Development of multi scale thermoforming process based on novel rapid prototyping mold cores

Doctoral Dissertation Development of Multi-scale Thermoforming Process Based on Novel Rapid-prototyping Mold Cores Department of Mechanical Engineering Graduate School, Chonnam National University Nguyen, Khoa Trieu February 2018 Doctoral Dissertation Development of Multi-scale Thermoforming Process Based on Novel Rapid-prototyping Mold Cores Department of Mechanical Engineering Graduate School, Chonnam National University Nguyen, Khoa Trieu February 2018 Development of Multi-scale Thermoforming Process Based on Novel Rapid-prototyping Mold Cores Department of Mechanical Engineering Graduate School, Chonnam National University Nguyen, Khoa Trieu Supervised by Professor Lee, Bong-Kee A dissertation submitted in partial fulfillment of the requirements for the Doctor in Engineering in Department of Mechanical Engineering Committee in Charge: Prof Lee, Dong-Weon Prof Kang, Hyun Wook Prof Park, Jang Min Prof Park, Sung Jea Prof Lee, Bong-Kee February 2018 CONTENTS Contents (Abstract) 1 Introduction and background 1.1 Research motivation 1.2 Literature surveys 1.2.1 Milli-scale thermoforming 1.2.2 Micro-scale thermoforming 1.2.3 Materials for thermoforming’s mold core 14 1.2.4 Fused deposition modeling 16 1.4 Research objectives and methodology 21 Application of FDM for thermoforming 24 2.1 Warpage problem in FDM process 24 2.2 Measurement of surface morphology and roughness 27 2.3 Heat absorption property 30 2.4 Measurement of dimensional accuracy 37 2.5 Emissivity of the aluminum coated surface 41 2.5.1 Emissivity calibration setup 45 2.5.2 Emissivity measurement results 49 2.5.3 Theoretical calculations for radiation heating of flat specimen 52 2.5.4 Thermal characteristics of the flat FDM specimen during radiation heating 54 i 2.6 Dimensional stability of FDM mold cores under cyclic heating and pressurizing 57 2.6.1 Cyclic heating and pressurizing experiment 58 2.6.2 Heat absorption experiment and analysis for flat specimen 60 2.6.3 Dimension variation of grooved FDM parts under cyclic heating and pressurizing 62 2.6.4 Theoretical calculation results for radiation heating of flat specimen 63 2.6.5 Thermal characteristics of flat specimen during radiation heating 65 2.6.6 Numerical verification for grooved FDM specimens 66 A simple lab-scale thermoforming system 73 3.1 Design and fabrication 73 3.1.1 Multi-well cell culture dish 73 3.1.2 Material selection 74 3.1.3 Design procedure 75 3.1.4 Fabrication of the apparatus 79 3.2 Descriptions of the thermoforming apparatus 81 3.3 Evaluation process for thermoformed sample 83 3.4 Thermoforming simulation using Ansys PolyFlow 84 3.5 Simulation verification – Bubble inflation method 89 3.5.1 For thicker PS film 94 3.5.2 For thinner PS film 97 3.6 Comparison between simulation and experimental results 99 3.6.1 Verification for the developed apparatus using metallic mold core 99 ii 3.6.2 Verification for the developed apparatus using FDM mold core 102 Development of a simple multi-scale thermoforming system 105 4.1 Milli-scale thermoforming using the currently verified apparatus 105 4.1.1 Thermoforming conditions 105 4.1.2 Using 50 µm BOPS film 106 4.1.3 Using 190 µm BOPS film 108 4.2 Micro-scale thermoforming using the currently verified apparatus 111 4.2.1 Thermoforming conditions 111 4.2.2 Using 50 µm BOPS film 113 4.2.3 Using 190 µm BOPS film 117 4.3 Multi-scale thermoforming using the currently verified apparatus 124 4.3.1 Preliminary tests 124 4.3.2 Feasibility of the current multi-scale thermoforming technique 132 4.3.3 Uniformity measurement 138 4.3.4 Repeatability measurement 143 Typical applications of multi-scale thermoforming 148 5.1 Multi-scale well plate 148 5.2 Multi-scale microfluidic hanging drop chip 149 5.2.1 Concept design 149 5.2.2 Bonding method selection 150 5.2.3 Thermal bonding experiment 151 Conclusion and future direction 154 iii Acknowledgement 157 References 158 (국문초록) 168 iv List of Figures Fig 1 A simple schematic of a pressure thermoforming process [5] Fig