Mold structure for forming a polymeric gasket around a predetermined portion of a sheet of transparent material such as glass, with the object of minimizing hand labor in the assemblying of seals and windows. The mold structure includes two cooperating mold sections (12 and 14) for defining a chamber for receiving the transparent sheet (16). A seal (18, 20) is positioned about the periphery of the chamber and is utilized to resiliently support the sheet within the chamber. Also, the seal cooperates with a predetermined portion of the transparent sheet for defining a gasket cavity having a configuration corresponding to the gasket to be formed on the sheet of transparent material. An inlet (25) communicates with the gasket cavity and is utilized to introduce a flowable gasket material into the gasket cavity.