A method for blocking an optical element on an insert of a blocking device comprising: - providing the insert of the blocking device, the insert being blocked in respect with the blocking device; - providing thermoplastic material in a solid state in a first particulate form, the provided thermoplastic material having a feature comprised in an operating range; - heating at least a part of the provided thermoplastic material at a temperature at which the thermoplastic material is in a melted state and flows under moderate pressure; - providing on the insert an amount of the heated thermoplastic material for blocking one optical element; - placing the optical element onto the thermoplastic material in the melted state; - allowing the thermoplastic material to solidify, thereby blocking the optical element on the insert; - converting the solidified thermoplastic material into a second particulate form, the converted thermoplastic material being intended to block another optical element, characterised in that the method comprises: - measuring the feature of the thermoplastic material; - comparing the measured feature with the operating range; - if the measured feature is not comprised in the operating range, mixing a first amount of thermoplastic material in the first particulate form and a second amount of the converted thermoplastic material in the second particulate form, wherein the measured feature is the viscosity of the thermoplastic material. - providing the insert of the blocking device, the insert being blocked in respect with the blocking device; - providing thermoplastic material in a solid state in a first particulate form, the provided thermoplastic material having a feature comprised in an operating range; - heating at least a part of the provided thermoplastic material at a temperature at which the thermoplastic material is in a melted state and flows under moderate pressure; - providing on the insert an amount of the heated thermoplastic material for blocking one optical element; - placing the optical element onto the thermoplastic material in the melted state; - allowing the thermoplastic material to solidify, thereby blocking the optical element on the insert; - converting the solidified thermoplastic material into a second particulate form, the converted thermoplastic material being intended to block another optical element, characterised in that the method comprises: - measuring the feature of the thermoplastic material; - comparing the measured feature with the operating range; - if the measured feature is not comprised in the operating range, mixing a first amount of thermoplastic material in the first particulate form and a second amount of the converted thermoplastic material in the second particulate form, wherein the measured feature is the viscosity of the thermoplastic material. - measuring the feature of the thermoplastic material; - comparing the measured feature with the operating range; - if the measured feature is not comprised in the operating range, mixing a first amount of thermoplastic material in the first particulate form and a second amount of the converted thermoplastic material in the second particulate form, wherein the measured feature is the viscosity of the thermoplastic material.