Long-Fiber Injection Advances Polyurethane Composites

Single-step molding technology lets fabricators enjoy the high modulus, low emissions and short cure times of thermoset polyurethanes.

Long fiber-reinforcement has been among the fastest growing fabrication techniques in the past five years. Long fiber-reinforced composites approach the performance of composites reinforced with continuous fibers, and offer production speed and manufacturing accuracy achieved with automation associated with chopped-fiber fabrication. Long-fiber reinforced thermoplastics (LFRTs) have replaced metals and other composite molding technologies in a number of applications (see, for example, CT February 2003, p. 20 and CT August 2004, p. 46). But long-fiber technologies also have been developed for thermosets — notably long-fiber injection (LFI) of fast-curing two-part polyurethanes in the automotive market. LFI pioneer Krauss-Maffei Kunststofftechnik GmbH (Munich, Germany and Florence, Ky.) has offered its LFI-PUR Technology for about a decade, and lays claim to about 90 percent of existing polyurethane LFI applications. Competing for market share is The Cannon Group (Milan, Italy and Cranberry Twp., Pa.), with its patented polyurethane LFI process billed as the InterWet system.

Augmenting Advantages With Automation

In an LFI system, chopped fiber and polyurethane components are combined and delivered to the mold cavity in a continuous process. A microprocessor-controlled mixing head incorporates fiber cutting units that chop the fibers to a specified length. Simultaneously, a polyurethane metering machine delivers the reactive polyurethane components, polyol and isocyanate, to the mix head, where they saturate the chopped glass fibers. A robotic arm directs the mixing head over the mold as the mixture is poured into the bottom cavity of a two-part heated mold, providing accurate and consistent material positioning and enabling processors to vary thickness in a way that meets localized structural requirements with no excess material usage. When the pour is complete, the mold is closed, applying pressure and, usually after two to four minutes of cure time, the part can be removed from the mold. (Editor's note: What's obvious by now is that the LFI process does not, in fact, actually employ injection as it is commonly known; there is no high- or low-pressure injection of PUR/fiber mix into a closed mold, as the term LFI implies. The phrase, however, has stuck among suppliers and processors, and so its use will persist here.)

In Krauss-Maffei's LFI-PUR system, fiber length ranges from 12.5 mm to 100 mm (0.5 inch to 4 inches) in increments of 12.5 mm/0.5 inch. (LFRT pellets, for comparison, feature fibers in the neighborhood of 0.5 to 2.0 inches, while conventional chopper fibers average 6mm/0.25 inch or less.) The company recently enhanced its system with a four-strand glass delivery unit, replacing the original two-strand system, to increase glass volume per unit of processing time. The new unit can produce glass loading up to 50 percent by volume. Cannon, which advertises a maximum fiber loading of 40 percent with fiber lengths of 25 to 50 mm (1 inch to 2 inches), highlights its system's versatility beyond long-fiber composites: the InterWet system is designed to mix either glass fiber or a range of fillers, including iron powder and mineral fillers. The equipment achieves high mixing efficiency, explains Barry Pile, a product manager at Cannon USA, by cutting the fibers with a specially designed chopper assisted by pressurized air. It also directs the fiber into the liquid polyurethane component stream just in front of the mixing chamber. "This patented arrangement facilitates a synchronized and instantaneous internal mixing of the materials," Pile explains. "This means that the kinetic energy generated from the turbulence of pressurized liquids wets out the glass thoroughly." This mixing concept also helps the InterWet system handle abrasive fillers without decreasing the life of the mixing head, he points out. (Mixing head cleaning systems are incorporated to prevent build-up of reacted material in the discharge tube of the mixing head between pour cycles.)

To ensure adequate fiber dispersion and precision lay-down of the composite mixture, LFI systems incorporate special deposition mechanisms. Krauss-Maffei's system uses focused air streams to randomize glass orientation. An oscillation mechanism (see photos, this page) produces a broader, more consistent dispersion of the mixture and a wider pour pattern. Similarly, the Cannon InterWet system uses a proprietary pneumatic deflector device at the outlet of the mixing head to optimize lay-down. With this device, called the Plus Feature, Pile says, "Glass lays down flatter, resulting in less air entrapment and fewer pinholes."

