What's Behind the Chrome Plate? Coinjection & Gas-Assist Molding

There's a lot more to Siegel-Robert Inc. today than just a shiny surface. Known as the world's largest electroplater of plastics, the St. Louis-based firm plates 300,000 sq ft of plastic each day at five facilities.


There's a lot more to Siegel-Robert Inc. today than just a shiny surface. Known as the world's largest electroplater of plastics, the St. Louis-based firm plates 300,000 sq ft of plastic each day at five facilities. "A big misconception people have is that we're just a plater of plastic parts," says David Rudder, executive v.p. of technology development.

In fact, the company is a big-time injection molder, running automotive, appliance, and consumer parts on over 160 machines from 30 to 3000 tons in six plants throughout the Midwest and South. What's more, the company has made a substantial commitment to advanced molding technologies like coinjection, gas-assist, and rotary multi-shot molding.

Much of Siegel-Robert's development work for these processes begins in the company's year-old technical center in St. Louis, which is outfitted with four injection presses of 44 to 500 tons from Battenfeld of America, West Warwick, R.I. "We established the center to come up with new solutions and innovative ideas," says Rudder.

Benefits of coinjection

For some molders, coinjection molding conjures up images of burying otherwise unusable scrap material under an attractive skin. Siegel-Robert, though, found a far more cost-effective use for coinjection: It recently allowed the company to re-engineer a problematic interior door handle for a Chrysler minivan. "For door handles, coinjection is a natural. It's a good example of how to solve a problem when existing technology isn't effective," says corporate molding engineer Cliff Riley.

The handle had caused molding difficulties from the moment it went into production. The nylon material, filled with 40% glass and mineral, wreaked havoc with the tooling. "We had to re-texture the tool and clean up the parting line every two to three days," recalls Rudder. And even nonstop maintenance didn't bring the scrap rates low enough to make the job worth molding.

Rather than back away from the job, Siegel-Robert engineers redesigned the handle as a coinjected part with an unfilled nylon skin over a 33% glass-filled core. "Coinjection gave us a part with a cosmetic skin and a strong core," explains Rudder. By design, the door handle has only core material and no skin in areas not visible in the finished assembly. This "controlled breakthrough" of core material gives the handle added strength in a critical area.

This material and process change made molding far easier. "Before, we had to fill very fast into a hot tool to keep the glass off the surface," says Riley. Thanks to coinjection, the mold now runs about 50° F cooler for a 5-10% savings in cycle time.

Riley also notes that coinjection lets the core material run cooler because it doesn't encounter the cold mold steel. By encapsulating the abrasive material in the core, the handle now runs for a month or more without maintenance.

More coinjection opportunity

Coinjection molding stands to become even more important for Siegel-Robert in the future. One reason, according to Rudder, is an automotive style trend: "We're seeing a swing back to more chrome plating, he says. PC/ABS, while plateable, won't meet some of the strength requirements needed for handles and other parts. "Coinjection allows you to couple PC/ABS with stronger core materials. You get strength and aesthetics," Rudder says. Riley says plated door handles will likely be coinjected with an ABS or PC/ABS surface and nylon/ABS core.

Siegel-Robert has done some work with "thin-wall coinjection." An example is a pager housing that has EMI shielding under an aesthetic skin while total part thickness is around 0.040-0.060 in. Future developments will also include foam-core coinjected parts.

Gas-assist, too

Siegel-Robert has also had success with gas-assist molding. An example is a recent Chrysler exterior door handle initially slated for zinc diecasting because the area around the lock mechanism was thought to be too thick to mold in plastic. Coring out the thick section with gas assist allowed plastic to do the job.

Gas assist also comes into play in components for exterior mirror assemblies. Rudder says gas assist has let the company use molded-in color instead of paint, even on parts that are chock full of internal features. "Gas assist stops any read-through on the Class A surfaces," he says.

Combining technologies

Siegel-Robert has produced automotive mirror assemblies consisting of both coinjected and gas-assist components. More recently, the company has started to combine processes within a single part. It has produced several hundred samples of a car door handle that uses coinjection to get a soft-touch exterior and gas assist to hollow out the rigid core.