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Thermoplastic composite extrusion earns an up-front position in the automotive world.
Volvo Car Components Corp., Body, IF Components Div. (VCCC/BCD) is using a manufacturing method developed by Composite Products Inc. (CPI, Winona, Minn.) where the low cost of inline extrusion is enhanced by the strength of long fiber technology. The grille molding that frames the headlamps and radiator of Volvo's model S70 and V70 automobiles is being manufactured from long fiberglass-reinforced thermoplastic by combining inline extrusion with compression molding. VCCC/BCD of Olofström, Sweden, is turning out the 7-lb, 60-inch long, 13inch high front-end modules at a rate of 75 parts per hour - including extrusion, compression molding, drilling and trimming.
The Direct Melt-Phase Forming (DMPF) is patented by CPI as a continuous process that mixes the raw materials, extrudes the compound into a preformed shape called a charge," and molds the part. "Our goal in the Volvo program was to get the preform from the extruder into the mold within seven seconds," said Ron Hawley, chairman of CPI, adding that this transfer can be accomplished in as little as two seconds. Traditionally, parts manufacturers buy materials in the form of pellets or glass mat thermoplastic (GMT) sheets. DMPF eliminates the costly middle step in which the extruded material must be cooled, packaged, shipped, and stored before it can be reheated and molded into a part.
Cost Reduced, Performance Increased
The performance of parts made by Direct Melt Phase Forming compares favorably with that of parts made by comparable processes, Hawley said. "Parts produced with DMPF are more uniform than those made with GMT. With DMPF, glass fiber is uniformly distributed throughout the material, resulting in better overall performance values."
"The CPI process is stable and, therefore, the properties of each component will be consistent," added Mats Ericson, manager of composites development for VCCC/BCD. "In a less stable process, stiffness could vary ±30 percent among parts. The extruded preform system also offers an economic advantage when compared to GMT. We've improved dimensional stability, stiffness, surface quality and impact resistance, while reducing material cost and weight
He added that while performance is up and costs are down, it is difficult to quantify cost/benefit contrasts between these two very different processes and kinds of materials.
VCCC/BCD tested and customized the process and equipment for body component production for two years before signing a license with CPI in September, 1994. Ericson said the main challenge in changing from GMT to DMPF was the "need to thoroughly understand the more complex technology of DMPF. Previously, we bought pre-formulated GMT material. For DMPF, we purchase the constituents individually from suppliers, then develop in-house formulations for the extruded preform."
Adopting preform technology does not necessitate redesigning the part or mold, explained Tuve Johansson, supervisor of composite production in VCCC/BCD's technical department. "We did change the design because the more stable process allowed us to optimize the function of the part. For example, we were able to integrate three brackets into the part that had previously been separate components that required assembly."
A Model Material
To manufacture the S70 and V70 front-end module, VCCC/BCD extrudes 0.5-inch glass fibers with its own formulation of polypropylene resin and recyclate. The polypropylene resin is pellet- or powder formulated with additives specified for the application. Most commonly available thermoplastic materials are compatible with the process, Hawley said. "Programs we have in place use polypropylene, but we also have successfully tested nylons, thermoplastic polyesters such as polybutylene terephthalate/PBS and polyethylene terephthalate/PET, and styrenic systems." Thermoplastic resins are not subject to the environmental regulations surrounding thermoset molding because they don't emit volatile organic compounds.
The part contains 15 percent (±3 percent) postindustrial recycled material. "Recycled materials are easily introduced into this process," Ericson said. "We chop our own glass/polypropylene scrap to make the recyclate and extrude the particles with 85 percent virgin resin. This reduces cost while caring for the environment."
Reinforcements, including fiberglass and carbon, can be used alone or in combination. It is possible to randomly disperse fibers measuring 0.5 to longer than 1 inch in the thermoplastic resin to make the preform compound. The fibers are supplied in the specified cut length and sizing (coating) to meet product application requirements. Richard L. Enochs, CPI's chief operating officer, said the company has "made technological advances while working on glass fiber development with Owens Coming in Toledo."
Making the Charge
The heart of the process is an extrusion system that consists of two connected and consecutive screws. The first extruder melts the resin. The process begins when preformulated resin, measured by a computer-controlled loss-in-weight feed scale, is dispensed into the barrel of the extruder and heated to the melt temperature of 400'F-700'F.
