Luận án TS: Chế tạo khuôn micro AA6061 ép nóng cho thiết bị vi lưu polyme

MAUFACTURIG AA6061 MICRO-MOLDS BY HOT EMBOSSIG FOR PRODUCTIO OF POLYMERIC MICROFLUIDIC DEVICES TRA HAT KHOA MAUFACTURIG SYSTEMS AD TECHOLOGY SIGAPORE-MIT ALLIACE AYAG TECHOLOGICAL UIVERSITY 2012 MAUFACTURIG AA6061 MICRO-MOLDS BY HOT EMBOSSIG FOR PRODUCTIO OF POLYMERIC MICROFLUIDIC DEVICES TRA HAT KHOA (B. Eng, Ho Chi Minh City University of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY I MAUFACTURIG SYSTEMS AD TECHOLOGY (MST) SIGAPORE-MIT ALLIACE AYAG TECHOLOGICAL UIVERSITY 2012 DECLARATIO I hereby declare that this thesis is my original work and it has been written by me and its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously.

Tran hat Khoa 02 July 2012 Acknowledgements This work could not have been done without contributions of many great people during my PhD study. What I have learnt in this four year will be very helpful not only in my future career but also in my life. I sincerely thank the following people for their kind help. First and foremost, I would like to give the deepest gratitude to my thesis advisor, Professor Lam Yee Cheong, for his tremendous support and careful guidance over the last four years.

I have achieved a lot through discussions with him about research as well as many other aspects of life. With me, he is always one of the best advisors. I would like to thank Professor Lallit Anand at MIT for accepting me as his student. I especially thank him for his guidance and supporting during my stay at MIT.

I would also like to take this opportunity to express my profound gratitude to Professor Yue Chee Yoon and Professor Tan Ming Jen for their valuable comments and suggestions. I am deeply indebted to Professor Tran Doan Son, Professor Pham Ngoc Tuan, and Professor Nguyen Huu Loc at Ho Chi Minh University of Technology in Vietnam, who have introduced and supported me to obtain SMA scholarship. I would like to thank my lab-mates Rajeeb, Roy, Saha, Jeffry, Hendra, Mohana, Lip Pin, and Phuong for their kind support when needed. I would also like to thank my lab-mates Shawn, David, Kaspar, Claudio for their i support during my stay at MIT.

I would particularly like to thank Shawn for sharing graciously his knowledge as well as many fruitful discussions. I would also like to thank all lab technicians from the School of Mechanical and Aerospace Engineering, Nanyang Technological University, who have supported me during my research. Last but not least, I would like to thank my family for their understanding and patience. Their emotional support is a great source of my strength.

My partner-to-be, Lan Anh, shared with me many wonderful experiences during my long journey and I would like to thank for her love and support. ii Table of Contents Acknowledgements. i Table of Contents. vii List of Tables.

ix List of Figures. x List of Abbreviations. xix Chapter 1 Introduction.3 List of publication related to this thesis .2 Conference and symposium papers .4 Organization of the thesis. 5 Chapter 2 Current concept on deformation mechanism of polycrystalline materials and methods to fabricate mold for microfluidic devices .1 Current concept on deformation mechanism in forming process of polycrystalline materials .2 Current methods in mold fabrication for making polymeric microstructures.1 High precision micromilling .2 Micro Electrical Discharge machining.4 Laser microcutting combined with laser microwelding .6 Micro powder injection molding .8 Photolithography and Deep Reactive Ion Etching (DRIE) .12 Hot embossing on metals .13 Hot embossing on polymer .14 Hot embossing on bulk metallic glass .15 Comparison of different mold-making methods.

