Core arranged to be inserted between mould cavities, suitable for performing a method for manufacturing a plastic fuel tank with improved creep strength, whereby: a) a plastic parison comprising two distinct parts is inserted into an open two-cavity mould; b) the core is inserted inside the parison, the said core bearing at least part of a reinforcing element capable of securing, e.g. creating a link, between the two parts of the parison; c) the parison is pressed firmly against the mould cavities by blowing through the core and/or creating suction behind the cavities; d) the reinforcing element or a part of the reinforcing element is fixed to at least one of the parts of the parison using the core; e) the core is withdrawn; f) the mould is closed again, bringing its two cavities together in such a way as to grip the two parts of the parison around their periphery in order to weld them together; g) a pressurized fluid is injected into the mould and/or a vacuum is created behind the mould cavities in order to press the parison firmly against the mould cavities; h) the mould is opened and the tank is extracted, characterized in that the core comprises a hydraulic ram for attaching at least one reinforcing element to a plastic parison, the hydraulic ram being capable of moving the at least one reinforcing element that is to be attached and of exerting pressure thereon, sensors for measuring parameters adapted to monitor the attachment of the reinforcing element to the parison and/or a camera adapted for watching and monitoring the quality of the fixing of the at least one reinforcing element through image analysis, whereby the sensors and camera are installed on the core. a) a plastic parison comprising two distinct parts is inserted into an open two-cavity mould; b) the core is inserted inside the parison, the said core bearing at least part of a reinforcing element capable of securing, e.g. creating a link, between the two parts of the parison; c) the parison is pressed firmly against the mould cavities by blowing through the core and/or creating suction behind the cavities; d) the reinforcing element or a part of the reinforcing element is fixed to at least one of the parts of the parison using the core; e) the core is withdrawn; f) the mould is closed again, bringing its two cavities together in such a way as to grip the two parts of the parison around their periphery in order to weld them together; g) a pressurized fluid is injected into the mould and/or a vacuum is created behind the mould cavities in order to press the parison firmly against the mould cavities; h) the mould is opened and the tank is extracted, characterized in that the core comprises a hydraulic ram for attaching at least one reinforcing element to a plastic parison, the hydraulic ram being capable of moving the at least one reinforcing element that is to be attached and of exerting pressure thereon, sensors for measuring parameters adapted to monitor the attachment of the reinforcing element to the parison and/or a camera adapted for watching and monitoring the quality of the fixing of the at least one reinforcing element through image analysis, whereby the sensors and camera are installed on the core.