Micro-injection molding of glass fiber reinforced parts

Abstract

Micro injection molding has recently gained tremendous attention owing to an increased demand for microelectromechanical systems (MEMS) and microsystems which encompass a wide range of industrial and engineering applications. Since the sizes of microinjection molded parts and devices are typically on the order of hundreds of micrometers, its flowability of molten polymer in a mold cavity becomes important to inject the molten polymers successively during the filling and packing stages during the injection molding. Particularly, the rheological properties of the injection molding materials play an important role in the flowability during an injection process due to the fact that the rheological properties are mainly affected by shear rate and temperature which are pragmatic approach to change injection molding conditions. The rheological properties affecting the flowability of molten polymers are extensively characterized by investigating fiber length distribution, volume fraction, and heat transfer of inclusions so as to determine the crucial parameters for the flowability inside micro-sized pitches and channels. Therefore, we firstly fabricated a thin tensile specimen and investigated the internal structures of the specimen by using Micro-CT to analyze the fiber length distribution of the samples. The data acquired by the Micro-CT was then processed by an image processing to analyze quantitative probability density functions, cumulative functions and probability functions. Also we compared it with several statistical fiber breakage models. From the fiber length distribution results, 3D internal structures of microinjection molded parts were obtained. The results of fiber orientation and flowability were compared with a numerical analysis by using a commercial software tool (MOLDFLOW). Both the experimental and numerical results indicated that long glass fibers, low volume fraction, and low thermal conductivity of inclusions show better flowability. Effective elastic modulus was measured and predicted by combining the theoretical modes.