Barrier type screw designs are the most widely used screw designs in the plastics industry (especially extrusion applications). The first barrier screw design was by Maillefer (Swiss patent # 82,535/59, filed 12/3/1959), the second and currently the most widely used barrier design is by R.F.Dray (Patent # 3,650,652, filed 5/5/1970).
How it Works
Barrier screws introduce an auxiliary flight, or barrier flight, within the transition section; this effectively separates a “solid channel” and “melt channel.” The solid channel is open to the feed section while the melt channel is open to the metering section. While the solid channel depth decreases along the length of the screw, the melt channel depth increases. As the solid bed melts along the length of the screw, the melted polymer flows over the barrier flight into the melt channel through a tight clearance. The barrier clearance prevents any unmelted pellets from flowing into the melt channel.
By separating the melt pool and solid bed, barrier screws increase melting efficiency and eliminate solid bed break up, allowing for more control and stability. Much like mixing sections, not all barrier designs are alike, a screw designer cannot simply add a barrier section to an existing screw and expect superior performance. The metering and feed sections of a given barrier screw must be properly designed in conjunction with the barrier section. When designed properly, barrier screws can accommodate a wide range of materials and provide a greater potential throughput.
There are many different names for versions of the original R. Dray barrier design by virtually all manufacturers and designers, although the basic foundation of the patent is common in all of these variations. In figure 1 you will note that the primary channel width is maintained allowing the barrier flight and auxiliary channel to be formed with the remaining width. By maintaining the primary channel width the R. Dray barrier screw provides improved melting efficiency over other barrier designs. Although some of these versions incorporate our original technology, they do not include new innovations by the original inventor.
The R. Dray barrier screw has proven to outperform competing designs in both trials and real world applications. Our barrier technology increases the maximum melt capacity of the screw while improving extrudate quality and stability. Both extrusion and injection molding customers rely on the R. Dray barrier screw to maintain their competitive advantage.