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Optical cure monitoring for process and quality control in DVD manufacture

An optical sensor system, which provides real-time process information during the curing of DVD adhesive within the DVD manufacturing process, is reported. Incomplete or non-uniform curing of the adhesive in the centre of the disk can cause variation in layer thickness or create stresses, which may cause warping of the disk. The monitoring of adhesive cure is then of vital importance to manufacturers, particularly if this can be achieved in situ and within the production cycle time for disk manufacture. The operating principle of the sensor is the detection of changes in the infrared absorption spectrum of the adhesive as it cures. The use of infrared absorption in itself is not straightforward as the adhesive is sandwiched between two polycarbonate disks. One of these disks is completely opaque throughout the infrared due to the aluminium layer deposited on it and the other disk has a semi-reflective layer of a material such as silicon, gold or silver. The sensor employs a reflectance based interrogation technique, which uses the highly reflective aluminium coated surface of the DVD. The work described here deals principally with (i) definition of the specifications of the sensor system and the measurement methodology, (ii) development and optimization of a laboratory prototype sensor and (iii) design of an industrial prototype system. Results are presented from both prototype systems.