The mix head's microprocessor controls enable LFI systems to adjust fiber length and fiber-to-resin mix ratio on the fly. Thus, the material composition of the part can be varied in selected areas of the component to tailor its load profile. This capability allows molders to adjust physical and cosmetic properties of various portions of the component. "Altering the length of the glass during the pour gives the operator a chance to match the lengths of the chopped strand to the mold geometry in an ideal way, thereby minimizing strand protrusion," explains Steven Willis, regional sales manager for Krauss-Maffei Corp., Reaction Process Machinery Div. "This is typically not possible in other glass composites, such as SMC and SRIM processes."

Krauss-Maffei developed a special software program, ShotWare, to interface the robot to the resin metering and glass delivery units. "This software, and the capability it imparts, is extremely useful when increasing or decreasing the glass and/or polyurethane content," Willis says, "since it focuses on the precise positioning and speed of movement associated with the lay-down robot."

Production of Paint-Free Parts

A significant advantage of polyurethane LFI is the potential to eliminate the need for paint. "Pops" or craters in the painted surfaces of compression molded composite exterior automotive body panels have made the use of SMC challenging for applications that must exhibit Class A finishes. While much work has been done to alleviate this difficulty in SMC Class A applications, polyurethane LFI processors can achieve Class A surfaces using new thermoplastic film technology available in a wide range of colors, such as GE Plastics' (Pittsfield, Mass.) Lexan SLX multilayer polycarbonate film or senosan film from Senoplast Klepsch GmbH & Co. KG (Piesendorf, Austria). Such films can be thermoformed to near-net shape and then placed into the mold, with the polyurethane/glass mixture poured directly onto the film. Krauss-Maffei's oscillating air blast and Cannon's Plus Feature (described above) help moderate the temperature of the composite mixture as it contacts the thermoformed film to prevent blistering of the thermoplastic shell.

Efficient, Economical & Versatile

Proponents say LFI offers an attractive alternative to both SMC and structural reaction injection molding (SRIM), not only for the automotive market but also for commercial vehicles, appliances and residential construction components. While LFI is essentially a one-step process, conventional SRIM, even when done with two-part polyurethanes, requires multiple steps, including cutting and/or preforming a glass mat, placement of the mat in the mold cavity and subsequent pouring of the polyurethane into the mold. Since LFI introduces the resin and reinforcement simultaneously, cycle times can be shorter than those achieved with other composite fabrication processes. Willis cites a tractor roof as an example: When it was made from SMC, the roof required a 300-second cycle time, while the LFI version requires only 245 seconds. Additionally, the LFI materials come directly to the fabricator from the glass and polyurethane manufacturers; no compounding or preforming is required. Further, LFI incorporates less expensive roving instead of the glass mat or preforms used in SRIM. It also eliminates waste associated with mat cutting. The result is a net reduction in material cost compared with SRIM and SMC processes.

Yet material selection still must be considered carefully, LFI providers caution. Choice of glass roving, for example, is an important aspect of LFI fabrication: excellent bundle integrity is needed as the roving navigates the high-speed delivery and chopping equipment, but the glass, once chopped, must disperse well in the polyurethane to ensure wet out of individual filaments and optimal physical properties in the finished component. This presents a challenge to glass manufacturers, who must tailor the glass sizing and other characteristics accordingly (see CT April 2006, p. 18).

Curt Thielker, product manager at glass manufacturer PPG Industries (Pittsburgh, Pa.) recalls preparing roving for one LFI fabricator. "The challenge was to allow our strands to survive the process effectively, lay down without lofting, but then during the heating process, get the bundle to open up and disperse within the polyurethane," he says. PPG started with an off-the-shelf product designed for polyurethane matrices and made some adjustments to tailor it to these requirements. The resulting product "allowed the glass to lay nicely on the mold without rolling or tumbling off some of the complex geometries this application presented," Thielker notes. Other roving manufacturers whose product has worked successfully in LFI are Mühlmeier (Bernau, Germany) and Saint-Gobain Vetrotex (Valley Forge, Pa.).

