Thermoformable Composite Panels, Part 1
Unlike thermosets, thermoplastics do not crosslink and cure, and therefore may be heated, formed and cooled several times without loss of properties. This distinction has prompted reinforced-thermoplastic materials suppliers to provide customers with premade, preconsolidated sheet stock, which subsequently can be thermoformed into shaped structures.
Thermoforming uses heat and pressure to transform sheet thermoplastics into the desired shape. In simplest terms, the sheet is preheated then transferred to a temperature-controlled mold and conformed to its surface until cooled. There are numerous variations of thermoforming, distinguished primarily by the method used to conform the sheet to the mold (See sidebar, p. 40).
FORMING A NEW CATEGORY
The advent of thermoformable panels is driving huge growth in the use of reinforced thermoplastics, particularly in the automotive world, where legacy materials, such as steel and aluminum, have predisposed manufacturers to materials with known, predictable properties available in standard thicknesses, which can simply be formed to shape.
Panel products have been developed from reinforced thermoplastic composites on both ends of the property and cost spectrum. On the low-performance, low-cost end, what the industry officially denotes as commodity plastics (e.g., polyamide), were initially modified with various fillers for automotive applications. As automakers sought to reduce vehicle weight, improve safety, reduce noise, add electronics and streamline manufacturing via modular assemblies, they fueled development of lightweight thermoplastics with progressively greater loadings of chopped glass fiber reinforcement (fiber length of 0.25-inch or less) that offered tailoring of properties, better impact and acoustic performance as well as complex shaping capability and flexibility in manufacturing. Recently, development of long fiber-reinforced thermoplastics (LFRTs) — thermoplastics, such as polypropylene, reinforced with increasingly longer glass and natural fibers (to 1.25 inch or more) — and effective methods for processing and molding them have improved performance and stimulated interest in other markets, including rail, bus and marine interiors, sporting goods and consumer products.
On the high-performance, high-cost end, thermoplastic prepregs were developed to replace thermoset prepregs in niche aerospace applications. The resulting unidirectional tapes and semipregs featured continuous glass, aramid or carbon fiber reinforcements combined, early on, with engineering thermoplastics, such as polyethersulphone (PES) and polyetherimide (PEI) matrices and, more recently, with higher performing polyphenylene sulphide (PPS) and polyetherketoneketone (PEKK).
Today, thermoformable panels produced by suppliers on both ends of the cost/performance continuum offer fabricators a lengthy list of processing and performance advantages: fiber/resin ratios and, where applicable, fiber orientation and architecture are the responsibility of the supplier, so fabricators can focus on a smaller set of forming variables. Manufacturing cycles are shorter (typically 2 minutes or less) and finished products have greater toughness and impact resistance than thermoset composites do — not to mention recyclability, both during manufacturing (recycling scrap) and at the end of service life. And thermoplastic reformability offers fabricators secondary forming options, such as forming parts in multiple steps or making corrections in improperly formed parts.
AUTO INTERIOR & UNDERHOOD APPS
Thermoformable panels are making inroads into automotive interiors. One of the main applications is headliners, which have become increasingly complex. Typically quite thin (as small as 1 mm) at the edges to facilitate attachment and load transfer, headliners must be thicker elsewhere to offer increased head impact protection and maximize noise abatement. Additionally, automakers desire to mold a finished component, including aesthetic covering, in a single cycle.
Three different fiberglass-reinforced polypropylene (PP) materials have been developed to meet these challenges. Each has glass content of 55 percent by weight for the standard product and helps fabricators achieve variations in part thickness in a single forming cycle through a phenomenon called lofting. Lofting is a mechanical process that increases the thickness and reduces the density of the sheet material when it is exposed to heat. During manufacture, a certain percentage of the fiber reinforcement is oriented in the z-direction and then compressed when the sheet in consolidated during manufacture. When heat is applied prior to thermoforming, the compressed z-directional fibers, like coiled springs, are released to loft the softened thermoplastic. Lofting permits the molders to mold a part with selectively varied thickness. Areas requiring high tensile strength are compressed to a thinner profiles with greater density. "To get a thicker section," explains Harri Dittmar, market manager for development composites at Quadrant Plastic Composites (QPC, Lenzburg, Switzerland), "you construct the tool in such a way that it doesn't compact the lofted material as much in the chosen areas." The thicker sections maintain a greater degree of the initial loft, which produces high stiffness and, at lower density, also contributes to more effective acoustic damping. Lofting reduces part weight and overall part cost, and eliminates the multiple production steps required with previous headliner materials.
AcoustiMax, developed by Owens Corning Automotive (Toledo, Ohio), is supplied in a range of weights, and features a scrim on one side and, on the other side, a customer-specified adhesive film used to co-mold the particular cover fabric required by the application. AcoustiMax sheets are slit to widths and cut to lengths, also to customer spec, and then shipped on pallets.
