurized gating system will cause molten metal to enter the mould cavity with less velocity leading to less turbulence and sand erosion in the mould.
The non-pressurized gating system is also called a free system or choke control system as it delivers molten metal in the mould cavity steadily and uniformly.
A non-pressurized gating system is suitable for non-ferrous metals such as bronze, aluminium, magnesium and light alloys.
Non-pressurized gating systems have lower yields as more molten metal is involved in the runner and gates.
Slag, dross, dirt, inclusions and unwanted substances in molten metal are easy to trap in the trapping system as the pressure in the system is low.
Non-pressurized gating systems are more prone to impurities and casting defects such as porosity.
The disadvantage of this gating system is that it will allow aspiration resulting in porosity and gas defects in the final casting as the gating system never remains full.
Gating Ratio For Non-Pressurized Gating System
The gating ratio for non-pressurized gating systems: Sprue area: Runner area: Ingate area
The ideal gating ratios for non-pressurized gating systems are 1:2:1, 1:2:3, 1:2:2 or 1:4:4, 1:1.2:2, 1:2:4, 1:1:3, 1:6:6 and 1:3:3.
Gate Design
Pouring Time
Pouring time can be defined as the time required to fill the mould cavity fully.
Molten metal should fill the mould cavity in the shortest time possible.
If the pouring time of molten metal is longer, the temperature of the molten during the pouring operation should be kept higher to maintain the temperature gradient.
If the pouring time is kept very less it will cause turbulence as more molten metal will be discharged in the mould in a shorter time.
Therefore, it becomes important that molten metal is poured at the optimum time during the casting process.
Pouring time is an important variable in determining the pouring rate of molten metal into the pouring basin.
If the pouring of molten metal is inappropriate it can cause mould erosion, casting defects, shrinkage in casting, defects such as cold shunt and misrun, metal penetration and mould drop.
Pouring time depends upon the following factors during the pouring operation:
- Solidification temperature of molten metal.
- Pouring temperature of molten metal.
- Size, shape and complexity and design of casting.
- The material used for casting.
- The sectional thickness of the casting.
- The rate at which molten metal loses heat.
Grey Cast Iron Pouring Time
The pouring time of grey cast iron mass of less than 450 kg can be calculated by the following formula:
`t=K(1.41+\frac{T}{14.59})\sqrt{W}`
'K=\frac{Fluidty of iron in inches}{40}`
K = Fluidty factor.
T = Average sectional thickness in mm
W = Weight of casting in kg
The pouring time of grey cast iron mass of greater than 450 kg can be calculated by the following formula:
`t=K(1.236+\frac{T}{16.65})\sqrt[3]{W}`
T = Average sectional thickness in mm.
W = Weight of casting in kg.
Steel Casting Pouring Time
The pouring time of the steel formula is as follows:
`t=(2.4335-0.3953\log W)\sqrt{W}`
Ductime Iron Pourtime Time
`t=K_{1}\sqrt{W}`
`K_{1}` = 2.67 for thinner sections of 10 to 25 mm thickness.
`K_{1}` = 2.97 for heavy sectional thickness.
Copper Alloy Casting Pourting Time
`t=K_{2}\sqrt[3]{W}`
`K_{2}`= 1.8 for a bottom gate.
`K_{2}`= 2.8 tin bronze.
`K_{2}`= 2.0 for brass.
Casting Above 450 kg and Thin Section Casting Pouring Time
`t=K_{3}\sqrt[3]{W^{`}}`
Steel Casting
t = `(2.4335-0.3953\log W)\sqrt{W}`
Shell Moulded Ductile Iron
t = `K_{1}\sqrt{W}`
`K_{1}` = 2.67 for the section between 10 mm to 30 mm.
`K_{1}`= 2.97 for heavy section.
Choke Area
When molten metal passes through the pouring basin metal moves towards the choke with high velocity.
The choke is located at the bottom of the sprue and is important to control the flow and velocity of molten metal in this area called the choke area.
The choke area is calculated by the Bernouilles equation.
The formula to calculate the choke area is as follows:
`A=\frac{W}{dtC\sqrt{2gH}}`
W= Weight of casting in kg.
g = Gravitational acceleration in `\frac{mm}{s^{2}}`.
t= Pouring time in seconds.
d= Molten metal mass density in kg/`mm^{3}`.
H= Sprue height or effective metal head in mm.
C= Constant Eiffency factor which is the function of the gating system.
H is sprue height or effective metal head in the casting process.
Sprue height (H) is calculated by the following formula:
H = h (Top Gate)
`H=h-\frac{c}{2}` (Bottom Gate)
`H=h-\frac{P^{2}}{2c}` (Parting Gate)
p = Height of mould cavity in cope in mm.
C = Total height of mould cavity in mm.
The efficiency of the gating system will depend upon various elements of the gating system.
Elements of the gating system should be circular in cross-section to improve efficiency and reduce friction during the flow of molten metal.
The location of the choke is in the sprue but at times it is preferred to have the choke in the runner.
In-Gate Design
In gate is the entry point of molten metal in the mould cavity.
For small casting, single in-gate is preferred but for larger casting and complex casting, multiple in-gates are provided as the quantity of molten metal making the casting is more.
In case of multiple in-gates care should be taken to see if molten metal reaching the mould cavity is uniform, and equally distributed without any turbulence.
Flow in in-gate is uniform when the runner area reduces progressively.
Width to the dept ration if in the gate is kept around 4.
The gate is designed with a smaller diameter for easy separation of the gating system from the casting.
Factors while considering in-gate design.
- In-gates should be not close to the core, pins and inserts.
- The location of in-gates should be at the longitudinal axis of the mould wall.
- The cross-section area of the in-gate should be smaller than the smallest cross-section of the final casting.
- In-gates should be at a distance from coreprint in casting.
Factor While Selecting & Designing Of Gate In Casting Process
- Single gate or multiple gates are used in the casting process. If multiple gates are used it will promote better metallurgical structure for metals having lower pouring temperatures and for metals that solidify faster. Injection moulding and die casting uses multiple gates for producing casting.
- Location of the gate in the mould. Improper location will increase sand erosion, hotspots, and casting defects in the mould cavity resulting in defective casting.
- The velocity of the molten metal through the runner before it enters the mould cavity through the gate. Metal entering into the mould cavity should have less turbulence for sand casting.
- Location of the gate with respect to the core reducing obstruction to the flow of molten metal in the mould cavity.
- Distance between the sprue and the gate.
- Uniform feed rate and solidification of the casting.
- The gate and ingate should be located near thick sections of the casting.
- The gate should not have any sharp edges and turns.
- Size of the casting to be produced. Larger-size casting will require multiple in-gates.
- The shape of the casting to be produced. For example, round-size casting will require a central gating system.
- Proper gate types should be selected in the casting process such as pin gate, submarine gate, valve gate, sub gate, cashew gate, tab gate, fan gate, spoke gate, edge gate and automatic trim gate for the injection moulding process.
- Pouring time and rate of pouring molten metal.
- The thickness of the casting section at the in-gate.
- Fettling of the gate from the final casting after solidification.
- Air aspiration during the metal flow should be minimum.
- Position of the gate with respect to the parting line.
- Length and opening of the gate.
- Deformation and shrinkage of the casting.
- Pressure control of the molten metal through the gate.