A screw rotates in a containment barrel, and comprises a shaft having an upstream end and a downstream end At least one flight is secured to the surface of the shaft and extends helically in successive turns around the circumference of the shaft. An upstream barrier member extends radially from the surface of the shaft and generally transversely between successive turns of the flight. The upstream barrier is disposed proximal the upstream end of the shaft. A downstream barrier member extends radially from the surface of the shaft and generally transversely between successive turns of the flight. The downstream barrier is disposed proximal the downstream end of the shaft.