Buyers' guide to twin-screw compounders.

Proliferation of suppliers of twin-screw compounding extruders makes choices more complicated than ever before. Here's summary of who offers what, and tips on making an informed buying decision.

Quite a number of new suppliers of twin-screw compounding extruders have entered the North American market in the last two or three years. This fact, combined with expanded product lines from established companies, has enormously widened the field of choice. This article presents the first up-to-date summary of what models are available from which suppliers and attempts to offer some means of comparison between models, pointing out some of the more important mechanical design features to consider when shopping for a machine.

Two types of twin-screw extruders for compounding are discussed here: corotating intermeshing and counterrotating non-intermeshing, or tangential. (A third twin-screw category - counterrotating intermeshing - which typically operates at low screw speeds and is primarily used in PVC processing, is not included here.) The second major group of machines considered are twin-rotor continuous mixers, which use "wing-type" rotors (similar to those in Banbury-type batch mixers) rather than screws to accomplish mixing and do not generate high pressure. Most machines in this group are based on the Farrel Continuous Mixer concept. The accompanying table is limited to twin-screw compounding extruders, because this is the machine category in which the proliferation of suppliers has mainly occurred, and in which distinguishing features between models are most evident.

BASIC CHARACTERISTICS

Twin-screw compounding extruders use varying degrees of distributive and dispersive mixing to combine two or more materials into a homogeneous mass. Distributive mixing blends the components uniformly in the mixture; in dispersive mixing, materials undergo changes in physical characteristics (reduction in agglomerate size). Both varieties of mixing require mechanical shear energy to be transmitted into the polymer, created by both corotating intermeshing and counterrotating tangential machines as their screws rotate against the barrel walls. Where these machine types differ the most is in what happens to the material in the shear field at the apex, where the two screws meet.

Screws of the corotating intermeshing type convey materials forward in a "figure-eight" pattern. In general, material alternates between moderate shear against the barrel wall and higher shear in the apex region. Shear-inducing kneading blocks can be added to increase dispersive mixing, and specialized screw elements can be added for distributive mixing.

Non-intermeshing screws, on the other hand, impart only minimal shear at the apex. Materials processed with tangential screws experience moderate shear against the barrel wall, then relax in the apex region. In tangential machines, the rotation of material in the screw channel and interchange of material between the screws is said to provide constant distributive mixing. Compounder sections can be added to provide higher shear for dispersive mixing.

Also using the counterrotating non-intermeshing principle are a number of twin-rotor continuous mixers. One claimed feature of these machines is that they separate mixing and pumping functions. Freed of the restrictions of combining pumping and mixing in the same barrel, this type of machine reportedly has the flexibility to optimize mixing for a wide range of materials. Forces on bearings and components are radial, not axial, as in twin-screw extruders, and shearing takes place mainly between the tip of the rotor to the barrel wall. Because there is little thrust, more energy can be put into mixing.

COROTATING INTERMESHING

By far the largest group of new suppliers and machines to appear on the market in recent years has been in the corotating intermeshing twin-screw extruders. One important overall trend in twin-screw compounding is the ability to process increasingly greater volumes for a given size machine. A number of manufacturers of corotating intermeshing twin-screws claim to offer machines that maximize free volume, or the free space in the barrel that determines volumetric processing capacity. Free volume may be defined by the centerline distance between the screw shafts and the OD/ID ratio of each screw.

APV Chemical Machinery, for example, claims that its MP-2000 series extruders have a greater free volume than competing high-torque corotating compounders. William J. Kleffman, manager of process engineering, says the line has "26% greater free volume than our closest competitor, assuming the same bore diameter and barrel length."

He points to three distinct benefits of the higher free volume. Feedstock fluidization can be minimized when feeding powdery materials, which permits higher throughput rates. Residence time can be maximized, allowing higher throughput rates on some chemical reactions and formulations of highly divergent viscosities. Lastly, vacuum entrainment can be minimized, allowing higher volatile levels to be drawn from the process. In addition, its clamshell barrel design can be an effective screw-design tool (besides facilitating cleanout). During lab testing, the machine can be stopped and the barrel opened while still full of process material, permitting "dead-stop" analysis of mixing.

