screw, thereby closing the way around the screws. The material
tlicrefore must stop its revolving motion, and as the screws keep ro-
tating, it slides within the channels, assuming an almost linear motion
toward the end of the barrel. This is positive pumping.
When the screws rotate in opposite directions, as we have seen,the
material is more or less sealed into chambers formed by the inter-
mesliing coryugated threads. The material muiit remain in these
separate chambers and, as the screws rotate, must advance forward.
The maximum possible output of these types of extrudcrs wouU
be, therefore, per each screw/fcqual to-lhc volume of one of these
chambers, already calculated before, by the rpm of the screws. For
two screws, we can write