Rule
No. 2:
"Do it s-l-o-w . .
."
When the melt is never poured (See Rule
No. 1), and never exceeds a speed of 0.5 m/s (about 20
inches/second), the casting can stay free from oxide cracks.
Gravity, Acceleration, Speed -- Thus the optimum filling
systems to avoid forming oxides in the casting are (1) counter-gravity,
and (2) properly controlled tilt-pouring (in which
the mould starts from horizontal or above the horizontal,
so that the melt is caused to transfer horizontally into the
mould cavity without flowing downhill).
However, most foundries use gravity pouring systems. Even
if the melt falls only 12 mm (about 1/2 inch!), it has already
reached a speed of 0.5 m/s. Thus all pouring introduces
problems to achieve a casting free from oxide cracks!
Design Features that ameliorate Pouring Problems --
To reduce these problems when pouring, many precautions need
to be built into the filling system design. The design is
often difficult, but the alternative is nearly always castings
that are impaired, if not actually ruined, by oxides, as recent
research has demonstrated. (Unfortunately, most foundries
remain unaware of these recent developments!)
Of the many possible design improvements, key beneficial
features include:
(1) an off-set weir basin;
(2) a correctly tapered sprue;
(3) a slim runner, correctly profiled
to slow flow, and distribute melt into gates; and
(4) gates entering the mould cavity
only at the lowest points.
In particular, the gates are the key control point for
making sure melt velocity is held to below 0.5 m/s. Velocities
higher than 0.5 m/s into the mould cavity will cause the melt
to jet into the mould, creating oxide and bubble damage in
the casting. Properties, leak tightness, and corrosion resistance
will all be randomly degraded in proportion to any excess
velocity.
Click here to
view diagram of oxide formation on the
surface of turbulent, high-speed melt.
Sources of Trouble -- Buyers should note that particularly
damaging features include: conical pouring basins, oversized
sprues and runners, and wells at the base of sprues. These
all contribute to the entrainment of bubbles and oxides, and
thus lead to the random degradation of properties. These features
must be avoided to successfully procure reliable castings.
Bottom Line: Rule No. 2 is closely related to Rule
No. 1 -- it is absolutely essential to fill below a top limit
of 0.5 m/s, and acceleration due to gravity is far too great
to control below this limit. The first step is to avoid pouring,
and the next step is to find other control points to limit
filling speed.
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