The long and short of an 
unscrewing mechanism

https://witmold.com/unscrewing-molds/

If necessity is the mother of invention, it certainly applies to the PERC system. Its inventor needed a way to solve a major problem for a customer—and so invented it.

Unscrewing molds can be challenging to operate. Conventional unscrewing molds are hydraulically driven, and the number and size of the threads on the part determine how many revolutions the core must rotate before the mold can open. The required number of rotations then dictates the length of the hydraulic rack and the size of the hydraulic cylinder.

That works well enough until a customer comes to you with a part that has so many threads that the rack—which is mounted to the mold—must be impractically long in order to unscrew the part from the core. "For example, a part that requires seven to ten rotations would need a rack 15 to 20 inches long, but a part that requires 20 turns would need a much longer rack!" comments Alan Petrucci, founder of B A Die Mold (BADM; Aurora, IL).

With a conventional unscrewing mold, gears would be used to reduce the required length of the rack. "This works with many threaded parts, but not all," says Petrucci. "The gearing necessary can take up a lot of space in the mold base, which becomes an issue and creates limitations when there are other actions needed for additional features on the part or if the processor needs to fit the mold into a particular size press."

Petrucci figured there had to be a better way when a customer came to him with a part that required precision positioning—not so much because of the number of threads, but because one core had to pass through another. "One core was threaded and had to reset again, while another went through it," Petrucci explains. "Also, it had to be flashless because it was a water filtration application. The customer was stuck having to make the part in two pieces, and I foolishly said, 'I think I can make it in one piece.' That's how I got the idea for the Programmable Electric Rotating Cores (PERC) System."

It's Official

In March, Petrucci and BADM received their formal patent for the PERC System. The company had been producing molds using the PERC System while awaiting patent approval, and even built systems for molds being manufactured by other shops. "Since then we've made molds with 30 turns to a core—conventionally you'd need a basement below the press and a retractable roof above just to run this," said Petrucci.

PERC eliminates the need for a rack by using a servomotor to rotate the core. This means parts with virtually an unlimited number of threads, intricate geometry, or special requirements are now possible and practical. Eliminating the hydraulic cylinder also eliminates the mess associated with hydraulic systems.

In one automotive application, the customer needed a rack for a mold that was too large for BADM to handle, so BADM built the system and then assisted the moldmaker with installation. "They needed the molded plastic bottle for the fluids under the hood," Alan says. "They were molding the top half of the bottle, then the bottom half, and sonic welding the halves together. They needed threads in the bottle, which required a very large rack and cylinder system, meaning they couldn't fit the mechanics into the press with the mold because they needed a tremendous amount of space. Someone at that mold shop found out about us and called, so we built the systems for four molds, trained them how to use them, and they'll never use rack and cylinder again."

Alan says the all-electric servocontrolled PERC System offers higher energy efficiency than its hydraulically driven counterparts. For this reason, some of BADM's customers specify PERC even when unscrewing can be effectively achieved with a conventional hydraulic rack and cylinder. He adds that another benefit is that the PERC System is fully programmable and extremely accurate. "You can program an unlimited number of profiles and repeatability is absolute. It's perfect for electric molding machines—the controls for the PERC System can be integrated or hardwired right into the press, much like a core pull function."