Design and Optimization of a High Temperature Microheater for Inkjet Deposition
Inkjet deposition has become a promising additive manufacturing technique due to its fast printing speed, scalability, wide choice of materials, and compatibility for multi-material printing. Among many different inkjet techniques, thermal inkjet, led by Hewlett-Packard and Canon, is the most successful inkjet technique that uses a microheater to produce a pressure pulse for ejecting droplets by vaporizing the ink materials in a timespan of microseconds. Thermal inkjet has been widely adopted in many commercial 3D inkjet printers (e.g., 3D Systems ProJet X60 series) due to its low cost, high resolution, and easy operation. However, the viscosity of the printable materials has been limited to less than 40cP due to insufficient energy provided inside the nozzle to overcome the viscous dissipation of energy. This paper presents a study on the design and optimization of a high temperature microheater with a target heating temperature of more than 600˚C (compared to ~300 ˚C for current printhead) to increase the energy supply to the nozzle. The benefits are fourfold: 1) higher temperature will lead to faster vaporization of ink and thus higher jetting frequency and print speed; 2) higher temperature will make it possible for jetting materials with higher boiling points; 3) higher temperature will reduce the viscosity of the ink and thus the viscous dissipation of energy; 4) higher energy supply will increase the magnitude of the pressure pulse for printing more viscous materials. In this paper, a high temperature microheater was designed with the following objectives: to reduce thermal stress in heaters, and to minimize uneven heat distribution. A literature survey was first conducted on design, fabrication, and operation of thin-film resistive microheaters. A multiphysics numerical model was then developed to simulate electrical, thermal, and mechanical responses of the microheater. The model was validated by comparison to experimental data and existing models obtained from literature. With proper parameterization of the design geometry, the geometry of the microheater is optimized using a particle swarm optimization method. Results show the optimized high temperature microheater successfully operates at temperatures in excess of 600˚C. The design optimization enabled better characteristics for even heat distribution and minimizing stress. The design approach can serve as a fundamental means of design optimization for microheaters.