Husky's New Clamping Unit for Two-Platen Machines

With the new E series (Figure 1), Husky departs from the usual three-platen clamping unit and eliminates the back stationary platen in a new hydromechanical two-platen design.

The impetus for Husky to join many other suppliers in two-platen design comes from the demands of users to reduce investment and operating costs in large-tonnage injection moulding without sacrificing product quality. The E series will be available in sizes from 900 to 4,000 metric tons (9,000 to 40,000 kN).

In the patented two-platen clamping unit (Figure 2), the tiebars are anchored in the stationary platen and guided through a simple locking mechanism in the moving platen. By minimizing tiebar load, this design offers advantages in speed and motion control with easy access to the injection unit at the stationary platen for purging and maintenance.

Clamp force is provided by a bayonet-type clamp and locking mechanism integrated into the moving platen. Four sections of sawtooth teeth on each tiebar are set 90° apart. Matching internal gear teeth on the rotary clamping pistons inside the moving platen (Figure 3) are rotated to engage the tiebar and build clamp tonnage.

To open the mould, the teeth are disengaged, allowing the tiebars to glide through the clamping pistons without actually touching them. Hydraulically linking all four pistons provides a uniform load on the tiebars.

Compared to machines using a hydraulic mould shut-height adjustment, the E series machines use only eight percent of their total oil volume. Accordingly, clamping forces build up quickly (in .35 second for 9,000 kN and in 1.3 second for 27,000 kN of clamping force). Combining the longitudinal piston movement and the pitch of the gear teeth provides continuous automatic mould shut-height adjustment.

To open the clamp, the hydraulic pressure is reduced and the rotating mechanism disengages the piston teeth. Up to this point, no noticeable movement in the mould's parting line is observed and this time can be used for mould cooling.

Under normal operating conditions, two high-speed stroke cylinders then open the mould. If a higher opening force is required, the small annular area of the clamping pistons can also be pressurized to provide a higher mould break force.

The clamp pistons' rotating device is located behind the moving platen (Figure 4). The two bottom pistons are rotated by a hydraulic cylinder while connecting rods simultaneously rotate the corresponding upper pistons. The action is controlled by four limit switches, two each on the upper pistons.

Clamping force is directed from the tiebars through the cast, one-piece platen (called a "Reflex" platen) to the mould area. This design produces a virtually deflection-free mould mounting area and preserves the mould guides and sealing planes to extend maintenance intervals.

The one-piece platen with cast-on shoes sits on four bronze shoes that glide on hardened steel bands on the machine bed. This provides a stiffer construction than any existing two-piece platen bolted to a sliding carriage.

The height and side position of these shoes can be adjusted to center and guide the mounting plate. Lateral guidance is accomplished by two hardened steel ledges. The clamp unit is produced with the parallelism of both platens better than .01 mm.

The reciprocal motion of the moving platen is produced by two traveling cylinders. In order to eliminate lateral forces in the hydraulic cylinders for the purpose of increasing the life of the seals, they are fitted with a double spherical bearing.

In addition to the new E series clamp design, a new hydraulic servovalve system from Hydrolux/ Moog combines an internal regenerative circuit with closed loop mould protection control for better motion control with fewer components.

Cost is about eight percent less than for comparably sized, more traditional three-platen models of machines with considerably longer clamp units. Husky reports that a 2,150-metric-ton E-series machine has been running three shifts solidly since January 1996 at an automotive moulder.