A centre gripping subassembly for injection molding machine, it is including (a) centre gripping cylinder case and (b) centre gripping piston, and the fixed tip to the connecting rod of this centre gripping piston slides in can clamping position, mould split position and position of engagement in the casing, and this position of engagement is arranged in the middle of the axial of clamping position and mould split position. This clamping position's centre gripping room is still pushed this centre gripping piston to this centre gripping subassembly when (c) is used for in the pressurization, (D, ) be arranged in at the pressure of this the centre gripping room return mechanism with this centre gripping piston push back position of engagement when releasing, and (e) is used for pushing thiscentre gripping piston to in when pressurization the mould split room of mould split position. This mould split room is axially restricted by the surperficial relatively of this connecting rod and this return mechanism.