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Avoid turbulent entrainment

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M. S. Ramaiah School of Advanced Studies 1 Metal casting and joining process Module leader: M.r. K.N. Ganapathi MSRSAS, Bangalore Avoid turbulent entrainment (the critical velocity requirement) Papineni.Satheesh BVB0911002 Bushan Yadav.B BVB0911003
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Page 1: Avoid turbulent entrainment

M. S. Ramaiah School of Advanced Studies 1

Metal casting and joining process

Module leader:

M.r. K.N. Ganapathi

MSRSAS, Bangalore

Avoid turbulent entrainment

(the critical velocity requirement)

Papineni.Satheesh BVB0911002

Bushan Yadav.B BVB0911003

Page 2: Avoid turbulent entrainment

M. S. Ramaiah School of Advanced Studies 2

Contents

Maximum velocity requirement

The `no fall' requirement

Surface tension controlled Filling

Filling system design

Gravity pouring of open-top moulds

Gravity pouring of closed Moulds

Pouring basin and down sprue design

Horizontal transfer Casting

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The key aspect of the critical velocity is that at velocities

less than the critical velocity the surface is safe. Above the

critical velocity there is the danger of entrainment damage.

The criterion is a necessary but not sufficient condition for

entrainment damage.

If the whole, extensive surface of a liquid were moving

upwards at a uniform speed, but exceeding the critical

velocity, clearly no entrainment would occur.

Maximum velocity requirement

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If the melt is travelling at a high speed, but is constrained

between narrowly enclosing walls, it does not have the room to

fold-over. Thus no damage is suffered by the liquid despite its

high speed, and despite the high risk involved. This is one of the

basic reasons underlying the design of extremely narrow

channels for filling systems (Gating system).

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The `no fall' requirement

the no-fall requirement applies to the design of the

filling system downstream of the base of the sprue.

The critical fall heights for all liquid metals are in the

range 3 to 15 mm.

For example, if liquid aluminium is allowed to fall

more than 12.5mm then it exceeds the critical 0.5m/s.

with a good sprue and pouring basin design this initial

fall damage can be reduced to a minimum.

The `no fall' requirement may also exclude some of

those filling methods in which the metal slides down a

face inside the mould cavity, such as some tilt casting

type operations.

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Narrow filling system geometries are valuable in their

action to conserve the liquid as a coherent mass, and so

acting to push the air out of the system ahead of the

liquid.

A good filling action, pushing the air ahead of the

liquid front as a piston in a cylinder, is a critically

valuable action. Such systems deserve a special name

such as perhaps `one pass filling (OPF) designs'

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Surface tension controlled filling

This is interesting situation that the liquid may not be able to enter the

mould at all.

This is to be expected if the pressure is too low to force melt into a narrow

section. It is an effect due to surface tension.

If the liquid surface is forced to take up a sharp curvature to enter a non-

wetted mould then it will be subject to a repulsive force that will resist the

entry of the metal.

Even if the metal enters, it will still be subject to the continuing resistance

of surface tension, which will tend to reverse the flow of metal, causing it

to empty out of the mould if there is any reduction in the filling pressure.

These are important effects in narrow section moulds (i.e. thin-section

castings) and have to be taken into account.

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Filling system design

The liquid metal as it travels through the filling system

indicates that most of the damage is done to castings by

poor filling system design.

The filling system design can be of two types:

Gravity pouring of open-top moulds.

Gravity pouring of closed moulds.

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Gravity pouring of open-top moulds

Generally moulds consists of cope and drag but in open-top moulds

only drag is required. This means the mould cavity is open so that

metal can be poured directly.

The skill of the foundry man plays vital role in the gravity

pouring system

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Page 11: Avoid turbulent entrainment

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Gravity pouring of closed moulds

Gravity pouring of closed moulds consists of pipes, channels

to guide the metal from the ladle into the mould.

In poor filling system designs, velocities in the channels can

be significantly higher than the free-fall velocities.

There fore it encourages surface turbulence, bubbles and bi-films.

In the gravity pouring system of closed moulds, bottom gating system

design is much efficient compared to top gating system.

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Page 13: Avoid turbulent entrainment

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Page 14: Avoid turbulent entrainment

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Pouring basin and down sprue design

The offset blind end of the basin is important in bringing the vertical

downward velocity to a stop. The offset also avoids the direct inline type of

basin, such as the conical basin, where the incoming liquid goes straight

down the sprue, its velocity unchecked, and taking with it unwanted

components such as air and dross, etc.

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An oversize sprue that has suffered severe erosion damage because of air

entrainment during the pour.

A correctly sized sprue shows a bright surface free from damage.

Greater the sprue diameter greater the turbulence.

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Runner

The runner is that part of the filling system that acts to distribute the melt

horizontally around the mould, reaching distant parts of the mould cavity

quickly to reduce heat loss problems.

For products whose reliability needs to be guaranteed, the arrangement of

the runner at the lowest level of the mould cavity, causing the metal to

spread through the running system and the mould cavity only in an uphill

direction is a challenge that needs to be met.

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Horizontal transfer Casting

Tilt casting is a process with the unique feature that, in principle, liquid

metal can be transferred into a mould by simple mechanical means

under the action of gravity, but without surface turbulence.

The problem of horizontal transfer is that it is slow, sometimes resulting in

the freezing of the `ski jump' at the entrance to the runner, or even the non-

filling of the mould. This can usually be solved by increasing the rate of

tilt after the runner is primed.

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References

1. John Campbell, Castings Practice, The 10 Rules of Castings,

published 2004.


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