Berstorff offers two different channel-depth options at the same screw diameter on its ZE and ZE-A lines. In the ZE-A series, the screw flights are deeper and the root of the screw is reduced. The larger free volume, coupled with high torque capability, results in higher throughputs, according to Gene Stroupe, product manager for compounding equipment. Other advantages of the ZE-A line are said to be reduced shear at a given screw speed, suitable for heat-sensitive materials, and large conveying volume.

Berstorff's latest introduction, the ZE-R series, has even deeper flights and larger free volume (although slightly lower torque) than the ZE-A. The deeper flights, coupled with venting provisions in the barrel, are said to make the ZE-R suitable for reaction and devolatilizing applications (see PT, July '91, p. 43).

Another important aspect of the free-volume issue is the ability to transmit power through the shaft. "It doesn't matter how much free volume you have if you don't have the power to put into the material," according to one machinery OEM. The relationship between free volume and power varies among different suppliers.

As one example, Werner & Pfleiderer claims that the 1.55 OD/ID ratio of its fifth-generation Super Compounders optimizes the balance between free volume and power for a wide range of materials. "The ratio of 1.55 is a midpoint based on specific mechanical energies we've seen in processing many materials over the years," explains Dr. Paul Andersen, director of process technology. "It provides high torque transmission and high free volume." An added benefit is that the power per unit of free volume is constant over the entire family of extruders. This consistency, which Andersen claims is unique to the Super Compounders, is said to make scale-up easier, because there is a consistent shear rate from one size to the other, assuming the same degree of channel fill.

A third related trend in corotating intermeshing twin-screws is the ability to run at increasingly higher speeds. According to Asmut Kahns, W&P's director of plastics machinery, "The trend is to push to higher rates and higher outputs whenever the process will allow it." W&P's Super Compounders, for example, can reach 600 rpm on some fairly large machines. At the higher speed ranges, the limiting factors are process-related - i.e., whether it's possible to maintain product quality at those high rates. Other limiting factors may be wear-related, as with abrasive fillers. But for nonabrasive applications, higher speeds may be achievable. Shear stress often can be minimized by screw design, so that even some temperature-sensitive materials, such as polycarbonate, can benefit from higher outputs, he adds.

MORE OPTIONS AVAILABLE

Several of the newer twin-screw nameplates have brought with them a number of distinctive features worth mentioning. These, combined with offerings of long-established suppliers, has resulted in a wide range of options from which to choose.

Theysohn Corp. showed its corotating twin-screw compounders for the first time in the U.S. at NPE '91. (PT, Jan. '91, pp. 21, 79; May '91, p. 173). Features include a simple rectangular barrel shape without flanges. The barrels are easier and more economical to manufacture, reportedly allowing a wider selection of alloys to be used in construction. Heating and cooling are achieved with cast-aluminum units attached onto the barrel, which reportedly eliminate stress cracks associated with having cooling coils bored directly into the barrel. Barrel segments are supported on a mounting bracket, which is said to offer better alignment than flange-type barrel segments. Screw elements and kneading blocks are provided with a twist-lock configuration for fast assembly/disassembly.

Davis-Standard has introduced its D-TEX line of twin-screws, based on technology from Japan Steel Works (PT, Oct. '90, p. 26). These convertible machines feature a gearbox that will change modes from corotating to counterrotating with no modification of its mechanical configuration. The machine's power capability is said to combine high torque and high speed for high throughputs. Its water-cooled barrels are said to have advantages in productivity and quality.

Japan Steel Works, as part of its licensing agreement with Davis-Standard, is not marketing its own TEX twin-screw extruders - on which the D-TEX line is based - in the U.S., Canada, or Mexico. But JSW does sell continuous mixers.

Welding Engineers has formed a partnership with ICMA of Italy (superseding ICMA's former relationship with Wayne Machine & Die Co.) to offer a line of corotating intermeshing twin-screws as a complement to Welding Engineers' well-known counterrotating, non-intermeshing type (PT, Dec. '91, p. 46). The new corotating intermeshing machines are available in two-lobe, three-lobe, and two-lobe/low-speed versions. Welding Engineers plans to eventually manufacture the intermeshing machines in the U.S., with some modifications to gearboxes and barrels.

