These machines are very insensitive to the pressure generatedd by the
die therefore, in equationtion of the output, there is not a term
simitur to the "back pressiirc" as in singlc-scrcw extruders. The output given by the fumula, however, is not the acluul output, as there
is always a "leakage flow around the screws. Furthermore, when
the screws are not conjugated or when the flights have a form that
docs not completaly fill the channels but leaves some passages, there
is also a leak within the intermeshing region.
In counterrotating screws, the drag effect makes this material flow
in opposite directions , so that, where thee screws meet at the intermesfiing, these flows also meet. The uneven pressure created within
the channels by these counterrotating flows squeezes part of the
material out of the C-shaped chambers. The larger the gap between
Channels and flights (for lower shear), the gruaterr is this leakage, and
therefore the greater is the reduction in the pumping efficiency of
the machine