Avoid turbulent entrainment

Metal casting and joining process

Avoid turbulent entrainment
(the critical velocity requirement)

Papineni.Satheesh BVB0911002
Bushan Yadav.B BVB0911003

Module leader:
M.r. K.N. Ganapathi
MSRSAS, Bangalore
M. S. Ramaiah School of Advanced Studies 1

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


Maximum velocity requirement

 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.


 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).


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.

 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'


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.


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.


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


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.


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.


 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.


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.


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.


References

1. John Campbell, Castings Practice, The 10 Rules of Castings,
published 2004.


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