Time to get reacquainted with single-toggle clamping.
Today's "workhorse" clamp for both high-performance and general-purpose injection molding is the double-toggle mechanical clamp. But molders may be surprised to learn that the features that make double toggles so popular--high speed, dependable performance, and rugged construction--are often surpassed in the single-toggle, or Mono-Toggle, clamping systems.
The Mono-Toggle (a trade name of Newbury Industries) is both old and new. It was originally designed for relatively large (e.g., 450-ton), high-speed packaging machines. The original patent was held by Improved Machinery Co. (Impco) until 1978. Several suppliers (including Newbury) have utilized the single-toggle clamp in vertical presses for specialized applications. But only for the last two years has the Mono-Toggle design been available in small, general-purpose horizontal injection machines.
One reason for that may be that this toggle design historically had a tendency to accelerate during the final stages of die opening. Thus, control of clamp deceleration required progressive mechanical cushions, either in the cylinder or in deceleration valves, operated by cams. But today's microprocessor controls and proportional valves give much finer control of oil flow, eliminating the need for complicated and less reliable mechanical controls.
Newbury now offers the Mono-Toggle in sizes of 60, 100 and 150 tons and a brand-new 200-ton version. Because its advantages are thereby available now to a much larger group of molders, the unique advantages of this design deserve a brief review.
THE SECRET IS SPEED
The Mono-Toggle's secret is its inherent design simplicity (see Fig. 1 for comparison with the double toggle). Compared with the typical double toggle, it offers cost-saving benefits to molders in higher operating speed, wear resistance and reliability; easier accessibility for servicing; and more compact size.
The Mono-Toggle can have as much as a 2:1 advantage in platen speed over double toggles, given comparable actuator speed. Longer toggle links are the principal reason. A shorter actuating-cylinder stroke on the single toggle provides the same clamp-stroke length that a double toggle achieves with a longer cylinder stroke. (For example, a 15-in. platen stroke is achieved with a 20-in. hydraulic actuator stroke on Newbury's 200-ton double toggle--and with a 10-in. actuator stroke on the 150-ton Mono-Toggle. The Mono-Toggle cylinder is of larger diameter, and thus requires the same amount of oil movement--but accomplishes the 15 in. of platen stroke 24% faster.) For a double toggle to achieve comparable actuator speeds would increase wear on its cylinder glands and seals and would require more energy to move its hydraulic fluid faster.
REDUCED WEAR AND FLASHING
With a double toggle, clamping forces are applied to the extremities of the mold. Forces are equal only in certain directions (see Fig. 2). Mono-Toggle clamp forces are concentrated in the center of the moving platen and distributed evenly over the entire platen. An extra "spider" plate behind the platen provides better alignment and greater bearing surface. (It also accommodates the ejector system.) Consequently, in molds that incorporate complicated shutoffs in the center of the cavity, the Mono-Toggle normally gives better performance. Under high injection pressures, a double toggle could allow some flashing to occur at those shutoffs.
With Mono-Toggle clamping forces concentrated at the center of the platen and distributed evenly to the edges, the mold can generally be expected to run-more reliably. Also, cavities may be located more toward the center of the block without danger of flashing.
Compared with a double toggle, the Mono-Toggle system is more tolerant of normal wear and of a mold that is slightly out of square or misaligned by up to a few thousandths of an inch. By clamping evenly across the entire parting line, the Mono-Toggle allows the mold to behave as if it were on a gimbal. It moves until the entire surface of the mold touches and then applies the load.
In contrast, a double toggle, with its two sets of links, applies the load at two locations near the outside of the platen. If the die is not perfectly square, because one set of links cannot extend farther than the other, the result is a pressure differential from corner to corner or side to side.
The same principle applies to normal wear of the linkage components. With a double toggle, wear can occur preferentially on one side. This can create a difference in clamping force from side to side. The double toggle will still develop the tonnage, but parts may be more likely to flash at the mold parting line. Even a slight difference in the level of wear on the two toggles can necessitate rebuilding a double toggle.
Not so with a Mono-Toggle. It's essentially self-aligning, or "forgiving." There is only one place wear can occur and it is always directly in line with the clamping forces--thus, its self-correcting aspect. Even as normal wear occurs, pressure is maintained consistently across the mold, allowing the Mono-Toggle to keep on making precision parts. If catastrophic wear should occur--for instance, if the lubrication system fails--the clamp's performance will not be appreciably affected (see photo).
Delbert Reynolds, machine maintenance supervisor for Mt. Vernon Plastics, Mt. Vernon, Ky., says his firm is especially concerned about how double-toggle machines concentrate their tonnage at the sides of the mold. "That can cause some deflection in the middle of the platen," he says. "And he parts we run are sensitive to deflection problems. They're very thin--containers, lids and the like. And we run very fast--typically on 3-sec cycles. That's why we're sticking with the Mono-Toggles. They're not only faster, but also uncomplicated and easy to service."
Mold wear is also reduced by eliminating the tendency of a double toggle to "wrap around" a small mold. In the case of the latest Mono-Toggle designs, mold wear is also reduced by the presence of eight tierod bushings (instead of the usual four), and extra-long, adjustable, cast-iron platen shoes that ride on replaceable, hardened wear strips on the machine base.
Mono-Toggle pins are larger than those of a double-toggle clamp, and they last longer. As noted, they are "free-floating" and are subjected to only half the toggle motion to accomplish the same stroke. The result is less wear, even though bearing loads are the same as those on a double toggle. Also, in Newbury's design, lubrication is supplied through noncritical wall areas of the links, rather than holes in the pins. The net result is longer life and stronger pins, because they haven't been weakened by holes drilled for lubrication.
With 33% fewer links, pins and bushings than a double toggle, there are fewer parts subject to wear and also fewer lubrication points. Fewer parts will need periodic replacement. (Newbury, in fact, provides a three-year warranty against wear-induced failure.)
Besides requiring less maintenance, the Mono-Toggle's design simplicity translates into easier access whenever servicing does become necessary. For mold setting and removal, for example, the back side of the Mono-Toggle's moving platen is accessible from the top, sides and bottom. On double-toggle systems, toggle-pin bosses can interfere with top or side access. Also, some older models have toggle pins in line with the tiebars, which consequently have to be pulled to change pins.
Mono-Toggle clamps significantly reduce overall machine size. That's because there's no long hydraulic cylinder projecting out of the back of the machine, a characteristic of double-toggle machines. Instead, the Mono-Toggle's actuating cylinder is positioned so that it projects down, beneath the links. Or, to provide open space in the machine base for conveyors, it can be projected up.
The difference can be 20-40% lower floorspace requirement. For instance, a typical 150-ton double-toggle machine occupies up to 86 sq ft of space, vs. only 52 sq ft for a 150-ton Mono-Toggle. One recent buyer of Mono-Toggles based his decision primarily on his ability to expand from two rows of machines to three rows in a small shop with very limited available floorspace.
This space-saving design, however, does not affect platen size or tiebar spacing, which are at least equal to those of most comparable double toggles available today.
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|Author:||Frohring, Glenn H.|
|Date:||Mar 1, 1990|
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