Holes in the Screw Flights Boost PS Foam Output.

* Unusual tapered openings through the flights of a deep-channeled cooling screw can raise the output of extruded polystyrene foam sheet and planks by as much as 70%. So says the inventor of the patented Turbo-Cool screw, Jim Fogarty, president of Plastic Engineering Associates Inc. in Boca Raton, Fla. Turbo-Cool is believed to be the first screw to use cross-flight holes, which Fogarty says contribute better mixing and heat transfer while using less energy.

The square holes in the flights have wider openings on the upstream flank and taper to smaller ones on the trailing side. Inlet holes occupy about 15% of the pushing side of the screw flank, while the discharge holes take up only about 4% of the area. This 4:1 reduction in area promotes elongational flow through the holes, stretching the polymer, which exhibits elastic recovery after exiting the other side. The walls of the cross-flight holes are also curved to give a scoop effect that promotes heat transfer, Fogarty says.

Proven in production

The Turbo-Cool screw can be installed as a direct replacement for any conventional foam cooling screw. It requires an operating license with a modest annual renewal fee. There are more than a dozen commercial installations in North America and Western Europe, plus one in the Far East. The first was installed over five years ago. The major market for the screw is PS foam sheet for clamshell and other food containers, but commercial users also make industrial packaging and insulation planks for construction. Although most applications are in PS foam, a few are in polyethylene. Commercial models range in size from 3.5 to 8 in. and 24:1 to 50:1 L/D, though the most common L/D is 30:1.

Production data show 25-70% higher output, plus a 10-20% energy savings on the drive motor compared with use of a standard cooling screw for foamed PS. The Turbo-Cool screw uses less chilled water per unit of production, adding to the energy savings. It also produces less scrap than a conventional foam cooling screw, Fogarty says, because the process flushes and stabilizes more quickly during product changes.

More tests to run

Cross-flight holes are located along the whole length of the screw. V.P. and general manager Dave Fogarty says, "The number, shape, and location of the holes are critical. We vary the pattern to suit the application." The company is conducting experiments with orienting the holes relative to the flights' helix angle, varying channel-to-channel spacing of the holes, and combining various channel sizes, flight depths, and thicknesses.

OUTPUT OF FOAMED PS SHEET & PLANK TURBO-COOL VS. STANDARD COOLING SCREW

(Both 6 in. diam. in a tandem foam line)


Product           Plank        Sheet        Sheet
Thickness, in.      4.0         0.16         0.10

Screw type        Turbo  Std.  Turbo  Std.  Turbo  Std.
Output, 1b/hr      2065  1414   1434   829   1478  1014
Screw speed, rpm   21.4  15.5   19.4  16.5   24.0  18.5