Direct-extrusion compounding: the savings can be worth the added complexity.
They're called compounding extruders, but these specialized corotating and counter-rotating twin-screw machines can make a lot more than just pellets. Today, they're hooked up to sheet, fiber, film, and profile dies with no intermediate pelletizing stage. So-called "direct-extrusion" compounding has experienced a mini explosion of new high-tech applications in the past couple of years.
"Over 50 companies in the U.S. practice direct extrusion now," estimates Charles Martin, sales manager of American Leistritz. He says that's a big jump from 10 years ago, when perhaps a half-dozen processors were making either wood-filled sheet for auto interiors and construction products or silica/oil-filled PE sheet for car-battery separators.
One reason for this growth is cost savings. Up front, the hardware expense is greater: A direct-extrusion line with a twin-screw compounder easily costs 50% more than one with a single-screw extruder. But savings come from replacing precompounded resins with less expensive raw materials. Leistritz estimates that a processor who pays 20[cents]/lb conversion cost for precompounded pellets and runs 3000 lb/hr saves over $14,000/day on resin cost.
There are also quality advantages because resins and heat- and shear-sensitive fillers degrade less in the one-step process. PET, for example, loses less intrinsic viscosity, resulting in higher strength, which could in turn allow products to be downgauged. Lesser amounts of reinforcements or modifiers may be needed to achieve desired physical properties. Both factors add to potential cost savings. "There's almost no product that isn't improved by eliminating an extra heat history," says James Charpentier, a business manager at Krupp Werner & Pfleiderer Corp.
Processors also gain the flexibility to vary compounds themselves. They can thus avoid inventorying different grades of precompounded pellets, and can produce their own variations as needed.
The sort of in-line compounding discussed here does not include everyday compounding and extrusion or calendering of PVC into film, sheet, or profiles. Nor does it include simply adding color concentrates or tackifiers in single-screw extrusion. The focus here is on more sophisticated compounding jobs involving high loadings of sensitive fillers and therefore requiring machines normally used just for pelletizing.
Machine builders say the reason direct extrusion has leaned heavily on twin-screw machines is that they offer gentle mixing and flexible configuration. Because twin-screws are commonly built in segments that can be combined for different functions, it's easy to introduce fillers like wood flour, carbon black, or fiberglass at the optimal locations along the extruder to achieve best dispersion with minimal fiber breakage or machine wear. Twin-screws can also gently devolatilize moisture-laden materials like wood flour.
"A twin-screw offers a lot more flexibility than a single-screw," says Charpentier of KW&P. "There's this big market out there of single-screw processors who could improve their products on a twin-screw. We're quoting on 15-20 lines now, while in all of last year we quoted on five."
Then and now
Among the early pioneers of direct-extrusion compounding, Lear Corp. in Southfield, Mich., and Sheboygan, Wis., and Andersen Corp. in Bayport, Minn., have compounded wood flour into PP and PVC, making sheet and profiles for automotive and construction applications. Lear, for example, has directly extruded PP sheet with 50% wood flour for auto interior trim since 1984, using a counter-rotating, intermeshing twin-screw extruder from ICMA San Giorgio of Italy.
The big battery-separator producers - Daramic Inc., headquartered in Burlington, Mass.; Entek in Corvallis, Ore.; and Exide in Corydon, Ind. - directly extrude compounds of polyethylene with silica and oil. Daramic has made coex battery separators commercially in-line for over 10 years on a corotating twin-screw.
Machine makers estimate that 30% of direct extrusion today is still devoted to battery separators and wood/polymer composites. Another 20% is in mineral-filled polyolefins, 10% in PET sheet or fiber, 10% in acrylic sheet, and the rest in unique products. Here are some of the newer players and the applications they're pursuing:
* Adell Plastics in Baltimore makes glass-filled polyolefin profiles to insulate oil pipelines.
* A manufacturer that prefers to remain anonymous has developed an in-line twin-screw coextrusion process to make a fiber-reinforced, multi-layer roofing sheet.
* Collins & Aikman Products in Charlotte, N.C., compounds and extrudes mineral-filled elastomers into automotive carpet backing.
* DuPont Co., Wilmington, Del., compounds and coextrudes PVB (polyvinyl butyral) film for car safety glass, using two corotating twin-screw extruders.
