Single-Screw Compounding Is Learning New Tricks
While twin-screws get most of the glory, a quiet revolution has been taking place in single-screw compounding. In the past six to 12 months, some half-dozen new dispersive mixing elements have gone into commercial production, and more are on the way.
For the first time in decades, several genuinely new single-screw mixing elements are emerging from R&D labs and going to work on the plant floor. Some are the result of advances in flow simulation and CAD design, others come from improvements in metal cutting. All but one are already in commercial use. In fact, there are over 60 applications, mostly at resin suppliers and at processors who are compounding in-line with extrusion, injection molding, or blow molding. For the moment, few of the new designs are being used in commercial compounding.
Most of the recent work has been aimed at remedying single-screws' notoriously limited ability for dispersive mixing (which typically involves breaking up agglomerates of fine particles). If single screws could do a better job there, they would have the advantage of being a lot less expensive than twin-screws.
Until recently, screw designers have tried to improve single-screw mixing with variations on decades-old designs like the Maddock mixer (a fluted cylinder), Saxton mixer (a densely flighted screw with a crosscut through the flights), and the "pin" or "pineapple" mixer, among others. Compounders who wanted more dispersive action could also add separate devices onto the end of an extruder--such as the cavity-transfer mixer, which has pockets in the rotor and barrel that push and pull the melt for extensional mixing; or a planetary gear mixer, in which a short screw is ringed by a half dozen smaller orbiting screws. But these devices don't disperse lumps so much as distribute them evenly. And they don't work in reciprocating-screw injection molding.
For the most part, the new mixing designs are not claimed to match twin-screw mixing performance. They are said to do a better mixing job than past single-screws, thereby boosting properties of finished parts, allowing higher letdowns of color concentrates, and permitting quicker color changes. They reportedly also improve homogenization of regrind, especially fluff from film and multi-layer scrap containing incompatible resins. Another claimed benefit is to raise output by 10-25%. "When you have an extruder that does better mixing, you can run faster," says Chris Rauwendaal, screw designer and president of Rauwendaal Extrusion Engineering.
The ring's the thing
Among the newest single-screw mixers are devices with floating rings. The last two examples cited below were reported for the first time at this year's SPE ANTEC meeting by Jeff Myers, engineering manager of Glycon Corp. His papers cite results of testing at Dow Plastics in Midland, Mich. Three ring-type mixers were compared for dispersive and distributive mixing, and their performance was compared with a Maddock mixer and a standard non-mixing screw. The tests ran ABS on a 2.5-in., 21:1 single-flighted screw. All three of the ring mixers performed much better than the Maddock mixer.
Twente Mixing Ring (TMR) was developed at the University of Twente in Holland and is sold here by Glycon. It was the first "floating ring" mixing element, patented in 1992. It has a smooth cylinder that "floats" around the screw and has rows of holes around its circumference. The ring is believed to rotate, but more slowly than the screw. The rotor inside the cylinder has hemispherical dimples or depressions corresponding to the holes in the ring. The combination of holes and depressions forces melt to move forward and backward.
Over 500 of these mixers are said to be running worldwide. Some are used on Krupp Kautex blow molders at the Visteon plant in Milan, Mich., where they help homogenize multi-material scrap from six-layer gas tanks. Van Leer Containers Inc. uses TMR devices to homogenize HDPE and color masterbatch at blow molding plants in Atlanta and Bradley, Ill. The mixers are said to speed color-changes.
Barr Sleeve Mixer (or Barr Mixing Ring, BMR) is another floating-ring device, developed by Robert Barr and Glycon's Myers. It resembles a deeply articulated version of the Twente mixer, but differs in having more holes and clearly defined barrier dams. Tests at Dow indicate that it does as good a job of dispersive mixing as the Twente mixer. The BMR mixer is not yet available commercially.
Barr Ring Mixer (also called the "Infuser"), co-invented by Barr and Myers, is the latest and probably most unusual of the new floating-ring devices. It has a number of disks attached to the screw, which alternate with free-floating disks or stators. Both have axial holes, which align temporarily as the rotors turn, generating radial and axial melt transfer. The floating stators turn also, but more slowly. The number of rotors and floating stators depends on the intensity of mixing desired. Two Barr Ring Infusers are in commercial use--one for blow molding, one for injection molding--at a large Midwest housewares firm. A third is in Dow's R&D lab.
"We're finding in our lab that the Sleeve Mixer gives more dispersive action but with greater pressure drop and higher melt temperature than the Infuser. The Infuser is more distributive, with a high degree of cross-cavity and axial material transfer and minimal temperature increase," says Myers.
New ways to mix it up
More rotor-stator systems, plus barrier designs and other types of mixing sections also number among recent developments.
CRD Mixer, designed by Rauwendaal has been commercial since last fall. It has been sold for over 30 single-screw applications and even for a few twin-screw uses. Most of the CRD Mixers have been built by Migrandy and Harrel.
The CRD Mixer has helical, wedge-shaped mixing and wiping flights. The mixing flights are lower, with a curved or slanted leading flank that forces the melt over the flights and creates elongational stresses. Trapezoidal slots in the mixing flights prompt additional elongational flows. "Elongational flow is much more effective in achieving rapid dispersion than shear flow," Rauwendaal says. CRD mixing sections can be of different lengths (usually 5-6D) and are located at the end of the first and/or second stage of a two-stage screw.
