Conveying in the feed zone will only occur if the plastic grips the barrel and slides on the screw.  Whenever it grips the screw surface and slips on the barrel, it will not move forward. Consistent feeding requires a consistent balance of friction between the barrel and the material, between the screw and the material, and between the particles of material themselves. Because a consistent balance of friction is difficult to achieve, many problems with surging, uneven output, and size variations actually begin in the feed zone.

Feeding and conveying take place a little differently in a grooved feed extruder. With a grooved feed throat, slippage between the plastic and the barrel is effectively zero. It must move forward as the screw turns, regardless of any variations in friction between the plastic and screw.

Therefor, companies who purchase grooved feedthroats to improve material conveyance. Please discuss the specific application with the machine manufacturer to ensure the feedthroat grooves accommodate the size of your pellets and regrind. 

One interesting variation on this is with a micro-extruder. Many of the small bench-top extruders will taper and groove the feedthroat since it is the only way for the pellets to fit into the barrel and convey down the screw to be properly melted.

Another area of interest for grooved feed throats is in the newer, high capacity shot-pot style injection molding machines. Many of these use the shot-pot two-stage configuration to increase melting capacity… manufacturers are testing more traditional extruder configurations including twin screw extruders and grooved feedthroats.

-Andy