WEB EXCLUSIVE:Structural Plastics gets innovative

If you can imagine a combination of processes and materials, a molder in the structural plastics industry has probably experienced it. The demanding applications that call for structural plastics often require equally demanding product development, resulting in the use of a multitude of molding processes. The parts seen at this year's Structural Plastics conference, sponsored by the SPI Structural Plastics Div. (SPD), are no exception. In all, 14 different processes, many used in concert with one another, were represented in the 63 parts on display at this year's event in Atlanta. Attendees saw everything from structural foam, injection molding, and gas assist to blowmolding, rotational molding, RIM, and SMC. Structural foam accounted for 27 percent of the parts; traditional injection molding for 16 percent; gas assist for 19 percent; and RIM for 13 percent. 

Improved efficiency, in terms of processing time and material cost, combined with the need for an appealing appearance, were the driving forces behind the manufacturing choices seen at the conference. Following are some of the more exceptional-or just plain intriguing-entries from the SPD's Design Competition. 

• Lawn tractor. The John Deere SST lawn tractor (Figure 1) is a good example of an effective union of process and material. The tractor combines straight molding and coinjection, drawing together complex component design into an appealing overall package. The low-profile hood (Figure 2), molded by Bemis Mfg., is highly stylized, yet functional; it supports the grille/light bar and acts as the air inlet and the air exhaust. Innovative placement of gates and knitlines under secondary decoration on both the top and sides of the hood help complete the cosmetic requirements of the part. 

The fenders (Figure 3) had major technical, cosmetic, and strength requirements, which were met with a special blend of glass-filled PP and other PP grades. FEA analysis was used to develop appropriate mounting details and load transfer information to allow the fenders to be load bearing, yet cosmetically pleasing. 

The SST tractor's steering wheel (Figure 4), perhaps the most complex component of the tractor, was singled out by the SPD for top honors as a single part. It combines three materials and a metal insert, requiring a three-barrel coinjection machine and a four-cavity steel mold with a rotary cavity plate. A skin of 20 percent glass-filled PP and liquid color cover a core of PP and liquid blowing agent. The wheel is rotated in the mold for overmolding of the third material; the texture is molded in. This combination provides the required strength, appearance, and weatherability, yet is cost effective. 

Involved in the project along with OEM John Deere and molder Bemis were moldmakers CDM Tool (Hartford, WI) and Triangle Tool (Milwaukee, WI), and designer Henry Dryfuss Assoc. (Warren, MI). 

• Loader/backhoe cab interior. One of the largest parts on display was the M Series loader/backhoe cab interior from Case Corp. (Burlington, IA). Molded by GI Plastek, based in Marysville, OH, the cab (Figure 5) pulls together 28 individual injection molded parts ranging in size from .05 to 7 lb and from 2 by 1.5 inches at 3 mil wall thickness to 48 by 29 inches with a 20-inch depth. The M Series replaces Case's existing L Series with a more aesthetic and cost-effective design. 

By consolidating componentry and fasteners, replacing metal hinges with molded-in alternatives, reducing secondary operations, and optimizing manufacturing processes, the company was able to reduce costs by $80/cab, reports Dave Adams, commercial manager for GI Plastek. And with approximately 20,000 machines being produced worldwide each year, $80/cab adds up quickly. Converting the fenders from RIM to IM amounted to almost half of this savings. 

Cost reduction was not the primary purpose of the redesign, however. Case wanted a cab that was wider and longer to provide improved ergonomics and visibility. Requirements of the redesign included a 23 percent improvement in user visibility, the addition of significantly more turnaround room for feet and knees, and more efficient placement of operator controls. 

Tooling played an important role in the development of the M Series. Case wanted tools capable of handling various options. Inserts are used to modify parts for user requirements. The tools were designed concurrently with the help of two tool shops-Active Burgess and Build-A-Mold, both of Windsor, ON. This approach allowed first parts to be produced only 20 weeks after the first PO. In all, 29 molds were produced. A Geoly ASA/polycarbonate blend is used for all parts except the fender extensions, which are made from TPO. No secondary finishing is required as parts are injection molded to color specification. 

No doubt, this will not be the last we hear about Case's M Series. Case has asked for continuous cost improvement, calling on GI Plastek to reduce costs associated with the cab by 3 percent/year during its production life cycle. 

• Kitchen and bathroom handles. Two gas-assist handles, both award winners, deserve mention. An oven handle (Figure 6) molded by Mack Molding (Inman, SC) for GE Appliance earned the award for best appliance part design. The one-piece, gas-assist plastic part replaces a three-piece plastic and metal handle. Two other options were also considered-a three-piece design with performance and perception issues and a thermoset design with high part weight issues-yet gas assist proved most effective. The new handle is produced from a glass-reinforced Valox PBT from GE Plastics that was chosen for its high strength and heat- and chemical-resistance properties. Gas assist helped overcome two major challenges involved in processing the handle: maintaining good aesthetics part after part with a high-flex-modulus, glass-reinforced material and reducing part weight to lower part cost. "With gas assist, we achieved 45 to 55 percent weight reduction, depending on the handle model, over a thermoset part," says Brian Sumpter, new business development director, Mack Molding. Yet, the new handle displays improved strength and reliability specifications. Expected to pass 30,000 door slam cycles (considered to be the lifetime requirement), it passed 120,000 cycles with no evidence of loosening, reports GE. Mack worked with Delta Mold (Charlotte, NC) to produce two-cavity, steel molds, which can be expanded to four cavities, in just six weeks. 

Another award-winning, gas-assist handle was entered by Mitsubishi Engineering Plastics Corp. The bathroom handrail (Figure 7), produced for Naka Corp., won best part design for the building and construction category. Gas assist, in combination with a core sliding technique, was chosen by molder Kokujoh Kanagata Industrial Co. Ltd. to reduce cycle time and part weight. According to the molder, the use of these processing techniques also allowed for constant residual thickness and good appearance. 

• Industrial emergency lighting. The first general-purpose industrial emergency lighting fixture in the industry to use rugged injection and compression molded components in its construction, Lithonia Lighting's Indura line (Figure 8) could be considered ground breaking. Molded by Underwood Mold Co. (Woodstock, GA), the Indura product line offers a form-flow-function design, adjustable beam pattern lamp heads, and quick-mount, rough-in brackets. The shape of the housing reportedly provides additional impact strength over the more traditional flat box shape. Captured fasteners and integral hinges provide easy maintenance at the product's typical 30-ft mounting height. Polycarbonate, ABS, and a 10 percent glass filler, selected for strength, are used to construct the lights. 

• Theft deterrent. Noteworthy for its eye-catching design is The Wrap from Blockit & Lockit Systems. Reminiscent of something from a Star Trek episode, the device (Figure 9) is injection molded by Semex in Mexico; Minco Tool & Mold (Dayton, OH) built the tools. The Wrap is meant to block and lock the steering wheel. It includes a visual LED warning light and an audio siren and flashing strobe light, motion-activated alarm system. Polycarbonate with impact-modified filler replaces the more traditional metal design. 

The next SPD conference will be held April 14-16, 2002 at the Hyatt Regency Dearborn in Dearborn, MI.