A promising injection molding technology on display at K 2010
is the projectile injection technology in which a plastic "bullet" powered by a
burst of inert gas is shot through molten plastic.
Röchling, a
German molder, developed the technology to improve uniformity of wall
thicknesses created in the gas injection process.
"The inner diameter is defined by the projectile," says Fabrizio
Chini, advanced projects manager,
Röchling Automotive. "The process enables us
to inject in the same process step additional features, such as lips and
brackets."
One of the new applications is integrated bi-material
tubular sealing used for a cowling grille on the Ford C-Max, a seven-seat
car built in Europe. The C-Max is being introduced in the North American market
next year.
"The function of the cowling grille is to
carry in air for climate-control in the car, drain water and to stop the hood
from fluttering," says Chini. "Steel
hoods are getting thinner and thinner for pedestrian safety, and as a result
they are beginning to flutter."
TPE
Plus PP
The PIT process starts by partially or completely filling
a mold cavity with plastic. The bullet-like projectile is then shot through the
cavity forcing polymer melt against the walls. In the Ford cowling grille,
polypropylene is first injected into the cavity. A second cavity opens for
injection of thermoplastic elastomer. The projectile is then fired. The result
is a part with a bulb-shaped hollow seal.
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Traditionally, the same type of part would be made with
an extruded EPDM (ethylene propylene diene monomer) rubber seal attached to a
hard plastic section. Chini says the PIT process cuts cost by 10 percent compared
to the traditional process, which requires manual assembly of the parts.
Expensive foamed EPDM rubber is also eliminated. The TPE is recyclable while
the vulcanized rubber is not.
A projectile designed with protruding ribs creates
notches in the elastomeric material that reduce permanent deformation and can
precisely control the compression force. Wall thicknesses in the notched areas
are 1 mm compared to 2 mm in other sections.
The bulb seal created by the projectile creates pressure
between the hood and the cowling grille to damp hood fluttering. Changing
dimensions of the seal allows tuning of the pressure. "The force can also be
tuned without any tool modification just by changing material hardness or the
projectile shape," says Chini.
In an interesting innovation, Röchling engineers developed
a chamber that recovers the melted core pushed out of the mold cavity by the
projectile. The melt is stored in the chamber and pushed back into the cavity
in the following injection cycle. "This avoids regrinding the purged
materials," says Chini.
Currently the projectile, which is made of polyimide, is
not re-used. Polyimide polymers are expensive, but very strong and resistant to
heat.
The Röchling process is a finalist in the 2010 Society of
Plastics Engineers Automotive Innovation Awards Program. Winners will be
announced
Nov. 9.
Wittman Battenfeld, a machine builder, also showed
projectile injection technology at their stand at K 2010. Its process, however,
propels the projectile with a blast of water, not inert gas.
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