Large injection presses combine rigid clamping & long strokes.

A new line of large hydromechanical-clamp injection machines from Engel Machinery Inc., York, Pa., reportedly combines key advantages of both toggle and hydraulic machines - i.e., rigid clamping and accommodation of long opening strokes. The ne short-stroke clamp cylinders are also designed for fast pressurization and low energy consumption, because only a small volume of oil is moved during operation. As reported here last month (PT, Jan. '92, p. 14), the new line will complement Engel's large toggle presses and initially will be available in sizes of 1500, 2000, 3000, and 4000 tons.

Systems involving short-stroke clamp cylinders on all four tiebars have been a growing trend in recent years among Italian and Japanese large-press builders in particular (see PT, Jan. '91, p. 61). Each builder's design is somewhat different. Here is how Engel's system works.

RAPID CLAMPING

The clamp is operated by two separate groups of hydraulic cylinders. The first group consists of two small-diameter "quick-clamp" cylinders, mounted on the top and bottom of the clamp unit, which open and close the clamp at high speed. The second group consists of four high-pressure locking cylinders, each located in the base plate of the moving platen, coaxially surrounding a tiebar. These transmit the locking and break-open force to the back of the moving platen, through the base plate, and a series of four force-transmission rods between the tiebars.

On closing, the quick-clamp cylinders quickly stroke the moving platen to its closed position. The system is fitted with a dedicated proportional valve to ensure accurate breaking and positioning. Platen speed and position are measured by a linear transducer, which is backed up by a proximity switch to signal the locked position of the clamp.

Four force-transmission rods, each the length of the clamp's maximum opening stroke, are attached to the rear of the moving platen. As the clamp closes, the rods are pulled clear of their bores in the base plate. Once they have pulled clear, two hydraulically activated shutters slide laterally to cover the bores. A retainer plate keeps the force transmission rods aligned and in place during closing and opening movements.

When the clamp is closed, clamping pressure is applied on the locking cylinders around the tiebars. Locking force is then transmitted through the base plate, shutters, and force-transmission rods to the back of the moving platen. During the injection and holding phases, locking force is maintained by a small pressure-controlled pump.

The clamp is unlocked during the cooling phase, which is said to slightly reduce overall cycle time. At the end of the cooling phase, the shutters are retracted and the quick-clamp cylinders rapidly retract the moving platen. If additional opening force is required, the locking cylinders can be programmed to apply a short, powerful opening stroke.

RUGGED DESIGN

The locking cylinders operate with only a small volume of hydraulic oil because they do not have to accommodate large opening and closing strokes. By minimizing the oil volume, less pumping is required and the system is stiffer, i.e., there is a greatly reduced cushioning effect as the oil is pressurized and compressed. Compared with hydraulic systems that must accommodate long opening strokes and have large oil cushions, here the base-plate movement for locking force build-up is at most 1/4 in. Build-up of full clamping pressure is also faster.

Heavy, box-type platens are said to ensure minimum platen deflection during operation. Engel recently increased the platen size on both its toggle and hydromechanical machines by up to 31%. Tiebar distance, opening stroke, and maximum daylight clearance were also increased significantly. These increases have been accomplished on the hydromechanical line with a simultaneous reduction in the machines' overall "footprint." Compared with other previous models, the new machines have a clamp length reduced by as much as 24%.

ENERGY-SAVING HYDRAULICS 

Energy conservation has been achieved by matching variable-displacement with fixed-displacement pumps to precisely match hydraulic output to demand. Because oil volume and pressure to each circuit can be precisely regulated, screw rotation and minor machine movements such as carriage, ejection and core-pull, can be controlled directly by the pumps without the need for conventional pressure-and flow-control valves. This eliminates hydraulic loss inherent in the valves, increasing energy efficiency. Major movements, such as platen motion and injection, are controlled by proportional or servo valves. Engel claims that average energy savings over standard hydraulic systems can be as much as 20%.

SIMPLE TIEBAR RETRACTION 

The clamp has been designed for rapid and easy retraction of one top tiebar. To remove a tiebar, a locking cylinder cover is removed. This is possible because only two locking cylinders are filled with oil in the rear piston chamber for mold-height adjustment and break-open force. The tiebar is then locked to the moving platen with an integrated fixing screw. Once the tiebar nut has been removed at the stationary platen, the opening of the moving platen will pull the tiebar out of the way.

Mold-height adjustment is accomplished by entering the change into the machine controller. The base plate then moves on linear bearings to increase or decrease the shut height, as measured by a linear transducer.

The line of hydromechanical machines is being manufactured for the North and South American markets at Engel's York, Pa., facility. Prices are comparable to those of the company's toggle line.