a half - section of a cassette casing having an embedded transparent wafer serving as a viewing window . the cassette casing section is formed by shearing the wafer from an elongated strip of transparent material , disposing and supporting the wafer within a cavity , and forcing a plasticized material into the cavity where it solidifies . the cavity is defined in an injection mold comprising a pair of relatively movable cavity plate assemblies with which are associated means for shearing and supporting the wafer .