The element increases output by up to 25% depending on material, says Migrandy president Tibor Menyhart. The first commercial installation was last October, at a large Canadian extruder of structural foam, which subsequently ordered three more CRD screws. The first compounder to use it was Uniform Color Co. in Holland, Mich., which uses two CRD mixers on one 3.5-in. screw. Uniform Color says the CRD has eliminated the need for dispersion aids in making color concentrates. The latest application is on a 320-mm, non-intermeshing twin-screw extruder at a resin producer, where it is processing 30,000 lb/hr.
Users report mixed results with the CRD for dispersing gels. Conwed Plastics in Minneapolis bought one to see if it could break up gels in a TPE compound used for netting. Conwed says the CRD gave better dispersion but didn't correct the gels. Southern Film Extruder Inc. in Fort Pierce, Fla., says the CRD Mixer did eliminate gels in PE film, thus permitting regrind levels to be raised from 3% to 12%.
'Left-Hook' Mixer from Scientific Process & Research is a new low-shear mixer said to cause very little pressure drop. It consists of a rotor with an embossed pattern of overlapping, right-angled corners or "hooks." Like the CRD mixer, it can be used in different locations--at the tip of a single-stage screw or before the vent in a two-stage screw (and possibly at the tip, as well). Two Left-Hook mixing sections can be used side-by-side on non-intermeshing twin-screws.
In early trials on flexible PVC, the Left-Hook reportedly produced 8% higher output and 10° F lower melt temperature than a Maddock mixer. Three of these mixing sections are running flexible PVC, including one at a major film producer. Seven others are still being tested on LLDPE/LDPE blends, PE foams, and PP.
Merritt Masticating Head from Merritt Davis is a separate device that bolts onto the end of an extruder to produce high dispersive and distributive mixing. The device alternates rotor rings that have gear-shaped outer diameters with stator rings having gear-shaped inner diameters. The latter are fixed to the inside of the barrel. The rotor and stator gears don't mesh, but are stacked closely together in series. As the screw turns, flow-channel windows open and close in quick succession. The number of ring pairs (outer and inner) and the clearances between them can be varied according to the material and the amount of mixing needed. Merritt Davis has built one device for testing in its lab and is looking for a development partner to continue R&D.
Triple-Wave Screw from HPM has a mixing section with three channels of different and varying depths, separated by barrier flights. Resin flows back and forth over the barrier flights between the channels. This screw has 15 commercial applications that include color masterbatches and 40% mineral-filled compounds.
VI 1, VI 2, and VI 3 (Velocity Interrupt) mixing sections, developed by Robert Dray, were commercialized three years ago and now have over 100 applications. The design has several cuts across the mixing flights at different helix angles to cause flows to intersect and recombine. The VI 1 configuration is for high-viscosity resins and distributive mixing. VI 3 has a dam that creates more shear for dispersive mixing, particularly with low-viscosity resins. VI 2 falls between the other two.
Zig Zag screw from HPM puts a reverse-angled mixing flight between barrier flights in a Z-shaped pattern. Patented in 1990, it was never used commercially because it was too expensive to make. Today's more advanced CNC milling technology makes the Zig Zag screw more affordable. It will be introduced later this year, and the first field trials were slated to run last month on a stretch-film line in California.
In addition to the above, New Castle Industries says it is working on an altogether new barrier-screw design with channels that fold resin back on itself to obtain more extensional mixing.
Davis-Standard also has a couple of brand-new mixer systems in development, but won't discuss them until patent applications are filed. Trials are scheduled for this summer.
Meanwhile, Kobelco Stewart Bolling is importing a new mixing technology from Japan called VCMT (Various-Clearance Mixing Technology). It has flights of three different heights and is said to improve throughput of polyolefins by 20%. VCMT has been used on twin-screw extruders but not yet on single-screws.
Longer multi-stage screws
Single-screw compounding extruders are also improving mixing capacity by adding more length, more stages, and more complexity. Typical L/D for a compounding extruder is still 20-30:1, but several recently built machines approach 50:1 L/D.
With longer extruders come more stages demarcated by zero-pressure zones at vents and feed ports. Single-screw compounders typically have two stages and one vent. But machines have been designed lately with up to four stages and three vents or feed ports.
An example of these trends is a single-screw line started up in April by Michael Day Enterprises in Wadsworth, Ohio, to reclaim nylon and other fiber scrap. The extruder has 49:1 L/D with three vents and four stages. Meanwhile, Merritt Davis is building an 8-in., 48:1, four-stage machine for reclaiming scrap polyethylene. It has two vents and a twin-screw side-feed port. Also, HPM is building a 4.5-in., 50:1 machine with three vents for compounding glass reinforcements.
There are many techniques known to operators and plant engineers for increasing the performance of a twin-screw compounding extruder.
Here’s a guide to specifying screws and barrels that will last under conditions that will chew up standard equipment.
Newer specialty slip masterbatches go beyond traditional capabilities to provide greater thermal stability, reliability, and ability to hold COF steady during laminating.