Sprue In Casting
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| Sprue |
Sprue In Casting Process
What is sprue and function of the sprue in the casting process?
Sprue is like a central vertical channel that connects the pouring basin and runner in the mould as shown below sprue in the casting diagram (a)
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| (a) Sprue In Casting |
The main function of the vertical sprue is to reduce the air aspiration of the molten metal passing at high velocity through the pouring basin towards the runner. Molten metal before entering the runner passes through the tapered large diameter vertical channel called sprue.
The primary function of the sprue in casting mould is to feed molten metal from the pouring basin to the gate via a runner.
Sprue in casting refers to the vertical channel that connects the pouring basin and runner together.
Cross-section areas of the sprue are shaped square, rectangular or circular depending upon the casting requirement.
If a larger sprue is to be made, a rectangular cross-section area is preferred and for a small-size sprue, a round cross-section area up to 20mm diameter is preferred.
Round cross-section area sprue has the lowest surface exposed to cooling and provides proper flow of molten metal throughout the vertical sprue towards the choke.
Rectangular cross-section sprue has higher aspiration due to negative pressure included in the sprue when molten metal flows from the sprue towards the choke.
When molten metal is poured in the pouring basin molten metal travels vertically at a higher velocity towards the base of the sprue.
Sprue has a larger diameter at the top and the diameter starts reducing towards the bottom.
Part of the gating system that regulates the pouring rate of molten metal at the end of the sprue in the casting process is called a choke.
The choke is the smallest section with the smallest diameter at the end of the sprue.
This molten metal flowing at higher velocity comes in contact with a splash core made of ceramic or baked sand as shown below in diagram (b).
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| (b) Splash Core In Casting |
The velocity at which molten metal travels in the sprue can be calculated with the formula `V^{2}`=2gh. In the velocity of molten metal formula, V= Velocity at which molten metal travels in the sprue. h=height of the sprue and g=gravitational force on the molten metal.
As seen from the formula as the height (h) of the sprue increases the velocity of molten metal increases. I have discussed this in detail in the sprue design topic below.
This splash core reduces the erosion of the mould when the molten metal comes in contact with the base of the sprue.
Once molten metal passes through the sprue it flows towards the runner. Molten metal enters the gate through the runner and goes into the mould cavity to produce the casting.
Sprue in the casting process is an element of the gating system in sand casting.
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Types Of Sprue In Casting
Sprue can be classified based on the position of the sprue (primary sprue, down sprue and side sprue) in the mould and the taper or non-taper sprue (straight).
Sprue based on position in the mould can be of three types.
- The Primary Sprue
- The Down Sprue
- Side Sprue
Primary Sprue
The primary sprue is vertical sprue which has a pouring basin at the top and molten metal travels vertically towards the base well with gravitational force.
The primary sprue is also called top sprue as molten metal travels from the top towards the bottom.
Primary sprue is also called top sprue.
Molten metal travels with high velocity creating turbulence and air aspiration causing defective casting. This sprue is best suited for ferrous metal.
This sprue should be designed as narrow as possible to avoid breakage of the mould surface as metal flows down at a higher velocity.
To solve this problem down sprue or bottom sprue is made to compensate for the turbulence and air aspiration to reduce the casting defects.
Down Sprue
Down sprue is exactly the opposite of primary sprue when it comes to the flow of molten metal. In a down sprue molten metal is passed in the mould cavity against the gravitational force.
Turbulence and air aspiration are reduced giving defect-free smooth surface finish casting. Molten metal travels vertically from the bottom towards the top of the sprue vertically.
Down sprue is also called bottom sprue.
The velocity of molten metal through the gating system is accurately controlled but it is difficult to maintain the thermal gradient of molten metal.
This sprue is best suited for non-ferrous metal and large-size casting having a complex structure.
Side Sprue
Side sprue has the advantage of both primary and down sprue.
Side sprue is best used for producing small and complex casting parts.
Sprue can be divided into two types, sprue with taper (Tappered Sprue) and sprue without a taper (straight sprue)
- Sprue With Taper
- Sprue Without Taper
Sprue With Taper
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| (c) Tapered Sprue |
The best sprue in sand casting is designed in such a way that the opening of the sprue at the top is wider as compared to the bottom of the sprue.
The way molten metal travels from the top of the sprue to the bottom due to gravity is called vena contracta and depends upon the surface tension and velocity of liquid metal due to the gravitational force.
If the sprue in the casting process is not tapered it will cause an aspirational problem, to eliminate the air aspiration problem sprue is tapered as shown in the diagram above (c).
Due to this change in area molten metal travels at a higher velocity reducing air aspiration and keeping the sprue full.
