Effective Medium-Volume Composite Parts
One of the problems with low-lot production runs is that they tend to be more expensive than higher-volume production. It’s the economies of scale effect. According to Bob Miller, general manager of Diaphorm, a division of Solectria Corp. (Woburn, MA), they’ve developed a process that permits the manufacture of fiber-reinforced thermoplastic composite moldings that are on the order of 20% to 70% less than compression moldings when producing from 1,000 to 100,000 parts.
The big difference is the cost of tooling. It’s much less for the “Pressure Diaphorm” process (for which a patent is pending). According to Vasilios Brachos, engineering and operations manager at Diaphorm, the tooling is based on a single-sided mold, which greatly reduces the cost. Typically, this mold half is produced with aluminum, but other materials can be used to produce it for purposes of prototyping. As for the other side, there is a rubberized diaphragm or bag. That’s what provides the pressure to press the material into the mold.
Miller says, “Tooling that might cost $100,000 or more for a 1,000-ton compression machine could cost as little as $25,000 or less in Diaphorm’s process.”
A real key to the process is the use of preconsolidated thermoplastic pre-pregs. Unlike some of the prepreg materials ordinarily used in composite processes, what they’re using is a material that’s like a fabric at room temperature. It combines the reinforcement (e.g., glass, carbon, or aramid fibers in a mat, unidirectional or woven) and the thermoplastic binder. This material is, Brachos says, “drape able.” It can be cut into the required size. This material is heated with infrared energy; the heating accounts for the greater portion of the three- to six-minute cycle time of the process. The material is put into the tool, which isn’t heated, and then the part sets up: the pressure causes the resin to form around the fibers.
Presently, they have developed equipment capable of producing parts that are as large as 3-1/2 x 7-ft; thickness can be up to 3/8 in. Diaphorm engineers are working on larger-capacity equipment.
Miller says that although the resulting surface finishes are “good,” they wouldn’t be considered Class A and therefore the process isn’t appropriate for producing exterior body panels. He says, however, that when it comes to structural or semi-structural non-cosmetic parts, the Diaphorm process provides a cost-effective means to achieve lower-volume part runs. For example, he suggests that a good application for the process is in bumper beams, where people are using advanced steels or aluminum so as to reduce part weight without compromising performance. Other automotive application areas include load floor, underbody shields, battery trays, and door inner panels.
While fiberglass/epoxy laminates or machinable polyurethane boards are often used to produce high-volume thermoform molds for forming ABS sheet, there are alternatives. Like machining aluminum. Yet that can be time consuming. Still another alternative is one that’s being used by Molded Plastic Industries (Holt, MI), which produces a variety of parts, such as interior and exterior components for trucks. According to company president Frank Phillips, Jr., they’ve found that using an advanced epoxy to do the job is working out well. Speaking of the molds produced with the Cast-IT material from the RenShape Solutions Tooling Group of Vantico Adhesives & Tooling Div. (East Lansing, MI), he observes, “For one customer, we’ve formed ABS sheet on the same tool for nearly three years, making 150 to 200 hits per week with no detectable surface wear. Another benefit of the cast epoxy molds is that they can be machined to make minor changes after they’re built. Alterations aren’t an option with laminated tooling.”
The molds the company produces measure from 5-ft long by 6-in. wide by 6-in. high to 4-ft long by 6-ft wide by 18-in. high.
When molding, pretextured ABS sheet (0.180- to 0.25-in. thick) is heated at a temperature ranging from 370°F to 410°F. Then the mold is pushed into the material. According to Phillips, “We try to run tools at a relatively constant heat using consistent cycle times to optimize tool life.”
Given that the VUE sport utility vehicle is a Saturn, it shouldn’t be at all surprising that there is the utilization of a considerable amount of plastic on it. One interesting application takes the form of roof appliqués. As shown in the accompanying photo these long, curved parts are fitted onto the vehicle’s steel space frame above the doors and behind the rear side windows (they are shown both attached to the vehicle as well as floating in space). Clearly, these appliqués are visible, which means that a Class A surface is needed. Which means that they need to be painted. Another aspect of the parts is that they serve as functional parts, as well, channeling rainwater and providing a smooth surface for sealing the doors’ weather stripping.
The parts are being produced for the VUE by A.P. Plasman Corp. (Windsor, Ontario) with Rynite RE5309, a PET thermoplastic polyester resin from DuPont Engineering Polymers. The material contains both glass fiber and mineral reinforcements. It is formulated so that there is dimensional stability, low coefficient of linear thermal expansion, and warp resistance, all of which are important in the case of body components, which are exposed to the heat of the sun.
The components are attached to the space frame with fasteners. What’s interesting is that a technique was developed by A.P. Plasman to mold parts with integral holes for fasteners. Says George Mitri, A.P. Plasman’s program manager, “The appliqués don’t need drilling, but the holes don’t have crack-prone knit lines around them, either.”
Big Part Mating
Need to join some sizeable thermoplastic parts? There is a 72-in. wide bed linear vibration welder now available from Sonics & Materials (Newtown, CT) that is capable of welding by time, distance or energy; up to four independent weld pressure, time and amplitude settings can be applied as required. A patented auto-tune function is said to be able to automatically detect, set and lock the upper tool’s optimum frequency. An industrial computer running Windows is interfaced with an Allen-Bradley programmable controller for machine operation and control.