Retractable core simplifies tooling

If you're a designer or toolmaker who struggles mightily with lifters and cores for undercuts, and you like to try relatively new technology, you might want to consider the retractable core from Cumsa. John Krieg did, and he says that if you don't have much room to move in the mold, it can save the day.

Krieg is the owner of PlasTec Engineering, a design and prototyping shop in Morristown, NJ. He was trying to develop a lifter design for an insert-molded part for a computer printer application. The mold had two lifters, back-to-back; the fit and tolerances were tight. Traditional lifters, he says, would have been quite difficult.

He opted for the retractable core for undercuts, a hardened steel core that uses the natural spring of steel to pull away from the part. It's attached to the ejector plate, and when pushed forward naturally bends to release the part. It's patented and manufactured by Cumsa in Spain, but sold in the U.S. by PCS (Frazier, MI). According to PCS, the cores are made from spring steel with hardness of 53 to 57 Rockwell C and can endure up to 5 million shots without significant fatigue. The specs show that the core "springs" .145 to .177 inch on stroke (Figure 1).

The cores, Krieg says, are easier to drop in and install. They don't require the tooling gymnastics that traditional angular lifters do; and wire EDM, he says, proved more accurate with the cores. For the part, "it really was the only option we had," he says. Cores cost $125 to $137 apiece. Krieg says a three-piece lifter that he would have used otherwise would have cost at least twice as much. Krieg has since designed the retractable core into three other tools. "In areas where fit is the issue, then this is a cost-effective way to do it," he says.

Physically the only limit Krieg has found so far is that welding can remove some of the spring from the core. Jeff Czarnecki, sales engineer at PCS, says the core is fussy when it comes to welding, but that a careful and experienced welder should be able to work with the tool without dramatically affecting the natural spring of the steel. The largest hurdle Krieg has encountered with the retractable cores is industry reluctance. "The downside was that nobody ever used it. Moldmakers are very reluctant to change."

David Guillette agrees. He's the owner of PencilLogic, a mold services company in Windsor, MA. He says he's specified the retractable core three times for three different customers, all of whom requested he take them out - they were too new and had too many unknowns for his customers. Customers were concerned about durability and what might happen if the spring in the steel wasn't strong enough to separate from the part. "The question is how strong is that spring," he says. Guillette says such concerns are valid but will probably prove moot eventually. He also agrees with the core's ease of installation: "I think for getting them in the mold, it's easier."

Meanwhile, over in Malaysia, Motorola Penang is building a mold that uses the retractable core to mold walkie-talkie housings from polycarbonate and PC/ABS. Clement Pakiam, a tooling engineer at Motorola, says the core "requires very little engineering design in the tool to form the undercut. It is also a very simple 'drop-in' assembly on the mold."