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Injected Projectile Boosts Design Options

Injected Projectile Boosts Design Options

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.

Rochling, 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,

Rochling 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."


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.

Injected Projectile Boosts Design Options_A

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, Rochling 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 Rochling 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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