Contemplating coinjection?Check with your tool firstContemplating coinjection?Check with your tool first

If you have a DaimlerChrysler minivan, you’ve probably grabbed the interior front door handles a thousand times without giving them a thought. But the engineers at injection molder Siegel-Robert have thought about them. A lot. The part, which has particular strength and dimensional requirements, started life simply enough: Inject a 15 percent glass/25 percent mineral filled nylon 6 into a four-cavity mold and—Shazam!—you have nice, durable, aesthetically pleasing parts (Figure 1).

The problem was that highly filled nylon wreaks havoc on hardened P-20 tool steel. Because of this, the tool had to be pulled every two to three days to be retextured. It doesn’t take a math genius to realize that such frequent downtime is a money-sucker. Also, because the material was so heavily filled, mold temperature and injection speed were elevated to maintain surface aesthetics. This caused flash and eroded the parting line, which also had to be fixed when the mold was pulled.

After much cogitation, engineers at Siegel-Robert decided the answer might be to try coinjection. Use a glass-filled core material for strength and rigidity, and an unfilled skin material to preserve aesthetics and tool texture. So, Siegel-Robert bought a 265-ton Battenfeld coinjection machine (Figure 2), threw the tool in, and it worked. An unfilled nylon 6 is injected first for the skin. Following it is a 33 percent glass filled nylon 6 for the core.

The benefits:

1. The handle can withstand 350 to 400 lb of force before breaking, more than twice the 150 lb required by DaimlerChrysler.

2. Aesthetics and texturing are preserved by the non-filled material used for the skin. Tool retexturing is done once a month, reducing tool repair costs and downtime.

3. Filled material is atthe core, which means it doesn’t come in contact with the tool steel, which means it cools at a different rate than the skin. Siegel-Robert compensated by lowering the core material melt temperature 50 deg F. The tool now runs cooler, with a 10 percent cycle time reduction (Figure 3, p. 78).

It sounds so simple. But this chain of events belies the adjustments moldmakers and molders must make before converting or building a tool for coinjection. In Siegel-Robert’s case, good fortune smiled on the molder. Says Cliff Riley, corporate molding engineer at Siegel-Robert, “Luck is a very good thing. Even a blind squirrel finds an acorn now and again.”

TOOL CHANGES

Where Riley and Siegel-Robert lucked out was in the tool. It was constructed in such a way that conversion to coinjection was relatively simple. But it’s not always that way. “The biggest downfall I’ve seen,” says Riley, “is that people get sold on the concept that coinjection is the best thing since sliced bread. But they don’t optimize the part and the mold to make it successful.”

Gate location. Traditional mold design, says Riley, dictates that a part be gated at or near a thick section, so that material flows from thick to thin part areas. “In coinjection, that flip-flops,” Riley says. “You want the material to flow from the area of greatest resistance to the area of least resistance so that the core material doesn’t show through the surface of the skin material [called a blowout].” In the case of the handle, it was already gated at a rib in the middle of the back of the part. Riley says this was originally done for balance purposes, but it proved fortuitous when coinjection was tried.

Also, because the core material is the last resin into the tool, and is a different color than the skin material, it may be visible at the gate. If possible, gate location should be on a non-visible side. On the door handle, Siegel-Robert intentionally allows core material to blowout through the skin at the cored-out hinge at the end of the part. DaimlerChrysler deemed that more than any other section of the part, this nonvisible area needs strength and wanted the glass-filled core to displace the skin here. The Siegel-Robert design and the versatility and timing of the process allow the molder to perform this controlled blowout.

Cavity balance. All good molders like to have balanced cavities, but for coinjection it’s essential. Riley says coinjection requires that an exact amount of two different materials be injected in a timed, sequential process. Any cavity that doesn’t get the prescribed amount of material is subject to a blowout. A multicavity coinjection tool should have some method to insure balance. Siegel-Robert has its own proprietary methods for achieving balance, but Riley says adjusting gate size, or using a device such as the Runner Flipper (see IMM, April 1998, p. 85) might work.

Keep it simple. The handle mold Siegel-Robert uses has four cavities and cold runners. The parts are subgated (although they could just as easily be edge gated). Riley says four cavities is his maximum for coinjection, as balance problems multiply beyond that. Hot runners, he thinks, would be a challenge and an unnecessary variable for Siegel-Robert’s coinjection efforts. In the end, he recommends molders carefully consider how coinjection will be used, and what benefits it will provide.

“You can coinject any part, but it may not have the properties you want,” he says. “It’s like any technology, it works great in more applications than we’ve come up with yet, but it’s not for every part.”

Similarly, know the compatibility of the materials you’re coinjecting. Certain resins bond better than others. And a crystalline/ amorphous material mix may not bond at all. Battenfeld and most materials suppliers can provide information on this topic.

THE EQUIPMENT

Tools aside, coinjection requires fairly specialized equipment. Siegel-Robert’s Technology Center in St. Louis has two conventional and two coinjection Battenfeld presses ranging from 44 to 496 tons, each equipped with gas-assist equipment. This setup allows Siegel-Robert to experiment with almost any manufacturing process. The coinjection machines use Battenfeld’s angled nozzles to reduce shear potential. They also have an interchangeable nozzle head for doing two-shot molding.

Most prototype and production-bound tools are tested at Siegel-Robert’s Technology Center before being sent to a molding facility in Wilson, AR; St. Louis and Portageville, MO; Ripley, TN; or Pompano Beach, FL. All together, the company, which serves the automotive industry primarily, has 160 presses and three tooling facilities. It runs six Battenfeld coinjection machines and is taking delivery of three more this spring and summer. The minivan handles are molded at one of the Tennessee plants in a Battenfeld automated production cell. The next generation of coinjected interior door handles will debut in 2001 model year minivans.