New muscle machines for compounding.

The new gang of high-output twin-screw compounders packs oversized motors that can rev screw speeds over 1000 rpm with tons of torque to match.

The first thing you'll notice that's different about these extruders is the outsized motor. Start the machine up with some resin in the hopper, and you'll see what all that power can do: Thirty percent higher horsepower and torque plus screw speeds of 1000-1200 rpm set apart a new generation of brawny twin-screw compounders capable of output rates double those of previous machines of similar size.

"There's always been high torque. The new wrinkle is adding high speed to the high torque," says Alfred Bartkiewicz, v.p. of sales and engineering at Davis-Standard Corp. His firm is one of several that are coming out with twin-screws that greatly exceed the 600-rpm maximum screw speeds typical of standard models. Not only can higher speeds push out more pounds per hour, but the increased torque lets a compounder take more resin into the screw flights, further raising output. (Barrel lengths are in the standard range of 28-40 L/D.) The combination of speed and torque gives processors two space-saving options: either raising throughputs without going to a bigger machine or maintaining current throughputs with a smaller machine.

Paul Anderson, director of process technology at Werner & Pfleiderer Corp. says that, for example, in a case where a conventional twin-screw unit operating at 400 rpm produces 1000 lb/hr, the firm's new "Mega-Compounder" will extrude 1300 lb/hr at the same rpm by virtue of its 30% higher torque (i,e., it can handle 30% more material filling the screw flights). If the screw speed is raised 15% to 460 rpm, output of the Mega machine would rise to 1500 lb/hr, a 50% increase over the conventional machine at 400 rpm.

"One chief benefit is that processors are not losing the normal capabilities of the conventional machines that they are used to. They can start off at an accustomed speed and can gradually move up to the higher rates," says Gene Stroupe, v.p. of the compounding division of Berstorff Corp. However, William Thiele, general manager at American Leistritz, says a machine specifically optimized for high-speed processing may not perform well at the slower speeds.

Processors should be prepared to pay a 10-30% premium for these new units, with most of the cost going toward the larger motor.

SCREWS UNDERGO CHANGE

Higher attainable screw speeds translate into shorter polymer residence times. Common residence times of 20-30 sec experienced when running at regular screw speeds can be trimmed to 6-10 sec at the higher speeds, says Berstorff's Stroupe. Shorter residence times are generally beneficial to the resulting properties of the material, he notes. On the other hand, higher rpm imparts higher shear and melt temperature, cautions Martin Mack, Berstorff v.p. of R&D. W&P's Anderson replies that a material may withstand a shorter residence time at higher temperature with no worse effect than a longer time at a lower temperature.

Although compounding extruders are typically starve-fed, the new high-torque machines can handle a bigger "bite" of material in the screw channels. The greater use of the free volume of the screw channel helps to keep the melt temperature down at the higher speeds, suppliers say.

In order to make effective use of the enhanced capabilities of the new machines while maintaining melt quality, equipment suppliers are recommending use of water-cooled barrels. They have also re-examined their screw designs, especially the mixing/melting portion. "The machine will have to be configured differently, whether that means adding diameters or changing the barrel or screw elements," notes Michael Hotchkiss, director of sales at Farrel Corp. Stroupe at Berstorff agrees: "The processing window is narrower at high speeds. The residence times are short so screw profiles are more critical."

W&P decided the best approach for high-speed mixing was to develop a brand-new patented three-lobed mixing element in place of the bi-lobed elements that have been the mainstay of its machines in recent years. The new tri-lobe kneading block reportedly provides gentler shear by means of a changed gap between opposing elements.

Other machine builders are sticking mainly with bi-lobal kneading blocks. Welding Engineers' new high speed Toshiba extruders use bi-lobal kneaders. Bi-lobal blocks on Berstorff's new Ultra Torque machines were modified for the higher speeds. Berstorff also is making the new screw elements of composite materials rather than through-hardened steels in order to provide increased strength and wear resistance. And for increased flexibility, the company now offers 4D and 6D barrel segments in place of 5D segments.

Farrel uses its patented polygon elements and bi-lobal continuous mixing elements (CMEs) to impart low shear with good distributive mixing, says Hotchkiss. Unlike a kneading block, the polygon elements are non-intermeshing. providing "lots of free volume," he says. The CME units are partially intermeshing. The combination creates a low-shear environment where there is no dramatic increase in melt temperature at high screw speeds.

B&P's new XLT screw design enhances the self-wiping efficiency and pumping capability of its MP 2000 series machines while helping to minimize the exit temperature of the melt at the die, says Greg Burns, product manager.

Davis-Standard is in the second part of a three-stage program redesign its twin-screw elements. The first stage allowed for processing at 400600 rpm, while the second (mid-range) stage is optimized for speeds of 600-900 rpm. The next step will be to perfect the screw design for speeds of 900-1200 rpm, says Bartkiewicz.

Berstorff, Century Specialties, Davis-Standard, Welding Engineers, and W&P all use a new or redesigned splined screw shaft to help impart higher torque and speed to the new machines. Farrel uses a two- or four-keyed shaft instead.

TORQUE COMPARISON OF W&P TWIN-SCREWS

ZSK       Super-          Mega-
Size      Compounder      Compounder
(mm)      (Nm/shaft)      (Nm/Shaft)

40        325             425
50        n/a             815
58        960             1250
70        1750            2275
82        n/a             3550
92        3850            5000
133       11,600          15,100

WHO NEEDS ALL THAT POWER?

The higher speed capabilities of the new machines may not be suitable to every processing application. though they seem to be far from a niche product. "Sales have primarily gone to makers of engineering resins." says W&P's Anderson. Applications include filled and reinforced compounds and alloying/blending. "Resins tailored for injection molding are suited to the higher speed process, says Berstorff's Mack.

A number of machine suppliers have noted surprisingly good results in high-speed compounding with chopped glass fibers. They say that the short residence times do not result in shorter fibers. W&P has run a 30% glass-filled polycarbonate on a ZSK 58-mm Mega-Compounder at rates of 3000 lb/hr using 1150 rpm. Welding Engineers ran a PC powder at over 1000 rpm and over 2200 lb/hr on its 58-mm high-torque machine.

Some suppliers note success with talc and carbon black as well. However, James Schak, director of sales and marketing for Welding Engineers, recommends using fillers with a minimum density of 30 pcf. "The heavier materials have a better chance of conveying in a high-speed screw. The slower machines handle the lighter Werner materials better," he says.

High-speed compounding of color concentrates and other masterbatches is one of the next frontiers to be explored. Welding Engineers, for example, has run an LDPE with 20% surfactant at 800 lb/hr and 800 rpm. High-rate mixing will become even more critical if the process is to find other applications. In a demonstration of its technology in Germany last month, W&P made a color compound by melting the resin first, then introducing the colorant, which helped to achieve higher feed rates without exposing the pigment to excessive shear.

Some materials don't lend themselves to the mixing/shearing characteristics of the new high-speed machines, concedes Fred Bartkiewicz of Davis-Standard. Heat- or shear-sensitive materials could be in for a rough time at the higher speeds. "Abrasive fillers, crosslinked materials, and reactive processes don't need to be exposed to the higher shear stress," says Farrel's Hotchkiss. TPEs, TPOs, reaction processes like TPU production, and other elastomers need longer residence times, be says, making them unsuitable for high-speed machines. These materials may be able to take advantage of the machines' higher torque, if not their speed, Anderson says.