Development of micro thermoforming 11 Fig Variants of the micro thermoforming processes: a) Thermoforming with a matching counter tool – micro matched-die molding, b) with an elastomeric counter tool, c) with a softened polymer – micro back molding, and d) with compressed gas – micro pressure thermoforming 12 Fig Classification of AM technologies 16 Fig Principal of a typical FDM process [39] 17 Fig Principle of warpage in FDM: a) top view, b) side view, (c) side view of deformed part 25 Fig 2 Photographs of two representative printed parts: a) typical part showing a large warpage after printing, and b) improved part fabricated by applying preprocessing 27 Fig Microscopic views of the flat specimen: a) as-printed, b) aluminum-coated only, c) acetone-treated only, and d) acetone-treated and aluminum-coated 28 Fig Surface roughness measurement: a) acetone-treated only, and b) acetone-treated and aluminum-coated 29 Fig Heat transfer mechanism of the present heating setup 31 Fig Measured temperature variations in the heating experiments 32 Fig Fitted non-linear models: (a) case of the highest equilibrium temperature (experiment #4) and (b) case of the lowest equilibrium temperature (experiment #5) 34 Fig a) Effect of the parameters on the heat absorption property and b) contributions of each parameter 35 v Fig Specimen with concave grooves: a) photograph of the fabricated specimen, b) cross-sectional schematic of the concave grooves, c) variations in width compared with the as-printed value, and d) variations in depth compared with the as-printed value 39 Fig 10 Specimen with convex grooves: (a) photograph of the fabricated specimen, (b) cross-sectional schematic of the convex grooves, (c) variations in width compared with the as-printed value, and (d) variations in height compared with the as-printed value 40 Fig 11 Thickness of aluminum-coated layer: a) concave grooved specimen and b) convex grooved specimen 41 Fig 12 Experiment setup for emissivity measurement for thin coated aluminum layer 46 Fig 13 Experiment setup for surface temperature measurement of flat specimen and parameter for view factor calculation 47 Fig 14 Heat transfer mechanism of the present radiative heating experiment 48 Fig 15 Emissivity calibration and temperature measurement using infrared camera 50 Fig 16 Temperature distribution with varying emissivity 51 Fig 17 Emissivity measurement for the far infrared ceramic heater: a) thermal image, b) emissivity determination 52 Fig 18 Theoretical calculation for heat absorption and heat loss of the flat FDM specimen under radiative heating 54 Fig 19 Temperature measurement using infrared camera 55 Fig 20 Measured temperature in the heating experiment for flat specimen (a) line measurement (b) average temperature 55 Fig 21 Experimental setup and grooved specimens in cross-section views for cyclic heating and pressurizing experiment 59 Fig 22 Experiment setup for surface temperature measurement for flat specimen 60 Fig 23 Heat transfer mechanism of the present radiative heating experiment 61 vi Fig 24 Dimensional variation of concave FDM part under cyclic heating and pressurizing: a) measure of depth, b) measure of width 63 Fig 25 Dimensional variation of convex FDM part under cyclic heating and pressurizing (a) measure of height, (b) measure of width 63 Fig 26 Theoretical calculations for heat absorption and heat loss of flat specimen under radiative heating 65 Fig 27 Measured temperature variations in the heating experiment for flat specimen 66 Fig 28 Numerical simulation for flat specimen (a) model and main boundary conditions, (b) temperature distribution result 67 Fig 29 Corresponding experimental measurement area in numerical simulation for flat specimen 67 Fig 30 Temperature distribution within grooved FDM parts: a) concave type, b) convex type, c) along center line of concave part, d) along center line of convex part 68 Fig 31 Deformation of grooved FDM parts under pressure at highest temperature: a) symmetrical concave model, b) symmetrical convex model, c) along center line of concave part, d) along