According to Tom Ketcham, product line manager for AcoustiMax, the product achieves superior noise reduction properties from its lofting ability — more than twice its original thickness. For example, a 1,000 g/m2 sheet of AcoustiMax starts out at a thickness of 3 mm to 4 mm (0.12 inch to 0.16 inch), lofts to 10 mm to 11 mm (0.39 inch to 0.43 inch) during preheating at 390°F (199°C), and then is molded at 110°F (43°C) and 70 psi (4.8 bar) to a final thickness of 5 to 6 mm, depending upon the geometry and compaction of the mold.
Although no AcoustiMax commercial headliner applications are yet in service, several Tier 1 automotive suppliers have completed prototypes and are in the final stages of material selection for specific vehicles. In addition, AcoustiMax is being developed for trunk liners, door modules, seat backs and package trays.
SuperLite is manufactured by Azdel (Shelby, N.C.), a 50/50 joint venture between GE Advanced Materials (Pittsfield, Mass.) and PPG Industries (Pittsburgh, Pa.). SuperLite forms at 375°F to 400°F (191°C to 204°C) and pressures of 30 psi to 45 psi (2 bar to 3 bar), with lofting capability approaching 200 percent. Gordon King, commercial director for Azdel Europe, says the material's light weight and low-pressure formability can reduce traditional headliner system costs by up to 20 percent.
SuperLite is currently used in headliners for 16 different production vehicles, in sunshades for five, as well as the rear parcel shelf for the Honda Accord and Toyota Camry and the back-panel for the Dodge Ram. It also is used in 11 different interior applications on the Ford GT limited-production sports car.
QPC's SymaLITE, can be made with a glass content from 20 to 60 percent and thermoforms at 50 psi (3.5 bar). Greater glass content produces increased lofting — five to six times its original thickness, according to QPC — and lower density, which translates into parts with higher stiffness-to-weight ratios. While the company's headliner product features glass content of 55 percent by weight, products for underbody shields, door inner panels and sandwich top layers use a glass content of 40 percent.
SymaLITE's first commercial application was the underbody shield for the BMW 5/6 series, comprising four components formed in less than 60 seconds in a single four-cavity mold and resulted in a 30 percent (4 kg/8.8 lb) weight savings. The center areas of the parts are 4-mm/0.16-inch thick, optimizing stiffness and noise reduction, while the perimeters are 1 mm/0.04-inch thick for attachment and seal strength. SymaLITE also forms the load floor/trunk separator for the 2005 Crossfire roadster by DaimlerChrysler. This large, complex one-piece structure is made using a 2,000 g/m2 sheet for the separator and an 1,800 g/m2 sheet for the floor. The 90-second process adds pressure-sensitive adhesive-coated carpet on one side and polyester scrim on the other via in-mold decoration. QPC says that the product is under evaluation for other automotive load floors as well as interior trim components, such as door panels, and OEMs are considering it for roof modules, hoods and trunk lids, where it would be used behind a Class A surface material, such as coil-coated aluminum or polypropylene film.
A fourth headliner panel, VolcaLite, is Azdel's GMT Lite product, made with polypropylene reinforced by long chopped basalt fiber. Basalt, an inert rock found worldwide, is the generic term for solidified volcanic lava. Basalt fibers, a comparatively new fiber reinforcement, are now marketed internationally by several recently formed companies, including Albarrie Canada Ltd. (Barrie, Ontario, Canada), Basaltex (Wevelgem, Belgium), Hengdian Group Shanghai Russia & Gold Basalt Fibre Co. (Shanghai, China), Kamenny Vek (Dubna, Russia) and Sudaglass Fiber Technology (Houston, Texas). Its main advantages are higher service temperatures, higher modulus and better chemical resistance compared to fiberglass. It is intended as a replacement for both glass and carbon fiber reinforcements in composites. VolcaLite has been targeted for use in headliners, where it is said to offer ultrathin profiles (down to 3 mm/0.12 inch, a 50 percent reduction, compared to conventional products. Currently, VolcaLite is being evaluated by multiple Tier 1 automotive suppliers.
In the past 18 months, ENSINGER/Penn Fibre (Bensalem, Pa.) has introduced a variety of short glass fiber-reinforced thermoplastic sheet materials for underhood and other applications, sold under three brand names.
Pennite 4512, a Nylon 6 (polyamide) with 12 percent glass fiber reinforcement, was developed for air dams, ducts and radiator shrouds. Parts made from the material can withstand 280°F (138°C) continuous in-service temperature. However, the company notes that polyamides are hygroscopic (susceptible to moisture absorption), and therefore must be pre-dried, using a desiccant or a recirculating-type oven prior to thermoforming to avoid surface blisters caused by outgassing.