Delaware Extruder Corp. is a relatively new firm offering corotating intermeshing extruders (PT, Nov. '90, p. 19). Again, the manufacturer emphasizes their high-torque capacity. Output shafts from the gearbox are balanced by contact with two larger gears, one above and one below. Energy of each larger gear is counterbalanced by the other and is directed inward toward the pinion gears turning each shaft. Higher torque is said to be transmitted to each shaft because opposing gears reduce deflection that normally occurs in designs using a single unopposed gear. The DEC 92, for example, is equipped with a gearbox that provides up to 50,000 lb of thrust per shaft at 200 rpm with a usable head pressure of 5000 psi.

W&P offers improved ZME distributive mixing elements in addition to its traditional TME mixing elements. ZME elements are pitched to be fully self-wiping on the screw root and barrel wall, eliminating possible dead spots. It also has experimented with a side vent to prevent material that has deposited on the vent from falling back into the melt stream. Also new from W&P is a radial barrel-valve option to complement more conventional axial barrel valves (PT, July '90, p. 64).

Toshiba Machine Co. America is free to reintroduce its line of corotating and counterrotating intermeshing extruders to the U.S., following last year's expiration of a three-year trade sanction.

A few suppliers combine the features of corotating twin screws and twin rotors in the same machine. Pomini uses such a concept on its relatively new LCM-AX model, designed for engineering resins. This single-stage, axial-discharge machine places corotating intermeshing screws in the feed and discharge sections. In the central mixing zone, wing-type mixer rotors transmit high shear into the material, providing high dispersion of components. L/D ratio for all sizes is 25:1 with one feed port and 35:1 with two feed ports. Similarly, Kobe Steel's KTX series extruders provide a rotor segment based on its Mixtron series continuous mixer. It's said to give more efficient compounding of fillers at lower temperatures than conventional kneading disks. Wing-type segments are available from other suppliers as well.

COUNTERROTATING

NON-INTERMESHING

Suppliers of counterrotating non-intermeshing twin-screw machines point to a number of advantages in feeding, mixing, and venting. The two primary suppliers of this type of extruder are Welding Engineers and, more recently, American Leistritz, the latter supplying machines that are convertible into all major operating modes.

One advantage of this type, says Brad Gannon, product manager at Welding Engineers, is maximum free volume, which is the result of two related design factors inherent in non-intermeshing screws. First, greater centerline distance can be achieved between screw shafts; second, flight occlusion - in which the wiping function of screw flights against the root of the opposite screw necessarily takes up some volume - does not occur in non-self-wiping screws. A third point relates to flexibility. Because the screws don't intermesh, it's possible to vary the root diameter to provide needed shear energy for the application. Gannon claims that, on average, counterrotating non-intermeshing machines may provide up to 30% higher output rates than corotating intermeshing types of the same screw diameter.

The non-intermeshing screw configuration also has implications for available power. Welding Engineers claims that more available horsepower for the same screw diameter enables its machines to handle greater speed, volume, and L/D. Most of the company's machines provide up to 500 rpm.

Gannon claims that counterrotating non-intermeshing machines are well suited to distributive mixing and provide a high degree of flexibility and control of the shear energy imparted. One advantage of the low-shear field in the apex is that it reportedly permits relatively high screw speeds in compounding shear-sensitive materials. Where more shear is needed, compounder sections may be added. Staggered screw flights provide cross-channel and radial back-mixing. An interesting point here, he adds, is that non-intermeshing machinery decouples mixing efficiency from screw speed, so that mixing remains relatively efficient at low rpm.

The counterrotating configuration also has advantages in feeding and venting, says Gannon. In feeding, the downward motion of the counterrotating screws is said to allow wider openings across both screws and to promote better feed acceptance. The same principle applies in venting, where wider openings are provided directly above the apex. Vents may be longer as well, he says. On Welding Engineers models, 6:1 L/D barrel segments may be equipped with 4:1 L/D openings.

CONTINUOUS MIXERS

Twin-rotor continuous mixers, first developed by Farrel Corp., are distinguished from twin-screw extruders in that they do not generate pressure. These counterrotating, non-intermeshing machines segregate mixing and pumping functions, the latter of which may be performed by a single-screw extruder or gear pump after the mixing section. They have a conventional screw configuration only in the initial feeding and melting section. Intensive shearing in the mixing section is created between the rotors and chamber walls, coupled with kneading between the rotors and a rolling action of the material itself.