center line of convex part 69 Fig Design of the multi-well cell culture dish 73 Fig Original PS foil for thickness measurement 74 Fig 3 Design of the thermoforming apparatus 79 Fig Photograph of the developed thermoforming apparatus 82 Fig Metallic mold core for preliminary tests 82 Fig Evaluation process for thermoformed sample 83 Fig Cross-sectioning principle for PDMS mounted thermoformed sample 84 Fig The fitted Cross-WLF curves in the range of forming temperature of PS 88 Fig A typical simulation result for 190 µm PS film 91 vii Acknowledgement The work presented in this dissertation was performed at M3LAB, Department of Mechanical Engineering, Graduate School, Chonnam National University under the supervisor of Professor Lee, Bong-Kee I would like to thank Professor Lee, Bong-Kee for his kindness and continues guidance and help in every step of my research and education And I would like to thank all other committee members, chairman - Professor Lee, Dong-Weon, Professor Kang, Hyun Wook, Professor Park, Jang Min and Professor Park, Sung Jea Thank you very much for spending time and providing valuable comments and questions I also thank to all the former and current members of M3LAB, Department of Mechanical Engineering, Chonnam National University who have been colleagues, labmates and friends Special thanks also go to all the Vietnamese students that I have had the pleasure of working with, going to market with, drinking with or living with during my lifetime at Chonnam National University And finally, I would like to express my gratitude to my wife, Luong Thi Tuyet Nga, and my daughter, Nguyen Luong Thao Nhi, my families for their continuous support, care and encouragement throughout my Ph.D time as well as my life 157 References 10 11 12 13 14 15 M Worgull, Chapter - Hot Embossing A2 - Qin, Yi, Micro-Manufacturing Engineering and 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열성형의 특성을 그대로 가질 수 있다는 점에서 매우 매력적이다 하지만 기존의 마이크로 168 열성형은 핫 엠보싱 장치를 기반으로 수행되었기 때문에 성형에 소요되는 비용의 증가와 마이크로 열성형의 활용 가능성을 제한하고 있다 다양한 스케일에서의 열성형에 대한 많은 연구가 진행되어 왔지만, 응용 분야의 확대를 위한 멀티스케일 열성형 공정의 연구가 여전히 요구되고 있다 따라서 본 연구에서는 새로운 멀티스케일 열성형 공정을 제안하고 실험적 연구 및 수치해석 검증을 진행하였다 첫 번째 단계로 융착 모델링(FDM) 기술로 제작되는 플라스틱 부품의 표면 특성 개선과 열 흡수 감소를 위하여 화학 처리, 건조 및 알루미늄 코팅으로 구성되는 새로운 공정 기술이 연구되었다 제안된 기술의 특징을 규명하기 위하여 먼저 평판 시편을 이용한 표면 구조를 분석하였다 시편의 열 흡수 특성은 다구치 기반 실험계획법과 변형된 집중 용량 모델을 이용하여 분석하였다 또한 양각 및 음각 홈을 가지는 시편을 이용하여 고온 조건에서의 치수 정밀도와 균일성을 측정하고 분석하였다 마지막으로 주기적인 가열 및 가압 조건에서 홈을 가지는 시편의 치수 안정성 연구를 진행하였으며, 이를 통하여 제안된 표면 처리 기술로 제작된 FDM 부품의 열성형 공정 금형 코어로의 적용 가능성을 확인하였다 다음으로 밀리 스케일의 표면 처리된 FDM 금형 코어와 마이크로 스케일의 금속 몰드 코어를 결합하여 멀티스케일 열성형 금형 코어를 구성하였다 이와 같은 공정의 연구를 위하여 단순화된 열성형 장치가 새롭게 개발되었다 ANSYS PolyFlow를 169 이용하여 열성형 공정에 대한 수치해석을 실시하였으며 실험 결과와 일치함을 확인하였다 개발된 열성형 장치와 멀티스케일 금형 코어를 이용한 열성형 공정을 통하여 성형품의 균일성과 반복성을 측정하였으며, 이를 통하여 멀티스케일 열성형 기술의 개발 가능성을 검증하였다 본 연구에서 제안하는 멀티스케일 열성형 공정은 기존의 마이크로 스케일 열성형 공정과 비교했을 때 많은 차이점과 장점을 가지고 있다 가변 온도 금형 방식의 핫 엠보싱 장치를 기반으로 하는 기존의 마이크로 스케일 열성형 공정과는 달리 복사 방식의 가열을 이용하는 열성형 장치를 개발하였다 이를 통하여 상온과 플라스틱 재료의 유리전이온도 사이의 일정한 값으로 금형 온도를 정교하게 유지할 수 있다 또한 기존 열성형 공정과 동일하게 본 연구에서는 금형의 냉각만 적용하였다 이를 통하여 본 멀티스케일 열성형 공정에서는 약 10분이 소요되는 기존의 마이크로 열성형 공정과는 달리 32초에서 40초 정도의 공정 시간을 구현할 수 있었다 즉, 가변 온도 금형 방식으로 구현된 현재의 마이크로 열성형 공정이 가지는 한계를 극복할 수 있었으며, 따라서 일반적인 열성형 공정과 유사한 대량 성형의 가능성을 확인할 수 있었다 이와 함께 멀티스케일 세포 배양 용기 및 멀티스케일 미세유체 칩과 같은 멀티스케일 열성형 공정으로 효과적으로 구현될 수 있는 응용 제품군을 제시하였다 이와 같은 신속하고 경제적인 멀티스케일 열성형 기술은 실험실 규모에서 170 제작뿐만이 아니라 원형 제작과 소량 생산 등의 생산 공정에도 적용될 수 있을 것으로 판단한다 171 ...Doctoral Dissertation Development of Multi- scale Thermoforming Process Based on Novel Rapid- prototyping Mold Cores Department of Mechanical Engineering Graduate School, Chonnam National University... Trieu February 2018 Development of Multi- scale Thermoforming Process Based on Novel Rapid- prototyping Mold Cores Department of Mechanical Engineering Graduate School, Chonnam National University... 133 xiii Development of Multi- scale Thermoforming Process Based on Novel Rapid- prototyping Mold Cores Nguyen Khoa Trieu Department of Mechanical Engineering Graduate School, Chonnam National University