Which Barrier Screw for You?

There is even a design originally patented by DuPont that starts the barrier by retarding the main flight pitch to form the barrier channel backward, which has been shown to reduce disruption of the solid-bed formation when that is an important consideration. The crossing barrier style typically has more head-pressure generating capability because of the wider, shallower melt channel compared with the narrow, deep melt channel of the parallel type. As a result, the parallel barrier style usually requires some metering-section length after the barrier section if a significant head pressure must be overcome.

By increasing the lead beyond the standard pitch of the main flight in the barrier section, some improvement in melting area can be made for both the parallel and crossing types. On a 3.5-in. screw, for example, increasing the lead in the barrier section by 25% to 4.375 in. (a 21.7° helix angle) for the main flight, and using the same axial length, results in only 8 turns vs. 10 turns of barrier flight. The increased helix angle will increase the melting area by less than 10% for both types, as shown in Figs. 3 and 4, because of the reduction in turns. Even much of that small advantage is lost as the viscous dissipation or melting per unit length is reduced with the longer helix angle.

The longer helix angle then is more appropriate to get more volume into the barrier section for higher output—but without significant improvement in melting capacity, unless the axial length is increased. Barrier screws of this type have been referred to as the “Dray/Lawrence type” after the inventor Bob Dray. Increased barrier-section lead does reduce issues with the effect of the barrier-flight and melt-channel introduction due to a wider solids channel.

Finally, the exit end of the solids channel for both types can be closed or open. Closed-end channels require that melting is largely completed, or the channel can plug with solids, causing pressure fluctuation, additional screw wear, and lack of thermal homogeneity. Conversely, open solids-channel ends require a design that completes most, if not all, melting before the end of the solids channel—or something else must be done beyond the barrier section to complete the melting. In either case, this requires some complex calculations to get it correct and to provide a large processing window.