Injected Projectile Boosts Design OptionsInjected Projectile Boosts Design Options
October 28, 2010
A promising injection molding technology on display at K 2010is the projectile injection technology in which a plastic "bullet" powered by aburst of inert gas is shot through molten plastic.
Rochling, aGerman molder, developed the technology to improve uniformity of wallthicknesses created in the gas injection process.
"The inner diameter is defined by the projectile," says FabrizioChini, advanced projects manager,
Rochling Automotive. "The process enables usto inject in the same process step additional features, such as lips andbrackets."
One of the new applications is integrated bi-materialtubular sealing used for a cowling grille on the Ford C-Max, a seven-seatcar built in Europe. The C-Max is being introduced in the North American marketnext year.
"The function of the cowling grille is tocarry in air for climate-control in the car, drain water and to stop the hoodfrom fluttering," says Chini. "Steelhoods are getting thinner and thinner for pedestrian safety, and as a resultthey are beginning to flutter."
The PIT process starts by partially or completely fillinga mold cavity with plastic. The bullet-like projectile is then shot through thecavity forcing polymer melt against the walls. In the Ford cowling grille,polypropylene is first injected into the cavity. A second cavity opens forinjection of thermoplastic elastomer. The projectile is then fired. The resultis a part with a bulb-shaped hollow seal.
Injected Projectile Boosts Design Options_A
Traditionally, the same type of part would be made withan extruded EPDM (ethylene propylene diene monomer) rubber seal attached to ahard plastic section. Chini says the PIT process cuts cost by 10 percent comparedto the traditional process, which requires manual assembly of the parts.Expensive foamed EPDM rubber is also eliminated. The TPE is recyclable whilethe vulcanized rubber is not.
A projectile designed with protruding ribs createsnotches in the elastomeric material that reduce permanent deformation and canprecisely control the compression force. Wall thicknesses in the notched areasare 1 mm compared to 2 mm in other sections.
The bulb seal created by the projectile creates pressurebetween the hood and the cowling grille to damp hood fluttering. Changingdimensions of the seal allows tuning of the pressure. "The force can also betuned without any tool modification just by changing material hardness or theprojectile shape," says Chini.
In an interesting innovation, Rochling engineers developeda chamber that recovers the melted core pushed out of the mold cavity by theprojectile. The melt is stored in the chamber and pushed back into the cavityin the following injection cycle. "This avoids regrinding the purgedmaterials," says Chini.
Currently the projectile, which is made of polyimide, isnot re-used. Polyimide polymers are expensive, but very strong and resistant toheat.
The Rochling process is a finalist in the 2010 Society ofPlastics Engineers Automotive Innovation Awards Program. Winners will be announcedNov. 9.
Wittman Battenfeld, a machine builder, also showedprojectile 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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