Plastics Take To The Fields

John Deere pioneered the use of large plastic components for agriculture equipment back in 2000 when the company developed SMC and polyurethane cosmetic body panels for its big STS Combines. Since then, the company hasn't rested on its laurels. Deere has recently taken big plastics even further with new tractor and combine components that meet important design goals such as function integration, structural integrity, aesthetics and cost.

To achieve design goals with plastics, engineers from Deere and its suppliers have embraced unconventional materials and processing methods for the production of its most innovative parts. Some of these parts were highlighted at the conference and new product design competition held by the Alliance of Plastics Processors in Columbus, OH. Here's a closer look at two of them:

Tractor cab roof

A tractor cab inner roof module for one of Deere's climate control utility tractors won the design competition's agriculture award. It's not hard to see why. The part's designers managed to integrate more than a dozen functional features into this single part — including housings and mounting points for HVAC components, the radio, interior and exterior lighting, the headliner and the exterior roof. "The roof module acts as the structural backbone of the interior cab system," notes Darin Grinsteinner, engineering manager for the part's molder and design partner, Composite Products Inc. (CPI)

Processing played a big role in making this complicated part come to life. CPI has developed a patented in-line transfer compression molding process, meaning that it runs both the compounding and molding steps as part of one integrated production line. Normally, compression molders start with a pre-compounded slug of plastic from a materials supplier.

This kind of integrated process, while not common in North America, gives CPI tight control over the materials it molds. In this case, the tractor roof used a polypropylene homopolymer with 30 percent by weight of chemically-coupled, half-inch glass reinforcements. The in-line transfer compression molding process, which CPI calls "Advantage," can also offer economic advantages — in part because the company compounds only what's needed for a given component and keeps no inventory of expensive custom materials.

The real processing challenge with this part, however, had less to do with the compounding than with the molding. Grinsteinner explains that the 4 x 5-ft part contains very thin, deep, hard-to-fill features — such as 9 x 0.04-inch ribs. "Many of the delicate deep-draw features simply could not have been molded using other long-fiber molding methods like standard compression molding," he says.

The part also had more than its share of thick-to-thin transitions given that its nominal walls ranged from 0.04 to 0.30 inches and had some sections as thick as 0.50. Finally, function integration also meant that the part contained numerous out-of-draw features that would have challenged many molding operations.

Despite the difficulties, CPI not only filled the part but did so with excellent distribution of the strength-enhancing glass fibers. Grinsteinner reports that CPI's tests reveal that glass distribution remains within 1 percent throughout the entire part. That figure includes the ribs and molded-in features.

Much of the company's success in making this difficult part came down to proprietary molding methods. The most important one involved using a large center gate in which the entire slug of raw material is incorporated into the finished part. Unlike the usual end-gate and runner system, this center gate consists of a "shooting pot," essentially a hydraulic platform beneath a gap in the center of the lower mold cavity. When it rises during the molding process to deliver the material, this platform actually makes up a portion of that lower cavity surface.

According to Grinsteinner, this center gate technique promotes low-shear filling of the deepest, thinnest ribs on the part. The gate also eliminated the excess material that an end-gate and runner would generate, and it simplified the tooling construction. Grinsteinner estimates the center-gated molding method saves about 10 percent in material costs per part, as well as shaving 10 percent off the cost of the tool itself.

Other clever tooling twists involved the use of two-stage ejection to get the fragile ribs out of the tool, the inclusion of side-pulls to produce the out-of-draw features, and the use of thickness-reduction techniques to save materials and cooling time.

While agricultural and automotive exterior parts have so far been the biggest outlet for CPI's Advantage process, other big part applications can also benefit from the process. For example, CPI has recently been producing golf cart bag wells, bumpers and battery covers.

A new choice for combine panels

While Deere's STS combine harvesters have sported plastic cosmetic body panels for more than five years now, the company's engineers recently had to take another pass at two of the panels and add an entirely new plastic component.

One reason for the change had to do with the tightening emissions requirements for diesel engines. According to Brian Miller, a supply chain integration leader and non-metallic design engineer for Deere, the new requirements resulted in the engine platform needing about 18 inches more room than in past models. This lengthening triggered changes to a couple of styling panels — those that make up the rear wall and the radiator cover.

What's more, Deere's functional testing had shown that the combines would work well with less air cleaning if the cooling air was taken in higher off the ground than in past designs. So Deere engineers added a new duct component, or "air scoop," to the machine.

To pick the best material for these three components, Deere engineers performed a competitive materials' analysis. Interestingly enough, the results of that analysis pointed to a different material than the company used in past panels. Miller explains that the first STS combines used both compression-molded, painted SMC and reaction-injection-molded polyurethane with an in-mold coating. The three new panels, though, all ended up in a lesser-known reaction-injection-molding material called polydicyclopentadiene (PDCPD).

He explains that the new panels were evaluated for both structural and aesthetic goals. On the first score, the panels have to stand up to impact forces while in use and also have the tensile properties to withstand substantial wind loads during truck transport. "A combine moving down the highway on a trailer might experience wind speeds as high as 100 mph," Miller says. As for aesthetics, the new panels had to match the machine's remaining painted SMC panels and steel structures in terms of color, gloss and finish.

In each case, painted PDCPD came out on top (see decision matrix charts). For the 10 x 6 x 2-ft rear wall, which was formerly two polyurethane RIM parts, Deere engineers found the high impact strength of PDCPD offset its lower modulus compared to SMC and polyurethane RIM. Likewise, PDCPD got the nod for the 6 x 3.5 x 1.5-ft air scoop because it had a better balance of impact and aesthetic properties than the competitive materials — in this case, thermoformed ABS with an acrylic cap and painted SMC.

With the 9 x 6 x 1-ft radiator door, PDCPD did not offer a strong technical edge over SMC. As Miller explains, the "panel is rigidly fixed to the machine so stiffness of the panel was not important beyond the deflection caused by a person leaning on it." Still, Deere went with the PDCPD in this case for "a number of commercial reasons," Miller adds.

And speaking of commercial reasons, Miller points out that PDCPD would not have worked from a cost standpoint had Deere and its suppliers not deviated from conventional wisdom about how to design parts in this material. Usually, PDCPD panels have a uniform wall thickness, while stiffening features are typically molded separately and bonded to the panel in a separate process step. "We determined this approach wouldn't allow us to meet our cost targets," he says, citing the extra tooling, molding and fixturing costs associated with the bonded stiffeners.

So Deere ended up molding in the ribs, bosses and other stiffening features needed to meet the panels' structural loads. These extra features did make the part more difficult to mold and risked read-through of ribs and bosses on the part's show surfaces. But Miller argues that ability to eliminate the extra molding and assembly costs justified the risk and let Deere take advantage of PDCPD's material properties. "We broke the design rules," Miller says. "But it worked for us."