Farrel marketing director Stephen E. Peterson points to a number of basic advantages of the Farrel Continuous Mixer (FCM). A wide range of materials can be run on the same configuration by adjusting certain process parameters, such as temperature of water circulating through the barrel jacket, rotor speed, or how full to run the machine. Ample clearance between the rotors and between rotors and the chamber barrel is said to be an advantage for mixing abrasive materials.

In addition to its conventional FCM, Farrel offers a UniMix high-output version, an LM unit for lab use and production of color concentrates, and side-discharge configurations. Most recently, it introduced an updated version of its Continuous Processor (CP), consisting of FCM above a hot-feed extruder on a common frame.

The FCM twin-rotor concept has been adopted by a number of other machine suppliers:

* Pomini offers its Long Continuous Mixer (LCM), with a longer mixing chamber and rotor length than the FCM.

* Kobe Steel  offers four series of continuous mixers for different applications. The K series has elliptical rotors offering a high degree of mixing and dispersion of different kinds of polymers. The N series has triangular rotors and longer mixing chamber for more intensive mixing. The L series performs two-stage mixing and degassing.

* Kobelco Stewart Bolling (owned by Kobe Steel) manufactures the Mixtrumat, which discharges through a second-stage, transverse-mounted single-screw extruder.

* JSW Plastics Machinery, sub. of Japan Steel Works, offers its CMP series of continuous mixers in models ranging from 90- to 400-mm rotor diameter in the mixing zone. Clearance between screws and barrel can be adjusted with a slot mechanism.

* Moriyama Corp., Farmingdale, N.Y., offers a twin-rotor continuous mixer, called the CM series, in 60- and 120-mm models, with a 6.5:1 L/D ratio and clamshell-opening barrel. The counterrotating machine has a patented combination of one tri-lobed mixing rotor and one four-lobed rotor.

There remains one unique type of machine that belongs in the continuous-mixer category rather than with the true extruders. Teledyne Readco offers what it calls a "low-pressure extruder" with corotating intermeshing twin screws. Most of the process length, after the initial feeding/melting section, consists of twin-lobed kneading disks. Although only about 10% of these machines go into plastics applications, product manager Irv Snider believes that the machine has some potential as a "preconditioner for a true extruder."

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SUPPLIERS OF TWIN-SCREW COMPOUNDING EXTRUDERS

APV Chemical Machinery, Inc., Process

Equipment Div., Saginaw, Mich.

(CIRCLE 20) American Leistritz Extruder Corp.,

Somerville, N.J. (CIRCLE 21) American Maris, E. Brunswick, N.J.

(CIRCLE 22) Amut, distrib. by Carl G. Brimmekamp &

Co., Inc., Stamford, Conn. (CIRCLE 23);

and Glacier Machinery Sales Corp.,

St. Paul, Minn. (CIRCLE 24) Berstorff Corp., Charlotte, N.C. (CIRCLE 25) Dr. Collin GmbH, distrib. by Carl G.

Brimmekamp & Co., Inc., Stamford,

Conn. (CIRCLE 26) Davis-Standard Div., Crompton & Knowles

Corp., Pawcatuck, Conn. (CIRCLE 27) Delaware Extruder Corp., Milford, Del.

(CIRCLE 28) Egan Machinery Div., John Brown, Inc.,

Somerville, N.J. (CIRCLE 29) Mitsubishi Heavy Industries America, Inc.,

N.Y.C. (CIRCLE 30) Pomini, Inc., Brecksville, Ohio (CIRCLE 31) Teledyne Readco, York, Pa. (CIRCLE 32) Theysohn Corp., McPherson, Kans.

(CIRCLE 33) Toshiba Machine Co. America, Elk Grove

Village, Ill. (CIRCLE 34) Welding Engineers, Blue Bell, Pa.

(CIRCLE 35) Werner & Pfleiderer Corp., Ramsey, N.J.

(CIRCLE 36)

SUPPLIERS OF TWIN-ROTOR CONTINUOUS MIXERS

Farrel Corp., Ansonia, Conn. (CIRCLE 37) Pomini, Inc., Brecksville, Ohio (CIRCLE 38) Kobe Steel, N.Y.C. (CIRCLE 39) Kobelco Stewart Bolling, Akron, Ohio

(CIRCLE 40) JSW Plastics Machinery, Houston

(CIRCLE 41) Moriyama Corp., Farmingdale, N.Y

(CIRCLE 42) Teledyne Readco, York, Pa. (CIRCLE 43)

[Tabular Data Omitted]