Stiffening of thermoformed plastics for transportation applications appears to be a new field of opportunity for reinforced-polyurethane dispensing techniques that produce low-density structural composites without costly glass mat or preforms.
Stiffening of thermoformed plastics for transportation applications appears to be a new field of opportunity for reinforced-polyurethane dispensing techniques that produce low-density structural composites without costly glass mat or preforms. These systems, which have emerged in the last five years, chop glass or other fibers directly at the mixing head, where the fibers are wetted by the PUR foam mix as it is dispensed into an open mold by a robotic traversing mechanism.
One successful commercial installation of such a system is at the Thompson Plastics Group’s Vehicle Industry Div. at Hull, England. This custom thermoforming and rotomolding company purchased an InterWet PUR “coinjection” system from Cannon of Italy in January 2000. Thompson’s experience indicates one direction this technology is taking.
Besides Cannon’s InterWet (supplied here by Cannon U.S.A., Cranberry Township, Pa.), two other similar systems are the LFI (Long Fiber Injection) system from Krauss-Maffei Corp., Florence, Ky., and the FipurTec system from Bayer Corp.’s Hennecke Machinery Div., Pittsburgh.
Suppliers peg the total number of installations at about 30 systems worldwide, most of them from Krauss-Maffei and Cannon. These include five systems in the U.S. and 15 in Europe, as well as several at R&D facilities of PUR materials suppliers. Commercial applications include parts for automobiles and heavy industrial and agricultural vehicles. Jim Riley, Cannon U.S.A.’s InterWet sales and marketing manager, sees the most interest in this technology for backing up a thermoformed shell of ABS or other plastic in smaller-volume applications of 5000 to 100,000 parts per year.
Thompson Plastics Vehicle Industry Div. supplies interior and exterior components for off-road, agricultural, and industrial vehicles, as well as public transportation. A sister firm called Borderfoam produces PUR cushioning, integral-skin, and SRIM products.
Richard Brophy, managing director of Thompson Plastics Group, says the company saw new opportunities for coupling InterWet technology with thermoforming in production of exterior engine side panels, access covers, and roofs, as well as interior parts like door panels, parcel shelves, and dashboards. “Noise absorption is emerging as a key product-improvement area for the next generation of tractors,” Brophy notes. “By applying InterWet PUR reinforcement to thermoformed ABS or vinyl coverings, we are able to offer parts with improved anti-squeak performance, vibration control, and noise absorption. Also of interest is a way to enhance internal styling with the ability to produce rigidly supported soft-touch materials.”
Thompson also found that InterWet PUR reinforcement can provide exterior components with structural rigidity, impact toughness, thermal insulation, and resistance to abrasion, solvents, and oil. These attributes are coupled with the advantages of thermoplastic skins, such as uv resistance, surface gloss, scratch resistance, and colorability.
Thompson Plastics sees InterWet technology as an advantageous alternative to the SRIM process, which can be expensive, messy, and labor intensive. According to Brophy, it is possible to switch SRIM tools to InterWet/ thermoplastic production and benefit from both lower manufacturing costs and improved quality.
“This advanced PUR reinforcement technique allows for greater glass content and it provides the opportunity to vary glass loading within the same part to accommodate different applied loads,” he says. “Moreover, the process provides design flexibility to integrate ribs and mounting fixtures.”
Unlike SRIM or thermoforming alone, Brophy adds, “You have a choice of what your exterior material will be, you can vary your formulation, and you are also using a lower pressure method.” Moreover, the InterWet process allows for the use of other fillers, including natural fibers, Brophy says.
He also lauds the reliability of the process. He notes that problems have been encountered when using glass mat owing to inconsistent bonding of the resin to the glass. With InterWet, the glass is wetted by the PUR inside the mixhead. As a result, Brophy says, “The consistency of wetting of the chopped glass is much better than when working with glass mat. This ability to wet the glass more effectively allows you to get more glass in—up to 60% by weight—than when using mat. We can make an extremely strong part by using more glass. Conversely, you can reduce the cost of a part by reducing or eliminating the glass where it is not needed.” The flexibility of the robotic dispensing process can deposit varying amounts of glass for selective reinforcement of different areas of the part.
Cannon’s Riley notes that in the U.S., glass mat sells for $1.35 to $1.65/lb, versus 65-85¢/lb for glass rovings. Moreover, the need to trim the mat to the part’s shape generates costly scrap amounting to anywhere from 3-4% up to 45%. In contrast, robotic spraying reportedly generates at most 2-4% scrap. Overall material cost savings can be 30-40%, according to Riley.
Thompson Plastics had a relatively short learning curve with InterWet. Brophy attributes this to support from Cannon as well as his firm’s prior experience with polyurethane processes and with thermoforming. “Learning to run the InterWet machine itself is not at all complex. But you need to know how to design the part,” Brophy says.
While Thompson did not experience any unanticipated problems, it has enjoyed unexpected advantages. Brophy says the firm has made parts that are 50% stronger than predicted.
Brophy sees another major opportunity for this new technology in headliners, where InterWet PUR would be coupled with a fabric covering rather than thermoplastic sheet. Thompson also plans to use the InterWet process without a thermoformed skin to produce composite mats for non-visual components. He notes that improved surface finish for exterior parts can be achieved by using in-mold film or foil coverings or even a spray-in-mold coating. The result, Brophy says, is not an automotive “Class-A” finish, but one that is acceptable for exterior components of recreational vehicles, buses, tractors, and other off-road equipment, as well as boats.
Cannon’s Riley acknowledges that wider use of the InterWet system and similar technologies will depend on further machine enhancements. “Increased throughput is the key, particularly for making larger parts, such as auto tailgates, which require up to 6000 grams of chopped glass. We are now introducing chopped glass at 100 g/sec, and we are aiming to go up to 150-250 g/sec. For a 50%-glass formulation, typical of large structural exterior parts, the ideal is 250 g/sec.