The polyurethane formulation also must be compatible with LFI technology, maintaining its liquid form long enough for LFI deposition over the entire mold surface, yet it must react and cure quickly to meet desired production rates. During the initial reaction, polyurethanes rise — that is, they expand in volume. The formulation must rise rapidly enough to achieve desired fabrication speeds, but not so rapidly as to be chaotic and produce trapped air bubbles. Furthermore, the reactivity profile must be modified in a way that does not compromise the resin's bonding properties, to ensure that the matrix effectively encapsulates the fibers and adheres well to outer film layers. Polyurethane manufacturers also highlight the styrene-free resin's environmental advantages. Manufacturers whose polyurethane systems have been used thus far in LFI processes include Bayer MaterialScience AG (Leverkusen, Germany and Pittsburgh, Pa.), Elastrogran GmbH (Olchingen, Germany, a subsidiary of BASF AG) and Huntsman Polyurethanes (Auburn Hills, Mich.).

Potential for Greater Part Quality

A driving factor in LFI development is its potential to improve part performance and processability. LFI molding can yield higher glass loading than typically achieved in SMC or SRIM processing. LFI also can accommodate a wider range of polyurethane formulations. "The variety of polyurethane formulations allows for lighter weight parts than SMC," Willis points out, "with densities ranging from 0.5 g/cm3 to 1.7 g/cm3." The result is optimized properties at the lowest possible weight. Elimination of hand layup inherently improves part-to-part consistency, and because the fibers are saturated prior to pouring, LFI is more amenable to large-part fabrication than SRIM, where consistent mat saturation can be difficult to achieve. LFI also helps processors avoid print-through problems that can afflict SRIM processing because of the improved laydown characteristics discussed earlier — both tailoring of fiber length and characteristics of the glass product used, such as low loft.

The temperatures and pressures needed to process long fiber-reinforced polyurethanes, typically 80ºC/176ºF and 5 bar/75 psi, are well below those used to process SMCs, typically 150ºC/300ºF and 70 bar/1,000 psi or more. And, according to Willis, the high cost of presses that have the tonnage required for SMC significantly increases capital expenditure compared to LFI. He also notes the critical requirements for mold parallelism (precise alignment of the upper and lower mold halves) during SMC molding, which puts press closure accuracy at a premium. In contrast, the low pressures of LFI help minimize tooling demands and has the potential for LFI fabricators to employ molds that were previously used in hand layup or compression molding of SMC.

Home, Car, Tractor & Earthmover

The recent expansion of long-fiber polyurethane into the housing construction market highlights the broad potential of LFI. JELD-WEN Inc. (Klamath Falls, Ore.), a major manufacturer of windows and doors, is using Krauss-Maffei's LFI-PUR system in its PURfiber technology, which the company developed to produce its Premium Fiberglass Door line of composite exterior doors. "When JELD-WEN decided to introduce this new line of composite doors," Willis reports, "they investigated various composite processes to compare the physical properties that can be obtained, the manufacturing costs, the cycle times, and their ability to mold realistic wood grain finishes."

LFI came out on top. Previously, such doors were made almost exclusively from polyester SMC. JELD-WEN, however, found that LFI could produce doors with as much as twice the fiber content of SMC doors — 35 to 40 percent loadings with LFI vs. 10 to 20 percent loadings with SMC. "As a result, the doors are four times stronger and 60 percent more dimensionally stable than previous composite designs," Willis points out, "yet they are cost-competitive with other fiberglass door products currently available on the market."

The automotive market was the first to embrace LFI in the manufacture of structural and semistructural panels, such as roof modules. For example, automaker smart GmbH (B�blingen, Germany, a business unit of DaimlerChrysler) has contracted with processor ArvinMeritor (Gifhorn, Germany) to make roof modules for its roadster and several other models. The smart roadster's removable hardtop is produced in two halves, each weighing only 5 kg/11 lb and capable of stowage in the vehicle's luggage compartment. Using a Krauss-Maffei LFI-PUR system, the roof incorporates — all via the one-shot LFI process — a thermoformed Lexan SLX outer shell backed by a composite consisting of Baydur STR polyurethane from Bayer MaterialScience and glass fiber from Mühlmeier or Saint-Gobain Vetrotex, plus an interior fabric (headliner) along with structural details, such as water channel, and all hardware and electrical system inserts.

