Rule
No. 3:
"DON'T STOP"
While the melt continues to rise smoothly in the mould, the
liquid front stays "alive", with the surface oxide continuously
breaking and sliding off the advancing meniscus to form the
skin of the casting. The thin oxide on the advancing liquid
front is not therefore a problem; this steady advance will
ensure a good filling condition and a casting free from oxide
cracks.
When filling the mould, the melt front must never be allowed
to come to a stop. If this happens, the stationary front becomes
covered with a thick oxide film, so that restarting its advance
may not be possible. The melt may break through and roll over
the oxide layer, trapping it in the casting as an "oxide lap".
If the arrest of the front is prolonged, the front may freeze,
creating a "cold lap". Laps of any sort can act as cracks.
Interrupted pours are therefore a NO NO!
However, stops can also occur as a result of part of the
melt arriving at an overflow. (Click
here to view diagram of overflow.) Such "waterfall" conditions
are to be avoided at all costs, and this is a further reason
for providing ingates at every low point in the casting. Stops
can even occur when, after filling thin walls, the advancing
front arrives at a large area expanse such as the top of a
box type casting (e.g. an automotive oil pan). The irregular
filling of such flat horizontal sections of castings can lead
to severe lap defects, often invisible and undetectable (except
to destructive mechanical testing such as bend testing of
the casting). Thus horizontal surfaces of castings should
be avoided by design if possible, or by tilting the mould
if possible, or finally simply filling these regions at a
speed sufficient to reduce the problem to an acceptable level.
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