Cast Metal & Diecasting Times July/ August 2016 July/August 2016 | Page 27

POURING | 25

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Figure 3. A slide gate affixed to the base of a bottom-teemed ladle.

pneumatic pump as part of a foundry layout in
which no pouring of metal occurs and bifilms of
all populations are eliminated by passive systems
such as avoiding the introduction of primary
oxide skins on charge materials via a dry hearth
furnace and reducing internal populations of
bifilms by sedimentation or filtration. Hydrogen
also should be eliminated by passive degassing,
avoiding the uncertainties of bifilm formation
in bubble forming processes, including rotary
degassing, in which it seems that bifilm
detraining and bifilm formation are both
happening at the same time but at uncertain
rates. It does not need to be emphasised that the
industry needs to move away from uncertainty in
casting production processes.
However, it is hoped that the counter-gravity
process in some enlightened and rational form
is a not-too-distant future target production
process for the foundry. It is not the target for
this presentation. The current target is contact
pouring, a potentially immediate, low cost action
to raise quality and profits simultaneously.
Contact pouring
The direct contact between the nozzle of a
bottom-teemed ladle and the entrance to the
filling system for the mould is designed to
eliminate the otherwise universal entrainment of
air into the filling system.
Early attempts to address this problem
included the provision of an argon shroud
around the falling stream at this point. However,
the argon would necessarily also entrain much
air and the argon/air bubbles would continue
to wreak unwanted damage, apparently filling
the metal with inclusions but in reality, much
worse, filling the metal actually with bifilms.
Although the bifilms would be scrambled and
would evolve into more compact shapes in
metals such as steels, the entrained unbonded
interface would never be completely eliminated.
The result would be a population of inclusions
with associated residual air, mostly probably in
the form of the 1% of residual argon in air, which
is insoluble in metals and therefore a permanent
bubble. These permanent pores would constitute
volume defects, which could serve as initiation
sites for cracks (otherwise it can be predicted
that inclusions in steels, for instance, would
not nucleate cracks), reducing the ductility and
fatigue resistance of the metal.
It is clearly of far greater effectiveness to
eliminate the ingress of any and all gases from
the filling system, which is why contact pouring
is so effective.

The mechanics of setting up the process are
interesting, being different for every foundry
seen by the author so far. The different
engineering solutions reflect the different
ladle designs; in particular, the detailed
geometry of the nozzle and its fixture in the
base of the ladle. For instance, it is helpful to
ensure that the full weight of the ladle does
not sit on the mould. Many moulds would
collapse and the ceramic nozzle could be
pushed up through the base of the ladle. Most
systems therefore do not rely on the dead
reckoning of height control by the crane but
provide a system in which the weight of the
ladle can be taken by a separate rigid frame
but the joint between nozzle and mould is
sealed by the sufficient compression of a
gasket of ceramic fibre.
The alignment of the ladle nozzle and
downsprue entrance is a second critical
feature of the process. So far, most foundries
continue to rely on the skill of the crane
driver, aided often by a local funnel device
to guide the final alignment to within a
‘rattling fit’ of a few millimetres. Some use a
lightweight alignment frame for moulds in
boxes, using the box pins, transferring the
frame from mould to mould along the line;
staying ahead of the pour. For the future, it
is not difficult to envisage the crane remotely
operated, possibly by laser triangulation,
delivering metal into the sprue entrance with
millimetre accuracy.
The technique has so far been used
exclusively with bottom teemed ladles
equipped with a stopper. It is not known
whether the technique can be applied to
those ladles equipped with a slide gate

Figure 4. Overhead launder, automatically topped up from a dosing furnace,
for contact pour with a vertical parting line mould. No valuable mould space
is taken up with a pouring basin and air entrainment is eliminated.

(Figure 3). However, if the slides are close
fitting, so as to be reasonably air tight and
there are no other pathways for air to enter
the metal stream in the assembly, there
seems no reason in principle why the slide
gate should not also be effective. Good
alignment of the middle slide between the
upper and lower plates is needed to ensure a
good integrity of the flowing jet.
The automatic moulding process using a
vertical joint to create a