Compounding.

Compounders can benefit from a number of introductions in both twin-screw and in other continuous compounders. Perhaps one of the more significant new concepts revolved around a one-step process that combines compounding and extrusion. At least two companies unveiled systems with these "direct extrusion" capabilities.

Berstorff, for one, showed a turnkey twin-screw system for in-line compounding and sheet production. The line at the show consisted of a ZE 90A 90-mm, 23:1 vented extruder, along with a screen changer, gear pump, sheet die, and horizontal roll stack. Capable of speeds of 98 ft/min, the line can yield an output of roughly 1760 lb/hr. It can accommodate sheet widths to 70 in. and thicknesses between 12 and 58 mils.

Similarly, Reifenhauser also unveiled a brand new twin-screw for in-line compounding and profile or pipe extrusion. The Reitruder RZE 85 relies intermeshing, corotating parallel screws to handle the compounding and extrusion duties with no intermediate steps. Unlike the company's Bitruder conical twin-screws, the Reitruder features modular barrels and screws. It can be both air and water cooled. At the company's lab, the 85 mm Reitruder was the heart of a system making winding cores from thermoplastic industrial scrap at 660-1100 lb/hr.

TWIN-SCREW COMPOUNDERS

Farrel Corp. showed off its newly-acquired line of corotating twin-screw extruders from the former S. Rockstedt OHG of Germany. Previously unavailable in North America, these extruders feature a unique screw design that doesn't employ kneading blocks. "We don't need them and don't use them," says Farrel v.p. Alberto Shaio. Instead, the screws feature mixing sections with a polygonal geometry. Shaio explains that these polygon sections don't intermesh, providing a "large-volume working chamber" that can increase residence times in a relatively short extruder--they typically have an L/D of 24:1. And because these polygon sections are symmetrical, they can be joined without regard to the alignment considerations associated with kneading blocks. The remainder of the screw does intermesh, though. Its elements are interchangeable, but the company says a standardized screw design can handle nearly all applications. Though air-cooled, standard Farrel-Rockstedt extruders can operate at temperatures up to 840 F. Sizes range from a 35-mm lab model up to a 115-mm extruder capable of outputs up to 4840 lb/hr.

Farrel also showcased its completely redesigned continuous compounder, the CP 500 Series II. Like earlier units, it consists of the company's FCM twin-rotor continuous mixer mounted over a hot-feed single-screw extruder on a common frame. Aside from making the unit more self-contained, the company has added a number of features: independently variable mixer and extruder speeds; centralized service connections reduced to one each for water and electricity; a new operator touchscreen station; and elimination of the lubrication system in favor of grease-packed bearings. Lastly, the company switched to a new three-piece rotor design with quick disconnect. Rotor changes that formerly required the removal of bearing blocks and took over an hour can now be done in 10 min, notes Shaio.

Berstorff showed the first production model of its multiscrew extruder (MSE). With its 10 screws in a planetary arrangement, the unit has been simplified compared with earlier versions (see PT, July '91, p. 43). For instance, the MSE now uses only one mechanical seal and stuffer box. According to U.S. v.p. Gene Stroupe, the MSE is aimed primarily at "lower-end" devolatilization tasks, those with solvent levels around 6-8%. For more intensive devolatilization and reactive applications, Berstorff recently introduced the ZE-R series with larger working volumes (PT, July '91, p. 43).

Werner & Pfleiderer introduced a new devolatilization or pre-concentrating" unit. The VS-30 is designed to be a pre-devolatilizer or "flash chamber" to remove as much solvent as possible upstream of a devolatilizing extruder. It's a vertical, thin-film evaporator with twin stirring rotors that wipe both each other and the chamber walls. The top-driven unit also has twin discharge screws at its bottom.

W&P also showed off a new look for its ZSK machines with new enclosed motors and barrel covers to minimize heat loss and improve worker safety. And, the company introduced a new water-ring pelletizer and a new non-stop screenpack changer, the ZSW.

Japan Steel Works (JSW Plastics Machinery), which makes the Tex line of twin-screw extruders, showed off its new CIM system. The JSW-Exanet is a distributed control system compatible with a variety of local-area networks.

If you're looking for a really tiny lab twin screw, Prism Inc. of England now offers its 16-mm, 15:1 compounder to the North American market through Welding Engineers. Like their larger production brethren, these tiny corotating machines feature segmented screws, variable-speed drives, and microprocessor control. It's also available with benchtop pelletizing system. The Prism extruder has output rates up to 22 lb/hr and a temperature range up to 570 F.

WHAT'S NEW IN SINGLE SCREWS

A new line of single-screw reciprocating kneader-extruders from Switzerland's Buss AG features numerous design changes. Called Advanced Performance Compounders (APC), the new machines feature performance advantages over the earlier MDK series. According to sales and marketing manager Michael Irish, the APC has 30% greater throughput capacity and max. screw speed increased from roughly 300 rpm to 500 rpm. Electric heating, rather than oil as in the past, allows the APC to process at temperatures up to 790 F, while the heat-up time has dropped substantially, Irish says. Side-feeding and increased venting capabilities are also new features on the APC. Available sizes so far are 70 and 100 mm, but other sizes will come out in 1993, the company says.

Buss also unveiled some new screw elements for all its Kneaders. A relatively narrow-flighted "equal-shear element" opens up the gap between the screw flights and barrel pins. When processing high-viscosity materials, the element can result in 30% less shear or 30% higher output rates at a given processing temperature, says Irish.

A still-experimental screw element from Buss reportedly lessens shear rates by 50% and may spur the company to renew its efforts in the thermoset market, adds Irish. Already, this new element being used commercially for other high-viscosity polymer systems, such as rigid PVC and semiconductor cable.

KSBI Inc. has switched to segmented rotors on its continuous mixers. The rotor elements fit together on a splined shaft, much the way twin-screw elements would, says sales engineer Kenneth Nekola.