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
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.
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.