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Aspiration Effect

The aspiration effect occurs when the pressure in a liquid metal stream drops below atmospheric pressure, allowing gases in the mold to enter the stream and cause porous castings. To prevent this, the shape of the sprue carrying the metal into the mold must maintain an equal or higher pressure. Specifically, the sprue area should decrease according to the reciprocal of the square root of the height to ensure constant pressure down the sprue. Additional causes of aspiration include sudden changes in flow direction and non-uniform velocity profiles, which can be addressed by rounding corners and accounting for friction losses in modeling equations.

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Savio Jain
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13K views13 pages

Aspiration Effect

The aspiration effect occurs when the pressure in a liquid metal stream drops below atmospheric pressure, allowing gases in the mold to enter the stream and cause porous castings. To prevent this, the shape of the sprue carrying the metal into the mold must maintain an equal or higher pressure. Specifically, the sprue area should decrease according to the reciprocal of the square root of the height to ensure constant pressure down the sprue. Additional causes of aspiration include sudden changes in flow direction and non-uniform velocity profiles, which can be addressed by rounding corners and accounting for friction losses in modeling equations.

Uploaded by

Savio Jain
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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ASPIRATION EFFECT

The

pressure in the liquid metal stream should not become less than (or should not fall below) the atmospheric pressure. If it falls
Gases produced due to baking of organic compounds in the mould enter the metal stream and result in porous castings It is called the Aspiration effect

In

other words, the shape of the sprue should be exactly as that of the metal stream did you ever observe the water falling from the tap under near laminar flow? What shape it acquires? let us determine the shape of the sprue to be made in the mould to avoid the aspiration effect.

Now

Mathematical model of aspiration effect

Applying the Bernoullis equation to the top gating system between points 2 and 3, we obtain that

p2 m gh2 if v2 v3
3

The shape of the sprue

By the above analysis, the design of sprue with straight cylindrical (uniform cross-sectional area from 2 to 3) is not acceptable. Then, what should be the shape? The shape should be as per the following equation:

A A
3

hc ht

Ideal versus actual shape of sprue


As

we can see, the sprue shape is such that the cross-sectional area varies as the reciprocal of square root of ht. is difficult to produce therefore a straight tapered sprue is used.

Ideal

Other cause of Aspiration effect


Sudden

change in flow direction The liquid metal contracts around a sharp corner, resulting in vena contracta Provide a round to accommodate the vena contracta d/d = 1.3 or r = 0.15d d = diameter at the entrance d = runner diameter r = radius of the round at the sharp corner
6

Prevent impurities in casting:


Pouring Strainer Splash Skim

Basin

core

Bob

Other effects to be accounted for


The

equations derived till now will be affected by the following. Non-uniform velocity distribution Frictional losses
Due to friction between the molten metal and mold wall Sudden change in flow direction Due to sudden expansion or contraction of flow cross-sectional area
8

Effect of non-uniform velocity distribution:


The

velocity of metal stream decreases from the centre (max) to the mould walls (zero) to account for the non-uniform velocity distribution? introducing a constant factor in the Bernoullis equation
9

How

By

Accounting for frictional losses: Between the molten metal and mold wall
The

energy loss due to friction in a circular conduit (per unit mass) is Ef1 equation for Ef1 can be obtained from the theory of flow through pipes friction factor f depends on the Reynolds number
10

The

The

Accounting frictional losses: due to sudden change in flow direction

Equivalent

(L/D) factor for the bend has to be used in the equation of E f1.

11

Accounting frictional losses: due to sudden expansion or contraction


The Ef2

loss is termed as Ef2

is determined by using average flow velocity factor ef in Ef2 is obtained from graphs
12

The

Modified

Bernoullis Equation for the top gating system

13

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