Thermoplastic composite preforms developed for automotive use.

Thermoplastic Composite Preforms Developed For Automotive Use

A West German firm says it has developed a high-speed method of making advanced composite preforms of woven glass fabric consolidated with polyetherimide (PEI) thermoplastic for use in structural automotive components, among other applications.

Key to the process developed by Mobik GmbH of Gerlingen, W. Germany, is a proprietary aluminum frame that grips some fibers in the glass fabric/PEI sheet while permitting others to move, maintaining the proper fiber alignment while allowing deep-draw thermoforming of the sheet. According to Mobik managing director Jurgen Koster, 3 x 3 ft preforms made using the CAD-specified frame have been vacuum formed in less than 30 sec at about 20 psi. Mobik's lab thermoformer, produced by a German company, is a single-oven unit set up to run continuously.

"This is high-speed fiber placement equipment. It's thermoforming, as the matrix must be `weak' to permit fiber alignment, but the key is rapid fiber alignment according to calculation of load transmittance. It's like steel-reinforced concrete - the steel is important, not the cement or stone," Koster says. He initially envisions using the preforms in RTM molds, which would then be filled with thermoset resins.

Mobik's installation consists of heating, forming and cooling modules (several cooling or heating modules could be combined with one forming module; for complex shapes, two or even three forming modules could be integrated, Koster says). The actual forming only takes a few seconds, with most of the cycle devoted to heating and cooling. Because the fiber alignment and drapability are crucial, the forming frame assures that warp and weft fibers can move relative to each other, and that wrinkling can be avoided (this is optimized with CAD, and vision controls will eventually be used on the Mobik system to supervise exact fiber placement).

The raw material for the preforming operation is 4 x 12 ft sheet containing several plies of oriented, woven glass fabric (45/90/45) that have been consolidated in a PEI matrix in a compression press at about 662 F (this takes about 15 min per sheet). Mobik is currently receiving this "semi-finished" material from a Dutch textile producer, Tencate Advanced Composites NV. Mobik is planning to open a pilot plant this fall, and expects to produce prototype automotive preforms. Initial applications will also include preforms for aircraft interior cabins.

Koster estimates the pilot plant will produce two preforms per minute, lowering the manufacturing cost for an advanced composite preform from four times the material cost to about twice the cost. "It's a two-step process, and we are only interested in making the finished preform," he says.

Koster says various thermoplastic matrices can be used, and available reinforcements includes aramid and carbon fiber as well as glass. Cycle times for the pre-finished sheet depend on the number of plies (two to six will be standard, but 50 plies or more are possible, says Koster). Tencate can also supply a sandwich panel with a Nomex core and consolidated prepreg face sheets.

SANDWICH PANEL HAS PEI FOAM

Mobik also is experimenting with a sandwich automotive body panel that consists of two glass/PEI preforms placed around a 1-mm-thick PEI film that has been treated with a blowing agent (the inner surface of the preforms have also been treated with the blowing agent). When heated to about 374 F in a flat tool that expands slightly to the needed dimensions, a "perfectly shaped" sandwich component is formed in less than 30 sec. The film expands to about 10 mm thickness, and the internal pressure of the foam is enough to form the part. "The beauty of it is that there is perfect adhesion, but you only have two types of material, glass and PEI," Koster says.

In a paper presented at the recent SME Composites in Manufacturing 8 Conference in Anaheim, Calif., and in a subsequent interview with PLASTICS TECHNOLOGY, Koster says that advanced designs such as the Porsche 959 and BMW Z-1 roadster demonstrate a trend toward a "continuous structure concept" for cars, in which horizontal panels such as doors and hoods become orthotropic load-bearing members without B-supports (similar to aircraft and bank-vault doors). These designs create a need for advanced composite preforms, he says, not only for high load-carrying capacity, but also because the integral designs are shaped beyond the limits of what can be achieved by metal forming.

Mobik was also spurred by the Chrysler CIV (composite-intensive vehicle) program, which attempted to create a composite spaceframe using a unique "hollow-bag" RTM molding method (see PT, Nov. '87, p. 41). According to Koster, the joint Chrysler/Budd Co. project was shelved primarily because of the lack of a high-speed method to make preforms for the structural components. Although a 36:1 parts integration was achieved through the CIV design, about 20 preforms were needed per car. This need for about 20,000 void-free preforms per day requires a new sheet-manufacturing technique similar to a steel roll mill.

"Some solutions use preforms of glass mat (which is not oriented and therefore low-profile), some use knitted or Brochier-type preforms (which are also not oriented), and others use 3-D stitched woven fabrics. The latter come near to what is needed, yet all of these sophisticated textile technologies are too slow. Drapability is limited, and therefore several preforms are needed to form one large component," Koster explains.

Koster also maintains that competing thermoplastic composite technologies, such as those based on comingled thermoplastic and reinforcing fibers, are too slow in consolidation of the shaped component and too expensive in the spinning of the thermoplastic matrix.

"Mobik will not compete with low-profile composites such as SMC, BMC, Azdel and the like - reinforced plastics in which the matrix carries the load and the fiber reinforces the matrix. We are investigating applications where the fiber carries the load and the matrix only keeps the fibers in shape and protects them. We do not reinforce plastics," Koster says.

Koster adds that Mobik is working on a primer and paint system for its thermoplastic preforms, and eventually hopes to achieve a Class A finish out of the thermoformer. He says the preforms are fully compatible with polyester or epoxy resins used in RTM.

Koster concedes that some mold-flow analysis is needed to develop large parts using the glass/PEI preforms, and that it may be necessary in some cases to "punch holes" into the preforms to facilitate mold flow. He also admits that the glass/PEI preforms, which have uniform reinforcement throughout, may be overengineered for many applications. But he insists that trading off extra weight and material cost in favor of high-speed manufacturing and greater structural strength will prove worthwhile, particularly in terms of part consolidation.

"In SMC systems, the load is carried by matrix, and you put in fibers to increase the load-carrying capability, but you come to a limit - you load up to 30-40% fibers and that's it. But if you have the fibers carry the load and the matrix does nothing but keep the fibers in line, then the whole engineering is different. My preforms are backbones," Koster says. (CIRCLE 90)

PHOTO : In Mobik's high-speed thermoforming system, proprietary aluminum frame grips glass fabric and maintains proper fiber alignment, allowing deep-draw thermoforming of composite sheet.

PHOTO : Mobik is experimenting with a sandwich panel, at right, that consists of two glass/PEI preforms placed around an Ultem film treated with a blowing agent. The materials are heated to 374 F in an expandable tool, producing the part in less than 30 sec. Without consolidated preforms, such a panel (left) might contain up to 10 different mat, fabric and foam components.