DUSSELDORF, GERMANY-When it comes to injection molding machines, the fewer the platens the better, say marketersof new, two-platen machines introduced at K'95. A platen is a heavy plate of steel that supports the mold assembly. Most traditional injection presses have three platens-one for each half of the mold, called the moving and fixed platens, and a third platen, often called the end platen. The end platen transmits clamping force, whether toggle, hydraulic or hydromechanical clamping, to the tie bars and moving platen.
The main advantage of eliminating the third platen - major reduction in the length of the machine - becomes more important in bigger machines with large clamping forces.
Krauss-Maffei KunststofftechnikGmbH said its new, two-platen versions of its existing C-range injection molding machines are about one-third shorter than comparable 3-ton presses. The
two-platen machines, using the MC brand name, are available in clamping forces of 1,100-4,500 tons.
The two-platen concept is actually areturn to an old idea, bolstered withnew technology, said Guido Radig, marketing manager for Krauss-Maffei. Injection presses used to have only two platens.
``The old problem was stability of the clamping force. So for stability, the three-platen machine was considered the state-of-the-art,'' Radig said at his firm's booth, where an 1,100-ton MC machine was operating.
Krauss-Maffei's clamping system features four locking nuts mounted on the fixed platen. The moving platen has been designed as a sandwich plate, made up of two interconnected plates. In one of the plates, the company has integrated a clamping piston from four short-stroke pressure cylinders.
Once the mold is closed, the locking nuts quickly move in to secure the columns on the fixed platen. Clamping pistons go to work, building up clamping pressure rapidly. During unclamping, clamping force release and unlocking of the guide columns happen almost simultaneously.
The guide columns never leave the guide bushings of the fixed platen, so the platen will not tilt even with very heavy molds, Krauss-Maffei said.
Krauss-Maffei of Munich, Germany, said MC machines are targeted to the automotive, waste disposal and furniture industries-all of which use large-tonnage machines.
Engel Vertriebsgesellschaft mbH of Schwertberg, Austria, said it already has sold 16 two-platen machines since the end of 1994. The company said it offers the machines, designated as DUO-line presses, in clamping forces of 562-5,000 tons.
At K, Engel showed two of its two-platen machines. One had a clamping force of 900 tons, the other was 1,686 tons.
Engel offers two types of clamping units on its DUO machines, depending on size.
Machines from 562-1,236 tons have a fully hydraulic clamp. Four high-pressure cylinders are attached to the moving platen, to build up, then reduce, the clamping force quickly. Tie bars anchored on the fixed platen act as continuous piston rods.
Two powerful, diagonally positioned cylinders enable rapid movement of the platen and strong force to open the mold. When the mold halves touch, a valve seals both chambers in the two-chamber cylinders and clamping force buildup results directly from releasing pressure in the left chamber. Clamping force is released by equalizing pressure, followed by a uniform pressure build-up in both chambers. For machines from 1,461-5,000 tons, Engel uses a hydromechanical clamp. The fixed platen has integrated pressure pads to apply force. Tie bars are supported fully by the fixed platen. The moving platen is guided on the machine's frame.
As in the smaller machines, the clamp has two diagonally positioned cylinders. Upon mold closing, four split nuts attached to the rear side of the moving platen are activated, to lock the molds in place. On the fixed platen, four pressure pads are put under pressure to build up clamping force. The pads are linked hydraulically, to guarantee even distribution of the tie-bar load.
At the cycle's end, pressure is released, the split nuts are unlocked and the cylinders open the clamp.
Hemscheidt Maschinintechnik Schwerin GmbH & Co. of Schwerin, Germany, makes a retractable tie-bar machine that uses two platens. The hydromechanical-clamp machine has been in production about 10 years, said owner Alexander Hemscheidt.
