Procedural Design of Imprint Rolls for Fluid Pathway Fabrication

This paper discusses a novel method for designing imprint rolls for the fabrication of fluid pathways. Roller imprint processes have applications in diverse areas including fuel cell manufacturing and microfluidic device fabrication. Robust design methods are required for developing imprint rolls with optimal features. In the method discussed in this paper, the rolls are designed procedurally with the fluid pathway design given as input. The pathways are decomposed into repeating features (or tiles), and the rolls are designed by first modeling a small set of unique tiles and then combining them to model the entire roll. The tiling strategy decreases the complexity of the model, and reduces the time taken for designing the rolls. The modular nature of the tiles also improves the efficiency of post-processing operations like feature identification and optimization, and the generation of toolpaths for machining the roll. INTRODUCTION Rapid manufacture of highly complex, precise parts requires a robust and efficient pathway between the design and manufacturing stages. This paper discusses the design of the roller imprinting manufacturing process for the fabrication of fluid pathways. We use procedural methods to rapidly design elements of the manufacturing process for a given fluid pathway design. Manufacturing process design, in this case, is limited to the design of the imprint roll, as the roll geometry plays the strongest ∗Address all correspondence to this author. role in the quality of the part. We have developed a system that can fully design an imprint roll for a given pathway geometry. The design system is extensible and can be easily integrated with CAM systems and other post-processors. There has been growing interest in the fabrication of fluid pathways for various applications, ranging from fuel cell bipolar plates in the macro-scale, to bio-fluidic devices in the microscale. These fluid pathways share common fabrication requirements: high form accuracy of cross-sections, controlled roughness in the walls and channels, cost-competitive technology and high throughput processes. Roller imprinting is a suitable candidate for fabricating these parts. In this process a hard cylindrical die (or roll) with raised surface features rolls over a soft workpiece imprinting flow channels onto its surface. Roller imprinting is more suitable than stamping as the stresses applied are significantly smaller and more uniform, resulting in controlled deformation of the workpiece. The precision of the imprinted features is a function of the features in the imprint roll, the dynamics of the rolling equipment, and the process parameters. Of these, the features in the imprint roll have the most significant effect. Precision is determined by the positional accuracy of the paths, the form error in the path channels, and the profile of the channel surfaces. Optimally designed imprint rolls are those that can create precise features for a given fluid pathway design and workpiece material. To design such a roll, a method is needed to first model the roll from a given set of input parameters. With this model, further analysis can be performed to guarantee precise imprinted path1 Copyright c © 2007 by ASME ways. The key challenge lies in developing tools to efficiently model and describe the roll. This paper discusses a novel method to model imprint rolls which exploits the repetitiveness of features found in fluid pathways. A given family of fluid pathways can be constructed from a small set of repeating units, and this information is used to efficiently describe the roll in a computer model. Apart from the obvious improvements in processing time, this approach aids in the development of more efficient post-processing operations, like machining process planning. Iterative optimal design of the roll is also easy to implement as the features of the roll are parameterized. While the scale of the features on the roll is limited by the manufacturing processes used, the modeling procedure itself is scale-invariant. This paper restricts to looking at rolls with features that can be created using micro-machining processes. Micro-machining processes have been used to create features as small as 20μm [1]. The next section highlights two application areas for roller imprint processes: bipolar plate fabrication and microfluidic device fabrication. APPLICATION AREAS PEM Fuel Cells PEM (polymer electrolyte membrane) fuel cells are electrochemical devices which generate electricity in the presence of a fuel (Hydrogen) and oxidant (Oxygen) [2]. The “heart” of the cell is the MEA (membrane electrode assembly), which preferentially allows protonic hydrogen to pass through from the anode to the cathode side, while forcing electrons to flow around the cell, generating electricity. The MEA is sandwiched between bipolar (or flow-field) plates which provide it a fuel rich environment. The bipolar plates are so called because they have hydrogen flowing on one side and oxygen on the other, and are used to collect electricity out of the cell as well. Bipolar plates are an important component of the fuel cell stack as they facilitate the diffusion of gases across the MEA. Surface quality and flow field design of the plates is critical in guaranteeing the performance of the cells. Bipolar plates are composed of a network of millimeter-scale flow paths. Figure 1 shows a representative bipolar plate design. Design of cost-effective and precise process technologies for fuel cell components is very important for the large-scale adoption of fuel cells. The DOE’s Fuel Cell Report [4] highlights the importance of manufacturing in achieving widespread use of these devices. The manufacturing processes must guarantee very precise parts, while being flexible enough to allow for quick changes in part design. It is also important that the process is cost-competitive. Currently, machining and stamping operations are being used to manufacture the bipolar plates [5]. Roller imprinting is a capable process for manufacturing these devices. It is also possible to pattern both sides of the bipolar plates simultaneously with this process, leading to a higher throughput. Figure 1. FUEL CELL BIPOLAR PLATE [3] Microfluidic Devices Microfluidic devices handle liquids on the nano-liter scale and have chemical, biological, and medical applications [6]. These devices have channels on the order of 10-100 μm with microand nano-scale features. Conventionally, silicon and glass were the materials of choice for the substrates of these devices, as they have established fabrication processes developed by the semiconductor industry. Though glass has good properties for use it is expensive to produce, because of this a variety of polymers are currently being developed as substrate materials. Common fabrication methods with polymers include imprinting, hot embossing, injection molding, lithography techniques, and laser photo-ablation [7]. Figure 2 shows microchannels fabricated in PMMA (polymethyl methacrylate) using the LIGA (Lithographie Galvanoformung and Abformung) process. Contact printing methods have also been used in the fabrication of microchannels [8]. Contact printing involves the transfer of features from an elastomeric stamp to a solid substrate by conformal contact. High-resolution features can be printed over a large area with this process. Although rolls created using micro-machining technology have limited ability to create sub-micron features and threedimensional features, they can be used in applications where large features need to be created efficiently. They can also be used in hybrid processes, with roller imprinting creating the large features, and a complimentary process creating the smaller features. This approach will decrease the overall fabrication time and cost of the devices. 2 Copyright c © 2007 by ASME Figure 2. MICROCHANNELS FABRICATED IN PMMA [7]