'd think they'd both add turbulence to the flow, as well as causing
the sprues to cool too quickly, since you're bleeding heat into the
walls as the metal swirls past them. They're not as thick as a
round sprue either, so they'll freeze before a round sprue would
have. I'd think that'd make porosity *worse*, not better.
Well, this is quite a complex topic. One disclosure is that my
comments will be true for gravity pours. I do not practice any kind
of forced castings, so it may not apply.
There are many other variable involved, but assuming that number of
sprues, metal temperature, mold design, and etc is taken care of,
spiral sprues do perform better.
There are basically 2 types of flows: laminar flow and turbulent
flow. There was a time when it was believed that laminar flow is the
thing to strive for, and that is why round sprues, 1930 airfoil
designs, and etc. Than some engineers noticed a bumble bee and
realized that based on what they knew at a time, a bumble bee should
not be able to fly, because her wings were too short and laminar flow
should not be able to produce enough lift. A closer look revealed
that bumble bee was triggering turbulent flow, which after very short
interval would convert to laminar flow and that is how lift was
generated. In another words, bumble bee was using back of her wings
to fly, but not the front, as airplanes were designed to do.
Flow which starts as laminar, due to surface imperfections, would
become turbulent in a short period of time and it would detach itself
from an airfoil, making only short section of a wing generating the
lift. When leading edge of a wing generates turbulent flow, first it
would detach, but then slow down and re-attach to the airfoil, and
flow for the rest of the surface, generating more lift than otherwise
would be possible.
Spiral sprues work on the same principal. By triggering turbulent
flow at the beginning, the design ensures laminar, or close to
laminar, flow at later stages. Round sprues would work better, if we
could insure perfectly frictionless surface on the inside, which we
One more thing. The twist of a spiral should correspond to the