Penn Fibre's two additional panel products are formed using thermoplastics developed by Ticona (Florence, Ky.). The first, a sheet material made form Ticona's Celcon acetal co-polymer resin and 15 percent glass fiber reinforcement, was developed for underhood and gasoline tank applications where the structural integrity and chemical resistance of the material meets end-use requirements. (Celcon and Pennite 4512 are both used in current production vehicles, however Tier 1 suppliers are protective of details, and thus, Penn Fibre is not able to discuss specifics.)
The second product is glass-fiber reinforced polyphenylene sulfide (PPS) sheeting, using Ticona's Fortron PPS. Sold as monolayer sheets and in rolls to 48 inches/122 cm wide and thicknesses between 0.010 inch and 0.25 inch (0.25 mm and 6.4 mm), the materials are available with optional backings that provide gluing surfaces in multilayer applications. The sheets are preheated to between 610°F and 625°F (321°C and 329°C) and can be formed using aluminum tools without a cooling system. The molds are preheated to 390°F/199°C and total cycle time from preheating sheet to forming and final cooling is between 60 and 90 seconds. The material's thermal performance, high strength, inherent flame retardant properties and chemical resistance makes it suitable for stationary and mobile chemical tanks, underhood automotive parts, and interior panels and other large, thin-walled elements in buses, aircraft and railcars, says the company.
Although each brand comes in standard sizes, most product is delivered to customer spec, in terms of thickness, width and length in order to provide optimum-sized blanks for thermoforming.
NATURAL FIBER PANELS
Natural fibers and wood flour are perhaps the oldest reinforcements used in plastics, dating back to Bakelite, the first plastic made from synthetic polymers (circa. 1909), in which they were used to reduce cost, control shrinkage and improve impact resistance. Although they were replaced with mineral fillers and fiberglass in the 1950s and 1960s, natural fibers are making a comeback. This trend started in Europe, where end-of-life recycling requirements have stimulated development of natural fiber composites that combine plant fibers, such as abaca, flax, hemp, kenaf, sisal or jute with polyethylene, polypropylene (PP) and other thermoplastics, offering properties comparable to glass-reinforced thermoplastics, but at 70 percent of the weight and at lower finished-product cost.
The U.S. is catching up. Most major suppliers now offer natural fiber-reinforced products and are working to develop applications (see CT February 2006, p. 32).
FlexForm Technologies (Elkhart, Ind.) has introduced FlexForm panels, featuring a variety of sheet composites made from plant fibers, such as kenaf, hemp, flax, jute or sisal, blended with thermoplastic matrix materials, such as polypropylene or polyester. According to Flexform VP of sales Harry Hickey, the first panel product was developed by request: "One of our automotive customers in the U.S. had seen this type of product in Europe and wanted to develop it here," he explains. "They wanted the environmental advantages, but also improved weight and strength while maintaining one-step processing and competitive part cost." According to Hickey, FlexForm products offer a lighter weight and more environmentally friendly alternative to wood flour-filled plastics, as well as a 25 percent improvement in strength in applications such as door panels and inserts, package trays, headliners, seat backs, sidewalls, pillars and center consoles. Flexform panels can be thermoformed at 392°F (200°C) and 55 psi (0.379 MPa). The materials are used in numerous production automobiles and are being marketed to RV and trailer manufacturers for sidewalls. Other markets include a variety of consumer goods, e.g., furniture, office partitions and ceiling tiles.
G.O.R. Applicazioni Speciali SpA (Buriasco, Italy), a subsidiary of Solvay Industrial Foils (Brussels, Belgium) has developed three classes of natural fiber-reinforced thermoplastics that can be extruded as thermoformable, recyclable flat sheets. The base product is Wood-Stock, a family of polypropylene (PP) composites with 5 to 55 percent wood flour loading by weight. It was developed to meet customer needs for an inexpensive, recyclable material for automotive interior applications, such as door panels, rear shelves and pillars. Gornaf, a new material, is comprised of polypropylene reinforced from 5 to 35 percent by weight with long sisal fibers (20 mm to 30 mm). The longer fibers reduce part weight and bring higher tensile and flexural strength to components with demanding requirements. G.O.R. has performed recycling tests on Gornaf door panels and confirmed that over 90 percent of the material (by weight) can be re-used. Therefore, G.O.R. buys manufacturing scrap from its customers, which then is reground and mixed with virgin materials for use in production of new panels. Tecnogor, the second generation of Gornaf, is reinforced from 10 to 40 percent by weight with even longer natural or glass fibers (40 mm to 250 mm). There are currently a total of 150 different variations among the three products, which have been developed to provide solutions for specific customer requirements.
