A process for the forming of a large sized container from a preform (Pa) made of an injection moulded material and having big thickness, the process comprising an initial pre-heat treatment step for the pre-heating of said preform (Pa), and a final blow moulding step of said treated preform (Pa) for the formation of said large sized container, and wherein it is provided an intermediate pre-blowing step of said preform (Pa) into a pre-blowing mould (1; 10), wherein said thermally treated preform (Pa) is inserted into a pre-blowing mould (1; 10) and a pre-blowing of said preform (Pa) is carried on until to a predetermined elongation ratio condition, and wherein said preform (Pc) after said intermediate pre-blowing step has predetermined geometric characteristics and thermal profile for the subsequent final blow moulding step of said large sized container, characterized in that said pre-blowing mould (1; 10) comprises a lower part and an upper part, and wherein in said intermediate pre-blow moulding step comprises the following steps: - a displacement of said pre-blowing mould (1; 10) onto a supporting frame and up to complete insertion of said preform (Pa) within said pre-blowing mould (1; 10), the subsequent closing of the pre-blowing mould (1; 10) by displacement means, wherein when the lower part of the pre-blowing mould (1; 10) is located at the lowest position, the upper part is detached from the former in an open condition, and wherein when the pre-blowing mould (1; 10) performs the vertical displacement by a vertical displacement actuator upwards and the upper part reaches the limit, the latter stops and a limit switch coupled to the vertical displacement actuator sends the input signal for the closing of the upper part; - when the pre-blowing mould (1; 10) is closed with the preform (Pa) housed therein, an upper seal means lowers onto said pre-blowing mould (1; 10) and the pre-blowing of the preform (Pa) starts; - when the pre-blowing is completed, air is discharged from the pre-blowing mould (1: 10) and the sealing means raises, wherein at this stage begins the descent of the lower part of the pre-blowing mould (1; 10) but the upper part stays by means of a respective actuator which bias the lower part away from the upper part of the pre-blowing mould (1; 10), and wherein the upper part of the pre-blowing mould (1; 10) is opened slightly before it reaches the maximum allowable distance between said two parts (5, 6) of the pre-blowing mould (1; 10) and before continuing the downward displacement of the pre-blowing mould (1; 10); and - at the end of said displacement the preform (Pc) results in a completely removed condition from the pre-blowing mould (1; 10) and it is conveyed to the area of the apparatus related to the blowing and final forming step of the large sized container. - a displacement of said pre-blowing mould (1; 10) onto a supporting frame and up to complete insertion of said preform (Pa) within said pre-blowing mould (1; 10), the subsequent closing of the pre-blowing mould (1; 10) by displacement means, wherein when the lower part of the pre-blowing mould (1; 10) is located at the lowest position, the upper part is detached from the former in an open condition, and wherein when the pre-blowing mould (1; 10) performs the vertical displacement by a vertical displacement actuator upwards and the upper part reaches the limit, the latter stops and a limit switch coupled to the vertical displacement actuator sends the input signal for the closing of the upper part; - when the pre-blowing mould (1; 10) is closed with the preform (Pa) housed therein, an upper seal means lowers onto said pre-blowing mould (1; 10) and the pre-blowing of the preform (Pa) starts; - when the pre-blowing is completed, air is discharged from the pre-blowing mould (1: 10) and the sealing means raises, wherein at this stage begins the descent of the lower part of the pre-blowing mould (1; 10) but the upper part stays by means of a respective actuator which bias the lower part away from the upper part of the pre-blowing mould (1; 10), and wherein the upper part of the pre-blowing mould (1; 10) is opened slightly before it reaches the maximum allowable distance between said two parts (5, 6) of the pre-blowing mould (1; 10) and before continuing the downward displacement of the pre-blowing mould (1; 10); and - at the end of said displacement the preform (Pc) results in a completely removed condition from the pre-blowing mould (1; 10) and it is conveyed to the area of the apparatus related to the blowing and final forming step of the large sized container.