Which twin-screw compounder is for you?

Thanks to recent design changes, performance differences between counterrotating and corotating extruders have narrowed for many applications.

The choice of a corotating or counterrotating twin-screw extruder for compounding may not be as easy as it once was. Formerly distinct differences between the processing capabilities of the two types of machines have become blurred in the past few years. In that time, some counterrotating twin screws have been redesigned to process a wider range of materials at rpm's and throughput rates that more nearly match those of corotating machines.

Today's counterrotating and corotating twin screws work equally well in approximately 70% of compounding applications, leaving only about 30% of applications where one machine may perform dramatically better than the other. If your spectrum of compounding needs includes even a few applications that would be better served by one machine more than the other, you can be confident that today's extruders whether corotating or counterrotating - have the versatility to handle a large majority of all compounding tasks efficiently.

American Leistritz sells compounding extruders that can be used in either corotating or counter-rotating mode (with an appropriate change of screws). Today, approximately 40% of the twin-screw machines Leistritz sells are outfitted for use in counterrotating mode and for processing a broad range of materials.

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WHICH WORKS BEST?

To determine which screw would work better in a particular application, a processor should ask three questions:

1. Which materials are to be compounded and what degree of mixing intensity (shearing) is needed? The theology of the materials to be compounded is a significant factor in choosing the type of machine to be used. If the material can tolerate the drag-flow or cross-screw shear forces associated with corotating screws, and if optimal mixing can be achieved with screw designs common to a corotating screw, then corotating may be best. For some compounds - such as those containing high levels of titanium dioxide or mineral fillers the degree of shear that corotating screws impart is more than adequate. But even in this application, production runs indicate that new counterrotating machines give output rates and dispersion quality equal to, and sometimes better than, corotating machines (see table at left).

Once used almost exclusively for mixing color concentrates and PVC, counterrotating twin screws are being used increasingly to process materials typically run on corotating machines. These include polyolefins and engineering thermoplastics containing high-viscosity and low-viscosity phases, such as high levels of plasticizers or slip agents (as in masterbatches). Other uses for counterrotating machines today include devolatilizing, reactive grafting, and compounding with organic pigments or resins such as PET, adhesives, medical-grade TP urethanes, and EVOH/PE blends.

Use of counterrotating twin screws can be desirable when processing high-viscosity materials, including polyolefins with a fractional melt index (below 1.0). Counterrotating machines also excel in applications that require precise control of residence time, low shearing, positive pumping, and/or constant pumping pressure.

Counterrotating twin screws are recommended for these materials: ones that are not mixed thoroughly by common corotating screw elements; ones that are heat-sensitive and degrade under the shear stress imparted by corotating screws; or materials that are too slippery to be handled by drag-flow conveying.

While counterrotating and corotating twin screws both rely to a large extent upon drag-flow (frictional) conveying, regions can be established in a counterrotating screw profile that provide positive conveying via tightly interlocking pump elements. Besides counterrotating twin screws, the only other extruder type that can provide positive forwarding is the ram extruder.

2. How many different processing functions must the machine perform? That number helps determine how long a machine is required to do the work. The design possibilities of counterrotating screws give better "function density," meaning they can perform more process functions - such as feeding, melting, mixing, pumping, and venting - in a given L/D than are possible in corotation.

3. Will I extrude a finished product directly from the compounding machine? In general, counterrotating screws deliver better pumping at the end of an extruder than corotating designs. Counterrotating types often need only standard geometries to conduct direct extrusion into sheet or film. This difference becomes insignificant if a gear pump is used between the extruder and die. Counterrotators often still use gear pumps, usually due to conflicting requirements of compounding, reacting, and devolatilizing versus simple pumping.

SCREW-DESIGN EVOLUTION

The counterrotating twin screw has seen recent design changes in its three basic elements - the pumping (forwarding) element, mixing element, and zoning element.

Pumping elements are generally located at barrel holes such as feed ports, vents, drains and at the die end of the screw. The mixing elements can be dispersive or distributive in their function and can act as a forwarding device or be neutral or even restrictive.

Zoning elements are used to isolate one function from another: They can be righted, non-righted, restrictive, or neutral and can even incorporate more or less mixing functionality.

In place of the old-style mixing/pumping elements, the last five years have seen counter-rotating screws incorporate dedicated mixing elements (without pumping flights) that consist of multi-lobed "kneading" elements similar to those long associated with corotating screws. New multi-lobed shapes can also be used. The mixing and pumping functions are now more effectively separated.

The trend is moving away from the older-style calender-type mixing element, which also incorporated directly intermeshing flights for positive pumping/forwarding action. This older type of element provides a powerful combination of dispersive and distributive mixing, but at the expense of lower screw speeds.

Because of the element's forwarding action, the calender-gap mixing area tends to stay full. However, the combination of mixing and pumping in one element tends to create high pressure in the calender gap and consequent screw deflection. To stay within screw-deflection safety limits, screw speeds of earlier counterrotating machines had to be held below critical levels, which in some cases meant running the machine at haft its rpm capability. This limited productivity with the counter-rotating screw.

The classical calendering/direct intermesh elements are now rarely used, except sometimes at the end of a screw to add pumping capability. However, some customers request a calender/direct intermesh comprising up to 15% of a screw's length.

Newer counterrotating screws run at 300-500 rpm, similar to corotating machines, vs. 100-150 rpm for classical counterrotating types. Productivity is usually greater with the higher screw speeds. Some customers report improved mixing with the new counterrotating designs. "Self-cleaning" ability and ease of color changeover remain good with the new designs.

BETTER MIXING ACTION

The newer counterrotating twin screws have become more like corotating machines by their transfer of the mixing functions from flighted forwarding-screw elements to dedicated mixing elements. However, in some cases the new counterrotators may have a real advantage over corotators.

As the two screws turn in corotation, the left and right lobes approach the intermesh region from the opposite direction. Corotating screws can only have one lobe in the intermesh region at one time so as to avoid collision. The lobed screws turn at different velocities and in a large angular sweep before the next lobe of the other screw can penetrate the intermesh region.

This factor restricts the number of lobes for a corotating system, usually to a maximum of two per screw. Though there are tri-lobal corotating screws available, they are rarely used due to poor throughput associated with shallow flight depths.

In corotation, dispersive mixing occurs largely outside he intermesh region by pressure-flow forces between the lobe and barrel wall. Higher shear is created as a result. (This is also true for many counterrotators.)

While lobal action against a barrel wall is similar in both rotating modes, lobed counterrotating behave differently in the intermesh region. Because the screws turn into each other at the intermesh region, more than one lobe can be in the intermesh region at one time. This allows for more mixing to occur directly in the intermesh region and at lower shear. In fact, the lobes mate like loosely meshing gears. The angular sweep between lobe penetrations can be much smaller in counterrotation than in corotation, allowing a higher lobe count and therefore more mixing points per turn of the screw.

Counterrotating screws can incorporate elements of two to eight lobes each, and the screws can turn at the same velocity without fear of collision in the intermesh region. A hexa-lobal counterrotating screw may impart less shear than a bi-lobal corotating screw.

Classical counterrotating screws are still successful in certain applications such as color masterbatch production, due to the highly dispersive mixing in the calender gap and quick cleanout. But the newer designs also process color concentrates well - often at higher screw speeds and throughputs. And the new counterrotating designs can provide throughputs of a wide range of products that are competitive with corotators. Their ability to zone regions for positive pumping and high lobal-count mixing also make them attractive for many applications.

William Thiele is general manager of American Leistritz Extruder Corp., Somerville, N.J. Mikell Knights is assistant editor at PLASTICS TECHNOLOGY.