A taper of 2° to 4° degrees downwards is provided to the sprue. Other dimensions such as the height of the sprue and the cross-sectional diameter of the sprue depend upon the time required to fill the mould cavity and the volume of the molten metal to be filled.
The longer the height of the sprue more will be the velocity of the molten metal passing through the vertical sprue channel.
Sprue Without Taper
If the sprue is designed straight without any taper as shown in diagram (d) above, a low-pressure area gets created causing air aspiration and the presence of air in the molten metal.
As the pressure in the sprue becomes negative it will cause a lot of air aspiration and turbulence.
Straight sprue will increase turbulence increasing the sand erosion in the sand mould. Air aspiration will cause an increase in gas defects in the final casting.
Along with gas defects straight sprue will increase oxidation of the molten metal which will give a rough surface to the final casting.
The heat loss of molten metal in the straight sprue will be high as compared to the tapered sprue.
Straight taper sprue will have a lower casting yield as compared to tapered sprue.
Sprue Design In Casting
If the sprue is designed by taper we can calculate the following values:
Velocity at the top of the sprue = `V_{t}` = `\sqrt{2gh}`
Area at the top of the sprue = `A_{t}`
Velocity at the bottom of the sprue (Choke) = `V_{b}` = `\sqrt{2gh}`
The area at the bottom of the sprue (Choke) = `A_{b}`
Velocity at choke (bottom of sprue) `V_{b}` will always be higher than `V_{t}` (velocity at top of sprue) for sprue that is the taper in shape.
Now, by using the equation of continuity we get:
`V_{t}` `A_{t}` = `V_{b}` `A_{b}`
`A_{t}=A_{b}{\frac{V_{b}}{V_{t}}}`
As mentioned above,
`V_{t}` = `\sqrt{2gh}` and `V_{b}` = `\sqrt{2gh}`
The final equation that we get,
`A_{t}=A_{b}\sqrt{\frac{h_{b}}{h_{t}}}`
When molten metal is poured into the pouring basin, molten metal travels at a higher velocity through the vertical sprue and then enters the horizontal passage called the runner.
The height (h) of the sprue is directly proportional to the velocity (V) of the sprue. The larger the height of the sprue more will be the velocity of the molten metal passing through the sprue.
Sprue will fill fast and full for the short sprue with the sprue to runner ratio is 1:1.
This can cause erosion of sand at the bottom of the sprue. To eliminate this problem of sand erosion splash core is placed at the bottom of the sprue.
Sprue is made in the mould with the help of a sprue pin and a sprue cutter.
Sprue should be designed in such a way that it can be easily removed or separated from the casting.
Sprue is never designed in parabolic shape but rectangular, bottleneck, circular, cylindrical, taper and square shape. The round shape has the lowest resistance to the flow of the molten metal flow.
Sprue Base Well
When molten metal passes through the pouring basin to the base of the sprue its velocity keeps increasing with respect to the height of the sprue gaining maximum energy.
This energy in the molten metal needs to be absorbed to reduce the erosion at the bottom of the sprue as steady molten metal needs to be passed through the horizontal runner channel.
This is achieved by placing a well at the end of the reservoir called a sprue base well. The sprue base well is used to reduce the energy at the end of the sprue so a steady laminar flow of molten metal can pass through the runner.
Dept of the sprue base well is designed with 2 times the runner.
Shown below sprue base well in diagram (d) below.
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| (d) Sprue Base Well In Casting |
The size of the sprue base well should be 5 times larger than the choke area which is at the end of the sprue.
The depth of the sprue base well should be equal to the runner's height and the sprue base well's diameter should be 2.5 times the width of the runner in the two-runner system and 2 times the width in the single-runner system.
When molten metal is poured into the pouring basin, molten metal travels at a higher velocity to the runner passage through the sprue.
This can cause erosion of sand at the bottom of the sprue. To eliminate this problem of sand erosion splash core, wire mesh or ceramic filters is placed at the bottom of the sprue.
The main function of a base well in sand casting is to absorb the velocity of molten metal and allow molten metal to steadily pass through the runner.
Factors While Selecting & Designing Sprue In the Casting Process
- Length, diameter and shape of the sprue. Cylindrical sprue has the least resistance to the flow of molten metal.
- Casting yield and taper on the sprue. Tapered shape sprue has the highest casting yield than non-tapered sprue reducing the use of molten metal in the process.
- Position of sprue in the mould. Primary sprue is best suited for ferrous metals while down sprue is best suited for non-ferrous metals.
- Size and complexity of the casting to be produced. Small complex shape casting can be accurately produced with side sprue.
- Ease to produce sprue during the moulding-making process using sprue pin.
- Thermal gradient during the casting process. Down sprue has low thermal gradient than the primary sprue.
- Location of sprue in the mould.