An optical sensor using fiber optics to access the mold cavity was used to monitor the mold filling and cooling phases of injection molding. The sensor consists of a sapphire window at the end of a sleeved ejector pin into which an optical fiber is inserted. For the application presented here, the molded product was a tensile specimen 16 cm in length by 3.175 mm (1/8) thick. The optical view with this sensor is through the thickness of the molded product. The measured optical signal was either from a fluorescent dye that was mixed with the resin or from light that transmitted through the resin, reflected off the opposite wall of the mold and retraced its path through the resin to the optical sensor. Both modes of sensing have been used to monitor molding of polyethylene, polystyrene and polypropylene during the packing and cooling phases of the process. Shrinkage of the molded product to dimensions less than the mold cavity can also be monitored because, when the product separates from the wall of the mold, the geometry of a Fabry-Perot interferometer is created. Interference fringes map the product shrinkage. Sensor behavior is described quantitatively by a processing model that includes a solution to the thermal diffusion equation, crystallization and glass formation kinetics, light transport properties, and principles of interferometry.
and Thomas, C.
An Optical Sensor for Polymer Injection Molding, Electrochemical Society Transactions, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=851715
(Accessed June 9, 2023)