Investigation for Optimization of Demolding Process based on Ceramic Injection Molding

Abstract

Piezoelectric ceramics are the most promising and unique materials because it is capable of energy transduction between a mechanical response including stress, vibrational impact and an electrical charge. They are mainly fabricated in the form of a microstructure array and mixed with a soft polymeric material to achieve a desirable acoustic impedance in the water. As a traditional technique, the dicing and filling method cuts the bulk piezoelectric ceramic to shape a square-pattern using dicing saw and fills the polymeric material into the kerfs. However, this method has the critical defects including property degradation by dicing saw, limitation of pattern shape to square and consuming a great deal of processing time. For this reason, casting, embossing, micro-pressing, additive manufacturing in a bottom-up technique, which can directly manufacture a microstructure array, have been introduced in recent years. Among the diverse bottom-up techniques, the powder-based injection molding process has the highest manufacturing rate and shape flexibility compared with other techniques. With the promising technology, many research institutes have built a piezoelectric microstructure array using a sacrificial and soft mold insert because of their easy replication and simple demolding process. However, severe demolding defects have emerged when using a metal mold insert. The microstructure array was vulnerably deformed or fractured because of the normal frictional force among the microstructure array and inner surface of the metal mold insert. The frictional force can be derived by the thermally contracted or expanded stress according to the temperature variation before demolding, and adhesive shear stress during actual demolding process. To overcome previous problem, three steps of development process have been introduced and performed in the present work. As a first step, Ni-Fe mold insert with 400 micro-cavities was replicated via X-ray based photolithography and electroforming process to increase the repeatability of mold insert up to 100,000 times. For this goal, the entire manufacturing process for Ni-Fe mold insert consists of fabrication of a gold mask by UV photolithography, photoresist and substrate preparation, synchrotron X-ray beam analysis and exposure, development and drying process, Ni-Fe electroforming process and final post-processing. Second, the systematic powder-based injection molding process was established for a piezoelectric microstructure array with a pattern dimension below 150 μm and aspect-ratio over 5, and it is comprised of characterization of powder and binders, design of binder system, determination of the optimum powder-binders volumetric ratio, choice of binder system, mixing and evaluation of homogeneity, analysis of rheological behavior and recyclability, especially for analysis of demolding process; simulation, experiment and verification of repeatability, debinding process for removal of binders and sintering process for densification of powder. The third step is the evaluation of piezoelectric composite performance. After formulating a piezoelectric microstructure array, it experienced an epoxy kerf filling, grinding, electric deposition and poling to fabricate a composite. Moreover, validation of the proposed process was conducted to comparing it with the diced reference samples for the performance, productivity and cost aspect.