A Drop In Time

CFD speeds development of MEMS-based printing technology

—Edited by Stephanie Gooch

Researchers at Eastman Kodak Company, Rochester, New York, are using computer simulation to speed the development of next-generation inkjet printheads based on MEMS technology. Micro Systems Technology (MST) has the potential to advance the quality of inkjet images and system productivity by permitting smaller ink orifices and larger arrays of orifices. In addition, the economies of scale of MEMS manufacturing will make printers more affordable by decreasing production costs. Kodak researchers are using computational fluid dynamics analysis to simulate drop formation by MEMS structures, because building and testing each generation of such devices is impractical. Plus, due to small geometric scale, measuring certain characteristics is impossible. “In addition to letting us visualize these microfluidic phenomena that can’t be seen in a test situation, the main benefit of CFD is that it lets us quickly explore many design alternatives,” says Christopher Delametter, senior research analyst in the Integrated Materials and Microsystems Laboratory at Kodak. “In this way, simulation is significantly reducing the time required to make a novel MEMS based printhead commercially viable.”

Problems with current technology 

Currently, Drop on Demand (DOD) inkjet printheads use one of two approaches for ejecting ink. One uses piezoelectric channels —the printer applies a voltage across the channels, which squeezes them and shoots out the ink. But limitations in the manufacturing process make it difficult to increase the size of the orifice array. “This type of inkjet printer is popular for home use, but it needslarger arrays to be fast enough for use in office and commercial applications,” says Delametter. The other approach to ejecting ink from the cartridge uses thermal bubble jet technology. This type of printhead heats ink with a resistive heater to create a vapor bubble. As the bubble grows, it pushes ink out of the orifice. The limitation here is that it requires a lot of energy to vaporize the ink. Because the printer must deal with the waste heat, it is difficult to increase the size of the arrays. Also, since it is less accurate at postioning ink than the piezoelectric method, thermal bubble jet technology printers must include methods to compensate.

Delametter and his colleagues are developing new printhead technology that has the potential to improve image quality and printing speed by replacing conventional printheads with novel MEMS approaches. “In one approach, the MEMS structures we are developing will control the motion of ink by exploiting its surface properties,” explains Delametter. “Heating a fluid meniscus non-uniformly induces a gradient in surface tension. This produces a tangential shearing force, often referred to as a Marangoni stress, on the liquid-free surface. When orifice dimensions are on the order of 10 microns, thermocapillary-driven forces can separate discreet droplets from the main fluid body and propel them rapidly through space.” Thedevice could be created using VLSI semiconductor manufacturing processes. “Through the leveraging of existing semiconductor infrastructure, these MEMS-based printheads would potentially have the same low production cost as that of the bubble jet printheads,” Delametter adds. “Yet they would produce less waste heat, so it would be possible to increase the size of the printhead arrays.”

To make MEMS technology viable for commercial use, Kodak researchers must design a printhead that produces very small drops (<2 pl) at high frequencies (>50 kHz), thereby enabling high-speed printing with photographic quality. This requires a thorough understanding of a number of microfluidic phenomena including drop creation via thermocapillary-driven forces. Many design variables influence thermocapillary flow including the size of the orifice, the size of the heater, whether the heater is embedded or at the surface, the best thermal and structural properties of materials for the ink and the body of the MEMS structure.

Benefits of CFD

Delametter and his colleagues are simulating the operation of the printhead on the computer using CFD. “Through simulation, we decrease development time tremendously,” he says. “It also helps us better understand the underlying mechanisms. We can turn some forces off, others on, and really dissect the system to understand its behavior.”

The biggest challenge in using CFD to simulate the inkjet printing process is keeping track of the free surface of the droplet interface. Modeling problems such as Delametter’s, where surface tension plays an important role, require accurate resolution and tracking of fluid surfaces. They also require an evaluation of surface curvatures and sensing where and how fluid adheres to solids. Delametter chose to use FLOW-3D from Flow Science, Inc., Los Alamos, New Mexico. “I had been doing CFD simulation for years using other codes but I couldn’t use them in this work because they didn’t handle the Marangoni driven flows,” Delametter explains. “This package is the only one I found that includes a thermocapillary flow model for predicting Marangoni forces.” It also provides algorithms that track sharp liquid interfaces through arbitrary deformations and apply the correct normal and tangential stress boundary conditions, an accuracy feature that distinguishes it from other CFD programs. It also saves modeling time because its FAVOR method offers the simplicity of a rectangular grid with the conforming properties of a body-fitted grid. “All in all, the FLOW-3D software combines ease of use with the ultimate in sophisticated physics,” Delametter adds.

Simulation details

Delametter began the CFD analysis by defining the geometry of the device within FLOW-3D. The geometry was fairly simple, consisting of an orifice, a taper back to a large open channel that holds the ink, an ink reservoir, a heater on the structure of silicon, and contacts for electrical connections. In addition to the orifice and the area immediately surrounding it, the analysis domain included part of the ink reservoir and far enough in the other direction so that the ink drop is able to detach from the bulk of the fluid. Boundary conditions included a pressure boundary representing back pressure on the ink reservoir and a continuative boundary condition where the fluid flows out of the domain. Most simulations are axisymmetric with approximately 50,000 cells. “That’s a relatively small number when compared to some CFD analysis being done today, but it’s sufficient for this simple geometry,” says Delametter. 

He verified the analysis model against experimental results. Then he ran a series of FLOW-3D models, modifying parameters such as orifice size, the size and location of the heater, ink properties and so on, observing the effect on drop formation and velocity. One advantage of simulation was that it showed temperature and velocities that could not be determined experimentally. Another was that Delametter could evaluate many different variable combinations in less time than it would have taken to build just one physical device.

These advantages of CFD allowed Kodak to achieve a workable MEMS printhead in a reasonable amount of time. “Without simulation, it would have taken years to get to the point where we understood this technology and had working devices,” Delametter says. Now they are at the point where they need to optimize drop-creation frequency and velocity to get the kind of performance needed to make this a viable commercial product. Simulation is also making that process faster. “With CFD simulation, the company has been able to understand the mechanisms of how novel MEMS structures can be used to form ink drops, optimize MEMS based printhead designs, and rapidly progress toward commercial application,” Delametter says.

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