Thermal Simulation and Experimental Comparison of a Printhead
Authors: Mulay Shirish 
Published: June 2009

Thermal inkjet technology (TIJ) has been the widely preferred method of inkjet printing since its market inception in 1985. Thermal inkjet technology continues to enjoy a greater unit market share for printers than any other printing technology. In this presentation history of inkjet printing will be discussed. In the recent past, thermal inkjets have made great strides. Speed of printing is a strong function of the swath size and can be increased by increasing the chip size. Photo quality printing has reached new levels by reducing drop size and spot size. This has been achieved by increasing nozzle density, increasing frequency of printing and using multiple pass printing to optimize quality. However, to improve speed of printing single pass resolution is an important parameter. The paper presents the simple working principle of the inkjet printhead. The physics of printhead operation will be shown briefly. A thermal analysis is carried out on the printhead. Simulations of printing on paper has been carried out for hypothetical print densities and compared with simple experiments. The scales that appear range from microns to millimeters. Short term and long term analysis has been carried out. Figure 1 shows the cross-section of a part of the printhead. The parts depicted in Figure 1 shows the plastic body, chip along with the vias for ink flow. In the analysis, die bond, encapsulant, tab circuit, nozzle plate have been included but not included in Figure 1 for clarity. Temperature as a function of time and was measured by the temperature sensing resistor (tsr). The parameter considered in this study was the density of printing, uni or bi directional printing and excess energy supplied to the chip. Current, voltage, time for printing and drop mass was measured. Temperature was monitored during the printing process. The effects of temperature on the viscosity, surface tension have been neglected in the study. A commercial code from Flow Science was used for the CFD analysis. The boundary conditions used were (i) Bottom boundary was set with velocity inflow rate and temperature (ii) top boundary conditions have heat transfer coefficient to ambient and exit pressure conditions (ambient pressure) and (iii) all sides have heat transfer coefficients. The appropriate properties have been used in the simulations, namely, density, specific heat and thermal conductivity of the different materials associated with the nozzle plate, tab circuit, die-bond and the plastic body is made of Noryl. The mesh sized varied from the smallest of 1 micron to the largest being a fraction of millimeter. Results of chip temperature response versus time are shown in Figure 2. Files of known density were generated by using Corel Draw package and printed. Figure two shows the response to seven pages of printing. It also shows the chip cooling off. The temperature response curve shows that the system can be divided in to three time constants. The first time constant is extremely fast and can be related to chip parameters. The third time constant is a very slow one and can be identified with the printhead body and its surroundings, like the ambient temperature, convective heat transfer to the surroundings etc. The intermediate time constant is a function of the assembly of chip to the printhead body and it’s surrounding cross section, i.e. the heat spreader. Figure 3 shows the temperature response (simulation) to a hypothetical printing case and an excess energy of 5%. Some part of the geometry has been changed for purpose of confidentiality. The energy levels in the standard case are not what one would use in regular printing. The temperature response is generally dependent on geometry and the package of chip encapsulation. For the geometry chosen, the temperature changes by the same amount in Celsius as the percent rate of energy changes at the heater level. For example, in Figure 3 the energy changes by 5 percent and the steady state temperature increases by 5 C.