Flexible molds have an elastic membrane supported by modules, arranged next to one another along a horizontal longitudinal axis of the mold. Each module has traction units, each forming a respective group with a respective thrust unit, with the groups arranged opposite one another at a preset distance. Pairs of traction units are arranged opposite one another in a direction perpendicular to the axis, and the respective thrust units thereof are arranged therebetween. The modules hold the membrane with the mold via reinforced eyelets distributed on both edges parallel to the axis. Each traction unit has linear actuators arranged horizontally and another linear actuator arranged vertically, which together enable movement. The membrane is previously deformed according to an approximation of the prior design of a part to be molded, and prestressed according to a mathematical prediction of the deformation thereof after receiving a conglomerate in liquid or plastic state.