In situ monitoring of product shrinkage during injection molding using an optical sensor

摘要

We have used an optical fiber sensor for in situ monitoring of product shrinkage during injection molding. The sensor, consisting of a bundle of opitcal fibers with a sapphire window at its end, is positioned in the ejector pin channel of the mold so that the sapphire window sits flush with the inside wall of the mold cavity. The optical view with this sensor is through the thickness (3.175 mm) of the molded product. The fiber bundle is divided into light excitation fibes and light collection fibers. Light from a helium neon laser is transmitted to the resin via the excitation fibers and the dtected response is light that reflects from any interface at which there is a difference of index of refraction. When the molded product shrinks, it separates from the wall and sapphire window, and establishes the geometry of a Fabry-Perot interferometer. During the molding of polystyrene, polypropylene and polyethylene, we observed optical interference fringes that were generated by the movement of the molded product away from the mold wall and window. By counting fringes, a measurement of shrinkage was made. Sensor behavior is described by a model that takes into account the reflection coefficient at each surface. the change in index of refraction of the resin and the coherence of the excitation light. The model has been used to describe asymmetric product shrinkage.