One of the best places to see trends in plastics’ design is the annual Innovations Conference and new product design competition held by the Alliance of Plastics Processors. You won’t see knock-your-socks-off all new technology because adoption of new approaches in plastics is evolutionary. But you will see what’s picking up speed. Where do the best designers want to take plastics?
Three trends stood out at last month’s conference, held in Memphis, TN:
Integration of metal and plastic structures,
Growing use of external gas molding and
Increasing acceptance of molded-in color.
One of the most interesting new designs was the first-ever use of a single-piece chrome tube directly molded in a composite structure for vehicle protection. The part is a brush guard used to shield the front end of GM sport utility vehicles and trucks. Possibly, even more importantly, the hybrid structure gives a strong and powerful look to the front ends of the vehicles.
New technologies in tooling and in-line compounding make the structure possible, foretelling even greater use of plastic and metal structures — particularly in cars and trucks. “We built a small transfer mold prototype because we wanted to verify the shutoff with the chrome tube without creating any (plastic) flash,” says Scott Ledebuhr, the vice president of sales engineering at Composite Products, of Winona, MN, the molder. Unique clamping and other special detail at the edge of the tool were developed.
“The tooling is also unique in that it allows the use of a 1.5-inch chrome tube to be fully protected from scratches and mars in a mold that is made of hard P-20 steel,” says Ledebuhr. “The tube has a dimensional tolerance or roughly 5 mm, while the tool has a tolerance of essentially zero.”
The chrome tube used in the guard weighs 17 lb and is 80 inches wide. Only the section that is overmolded must fit in the tool, which weighs 50,000 lb.
Previously these grilles were made by two methods. In the first, an assortment of chrome tubes are cut, bent and welded and then painted or powder coated. In the other approach, a tube assembly is attached to a steel substructure that is subsequently covered with a rubber bumper or a plastic beauty cover.
“We also wanted to prove out what the rib design would look like,” said Ledebuhr. Rib placement greatly influenced vibration resistance.” One of the keys to the approach is the strength and flowability of the 40 percent glass-reinforced polypropylene composite, which must also have a Class A texture finish. In the CPI process, gravimetric fiber feed systems deliver controlled amounts of one half-inch (up to one-inch) chopped fibers to the feed throat of a compounding extruder. This approach is significantly less expensive than using precompounded material, and also creates a uniform distribution of glass reinforcement, allowing more design flexibility. “Because this material flows so well, the design incorporates many undersize ribs that were optimally placed to reduce the deflection of the part in normal operating use,” said Ledebuhr. “This was very important because the attachment points are at and underneath the front bumper, creating a large cantilevered load that requires very strong materials and design.”
Use of the hybrid approach saved money — about 50 percent — and time — 2.5-min cycle time versus hours.
The mold maker for the project was Delta Mold of Charlotte, NC and the designer was Algonquin Automotive, of Huntsville, Ontario. The grille guard is an accessory on Chevrolet Avalanche, Suburban and Tahoe.
Most metal-plastic structures are created by overmolding, heat staking or riveting.
An adhesive bonded approach was presented by Ashish Kotnis, a development engineer at Dow Automotive in Auburn Hills, MI. The structure also uses 40 percent glass-filled polypropylene. The adhesive is a two-component acrylic-based material that can bond low surface energy components without using primer or pretreatment. The bonded front-end box is used on recently introduced Dodge Nitros and Jeep Wranglers.
Different designs can be used. In one, an upper steel cross member is stamped and bonded to the upper section of the plastic carrier to form a closed box section along the carrier’s top. “The steel cross member is attached to the outer shotgun sections to carry the latch loads,” says Kotnis. “The resulting front end module holds the radiator, headlamps, grille and attachments for other components of the front end of a vehicle.”
For vehicles requiring stiff upper and lower cross members, the steel cross members can be connected via an adhesive bond to an injection molded two-piece plastic carrier. In a third design, the plastic carrier is designed with no lower cross member connecting the vertical carrier supports. That improves easier access to the radiator, which can be lowered from the underside of the vehicle.
The combination of plastic and metal in vehicles is often driven by a need to reduce weight and fuel consumption.
For its 365/367 trucks, Peterbilt Motors Co. of Denton, TX, uses a polished 0.100-inch aluminum outer skin bonded to a gas injection molded structural frame that cuts weight and eliminates fastener heads on the cosmetic surface of a front grille. Previously, Peterbilt used stainless steel and aluminum for the front grillwork. The structure was heavy, and expensive, particularly considering recent nickel-driven increase for stainless steel. The plastic used is glass-filled polyethylene terephthalate/polycarbonate. The gas-assist process provides structure and hollow internal ribs, adding hoop strength to the perimeter of the part. The other benefit of gas assist is the ability to a significantly smaller press (one ton per square inch versus three tons per square inch).
An Assist from Gas
Gas assist has been an important theme at the conference for several years, but use of external gas is a new twist. External gas molding (EGM) applies nitrogen behind the non-cosmetic surface of a part after plastic is injected into the mold cavity. The gas creates a blanket on the core side, pushing the core against the cavity and eliminating sink marks. The result is a solid part with no gas holes, voids or marks from gas lines on cosmetic surfaces.
Cinpres Gas Injection of Cheshire, U.K., and Ann Arbor, MI, licenses the external gas molding process in North America under patents owned by Asahi Kasei of Japan. Cinpres General Manager Lee Pereces told Design News, Cinpres has sold five licenses in the past two years.
One of the licensees, Mack Molding of Arlington, VT and Statesville, NC, showed a new design for a decorative cover on a standalone ATM. “EGM was used to provide extensive product design freedom, combine system components for overall part number reductions, reduce molding press clamp requirements, and ultimately produce a cosmetic, textured, molded in color exterior fascia,” says an engineer from Mack Molding. Normally the amount of structural ribs on the inside of the cover would have caused a significant amount of sink to show on the cosmetic surface. Sink marks are indentations on the surface caused by non-uniform shrinkage of the plastic, particularly opposite thick sections, which shrink more slowly than thin sections.
“This (EGM) will allow me to do things I couldn’t previously do,” Gary Vande Berg, director engineering-injection molding at Bemis Manufacturing Co., told Design News as he held a test panel near the Bemis booth at the design show. The EGM provides a lot more design latitude in designing structural ribs for large parts. The process is not size dependent and has been used for cell phone bezels. “It opens up your design window quite a bit,” said Vande Berg.
Bemis showed a new engine enclosure for the John Deere 7000 series that features two-shot, or coinjection, molding that includes a regrind layer to lower costs and improve mechanical strength. The entire hood enclosure assembly can be recycled and used as a core material for future products, after metal parts are stripped. The enclosures are made with unusually complex tooling. Lifters are placed inside of larger lifters to create the attachment points for the assembly and reduce potential for sink marks. That’s a problem that could potentially be solved with external gas molding.
The parts are now being painted, but Vande Berg said Deere may switch in the future to molded-in color. “All tooling is molded-in color capable,” Vande Berg says. Molded-in color parts are being tested in the field for wear and tear and customer reaction.
An electric Maxi-Scooter developed by Vectrix Corp. of New Bedford, MA, features high-gloss molded in color with polycarbonate/PBT resin. Tooling for the scooter was developed by Minco Tool and Mold of Dayton, OH.