Feature Article: Hybrid Injection Clamps The Best of Both Worlds? - 04/03
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proven technology and offer the user reliability, simplicity, and rigidity. We have tests that show an electrically driven toggle clamp can achieve ±0.0005-in. positioning accuracy. That means that the molder who is adding automation can be sure that the robot will find the part in the same place every time.”
“Toggle units have a shorter locking time and pressure-generation time, since the design or kinematics leverage a larger force with less energy,” says John Ward, technical sales manager at Van Dorn Demag. His firm’s El-Exis machines are the only members of the hybrid category that have toggles powered by an electric servo-driven hydraulic pump.
Toggles may have a slight edge in clamp-tonnage build-up time. “A toggle has an amplification ratio at the end of the stroke, meaning it locks up faster than a hydraulic system, says Ward. Lock-up can be as quick as 0.2 sec with a toggle system vs. 0.5 sec for a hydraulic system, he claims.
“The toggle has greater opening force than a hydraulic clamp, and thus the toggle lends itself well to deep-draw parts,” adds Ward. Cycle time in the molding phase is also an area of debate. “With an electric driven system a user can start injection while the clamp is closing, so functions can be overlapped for cycle time savings,” says Milacron’s Royer. “Likewise, during the cooling phase, we can unlock and decompress the tonnage without opening the clamp. This can minimize the delay and shave half a second of cycle time. This unlocking step can be done with a hydraulic system, but there is a question of accuracy,” says Royer.
Like other two-platen, hydromechanical machines, most of the new hybrid clamps reduce the overall machine length vs. toggles or fully hydraulic machines. Compared with hydraulic two- platen machines, the hybrids are said to offer increased precision and accuracy and reduced maintenance.
Hybrids are said to be more suitable than toggles for coining or injection-compression processes, which require injecting into a partly open mold, followed by full closing and lock-up. Thermosets or optical lenses are applications that commonly use injection- compression, notes Paul Caprio, v.p. for injection molding at Krauss-Maffei.
Another advantage claimed for hybrids is the “closed- loop” nature of their hydraulic systems. The hydraulics are completely encapsulated within one of the platens— instead of being recirculated to a central tank—so leaks are much less likely. Only a small amount of oil is used in these systems, which are designed to require no servicing by the molder. “The oil reservoir for the clamping units holds just 1 gal of oil,” says Henry Chang, Meiki’s assistant manager of customer service. “The amount is so small that the machine ships with the oil in it.” Sodick Plustech’s Eclipse and Krauss-Maffei’s Eltec machines use only about 1.5 gal of oil. The Eltec hybrid from Krauss-Maffei uses electric ballscrews integrated with the tiebars for opening and closing. A self-contained, “closed- loop” hydraulic system buried inside in the fixed platen delivers 1.5 liters of oil to each tiebar for clamping. The “power nut” engages to generate clamp pressure. A “travel nut” is engaged during platen movements.
For the most part, machine builders make no claims for increased speed or energy efficiency of hybrid clamps relative to standard hydromechanical presses. However, Meiki points out that some cycle-time savings can be achieved in mold opening and closing with servomotors and ball screws to drive the moving platen. This system has low inertia and very quick acceleration and deceleration—only 100 millisec from zero to maximum speed—as well as a position accuracy of 0.05 mm (0.002 in.) for the switchover position to decelerate from high speed.
Diversity of designs
Krauss-Maffei’s two-platen Eltec hybrid line spans a range of 55 to 165 tons. These models