The Evolving Barrier Screw

The use of barrier screws for plastics processing is now a well-established practice for both smooth- and grooved-bore extruders. At the same time there has been a gradual transition from the earlier Maillefer designs used in Europe with the crossing barrier design (Fig. 1) to the Dray/Lawrence design using the parallel barrier design with an increased helix angle to accommodate wider channels (Fig. 2).

The crossing barrier design starts with a full-width solids channel and ends with a channel having no width. The parallel design with an increased helix angle has been found to have several potential advantages over the crossing barrier type.

First, the use of an essentially constant-width channel from the feed section through the entire barrier section eliminates the continuous distortion of the solid bed that is necessary with the crossing barrier design. The pressure required to facilitate that distortion in the crossing barrier can be carried back into the solids-feeding section, reducing the feed efficiency and output.

Also, the depth of the solids channel in a crossing barrier often must be kept essentially constant in order to maintain sufficient transport area to meet the output requirement. This changes the melting pattern in a number of ways. Melting in tapered-depth channels is generally faster than in constant-depth channels because of higher shear rates in the melt film. Constant channel depth makes distortion of the solids bed, rather than reduced channel depth, the mechanism to force unmelted resin against the barrel wall. The continuous distortion increases the likelihood of a breakup of the solid bed, which would reduce the melting rate—particularly with hard polymers, which do not have a strong solid bed. Finally, melting rate is closely related to the area in which the unmelted polymer is in contact with the barrel.

By increasing the helix angle in the Dray/Lawrence type design, the channel is wider, allowing for introduction of the barrier and melt channel without restricting the width of the solids channel and the solid bed. Conversely, the crossing barrier design reduces the area exposed to the barrel in the solids channel of the barrier section to less than half as much as a parallel barrier of the same length. The only way to overcome that restriction is a much longer barrier section, which is usually not possible except with exceptionally long L/D extruders.

Screws having a parallel barrier but no increase in helix angle (Fig. 3), fall somewhere in between the Dray/Lawrence and Maillefer concept with respect to melting rate per unit length. However, they have worked well in applications for softer polymers and/or polymers having a low energy requirement. Their narrower channels can provide lower melt temperatures in many instances.

This is not a blanket indictment of either the crossing barrier design or the parallel barrier design without the increased helix angle. Many extruders are limited in torque, have sufficient L/D, and do not require a maximized melting rate because of feed limitations. As is usually the case with polymer processing, there is no single design to fit all processing situations.