63 Chapter 3 Deformation phenomenon for micro-hot-formability of polycrystalline materials .2 Manufacturing silicon master by photolithography and DRIE .2 Etching AA6061 specimen for grain size determination .3 Deformation mechanism of AA6061 in micro hot embossing experiments. 72 Chapter 4 Hot embossing with silicon master for fabrication of AA6061 micro-mold and its use for hot embossing polymeric micro-channels .1 Fabrication of AA6061 micro-mold via hot embossing with silicon master .2 Hot embossing on TOPAS 8007 using an AA6061 mold .3 Evaluation of hardness, roughness and strength of AA6061 micro-mold .2 Surface roughness measurements. Tensile strength measurements .4 Fabrication of different complex micro-features on AA6061. 94 Chapter 5 Analyses of aluminum alloy 6061 micro-mold fabrication .1 Effect of AA6061 orientation (rolling direction) on filling of silicon master during hot embossing .2 Large-deformation theory of isotropic elastic-viscoplastic materials 105 5.2 Finite-deformation theory of isotropic elastic-viscoplastic solids (This is adopted from Anand [58-59] ) .3 Material parameters for hot forming model of AA6061 .4 Validation experiments and simulation .3 Comparison of numerical results of hot embossing process with corresponding experiments .2 Recommendations for future work.

145 Appendix A Grain size calculation of AA6061.1 Sample and reagent preparation .2 Etching aluminum alloy 6061 .3 Grain size calculation. 162 v Manufacturing AA6061 micro-molds by hot embossing for production of polymeric microfluidic devices by Tran Nhat Khoa Submitted to the School of Mechanical and Aerospace Engineering on 1st July, 2011, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract In the manufacturing of microfluidic devices, the micro-molds will not only affect the overall manufacturing cost but also determine the quality of the molded micro-parts. Thus, the focus of this research is to investigate if an aluminum alloy micro-mold could be fabricated by hot-embossing using silicon (Si) master with acceptable cost, quality, and life span. By employing the appropriate processing parameters, experiments conducted indicate conclusively that the deformation mechanism of aluminum alloy 6061 (AA6061) in micro hot embossing is the plastic deformation of the grains themselves.

As such, contradictory to conventional wisdom, this investigation shows that grain size is not a determining factor on the smallest feature that could be reproduced for a polycrystalline material. Using the processing methodology developed, AA6061 micro-molds were successfully fabricated and the effectiveness of these molds was examined by hot embossing on TOPAS 8007 substrates. Finally, Anand’s large deformation theory for isotropic plastic solids has been demonstrated to be adequate in predicting the forming process of AA6061 in micro hot embossing. Lam Yee Cheong, SMA Fellow, NTU 2.

Lallit Anand, SMA Fellow, MIT. vi Summary The main focus of this thesis is the fabrication of AA6061 mold for the manufacturing of polymeric microfluidic devices. In the forming process of polycrystalline metallic material, grain size is believed to be the limiting factor that determines the minimum feature size fabricated. This investigation showed conclusively that the deformation mechanism of AA6061 near its solidus temperature is plastic deformation of grains themselves.

As such, grain size ceases to be the limiting factor on the minimum feature size. With a proper choice of a set of embossing parameters, it was demonstrated that micro-features much smaller than the grain size can be fabricated on AA6061 substrate. AA6061 molds containing fine features were fabricated with excellent fidelity by hot embossing using a Si master. The ability of embossing different features with complex geometry on AA6061 has also been demonstrated through the successful replication of T-shaped, I-shaped or micro-mixer shaped features.

Subsequently, the AA6061 mold was employed to emboss on TOPAS 8007 substrates to illustrate that a mold so fabricated could be used for mass production. Temperature cycling during the hot embossing step in AA6061 mold manufacturing process reduces significantly the original tensile strength and hardness of the mold, which is not desirable. As such, in this study, a tempering process was carried out to recover the tensile strength and hardness of the embossed mold. Surface roughness, tensile strength, and hardness values were measured in each stage: (i) before hot embossing, (ii) after hot vii embossing and (iii) after T4 tempering and T6 tempering.