The roadster roof weighs 20 percent less than a comparable steel roof, thus improving fuel economy and reducing rollover risk by lowering the roadster's center of gravity. It also offers more than double the stiffness of comparable aluminum or glass/thermoset roofs, eliminating the need for structural beams while offering greater resistance in side-impact crashes (see CT October 2004, p. 32).

The roof for the General Motors Opel Zafira also is manufactured from a Baydur polyurethane composite with a polycarbonate-based outer film. Fabricator Webasto AG of Stockdorf, Germany makes the support frame of the roof module, which incorporates four large glass elements, using LFI-PUR to produce a component with 22 percent glass content. Wall thickness is varied within the roof frame, and the frame incorporates the thermoformed polycarbonate outer shell to harmonize with the glass elements.

A finite element analysis (FEA) study at Bayer MaterialScience, according to a paper presented at the 6th Annual SPE Automotive Composites Conference (Sept. 12-14 in Troy, Mich.), indicates that a long fiber-reinforced polyurethane roof module, 3 mm/0.1 inch thick with 45 percent glass content, is 82 percent stiffer than a comparable steel roof in knee-load simulations. In the same study, wind loading deflection was almost identical to that of steel, while torsional stiffness was 40 percent greater in the composite module. Finally, the polyurethane composite roof was 5.5 percent lighter than the steel roof, providing weight savings of 5.6 kg/12.3 lb.

LFI also has moved into agricultural and commercial vehicles. Using Senoplast's senosan thermoformable sheet for the outer film and Elastogran polyurethane, molder Parat Automotive (Remscheid, Germany) is making hoods for the CVX line of tractors from Case Corp. of Racine, Wis. (see CT August 2005, p. 52). Romeo RIM (Romeo, Mich.) uses Krauss-Maffei equipment, Bayer polyurethane and PPG glass to produce panels of various sizes for the interior of Freightliner's LLC Century Class heavy truck. Cycle times as short as five minutes are achieved in this application (see CT June 2005, p. 33).

Thompson Plastics Ltd. (Manchester, U.K.) has used the Cannon InterWet system to fabricate an external body panel for a Caterpillar (Peoria, Ill.) earthmover. This panel includes a thermoformed PMMA (polymethyl methacrylate) outer shell with glass-reinforced polyurethane for its structural layer. Cannon reports that a variety of other heavy-vehicle applications, including door panels, parcel shelves, tractors hoods and fenders, are in commercial production. Further, several fabricators use the InterWet system to mold external housings for exterior air-conditioning units and luggage racks for buses.

Massive Market Potential

As these applications gain wider publicity, the market for polyurethane LFI is expected to grow in the next five years, just as LFRTs did in the past five years. The high performance that long fibers offer, combined with automation, short cycle times, and reduced environmental impact is proving to be a winning package for a variety of applications.

Karen Fisher Mason, Contributing Writer

Long fiber injection (LFI) of polyurethane has made inroads into automotive manufacturing. The hood on this Case CVX tractor was molded by Parat Automotive (Remscheid, Germany), using the process and a thermoformed senosan thermoplastic film for the exterior surface, provided by Senoplast (Piesendorf, Austria). Source: Senoplast

Krauss-Maffei's microprocessor-controlled LFI-PUR mixing head chops glass fibers, mixes them with two-part polyurethane resin and deposits the mix into the mold cavity in a continuous process. As demonstrated above, oscillation during deposition better disperses the materials and minimizes air pockets.

The Cannon InterWet mixhead is designed to ensure saturation of glass fibers as they meet the polyurethane components prior to pouring.

Cannon's InterWet system in use, during manufacture of a cargo box for a GM pickup truck.

A switch to long-fiber-reinforced polyurethane enabled residential door manufacturer JELD-WEN Inc. of Klamath Falls, Ore. to double the fiberglass content and, thus, the strength and durability of its premium composite entry doors.

From panels for air-conditioning units to tractor hoods, long-fiber polyurethane composites have demonstrated their applicability to a broad range of components.