Flexible and rigid plastics join forces-inside a blow mold

Hybrid injection molding technology, which joins metal and plastic inside the mold to produce structural components that offer the best of both materials, has been around for more than a dozen years now. But Bayer Polymers, a pioneer in this technology, has now come up with a brand new kind of hybrid based on extrusion blow molding. This patent-pending process can combine rigid and flexible materials inside the mold, creating components such as seat frames with integrated air-bladder cushions.

"Think of it as an evolution of our injection molding hybrid technology," says Chris Cooper, senior process engineer for Bayer and one of the inventors of the new process. Like the company's earlier technology, the new blow-molding hybrid starts with a rigid substrate placed inside the mold, usually robotically. This rigid inserted part would then be overmolded with a flexible plastic, which forms the bladder and other molded in features, And like the earlier hybrid technology, molded-in part features form a mechanical interlocks between the insert and overmolded plastic. As Cooper describes it, the flexible plastic flows through holes in the rigid inserts during the molding process, forming "chamfered buttons" that join the insert and overmolded bladder. Though their exact size and geometry would depend on the application, the buttons would typically range from 0.25 to 0.75 inches across. In early testing of one button design, Bayer found that each offered about 43.5 lb of pull-through strength. By varying the number and spacing of the buttons, engineers could meet a variety of mechanical requirements, Cooper reports.

Aside from the fact that it's based on extrusion blow molding, the new hybrid also differs from the injection molding technology in another key respect. Whereas hybrid technology has traditionally relied only on metal stampings for the inserts, the blow-molded hybrids can use metal or rigid plastic inserts. In fact, Cooper believes injection molded plastics-including glass-filled nylon, PC/PBT, or others with good structural properties-will be more common than metal in the seating and cushioning applications that the new technology targets.

Though it could also work with a variety of elastomers for the flexible material, thermoplastic urethane (TPU) grades will likely dominate. Gerry DiBattista, Bayer's market channel manager for TPU and another one of the inventors, notes that TPU has well-recognized combination of tear, puncture, chemical, thermal, and UV resistance. These make it well suited to all sorts of rugged seating applications. Consider, for example, the guy who plunks down in his tractor seat with a screwdriver sticking out of his back pocket. Seats on lawn-and-garden equipment, recreational vehicles, and personal watercraft can also benefit from the same TPU attributes, DiBattista says. And the blow-molding hybrid doesn't just lend itself to seating. "It could be used in any area needing cushioning," he adds.

Size wise, the blow-molding hybrid can tackle some pretty big parts. Bayer has tried it with components measuring as much as 6 feet long and about 2.5 feet in diameter. And DiBattista believes that even larger parts are possible. The only limitations he cites relate to the size of the molding machine and to TPU's possible melt strength limitations in truly gargantuan parisons.

This seating concept shows the integration of a flexible TPU cushioning bladder (lighter color) would be integrated with a rigid thermoplastic substrate (black).

Capable of running on standard blow-molding machines and requiring mostly conventional processing expertise, the blow-molding hybrid promises several important advantages. The most important has to do with cost avoidance. Traditional seating or cushioning components need some assembly-such as attaching frame to a foam cushion and adding a covering. The hybrid process turns out an integrated frame and bladder without the need for foam or a covering. "The bladder and seating surface can be one and the same," DiBattista says, though he adds that the method can incorporate fabric coverings if the application requires them. "We can add fabrics right in the mold," he says. Bayer has also experimented with foam filled bladders as an alternative to air.

Other components can be assembled inside the mold as well. The process can mold in nozzles for inflating the bladder, to take one example. And the inserts can be molded with built-in attachment points, such as boltholes.

A cross section of a blow-molded hybrid part shows how the molded-in buttons lock together the flexible TPU bladder with a rigid substrate.

The process could also enhance design flexibility. In one study, for instance, Bayer engineers come up with hybrid parts that use a living hinge-a molded-in hinge made from the TPU itself-to join a seat bottom insert to a seatback insert. The same approach could work with a storage console, using the TPU to form both a cushioned air bladder for the top of the console and a living hinge to join the top the rigid box underneath.

For general information Bayer Polymers, visit www.bayerpolymers.com. Email TPUinfo@bayerpolymers.com for more information about the hybrid blow-molding technology.