Feature Article: Hybrid Injection Clamps The Best of Both Worlds? - 04/03
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Meiki's Nadem hybrid machine uses two servomotors and ballscrews for rapid-traverse movements and to help build tonnage.
Hybrid presses are made by the same firms that build hydraulic and all-electric machines, so clearly they feel the hybrids have a place. Yet they still must convince molders why they should spend more for this design than on a hydraulic model, and why they should not go all the way and get an all-electric machine.
Why go hybrid?
All-electric machines have made considerable headway in the marketplace, despite their higher price, and have accounted for 20% to 25% of U.S. machine sales in the last three years. All-electrics are said to provide energy savings, greater speed and precision, and cleaner, quieter operation than hydraulic- powered machines. However, one aspect of all- electric machines that may not appeal to some molders is that they are toggle-clamp machines. The rare exceptions are some small (3.2 to 7.5 tons) non-toggle electric machines built by Nissei. According to Nissei sources, this non- toggle approach is limited to such small presses because it requires larger servo motors, which add to the price premium for electric machines.
Sodick Plustech’s Eclipse line uses a central servomotor and ball screw for mold opening and closing. After clamping locks engage the central ram, a central hydraulic “pancake” cylinder pulls on the tiebars for clamping.
Most, though not all, of the new hybrids are two-platen clamps with a hydromechanical style of locking mechanism. Hydromechanical clamps are characterized by split nuts or other means of locking the position of the moving platen before applying clamp tonnage with one or more short-stroke or “pancake” hydraulic cylinders. In the hybrids, electric servomotors instead of hydraulics are used for mold traverse movements.
Hybrid systems are designed to appeal to molders who like some of the features of all-electric machines but do not favor toggle clamps. Debates over the relative merits of toggles and fully hydraulic clamps have persisted for decades. Among the drawbacks cited for toggles are their more limited range of stroke adjustment and the risk that die-height adjustment when the mold is closed can lead to overclamping after the mold warms up and expands. Toggles also reportedly make it difficult to use less than maximum tonnage or to ramp tonnage up and down during a cycle. Some machine builders say that toggles also require more time for mold set-up (die-height adjustment) than a hydraulic or hybrid unit.
Toggle critics also raise questions about maintaining mold parallelism if the linkages don’t all perform or wear uniformly. There is also potential for higher platen deflection since most toggles generate force on the corners of the platen rather than the center. (Nissei recently came out with an all-electric three- stage toggle clamp that answers this criticism by applying force at the center of the platen.) Finally, the need for grease to lubricate toggle linkages presents potential contamination issues that counteract the cleanliness argument for all-electrics. Hybrid presses are well-suited to injection-compression molding. Sodick Plustech also offers the option of this novel “ejection-compression” function using its electric-powered ejector.
Notwithstanding these criticisms, toggle clamps remain popular and they have plenty of defenders. At least 17 firms offer all-electric presses with toggle clamps. Says Kent Royer, product sales manager for Milacron’s Roboshot electric-toggle machines, “Toggle clamps are a