Hydraulic device for back and forth movement of a machine part, and a clamping unit of an injection molding machine fitted with such a hydraulic device

A clamping unit for an injection molding machine, comprising:

• a support platen;
• a fixed platen;
• a moving platen;
• and
• a hydraulic device for operating the moving platen, said hydraulic device including a cylinder having a first pressure space with a pressure medium, a primary piston which includes one or more piston rods and is constructed to float in the pressure medium in the first pressure space, with a one of the one or more piston rods being connected to one of the platens for a half-mold of an injection molding machine, and a secondary piston axially movable in the cylinder and sized to bound the first pressure space on one side thereof in any axial disposition thereof, said secondary piston having a recess in which one of the one or more piston rods of the primary piston is movable, said primary and secondary pistons having opposing sides to define confronting contact surfaces which contact one another when the fixed and moving platens are clamped.

A clamping unit for a two-platen injection molding machine, comprising:

• a fixed platen;
• a moving platen;
• and
• a hydraulic device for operating the moving platen, said hydraulic device including a cylinder having a first pressure space with a pressure medium, a primary piston which includes first and second piston rods and is constructed to float in the pressure medium in the first pressure space, with one of the first and second piston rods being connected to one of the fixed and moving platens, and a secondary piston axially movable in the cylinder and sized to bound the first pressure space on one side thereof in any axial disposition thereof, said secondary piston having a recess in which the other one of the first and second piston rods of the primary piston is movable, said primary and secondary pistons having opposing sides to define confronting contact surfaces which contact one another when the fixed and moving platens are clamped, wherein the one piston rod is defined by a diameter which is smaller than a diameter of the other piston rod.