14 Barrier screws.

Barrier screws were invented in B1959 as a way to increase throughput of single-screw extruders and to improve melt quality. They keep unmelted solids in the primary screw channel by increasing the flight clearance on the downstream side, while letting the fluid melt escape over the barrier flight into the secondary channel. That way, the solids don't randomly disperse into the metering section. This allows for extrusion at higher pressures and higher rates without sacrificing melt quality.

Barrier screws proved to be a highly active, and at times hotly contested, area of technical development. The original inventor, Dr. C.E. Maillefer in Switzerland, applied for a Swiss patent in 1959, which was granted (#8253559). He then applied for a U.S. patent in 1960; it didn't issue until seven years later (U.S. #3358327, Dec. 19, 1967).

Meanwhile, in 1961 Uniroyal's Paul Geyer applied for and received another barrier patent (U.S. #3375549). In 1966 a patent lawyer named P.K. Lacher got a patent for a long barrier section on behalf of NRM Corp. (U.S. #3271819).

These companies vied for the primary U.S. patent, which Uniroyal won and vigorously defended, licensing virtually all U.S. machinery and screw manufacturers. NRM's version, called the Plastiscrew, was used primarily as a dispersive mixing section, not a barrier screw. The Maillefer screw found use mainly in Europe.

Robert Dray patented his own barrier screw design (U.S. #3650652, March 21, 1972), which increased melting area by using a longer lead and keeping the primary channel a constant width, instead of narrowing it as in the Maillefer, NRM, and Uniroyal designs. It was assigned to Feed Screws Inc. and Owens-Brockway, Variations are still widely used today.

Another barrier design with parallel channels alternating in depth while maintaining a constant lead was the subject of two almost simultaneous patent applications. One was filed in June 1971 by Robert Barr on behalf of Midland-Ross Corp. for a barrier design with alternating-depth channels (U.S. #3698541, Oct. 17, 1972).

Two months earlier, H. Schippers, president of Barmag Barmer Maschinenfabrik in Germany, had filed for a similar patent on a main channel that gets shallower as the auxiliary channel deepens. That patent issued two weeks after Barr's (Oct. 31, 1972).

Barmag didn't pursue the U.S. patent, but Barr developed further versions of his alternating-depth channel design. Also, Dr. C.I. Chung patented a variation on transposing the primary and auxiliary flights (U.S. #4000884, July 25, 1975). George Kruder at HPM also patented a double-wave design, primarily as a mixing device, not as a barrier.

In 1980, N.C. Wheeler from Davis-Standard Corp. received a patent (U.S. # 4341474) for the DSB-1 barrier screw, which uses three discernable lead changes at the end of the feed section. By the early 1980s, barrier screws were popular in applications like blown film that require low melt temperature and high output rate with consistent melt quality. Today many barrier designs also incorporate a mixing section to maximize extrusion quality. In injection molding, barrier screws reduce recovery times and improve melt quality.