Feature Article: Hybrid Injection Clamps The Best of Both Worlds? - 04/03
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use electric ballscrews integrated with the tiebars to drive rapid-traverse open/close movements and a hydromechanical system for lock-up. Once the split-nut locks around each of the tiebars, the system delivers oil to four small short-stroke cylinders on the tiebars in the stationary platen. Clamping occurs in the four corners of the platen, like a standard hydromechanical or toggle press. The cylinder movement is only 6 mm and uses about 1.5 liters of oil for each tiebar. “Our standard C series unit would require about 180 liters [45 gal] of oil,” says Caprio. Tonnage build-up is as quick as 0.5 sec, he says. A servo pump drives the hydraulic system. Electric motors drive injection, ejection, and plastication.
Meiki developed what it calls a pumpless clamping device for its New Advanced Direct Electric Machine (Nadem) series of hybrid-clamp machines. The line currently ranges from 88 to 1430 tons, but Meiki plans to add models up to 2000 tons in the next year. This design uses two servomotors and ballscrews on diagonal corners of the platen for rapid-traverse clamp movements. What’s unique about this design is that the ballscrews—which have their own locking/unlocking mechanisms separate from that on the central ram—continue turning during lock-up. Their motion compresses cylinders built around the ballscrews, moving a small amount of oil out of those cylinders and into the central clamping cylinder to build hydraulic pressure. The result is to push on the corners of the platen and with the center ram, applying more widely distributed pressure than with other designs.
“Clamp-tonnage build-up will be two to three times faster than a straight hydraulic—and may be as quick as an all-electric,” says Meiki’s Chang. The unit can achieve mold/clamp movements up to 800 mm/sec. Nadem machines also have electric servo-driven ejection, injection, and plastication.
The new EM line from Mitsubishi (MHI) generates hydraulic clamp force on the four tiebars in the corners of the platen. An electric inverter motor drives the hydraulic system. One servomotor drives a single central ballscrew mechanism to move the platen back and forth. The drive system drives opening and closing motions up to 1.3 times faster than a conventional hydraulic Mitsubishi press. Clamp-speed accuracy is so tight that a zero setting is possible for clamp-slowdown areas, says MHI. The unit provides a mold stopping accuracy of 0.0012 in., which compares favorably to 0.039 in. for MHI’s hydraulic machine, says Tom Geddes. He adds that the platen design helps reduce deflection by at least 50%. Demag’s El-Exis Series uses an electric motor to power a “hydrostatic” closed-loop hydraulic system to power a toggle. This arrangement downsizes the hydraulic accumulator and pump package by 66%.
MHI’s EM machines also have electric plastication, injection, and ejection. The line now spans 720 to 1200 tons, though plans are being considered to add a 1500-tonner and maybe a 2000-ton model. The line may eventually extend to 3000 tons.
An electric-driven center ballscrew performs rapid-traverse motions on the Eclipse series of hybrid presses from Sodick Plustech (sold here by Yamazen). After the center ram is locked in position, an accumulator-assisted, doughnut-shaped, central “pancake” hydraulic cylinder surrounding the ballscrew pulls on the tiebars to create clamping force. These hybrids come in sizes of 5.5 to 385 tons, including a vertical 40-tonner. Servomotors also actuate the ejector and plasticating screw, while injection is accomplished hydraulically with the assistance of a gas accumulator for high speed.
Sodick Plustech sources say electric drive provides faster acceleration and deceleration of mold travel, as well as greater position accuracy. Meanwhile, direct hydraulic clamping reportedly provides finer pressure control, more uniform pressure distribution, delicate mold protection, and greater die-height range.
Demag Plastic Group’s El-Exis S series (sold here by Van Dorn Demag) is designed as a high-speed packaging machine. It currently comes in sizes of 66 to 462 tons, though models up to 1000 tons are planned.