The results obtained demonstrate that the original strengths and hardness can be fully recovered by a post-tempering process after hot embossing, but with an increase in surface roughness. Accelerated testing was carried out to evaluate the changes in hardness and roughness of AA6061-T4 and T6 molds under the typical hot embossing temperature cycles of manufacturing polymeric devices. The results obtained indicate that these temperature cycles have only a minor effect on the roughness of both T4 and T6 molds and will increase the hardness of T4 molds to T6 temper, and have a negligible effect on the hardness of a T6 temper mold. This study shows that when the embossing temperature is near the solidus temperature, AA6061 behaves as an isotropic material, i.

the forming ability is the same in all directions. As such, it is appropriate to employ Anand’s constitutive model for hot deformation of isotropic materials to predict the filling process of AA6061 in micro hot embossing. The material parameters for the model were obtained from constant true strain-rate compression experiments. The predictive capability of this constitutive model was first validated by comparing predictions against macro-scale experimental results such as plane-strain cruciform forging and axi-symmetry forging.

Subsequently, obtained results for micro-hot embossing demonstrated that Anand’s constitutive model predict well the form process of micron-scale features near the solidus temperature of AA6061. viii List of Tables Table 1: Embossing parameters for Silicon and PC molds, PC, PMMA and COP substrates [47]. 60 Table 2: Comparison of different methods of making micro-molds. 65 Table 3: Composition of AA6061.

67 Table 4: Lubricant table for compression tests of AA6061. 128 ix List of Figures Fig. 1: Illustration of micro-formability of coarse polycrystalline, ultra-fine grained and amorphous metals [9]. 2: (a) Machined aluminum wafer containing negative channel relief features created by CAD program.

(b) Acrylic mold created from aluminum wafer. (c) PDMS channel profile at T-section. (d) PDMS channel geometry. 3: SEM of micro structures milled in brass (a) High aspect ratio wall of 20 µm wide and 400 µm tall (20:1) (b) Cross structure finished with a 100 µm radius milling bit (c) Cross structure finished with a 25 µm radius milling bit [12].

4: (a) Micro-milled mold master. (b) Sidewall of mold. (c) Molded PMMA substrate. 5: SEM image of as- µEDMed Ta insert of 12 regular protrusions with length, width and height of ∼9,500 µm, ∼170 µm, and ∼400 µm respectively and ∼750 µm center to center spacing [14].

6: (a) Embossed feature on Al. 7: (a) Embossed feature on Cu. 8: SEM images of (a) Ni electrode and (b) as-µEDMed Ta blank [15]. 9: SEM images of (a) Ni electrode with an array of micro gears with teeth on external diameter and (b) as-µEDMed Ta blank [15].

10: SEM images of (a) Ni electrode with an array of micro gears with teeth on both external & internal diameters and (b) as-µEDMed Ta blank [15]. 11: Holes machined in SD plates of different diamond particle sizes [16]. 12: Different stages of prototype fabrication: initial design by CAD module; filling strategy for each of 40 layers (CAM module); laser patterned steel substrate; demolded PMMA part [20]. 14: Mold insert made of PI (left) and molded part made of PMMA (right) [20].

15: Micro-mold fabrication process by laser microcutting and laser microwelding [23]. 16: Experiment setup of simple Y channel mixer, 75 µm width and 50 µm height [23]. 17: Micro channels fabricated by ECM die-sinking method [24]. 18: Five ribs manufactured by electrochemical milling on stainless steel [19].

21: SEM micrographs of freestanding micro-parts made by (a) ZrO2 and (b) Al2O3 (b) [28]. 22: Process steps of LIGA [30]. 25: Fabrication process of silicon mold [32]. 27: Before TMAH etching (left) and after TMAH etching (right) [32].

29: Schematic diagram illustrating fabrication planar (left) and orthogonal 3D (right) embossing tool [36]. 30: SEM images of channels of various dimensions -- 5 µm deep, 40 µm wide and 90 µm center-to-center (a), 90µm deep, 300 µm wide and 500 µm center-to-center (b) and 250 µm deep, 600 µm wide and (c) 1mm center-to- center [36]. 31: (a) Orthogonal 3D PDMS mold. 32: Deformation of PDMS: pairing (left) and sagging (right) [38].