An injection assembly for a die-casting machine, comprising: - an injection piston controllable in translation to operate on molten cast metal in a machine die for which a desired speed (Vset) is defined during a metal injection step; - a main pressure chamber, upstream of the injection piston, for containing and pressurizing the fluid intended for the outward translation of the injection piston, wherein said main pressure chamber is further connected to an injection drain connected to the drain, along which an injection return drain valve operates; - a proportional injection delivery valve for feeding the fluid to a main inlet and from said main inlet to the main pressure chamber as a result of the opening of a main shut-off valve; - a main back-pressure chamber, downstream of the tail end of the injection piston, connected to a return inlet for supplying pressurized fluid for the return translation of the injection piston, wherein the return inlet is connected upstream with a pump delivery, upstream of which a pump is located; - an injection return valve arranged between the delivery pump and the return inlet; - a proportional injection drain valve for draining the fluid from the main back-pressure chamber, so that the action of the fluid in the main pressure chamber and the opposite action of the fluid in the main back-pressure chamber generate an outward thrust for the actuation in translation of the injection piston, controlled in opening according to a tuning speed (Vreg); - an auto-tuning injection valve opening system configured so that the opening of the injection valve during a predetermined die-casting cycle is tuned according to the tuning speed (Vreg), determined as a function of the difference between said predetermined desired speed (Vset) of the injection piston and an effective speed (Veff) of said injection piston detected for the previous die-casting cycle. - an injection piston controllable in translation to operate on molten cast metal in a machine die for which a desired speed (Vset) is defined during a metal injection step; - a main pressure chamber, upstream of the injection piston, for containing and pressurizing the fluid intended for the outward translation of the injection piston, wherein said main pressure chamber is further connected to an injection drain connected to the drain, along which an injection return drain valve operates; - a proportional injection delivery valve for feeding the fluid to a main inlet and from said main inlet to the main pressure chamber as a result of the opening of a main shut-off valve; - a main back-pressure chamber, downstream of the tail end of the injection piston, connected to a return inlet for supplying pressurized fluid for the return translation of the injection piston, wherein the return inlet is connected upstream with a pump delivery, upstream of which a pump is located; - an injection return valve arranged between the delivery pump and the return inlet; - a proportional injection drain valve for draining the fluid from the main back-pressure chamber, so that the action of the fluid in the main pressure chamber and the opposite action of the fluid in the main back-pressure chamber generate an outward thrust for the actuation in translation of the injection piston, controlled in opening according to a tuning speed (Vreg); - an auto-tuning injection valve opening system configured so that the opening of the injection valve during a predetermined die-casting cycle is tuned according to the tuning speed (Vreg), determined as a function of the difference between said predetermined desired speed (Vset) of the injection piston and an effective speed (Veff) of said injection piston detected for the previous die-casting cycle.