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June 1, 2001

12 Min Read
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. 



Figure 1. The John Deere SST lawn tractor incorporates a variety of materials and processes to provide an appealing product for the consumer market.

Figure 2. Innovative placement of gates and knitlines under secondary decoration help complete the cosmetic requirements of the low-profile hood of the SST tractor.



Figure 3. Fenders for the John Deere lawn tractor use a special blend of materials to meet demanding strength and cosmetic requirements.

Figure 4. Winner of a design award, the steering wheel for the SST tractor combines three materials and a metal insert for strength, appearance, weatherability, and cost effectiveness.

• 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). 


Figure 5. Significant cost savings was achieved with a redesign for the Case loader/backhoe cab. Though the cab is larger and more ergonomically designed, material and process replacements helped to reduce cost as much as $80/cab.

• 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. 



Figure 6. Gas assist was employed to successfully replace a three-piece plastic and metal oven handle with a one-piece plastic part.

Figure 7. Gas assist helped reduce cycle time and part weight in this award-winning bathroom handrail.

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. 


Figure 8. Lithonia's Indura line of general-purpose industrial emergency lights represents the first time injection molded components have been used in such an application.

• 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. 


Figure 9. Structural plastic replaces the more traditional metal approach to automobile theft deterrent devices with this high-tech design.

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

Editor's note: Several noteworthy parts from the computer and business equipment category, including a six-part cabinet door molded by Mack Molding Co. and a high-speed paper transport molded by APW Plastics, which won the award for part design in this category, were covered in the May issue of IMM (See "Market Focus: Computers & Business Equipment," May 2001 IMM, pp. 60-68.) A list of new product design and best paper winners is available at the SPD website (www.plasticparts.org). 

Society of the Plastics Industry
Structural Plastics Div.
Washington, DC
Karen Miles
Phone: (202) 974-5247
[email protected] 

Gas Assist vs. Structural Foam

Structural foam has become a strong business for Preproduction Plastics Inc. (PPI, Corona, CA) over the past 20 years, but when it comes to gas-assist molding, the company has been reluctant to test the waters. That is, until recently. "We've been watching the gas assist battle from the sidelines all this time," says Kevin Rafferty, gm at PPI. "Litigation was a real deterrent to jumping into it." However, PPI is confident that those issues are past. "We just purchased two new 1100-ton Battenfeld presses with gas-assist systems. We decided to get on board because a lot of our customers are looking at cost reductions while improving the cosmetics of the part," says Rafferty.

There are generally two reasons for using gas assist: improved productivity and design flexibility. "It's a natural for many products in structural foam to be converted to gas injection," says Steven VanHoeck, a principal and officer for Alliance Gas Systems (Chesterfield Twp., MI). "However, gas injection does not logically replace structural foam. It depends on the product's design and its needs."

Gas assist offers the same functional characteristics of structural foam-rigidity and strength-while providing additional benefits like no molded-in stress and better surface finish, which means fewer secondary operations. Gas assist also allows for ribs and bosses and extended flow lengths. The biggie however, is dramatically reduced cycle times.

Practical application 
A typical business machine component such as a panel is a good structural foam application because it needs strength and rigidity without warping due to heat. However, the process is relatively expensive because it's slow, explains VanHoeck. With gas assist you can achieve the same characteristics, but reduce the cycle time 75 percent. VanHoeck was involved with Xerox in several part conversions in the late 1980s. The conversion from structural foam to gas assist allowed Xerox to get the same functional panel down to a 60- to 70-second cycle time vs. 3 minutes. Elimination of molded-in stress provided a better surface finish, reducing secondary operations.

Applications for gas assist are broadening to encompass more markets and a wider range of parts, moving from more simplified, straightforward, or flat parts to smaller, more complex, thin-walled parts. This will open the doors for more widespread adoption of the technology. Still, the lack of a major cheerleader to promote gas assist has contributed to its slow adoption. Typically these promoters are raw materials producers seeking to capture sales through educating product designers in applications using the technology. "Ten to 15 years ago, our best leads [for structural foam molding work] came from GE when they were trying to get their market established," says Rafferty. "I don't think there are enough people who know what gas assist is who can teach designers how to use this, like GE did years ago with structural foam."

Gas assist, however, is a different animal. "We had specific resins that had an excellent fit for the [structural foam] process," says Jack Avery, manager of operations for GE Plastics. However, while structural foam is a resin-specific process, gas assist uses any material, he adds. Rafferty says that PPI spends time educating customers about gas assist. In fact, gas assist is primarily a customer-driven technology, VanHoeck explains. "Molders aren't going to adopt a technology if they can't sell it to their customers," he adds.

GE Plastics' Avery is quick to point out that structural foam molders can't just jump into gas assist overnight. "There's a definite learning curve," he says.—Clare Goldsberry

Contact information
Preproduction Plastics Inc.
Corona, CA
Kevin Rafferty
Phone: (909) 340-9680
[email protected]

Alliance Gas Systems Inc.
Chesterfield Twp., MI
Phone: (810) 948-5000

GE Plastics
Pittsfield, MA
Jack Avery
Phone: (413) 448-4816



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