The melted resin is conveyed through a connecting channel into the second extruder, where the fiber is introduced. A computer specifies the total input of fibers and resin in pounds per minute, as well as the percentage of resin to fiber, typically 40 percent fiber by weight. "An accurate blend of ingredients is crucial to avoid compromising the molded part," Hawley emphasized. "Every handful of material that flows through the system must have exactly the same formulation."
Maintaining Maximum Fiber Length
The system is designed to preserve the length and integrity of the fiber as it is mixed with the thermoplastic resin. Enochs stressed that a key element of the patented machine is the gentle handling of the fibers, both by the second extruder screw and in the way they are introduced to the process. As they enter the extruder, the fibers are gently heated and stirred by the screw before they are merged with the resin.
"Introduction of cold fibers can cool the resin, increasing its viscosity and propensity for degradation, as well as the likelihood of breaking up the fibers at the moment of contact," Hawley explained. Heating the fibers allows the resin to maintain consistent heat and viscosity. He added that CPI's approach enhances thorough resin wetout of the fibers and distributes them evenly as the power screw stirs the materials.
The fiber/resin compound is then extruded into a heated chamber that accumulates the charge to make the preform. Preform temperature, typically 380'F-420'F for polypropylene, depends on the resin system. Size can vary from a 4-inch diameter by 12- to 16-inch long cylinder for the Volvo bumper module to a 2-lb loaf used to mold the interior door panels. The size of charge can be optimized by adjusting the preform chamber's length and diameter. When the compound reaches its programmed configuration, a knife cuts the preform to length and a plunger pushes it out of the chamber onto a shuttle or conveyor belt. The glass fiber of the preform, which has the consistency of modeling clay, holds the charge's shape on the conveyor. The polypropylene absorbs little moisture from the air and doesn't require drying before it is extruded.
Molding the Part
DMPF molding time depends on the resin and part size, with DMPF currently being used to mold parts as heavy as 22 lb and as light as 1.5 lb. The finished charge is moved by a pick-and-place robot from the conveyor to the mold. The mold is chrome plated to reduce abrasion by the fiberglass, but no special mold preparation is necessary to make the grille part, Johansson said.
The press closes, forcing the 395'F charge through a mold heated to 150T. Under pressure, the hot preform material flows easily into the tool recesses. Cooling water cycled through the mold draws off the heat and the part sets in 24 seconds. At cycle's end, the press opens, the finished part rises with the upper die, and a sleigh simultaneously slides between the open dies. A plunger pushes the part out of the upper die onto the sleigh for transport downstream. The front-end part features integrally molded inserts for assembly hardware. As the sleigh retracts, 10 inserts pop into position in the lower mold, the robot places a new preform in the lower mold, and the press closes again.
The sleigh delivers the still-cooling part to the next station, where it is weighed and measured. The module is then conveyed to the milling station, where holes are drilled for the radiator inlet and headlamps, and the part line between the two mold halves is robotically deflashed. 'The surface is perfect so we don't use coatings, and no surface finishing is required. After drilling and deflashing, the part can be placed on the car," Johansson explained.
DMPF Ideal for Automotive Applications
"The economics of the CPI system favor larger parts," Enochs said. "And the weight/strength advantage gained by its uniform glass distribution makes the process very attractive for automotive applications." By adopting a process that can produce one front-end molding every 54 seconds, Volvo emerged as one of the innovative automobile manufacturers expanding the range of applications for CPI direct extrusion.
Other automobile manufacturers are adopting CPI's technology for different applications. In-mold coating and other surface materials are being successfully used. Dodge uses DMPF to produce the interior cab back panel of the Ram T-300 pickup truck by placing fabric into the tooling and integrally molding it into the preform.
In the VCCC/BCD front-end part production, the extruder serves one press; but Johansson noted that it could be linked to another press to increase the production rate. VCCC/BCD plans to use CPI's direct extrusion process to produce several other body components, starting with an underbody splash shield and the "personal box' common in most trucks. Volvo‘s drawing board also features a proprietary component that calls for extrusion using 1-inch long fibers.
For molding multiple parts in a single die or complex shapes, CPI has patented the tooling needed for transfer molding of its long-fiber preforms. Borrowing from injection molding procedures, the process places the preform in a transfer pot and hydraulically pushes the material from the pot through a runner system into a closed mold.
From – Composites Technology
November/December 1998
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