Hydraulic device for back and forth displacement of a machine piece and a closing unit on an injection moulding 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 at least one piston rod constructed to float in the pressure medium in the first pressure space, and a secondary piston axially movable in the cylinder and having a recess in which the piston rod of the primary piston is movable, said primary and secondary pistons having opposing sides to define confronting contact surfaces which can be brought to impact one another.

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 at least one piston rod constructed to float in the pressure medium in the first pressure space, and a secondary piston axially movable In the cylinder and having a recess in which the piston rod of the primary piston is movable, said primary and secondary pistons having opposing sides to define confronting contact surfaces which can be brought to impact one another, wherein the primary piston has a side which is distal to the secondary piston and has a second piston rod defined by a diameter which is smaller than a diameter of the first piston rod, said further piston rod projecting beyond the cylinder.