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THE TEN
RULES:
(1)
"NO POURING!"
The best quality and most reproducible castings are those
produced by those few foundries designed to avoid the pouring
of metal. Hard to believe? Not when you think about what happens
to metal when it is poured, and how that affects it after
solidification.
(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.
(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.
(4) "BUBBLES
ARE BAD"
Bubbles are the most common source of porosity in
castings -- but their effect is usually mistaken for shrinkage,
which it closely resembles, because the bubbles and their
trails are often irregularly shaped.
(5) "BIG BUBBLES
ARE WORSE"
The outgassing of cores can lead to huge defects, filling
whole areas of the tops of castings. However, even a small
blow from a core can leave a bubble trail that can create
a leak defect. To avoid blows from cores, the core must be
vented to the atmosphere.
(6) "AVOID SHRINKAGE"
Follow the three steps provided to assure that feeder design
is as reliable as possible.
(7) "ROCK OR
ROLL"
The great majority of castings in the market place have a
freezing time of several minutes. This is similar to the time
taken for the convection of hot and cold liquid metal in the
solidifying casting to build up, and for the resulting convection
currents to start re-melting their way through the casting
as it attempts to solidify. Four steps can be taken to alleviate
the problems this causes.
(8) "SEVERE SEGREGATION"
All freezing will cause some segregation of alloying elements,
and some alloys segregate seriously, to the point at which
parts of the casting will be well outside chemical specification.
In principle the problem can be predicted by computer packages,
and therefore allowed for in the design.
(9) "NO QUENCHING"
Water quenched castings are effectively pre-loaded to approximately
50 % of their failure stress before being put into service.
Polymer quenchants or air quenching are much more reasonable
alternatives.
(10) "THE TOOL
RULE"
The best castings start by thinking about the
work that happens after casting -- and that
means providing pick-up points needed in machining and other
finishing operations. Only then will a trauma-free, integrated,
supply of castings be achieved, all accurately within dimensional
specification.
About John Campbell
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