Molding for quantity and quality

Back in November of 1998, Gary Vande Berg, director of engineering for Bemis Mfg. Co., took John Deere's initial proposal as a challenge. The agricultural machinery OEM was looking to replace the stationary steel hood on its 8000 Series tractors with a lighter apparatus that would pop open like a car's. John Deere had approached Vande Berg and Bemis after weighing several different options in materials and processes.

"[John Deere] had looked at steel, SMC, and RIM," Vande Berg says. "All three of the processes had deficiencies or problems either in tool cost or part performance, or [the process] couldn't meet the form, fit, and function."

Vande Berg and his team at Bemis countered with an injection molding proposal that impressed John Deere. A long examination process ensued as the involved parties tried to determine what it would take to make the project happen.

"We went through about an eight-week intense study looking at cost, options, design, problems, and tooling issues," Vande Berg explains. Consultations with GE Plastics helped decide the materials that would be needed, and flow simulations indicated the machine size necessary to mold the backbone component for the hood assembly—6600 tons.

Bemis had taken on some large-part programs, but nothing of this scale. The prospect of the massive project gave Vande Berg and Bemis pause, if only momentarily.

"It's not to say that there wasn't some concern along the line," Vande Berg admits, "but we approached it as a challenge."

Nearly three and a half years later, Bemis answered that challenge and was honored by the SPI Structural Plastics Div. at its annual show in Dearborn, MI from April 14-16. The hood assembly took top honors for an agricultural application, and the space frame that served as the assembly's backbone was awarded best individual part. In the time between the initial proposal and accepting two awards on the Structural Plastics podium, Bemis and John Deere took a project from rough concept to rigorously tested assembly that would be recognized by their peers.

Radical Redesign
The original structure was really only a hood in name. The fixed steel assembly could be better described as an engine cover, since farmers who wanted to service their tractors were forced to unscrew and remove various access panels to get at the motor. Popping the hood wasn't an option.

"[John Deere] was updating the machine entirely for the next generation," Eric Keen, a John Deere design engineer, explains. "Our customers wanted the increased service access from a tilt function."

The new hood assembly would have to be lighter to make that tilt function feasible, but it would also need to be rugged enough to handle work in the field and have high enough temperature resistance to enclose a hard-running tractor engine. Keen says underhood temperatures run continuously at 100C with spikes reaching as high as 130C.

John Deere and Bemis began simulation work on a main backbone part that would have the bolt and latch hardware attached to it as well as decorative body panels. Ultimately, the backbone would not only have to support all of the exterior components, but also act as a heat shield and protect them from the engine.

"We needed tremendous rigidity because this is kind of the backbone structure for all the green cosmetic panels," Keen explains, "and it shields them from the heat, because they can't take 100C continuously."

So, not only would the backbone structure be stressed by abuse in operation, but with two side panels, a plastic hinge block, a lift and latch system, headlight bezels and assemblies, and a grill frame mounted to it, the part would have to be, in a word, sturdy.

"You know the Saturn commercial where the kid's throwing the baseball against the side of the car?" Keen asks. "We were looking for that kind of toughness." Keen says Bemis and John Deere needed a material with a rigidity of 800,000-plus-psi flexural modulus. Given that and the temperature resistance requirements, Bemis opted for a 30 percent glass-filled PBT from GE for the backbone and a PC/PBT blend for the body panels. John Deere's trademark green was molded-in and painted over the body components to protect against harmful agricultural chemicals.

We're Going To Need a Bigger Machine
With Bemis and John Deere settling on materials and design, they now needed a machine that could mold the underhood space frame, which, according to designs, would weigh 60 lb and measure 9 ft tall and 30 inches wide.

Simulations suggested 6600 tons of clamping force would be needed, which is what Bemis asked of its machinery supplier, Milacron. Bemis wanted to use coinjection to save on materials, but Milacron had never made one that large. After consultation, Milacron delivered with a 6600-ton coinjection machine, but first it had to wait for Bemis to find somewhere to put the colossal press.

"We put on a 60,000-sq-ft addition to house this thing," Bemis Market & Business Development Manager Steve Kolste explains. "This was partly because we didn't have the space, but mostly because we needed 5 ft of concrete footings under the machine."

A more fortified infrastructure was also needed to support the 80-metric-ton crane that would pull the massive 121,000-lb space frame mold. Four tiebars with just less than 10 ft of spacing support the machine's 140-inch platens, which, opened fully, expose 17.5 ft of daylight. This all fits neatly within a 2100-sq-ft footprint.

The new space also had to accommodate a six-axis 6400 Series articulated-arm robot from ABB. The robot is the largest made by ABB, and it uses its maximum end affector capability of 222 lb to pull the backbone piece from the mold. Tracks were built so the robot could move between the platens to remove the part at its center point.

Final Exams
With the machine, mold, and materials in place, completed assemblies were integrated onto the tractors and given rigorous testing.

"We do full-vehicle acceleration or shake test where we've taken measured field loads, and we hook hydraulic cylinders up to each corner of the vehicle," Keen says. "We replicate our worst fields conditions." According to Keen, a month of accelerated testing can simulate 6000 hours of customer use. "Essentially we test the life of the vehicle in 30 days. It looks terrible because we've picked out all the damaging loads, and we just run them one after another. You'd never really see this kind of abuse consecutively—normally they're spaced one every 10 to 20 hours."

Eighteen months of field tests followed, and the end result is a very successful product. The original assembly weighed in at 220 lb, with 56 parts, while the new hood is 160 lb with only 29 parts. The agricultural community might seem a reluctant participant in such a massive encroachment by plastics in an application traditionally filled by steel, but Keen says they've been very open to the redesign.

"It's been very well received," Keen says. "The customers are perceiving the plastic as tough and less likely to be damaged. They've really been pretty accepting, and they'll all agree that it allows us improved styling."

After tackling such a huge part, Vande Berg says there are only two hindrances to the size and end function that molded parts can achieve. "My opinion is, basically, that with parts like this, you're limited only by your imagination and your pocketbook," he muses.