Sol-gel process for the manufacture of moulds having at least one portion transparent to UV rays, comprising the following stages: a) providing a container which is a negative of the said at least one portion transparent to UV of the mould which is to be manufactured and internally defining a chamber, the said chamber having at least one first surface which is substantially the negative of a corresponding outer surface of the said at least one transparent portion of the mould being manufactured, and at least one second surface which is substantially a negative of at least one corresponding inner surface of the said at least one transparent portion of the mould being manufactured,; b) filling the chamber with a sol; c) gelling the sol, obtaining the so-called gel; d) heating the gel and a corresponding solvent for a programmed period of time consequently achieving predetermined temperature and pressure values preferably higher than the critical values for the solvent present in the gel, and evaporation of the said solvent, the said heating giving rise to drying of the gel; e) densification and/or sintering of the dried gel through heating to a predetermined temperature, preferably higher than the temperature at which drying stage d) takes place, with the consequent formation of the said transparent portion of the mould, with a vitreous consistency and transparent to at least UV rays, the said mould being suitable for the formation of pultrusion and/or extrusion products and/or the moulding of plastics materials through a process of photocatalytic synthesis characterized in that the container comprises at least one insert defining the said at least one second surface of the chamber. a) providing a container which is a negative of the said at least one portion transparent to UV of the mould which is to be manufactured and internally defining a chamber, the said chamber having at least one first surface which is substantially the negative of a corresponding outer surface of the said at least one transparent portion of the mould being manufactured, and at least one second surface which is substantially a negative of at least one corresponding inner surface of the said at least one transparent portion of the mould being manufactured,; b) filling the chamber with a sol; c) gelling the sol, obtaining the so-called gel; d) heating the gel and a corresponding solvent for a programmed period of time consequently achieving predetermined temperature and pressure values preferably higher than the critical values for the solvent present in the gel, and evaporation of the said solvent, the said heating giving rise to drying of the gel; e) densification and/or sintering of the dried gel through heating to a predetermined temperature, preferably higher than the temperature at which drying stage d) takes place, with the consequent formation of the said transparent portion of the mould, with a vitreous consistency and transparent to at least UV rays, the said mould being suitable for the formation of pultrusion and/or extrusion products and/or the moulding of plastics materials through a process of photocatalytic synthesis characterized in that the container comprises at least one insert defining the said at least one second surface of the chamber.