G.O.R.'s technical marketing manger Ariano Odino says thermoforming of these products can be done with a relatively inexpensive infrared oven, a 200-ton hydraulic press and a steel mold with water-cooling system. "In the standard process we use, sheets are heated at 180°C to 190°C (356°F to 374°F) for 50 seconds and then transferred automatically to the mold where pressure of 8 to 10 kg/cm2 [114 to 142 psi] is applied for 40 seconds." he explains. "You can obtain a finished part with coverstock on both sides (fabric or vinyl) and integrated plastic brackets, all molded in one step, in a very fast cycle time."
Gornaf has been used in the front and rear door panels for the Peugeot 406 Coupe. Newer Tecnogor currently is being evaluated by potential customers.
Recyclability is a primary — but not the only — driver in the development of self-reinforced plastics (SRPs). The following three SRP sheet materials were developed initially for automotive applications, but also are proving useful in industrial, sporting goods and consumer goods applications. The resulting composite, basically a mono-material, offers no impediment to recyclability, and attains specific strength and stiffness properties comparable to fiberglass-reinforced composites. This is due to polypropylene's very low density, which, SRP manufacturers say, offers 40 to 60 percent weight savings versus glass mat thermoplastics (GMTs) and other traditional GRP materials. Additional benefits include the soft crash behavior imparted by the ductile failure mode of polypropylene, which is said to prevent splintering, and the product's impact resistance, which it maintains even at very low temperatures.
Curv is manufactured by Propex Fabrics (Gronau, Germany). Extruded polypropylene film is stretched into tapes with exceptionally high stiffness and strength. These tapes are then woven into fabrics and undergo a patented hot compaction process in which the surface of every tape is partially melted, creating a matrix which bonds the tapes into a self-reinforced composite.
Thermoforming is achieved using pressures of 5 bar (73 psi) and up, depending on the complexity of the part, and temperatures of 150°C to 160°C (302°F to 320°F). Typical cycle times are less than 60 seconds. The consolidated sheets are slit to standard or customer specified lengths to 3m/9.8 ft and widths to 1,360 mm/53.5 inches, in thicknesses from 0.3 mm to 3 mm (0.01 inch to 0.12 inch).
Automaker DaimlerChrysler has evaluated Curv for underbody shields, and several processors are testing it for use as a local reinforcement material for injection-compression and compression molded parts. The product also is being marketed for bumpers, body and underbody panels, interior headliners, door liners, load floors, pillar trim and rear parcel shelves. Other applications include building cladding, personal protective equipment, sporting goods, briefcases and luggage, audio speakers and even shoe inserts.
Milliken & Co. (Spartanburg, S.C.) produces Moldable Fabric Technology (MFT) sheet stock using PURE technology through a licensing agreement with PURE's manufacturer, Lankhorst-Indutech (Sneek, The Netherlands).
Lankhorst-Indutech uses a patented process to produce tape-like yarn using three flat coextruded polypropylene layers. The middle layer has been drawn out under tension and heat to orient the polymer chains along the tape's 0° axis to optimize high strength and high modulus. This layer is sandwiched between two thin outer layers specially formulated with a lower melting point. The tape-yarns can be woven into fabrics. According to Lanhorst-Indutech's Astrid Wijninga, "PURE's specific impact performance is so good that it is now being considered for use in protective applications in sporting goods and anti-ballistic applications in the defense industry." (Commercialization is expected, respectively, in summer and fall of this year.) Additional targets include liners for truck trailers, kayaks, canoes, body panels for personal watercraft and snowmobiles, car-top carriers, recreational vehicles, piping and construction.
Milliken uses PURE fabrics to create its MFT sheet stock. The sheet is comprised of multiple layers of twill or plain-weave fabric woven from PURE yarn. During processing, the company's hot compaction process applies heat at a temperature above the melt point of the yarn skins, but below that of the middle layer. This permits consolidation of the fabric layers, as the low-melt-point outer layers of the yarn melt and impregnate the woven fabrics, but maintains the integrity of the oriented polymer chains in the middle layer — a key to optimizing properties such as impact resistance.
MFT parts are thermoformed at relatively low pressures, starting at 3 bar (44 psi) and temperatures from 150°C to 160°C (302°F to 320°F). Typical cycle times are less than 1 minute and can be as low as 25 seconds, depending on the complexity of the part. Finished sheets can be supplied in 0.3 mm to 3 mm (0.01 inch to 0.12 inch) in thickness (greater thicknesses are available on request) and are cut to customer-specified width and length and shipped on spools.
In Part II (CT June 2006): Thermoformable panels are also available for load-bearing applications — semi-structural and structural — using continuous reinforcement, including not only glass but advanced carbon and aramid fibers as well.
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