Rule
No. 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. This little-known problem can
lead to unsuitable temperature gradients (for instance from
bottom gating in an effort to promote good filling) and so
can undermine the effectiveness of feeders, and lead to segregation
and a kind of shrinkage damage that is difficult to eliminate.
The problem is a source of concern, because it is little
understood, and little researched. Most computer packages
cannot simulate it, and thus predict the wrong shrinkage pattern
in the casting.
Problems of convection are eliminated by "rock or roll,"
i.e.
(1) Careful horizontal transfer by tilt casting operations
(requires a start tilt condition above the horizontal, and
a slow tilt speed);
(2) Counter-gravity filling followed by immediate
roll-over of the mould through 180 degrees.
(Click here to view diagram of
a counter-gravity filling as applied in the Cosworth Process.)
Usually, however, neither of these solutions is possible
because most foundries work only with static moulds. In such
cases the problem is reduced, but not necessarily eliminated,
by
(3) Feeding by oversized feeders placed on the top
of the casting and feeding under gravity.
(4) Avoiding convection loops, especially in the
rigging of investment castings (where the problem is especially
common).
As an aside, the problems of convection are automatically
avoided in very thin or very thick section castings. Thin
section castings freeze quickly before convection can build
up to become important. Thick section castings take so long
to freeze that there is plenty of time prior to freezing for
hot metal to convect upwards into the feeders and for the
cold metal to sink to the bottom. Thus the melt can redistribute,
favourably arranging the temperature gradients before freezing
starts.
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