A back mix, drag-flow apparatus that provides both radial and axial mixing of materials comprising at least two worms in screw form and parallel and tangential to one another that are of opposite hands and are rotatable only in the same direction or are of the same hand and are rotatable only in opposite...http://www.google.com.au/patents/US5372418?utm_source=gb-gplus-sharePatent US5372418 - Back mix drag-flow apparatus