* Lawson Mardon Thermaplate in Piscataway, N.J., direct extrudes PET with impact modifiers and pigments using both twin- and single-screw extruders in a proprietary wheel process for making dual-ovenable CPET food trays.
* Plaskolite in Columbus, Ohio, direct extrudes glass-filled acrylic sheet for light fixtures.
* PPG Industries in Barberton, Ohio, extrudes silica-filled PP in-line to make synthetic paper.
* Raychem Corp. in Menlo Park, Calif., compounds and extrudes 4050% carbon black and HMW-HDPE into electrically conductive plastic film for polymeric circuit breakers.
In addition, several companies are making profiles of wood flour and HDPE, PVC, PP, or PS using PVC-type, counter-rotating, conical twin-screws from Cincinnati Milacron. (This is an exception to the other examples of direct extrusion mentioned here, which employ specialized twin-screw compounders.) One example is Crane Plastics, columbus, Ohio, which extrudes 50% wood flour with HDPE. Other firms use up to 70% wood flour in a process licensed from Strandex Inc., Madison, Wis. (see PT, Jan. '98, p. 67). They include Comptrusion Corp., Toronto; Eaglebrook Products, Chicago; Fiber Composites Corp., New London, N.C.; and Hoff Forest Products, Boise, Idaho.
Other direct-extrusion products in development include exotic coextrusions for electrodes in lithium batteries using as little as 10% fluoropolymer as a binder for high loadings of carbon or metal oxides.
A more complex process
Whatever the end product, direct or in-line extrusion requires high enough volume to dedicate a line to making one compound or a related family of compounds. The complexity of these systems makes it awkward to reconfigure them (although tweaking ratios of ingredients is easy). Powders, fillers, pigments, oils, plasticizers, or other additives may be added at a half-dozen points on the extruder barrel. Changes to these feeding ports (and corresponding screw segments) are possible but cumbersome.
The overall increase in process complexity is a significant challenge to processors who approach direct extrusion. "If you run a compounding line making pellets, all your complexity is at the front end feeding raw materials. You can tolerate variation on the pellet end," says Leistritz's Martin. "When you run a conventional extrusion line, all your complexity and close tolerances are in the dies and takeoff equipment. You don't have to pay as much attention to material feeding. When you put the two together, you have a lot of complexity at both ends."
There is so much prior art going back so many years in direct extrusion that little if any technology on the machinery side is blocked by patents. But as for processing, the tendency has been to keep recipes and know-how strictly proprietary.
However, assistance is available from equipment and materials suppliers. For example, Duane Peterson, senior application specialist at Advanced Elastomer Systems, has worked with several processors on getting started in direct sheet extrusion of olefinic TPEs. He says processors typically start with relatively simple compounds and colors. They learn how to make elastomers from [TABULAR DATA OMITTED] 65 Shore A up to 40 D hardness using the same four ingredients - plastic, filler, oil, and rubber concentrate.
Besides American Leistritz and KW&P, other makers of twin-screw compounding extruders are pursuing in-line applications:
* Berstorff is active in Europe in direct extrusion of packaging materials with corotating twin-screws.
* Davis-Standard just installed two corotating twin-screws for a direct-extrusion application at an unnamed customer.
* Century Extruders is working with customers to develop in-line uses for its corotating twin-screws.
* Farrel is working on direct extrusion that pairs its counter-rotating twin-rotor continuous mixer with a short single-screw discharge extruder.
* Toshiba Machine Co. in Japan sells roughly 25% of its extruders for in-line applications, says James Shack, sales manager at U.S. representative NFM Welding Engineers. Toshiba has two in-line systems in the the U.S., both for automotive applications.
A twin-screw compounder requires some modifications for in-line extrusion because dimensional control of the extrudate is more critical than when making pellets. Extruding through a sheet or profile die also takes more melt pressure than making pellets. However, twin-screw compounders are generally better at mixing than at pumping, and uniform pressure generation is a key to producing a uniform extrudate. A twin-screw for in-line extrusion therefore requires a longer barrel (40-48:1 L/D) than a pelletizing twin-screw (32-36:1) and also usually needs a melt pump.
Machine suppliers have developed various strategies for closed-loop control in order to keep melt pressure constant at the die. For example, Leistritz, KW&P, and Davis-Standard lock in the gear-pump rpm while varying the material feed rates and screw speed to alter mixing intensity. Some products like battery separators don't use gear pumps, but control sheet thickness with calendering rolls instead.