To eliminate as many variables as possible in the simulations, each barrier screw is considered to have an entry pressure that assumes a fully compacted solids bed. In the real world, however, the solids bed isn't fully compacted. Also, all screws were assumed to have a standard diameter of 4.492 in. and an axial barrier length of 45 in. or 10 turns. The screw speed used for the calculations was 100 rpm. Channel width for the unwrapped barrier charts is measured perpendicular to the flight, not circumferentially. Circumferential width—i.e., when the flight is wrapped around the screw—is wider than the perpendicular channel width used in these calculations.

To check the assumptions in the computer simulation, the program was verified with data from experiments run on a 3.5-in. diam., 30:1 L/D lab extruder at R. Dray Mfg. The resin used for both the simulations and lab tests was a 1-MI LDPE.

Basic & Improvement Barrier Patents

The nine historically significant barrier screw designs include three basic patents—Maillefer, Schippers, and Dray—and six improvement patents. Barrier feed-screw technology began with Maillefer's basic Swiss Patent and subsequent U.S. patent #3,358,327, filed in 1960 for a barrier flight that retains unmelted solids in a primary channel while melted resin goes downstream in an auxiliary channel.

Schippers' basic U.S. patent #3,701,512, filed in April 1971, has two different configurations. The first has parallel channels with the primary or entry channel reducing in depth while the auxiliary or melt channel (on the other side of the barrier flight) increases in depth. The second configuration adds distributive mixing by transposing the primary and the auxiliary flights. This causes the resin in the melt channel to mix with the resin on the trailing side of the primary channel.

The Dray basic U.S. patent #3,650,652, filed in 1970, increases the melting area with a longer lead at the end of the feed section. The longer lead allows the auxiliary channel to be included, while the width of the solids bed in the primary channel remains unchanged.

Improvement patents are patents that have precedent. They are intended to provide new technology that improves the prior art. Six improvement patents add new technology to these three basic designs. The first two basic European design patents were never used commercially in the U.S., but came here via improvement patents on them. Geyer's U.S patent #3,375,549, filed in 1961, and Lacher/NRM's U.S. patent #3,271,819, filed in '62, improve on Maillefer. Barr's U.S. patent #3,698,541, filed in 1971, and Chung's U.S. patent #4,000,884, filed in '75, improve on Schippers. Kim's U.S. patent #3,867,079, filed in '72, and Wheeler's U.S. patent #4,341,474, filed in '80, improve on Dray. (However, the Wheeler/DSB-1 barrier from Davis-Standard Corp. is more a hybrid—i.e., combining elements of Maillefer, Schippers, and Dray.)

Because the improvement patents have melting-channel configurations similar to those of the basic patents, only the basic designs need to be simulated. An exception is the Wheeler patent, which combines elements of all three basic patents in a melt channel with three different lead dimensions. Therefore four simulations were performed: the three basic designs and the Wheeler hybrid.

These four barrier sections were "unwrapped." The geometry of the barrier section with primary and auxiliary channels was simulated in a flat plane, as if it had been unwound off the screw and flattened out. The surface area of the melt channel was then calculated.

This method can be used to determine the melting capacity of any barrier-screw design. Total melting area is calculated as the surface area of the primary channel where solids contact the barrel wall and are melted mainly through surface friction. The size of this area determines melting capacity. This contact area in sq in. is converted into lb/hr of melting capacity using a simulation model for heat transfer.

As a control, a conventional non-barrier screw design with a square pitch was also evaluated for melting capacity. This channel section was unwrapped, and heat transfer between the melt pool and the solids bed was calculated.

Simulation result: The conventional non-barrier screw control shows a melting capability of 482 lb/hr.

Maillefer Barrier Family

Tadmor and Klein's original computer simulations modeled the break-up of the solids bed. In a conventional non-barrier screw, when the internal melt pressure is great enough, it penetrates the solids bed and disperses the remaining solids randomly into the metering section. This phenomenon mixes melted and unmelted plastic, reducing melt quality.

The Maillefer barrier, patented here in 1967, was the first barrier or melt-separation device used in a feed screw. It increased melting area relative to a conventional non-barrier design and eliminated the break-up of the solids bed, thus improving melt quality. The Maillefer design elongates the lead that starts on the pushing side of the primary flight and ends on the trailing flight. This flight becomes the barrier separating the solids bed from the melt and preventing solids from dispersing into the metering section.