Feature Article: Hybrid Injection Clamps The Best of Both Worlds? - 04/03
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The El-Exis S has a different kind of hybrid clamp from all the others. It uses self-contained hydraulics (which Demag calls a “hydrostatic drive”) to drive a toggle. The hydraulic pump is driven by an AC frequency motor. The system has response time of 12 millisec, platen speeds up to 25 in./sec, and position control as tight as 0.01 mm, says John Ward, technical sales manager. “We felt that a self- contained hydrostatic drive was a better option than using components such as belt drives or rack-and-pinion systems. The latter may be more sensitive to wear and stretching and can yield variations in accuracy and be less responsive,” he says.
Hybrid clamping designs typically start with hydromechanical locking. This EM unit from Mitsubishi uses a split-nut lock system. The toggle clamp is styled to generate clamp forces at an angle to the mold face. Along with the platen design, it helps to drive the clamp force toward the center of the platen for uniform distribution across the mold face, says Ward. He also notes that the motor and pump work only as hard as is required to meet the instantaneous demand of the process and stop during idle portions of the cycle, rather than spilling oil over a relief valve back to the tank. “This hybrid system saves energy,” he claims. “A 200-ton conventional hydraulic machine may need 100 gal of oil, but our unit needs only 1.5 liters.” The hydrostatic system is mounted atop the clamp for ease of access.
Using a strain gauge on the tiebar, mold-protection sensitivity can be set as low as 5 bar (72.55 psi), and response within 0.2 mm of travel is reportedly achievable.
Other machine functions—injection, core pull, ejection, and carriage movement—are powered by a gas accumulator and variable-volume hydraulic pump. This arrangement provide high precision, low noise, and energy consumption half that of a comparable hydraulic machine without compromising performance, says Ward.
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