Rule
No. 4:
"Bubbles are Bad .
. ."
In addition to making holes in the casting, bubbles leave
trails of oxides, making double trouble.
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. (Click
here to view diagram of bubble damage.)
They are manufactured with awe-inspiring efficiency in badly
designed filling systems. Thus all those damaging features
of filling systems listed for Rule
2 apply here also. Good filling systems (counter gravity,
tilt pouring especially if started from above the horizontal,
and well designed gravity systems) all reduce bubble damage.
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