Thermoformable Composite Panels, Part 1
Preconsolidated fiber-reinforced thermoplastics offer short cycle times, tailored properties, recyclability and lower cost.
By Ginger Gardiner, Contributing Writer | April 2006
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).
Source: Owens Corning
Automotive headliner (above), made from AcoustiMax, one of several thermoformable fiber-reinforced polypropylene composite panel products available to automotive part molders, features cutouts to accommodate DVD monitors and sunroof. Side view of a molded headliner (at right) shows part thickness achievable with the panel's "lofting" capability.
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.
Source: ENSINGER/Penn Fibre
Pennite 4512, a 12 percent glass-reinforced Nylon 6 panel product, was developed for underhood automotive parts, such as this radiator shroud, on the strength of its ability to withstand up to 280°F/138°C continuous in-service temperature.
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.
