a method and system for providing for the production of preforms of thermoplastic resin by filling a plurality of multiple - cavity molds with molten resin for forming the preforms , holding and cooling down the molten resin inside the molds , removing the molded preforms from the molds , subjecting the preforms to a subsequent temperature conditioning stage , and transferring the preforms to a blow - molding stage . each of the multi - cavity molds includes a plurality of cavities defined in several distinct clusters and each cavities belonging to the same cluster is cooled in a distinct and differentiated manner with respect to the cavities belonging to other clusters provided in the same mold .