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Expanding Cavity Replaces Old Tool
December 27, 1999
4 Min Read
Greg Basque says the tool was at least 30 years old. “The paperwork on the mold goes back to the ’60s,” he says. “It’s probably older than that.” Basque is the vice president of Basque Plastics, based in Leominster, MA. His custom shop has 20 presses, ranging from 50 to 400 tons, molding products for high-tech, agricultural, and other applications.The aged mold made small, clear PC, vial-like parts, about 3 inches long and .75 inch in diameter, for use in float gauges in oil drums. The mold was three plates, with a stripper, to accommodate the threads on the outside of the vials. It was taking Basque a full week to run 15,000 parts, with cycle times of about 56 seconds. He was also losing about 200 lb of polycarbonate a week to sprues and runners that could not be reused due to part clarity requirements.Basque finally talked his customer into investing in a new mold, one that would incorporate modern toolmaking techniques. He knew that the expense of building a new mold would have to be more than offset by efficiencies in cycle time and material use. To do this, he invested in hot runners to cut down on scrap lost to the cold sprue and runner in the old mold. To accommodate the threads on the polycarbonate, he invested in expandable cores from Roehr Tool in Hudson, MA.The expandable cavity is an A-2 steel tool with four segments that expand radially outward from the center axis of the tool under their own spring force. Threads are machined along the inside of the end of each radiating arm; inside a closed mold, the arms are squeezed together to provide a machined, flash-free enclosure (Figure 1). Upon mold open (Figure 2), the “A” side striker is removed, allowing the radiating arms to naturally flex outward, freeing the part for ejection.The expandable cavities are also available in asymmetrical configurations: for example, two segments at 90° angles with three segments at 60° each. Jim Cullison, director of engineering at Roehr, says the cavities can mold threads, snapfit covers, O-ring grooves, barb connections, and luer connections. Tolerance of fit between each arm is .0005 inch. At Basque Plastics the results were significant. Coupled with the hot runners, the expandable cavity helped reduce the 15,000-part run from a week to 16 hours. The cycle time dropped from 56 seconds to less than 30 seconds. And Basque says he reduced his scrap rate from 5 percent to 1 percent.Basque says the overall cost of the expandable cavity is about a third less than other tools for molding threads, mainly because he didn’t have to tool the eight actions and locks traditional tooling would have required. Also, the tools delivered by Roehr are machined to specification, meaning Basque only had to machine a cavity to receive the tool.Cullison says prices for the expandable cavity vary depending upon tool size and application. He says a typical multicavity order ranges from $700 to $800 per tool; single cavity orders can run up to $2000.Combining Roehr’s expandable cavity with a Roehr collapsible core is Basque’s current challenge. The proprietary application uses internal and external threads. The internal threads are 4.75 inches long and are molded by the collapsible core. The external threads are .75 inch long and are molded by the expandable core.Basque says the mold is particularly challenging because both mold halves have moving parts. The expandable cavity is pushed out upon mold open; the collapsible core retracts during eject. He says the collapsible core is most difficult, simply because it must travel the 4.75 inches out of the part before the part can be released. The material to be used in the mold is 60 percent glass filled, one that will challenge the wear resistance of the tools, says Basque.Basque does say that he’s a little more vigilant with cleaning and maintenance of the expandable cavities, but that in general, they run cleaner and faster than traditional tooling for threaded inserts. “Expandable cavities give us less maintenance over the life of the mold as compared to normal actions,” he says. Also, he says the cavities are slightly more susceptible to damage, as the radial arms can trap stray plastic and small parts. “If the tool flashes at all, our operators know to make sure there isn’t any plastic down inside the expandable cavity,” says Basque.
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