A method of loading and unloading a mold on a foam plastics molding machine; the method comprising: - providing a machine comprising a fixed frame having a window for receiving the mold and located at a higher level than the supporting surface of the machine; - a first step of setting up and connecting to the mold an interface frame interfacing the machine and the mold, to form a molding unit, which comprises the mold and relative interface frame, and has a number of plastic molding material and operating fluid inlets (8, 9); the interface frame having vertical ribs for its coupling to the machine; - a second step of advancing the molding unit towards the fixed frame; - a third step of lifting the molding unit to the level of the window; and - a fourth step of loading the molding unit into a work position physically and functionally engaging the fixed frame at the window; and locking the molding unit to the fixed frame by first locking means; and the method being characterized in that the second step comprises the sub-steps of: - positioning the molding unit in a given fixed first position on the bed of a trolley; said given first position being defined by locators carried by the trolley and specific for the molding unit; and - moving the trolley into a given fixed second position astride a platform defining a horizontal supporting surface for the bed of the trolley; in said given fixed second position, a front edge of the trolley contacting a fixed locator on a top plate of the platform; the bed having been configured so that, when the molding unit is in said given fixed second position, the vertical ribs of the interface frame are aligned to respective vertical guides of the machine; and the third step comprises lifting the molding unit resting on the trolley, and comprises the sub-steps of: - raising the platform with the molding unit positioned in the given first position on the trolley, and the trolley positioned in the given fixed second position on the platform, so as to move the vertical ribs of the interface frame axially to slidably engage the respective vertical guides on the machine; - locking the ribs to the respective guides by means of second locking means, when the molding unit is positioned coaxial with the window; and - releasing the molding unit on the guides by lowering the platform and the trolley to detach the trolley from the molding unit. - providing a machine comprising a fixed frame having a window for receiving the mold and located at a higher level than the supporting surface of the machine; - a first step of setting up and connecting to the mold an interface frame interfacing the machine and the mold, to form a molding unit, which comprises the mold and relative interface frame, and has a number of plastic molding material and operating fluid inlets (8, 9); the interface frame having vertical ribs for its coupling to the machine; - a second step of advancing the molding unit towards the fixed frame; - a third step of lifting the molding unit to the level of the window; and - a fourth step of loading the molding unit into a work position physically and functionally engaging the fixed frame at the window; and locking the molding unit to the fixed frame by first locking means; - positioning the molding unit in a given fixed first position on the bed of a trolley; said given first position being defined by locators carried by the trolley and specific for the molding unit; and - moving the trolley into a given fixed second position astride a platform defining a horizontal supporting surface for the bed of the trolley; in said given fixed second position, a front edge of the trolley contacting a fixed locator on a top plate of the platform; the bed having been configured so that, when the molding unit is in said given fixed second position, the vertical ribs of the interface frame are aligned to respective vertical guides of the machine; - raising the platform with the molding unit positioned in the given first position on the trolley, and the trolley positioned in the given fixed second position on the platform, so as to move the vertical ribs of the interface frame axially to slidably engage the respective vertical guides on the machine; - locking the ribs to the respective guides by means of second locking means, when the molding unit is positioned coaxial with the window; and - releasing the molding unit on the guides by lowering the platform and the trolley to detach the trolley from the molding unit.