Macromolding alternatives

Using thermoplastics molding technologies mastered while serving more conventional markets like medical and consumer electronics, this inventive company competitively manufactures big structural parts as well—truck bumpers, for instance.

Imagine watching a seven-axis robot remove a truck bumper over the tie­bars of a 4000-ton molding machine. Or, imagine transporting a 100,000-lb mold to your tool shop for maintenance by hauling it on the same type of tug usually used to move around small airplanes.

If you’re working in a certain plant expansion, one dedicated to molding super-large parts for heavy trucks, recreational/utility vehicles, HVAC applications, and commercial home products, you don’t have to imagine such things—it’s just another day in the shop.

Mack Molding Co., an 88-year-old molder headquartered in Arlington, VT, is putting the finishing touches on a brand-new 18,000-ft² addition to the $10 million large-parts molding facility added to its Inman, SC plant in 2001. When completed, the large-parts facility will measure 38,000 ft², house a total of eight big molding machines ranging from 2500-4000 tons, and bring Mack Molding’s entire SC facility to a total of 270,000 ft² with a total of 18 machines, ranging down to a meager 950 tons.

At the dawn of the 21st century, Mack Molding bet heavily on the growth opportunities in molding large parts as a means of offsetting the competitive threat posed by global competition. It believed the freight costs involved in shipping big parts from overseas would outweigh the cost advantages of such parts being manufactured in cut-rate countries (see imm.plasticstoday.com/?q=articles/10524 for more).

Judging from the continuing growth at its Inman facility and at its equally successful sister molding plant 2 hours away in Statesville, NC, which runs machines up to 1100 tons, Mack’s gamble is paying off . . . big time.

Yet, according to Raymond E. Burns, president of Mack’s Southern Div., which includes both the North and South Carolina plants, the company’s growing success in servicing large-parts customers isn’t just due to the size of the parts it manufactures. Burns and his associates agree it has just as much to do with how it manufactures such parts.

Creative crossbreeding

“Our market is all about replacing other technologies used to manufacture large parts—technologies like rotomolding, compression molding, blowmolding, reaction injection molding, and metal stamping,” says Jack Katilius, business unit director. “There are thermoplastic materials today that can match the properties of, say, SMC, when they’re used properly.

“Our molds are expensive, but the tooling becomes justifiable over the range we work,” he adds. “Injection molding and the use of technologies like external gas and gas assist can maximize the performance properties of the parts we mold. With them, we can put the strength reinforcement precisely where it’s needed.”

“Ninety percent of the time we get involved in our projects up front,” says Brian Sumpter, business development director. “So we provide a considerable degree of design assistance. We have to match the process with the material for the jobs we run, because of the hybrid processes we use. Also, we often find we have to ‘design’ materials for different applications. So, we maintain a very close relationship with our materials suppliers. Finding, say, a PC/ABS that knocks off SMC isn’t easy—it’s not an on-the-shelf formulation.”

Borrowed brilliance

A variety of different materials are run at Mack Inman, including PC/ABS, PP, filled PP, PC, PC/PBT, and ASA—about 40 million lb of material a year. But, again, the materials it uses are often just the beginning of the story.

Mack has transported the applicable design, engineering, tooling, and manufacturing methods it’s mastered over the decades in serving other markets into the production of large parts. For example, it has borrowed some of the same approaches in robotics, adhesive bonding, and painting that it used to produce the Bose Wave music system and, believe it or not, adapted them to producing PC/ABS truck bumpers for Volvo, according to Katilius.

In addition to Volvo, Mack Inman manufactures parts for several of the major U.S. heavy truck OEMs. Its big machine expansion has three silos, each with about an 80,000-lb capacity. One holds glass-filled PP; another, PC; the other, PC/ABS.

Class-A constructions

After they’re molded, the Volvo bumpers’ interior reinforcements and their primed and painted molded exteriors are racked, a steel plate is inserted, and they’re assembled together. A special-purpose robot is used to deposit methyl methacrylite glue before the components are clamped together for permanent bonding.

After a prescribed cure cycle in a holding fixture, the bumpers are labeled and bar-coded. Mack Molding’s manufacturing know-how allows it to produce these structurally sound thermoplastic parts with Class-A surfaces.

The holding fixtures are engineered to ensure proper assembly and alignment, and the accurate tolerancing required, so the bumper brackets will properly fit to the trucks’ frame rails. What’s more, there’s no 3D geometry on the structural interior of the bumpers, ensuring there’s no postpainting read-through on the bumper fascia’s exteriors.

“With this semiautomated process, we’re building a big, beefy, structural part within a cosmetic exterior,” says Katilius. “In doing so, we’ve converted this application from a one-piece SMC bumper into a cost-effective thermoplastic part. And the manufacturing process we use is all based on the experience we gained working on that Bose job.”

Foamed floorboards

Another interesting example of its creative crossbreeding of manufacturing technologies is how it manufactures floorboards for the E-Z-Go golf car. The customer originally used thermoset composites. Mack Molding had a better idea.

It runs these 46-by-48-inch, 30-lb parts in a 50,000-lb mold on a single-nozzle 1500-tonner, with a 42-lb shot capacity. And it manufactures them using the structural foam process—a suggestion it made to its customer.

Structural foam allows Mack Molding to produce lighter-weight parts with excellent structural strength and minimized knitlines. And these thermoplastic parts are just as resistant to fertilizers and pesticides as are thermoset composites.

The manufacturing cell’s press robot removes the parts and positions them for transfer into a station where three crushable rivets are automatically inserted. Mack’s redesign of the part reduced costs, and its engineering savvy reduced the part’s weight by 6 lb.

Mack Inman fast facts
• Mack Inman prefers targeting medium- to low-volume molding opportunities—10,000- to 100,000-piece runs—though it has run jobs producing 250,000 parts per year from a single mold.
• Company sources say divisional sales are “somewhere north” of $75 million—20-30%/year growth is projected.
• Capital investment is about $12 million.
• Heavy truck customers include Volvo, Daimler Trucks North America, Peterbilt, ­Kenworth, Navistar, and Mack Trucks.
• Molding machine lineup includes presses from Toshiba, HPM, Engel, Demag, and Cincinnati Milacron.
• Its 2600- and 4000-ton Maxima MG 2-Platen Series Milacrons were just added earlier this year, each with a seven-axis Kuka robot.
• It uses SolidWorks and Moldflow CAE regularly as part of its initial quoting process.
• Mack Inman is ISO 9000-certified, and is implementing TS-16949 registration.

Web extra: A photo tour of Mack's facility—massive machines, drying hoppers, and molds.