Development of Multi-scale Thermoforming process Based on Novel Rapid-Prototyping Moldcores :Doctoral Dissertation - Major: Machanical Engineering

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

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CONTENTS

Contents

(Abstract) ............................................................................................................................. 1

1. Introduction and background ............................................................................................... 4

1.1. Research motivation...................................................................................................... 4

1.2. Literature surveys.......................................................................................................... 5

1.2.1. Milli-scale thermoforming ..................................................................................... 5

1.2.2. Micro-scale thermoforming ................................................................................... 6

1.2.3. Materials for thermoforming’s mold core............................................................ 14

1.2.4. Fused deposition modeling .................................................................................. 16

1.4. Research objectives and methodology ........................................................................ 21

2. 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

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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

3. 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

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3.6.2. Verification for the developed apparatus using FDM mold core....................... 102

4. 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

5. 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

6. Conclusion and future direction....................................................................................... 154

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Acknowledgement ........................................................................................................... 157

References........................................................................................................................ 158

(국문초록)....................................................................................................................... 168

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List of Figures

Fig. 1. 1 A simple schematic of a pressure thermoforming process [5]................................... 6

Fig. 1. 2 Development of micro thermoforming .................................................................... 11

Fig. 1. 3 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. 1. 4 Classification of AM technologies .......................................................................... 16

Fig. 1. 5 Principal of a typical FDM process [39].................................................................. 17

Fig. 2. 1 Principle of warpage in FDM: a) top view, b) side view, (c) side view of deformed

part ......................................................................................................................................... 25

Fig. 2. 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. 2. 3 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. 2. 4 Surface roughness measurement: a) acetone-treated only, and b) acetone-treated and

aluminum-coated.................................................................................................................... 29

Fig. 2. 5 Heat transfer mechanism of the present heating setup............................................. 31

Fig. 2. 6 Measured temperature variations in the heating experiments.................................. 32

Fig. 2. 7 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. 2. 8 a) Effect of the parameters on the heat absorption property and b) contributions of

each parameter ....................................................................................................................... 35

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Fig. 2. 9 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. 2. 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. 2. 11 Thickness of aluminum-coated layer: a) concave grooved specimen and b) convex

grooved specimen .................................................................................................................. 41

Fig. 2. 12 Experiment setup for emissivity measurement for thin coated aluminum layer.... 46

Fig. 2. 13 Experiment setup for surface temperature measurement of flat specimen and

parameter for view factor calculation .................................................................................... 47

Fig. 2. 14 Heat transfer mechanism of the present radiative heating experiment .................. 48

Fig. 2. 15 Emissivity calibration and temperature measurement using infrared camera ....... 50

Fig. 2. 16 Temperature distribution with varying emissivity................................................. 51

Fig. 2. 17 Emissivity measurement for the far infrared ceramic heater: a) thermal image, b)

emissivity determination........................................................................................................ 52

Fig. 2. 18 Theoretical calculation for heat absorption and heat loss of the flat FDM specimen

under radiative heating........................................................................................................... 54

Fig. 2. 19 Temperature measurement using infrared camera ................................................. 55

Fig. 2. 20 Measured temperature in the heating experiment for flat specimen (a) line

measurement (b) average temperature ................................................................................... 55

Fig. 2. 21 Experimental setup and grooved specimens in cross-section views for cyclic

heating and pressurizing experiment...................................................................................... 59

Fig. 2. 22 Experiment setup for surface temperature measurement for flat specimen........... 60

Fig. 2. 23 Heat transfer mechanism of the present radiative heating experiment .................. 61

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Fig. 2. 24 Dimensional variation of concave FDM part under cyclic heating and pressurizing:

a) measure of depth, b) measure of width.............................................................................. 63

Fig. 2. 25 Dimensional variation of convex FDM part under cyclic heating and pressurizing

(a) measure of height, (b) measure of width .......................................................................... 63

Fig. 2. 26 Theoretical calculations for heat absorption and heat loss of flat specimen under

radiative heating..................................................................................................................... 65

Fig. 2. 27 Measured temperature variations in the heating experiment for flat specimen ..... 66

Fig. 2. 28 Numerical simulation for flat specimen (a) model and main boundary conditions,

(b) temperature distribution result.......................................................................................... 67

Fig. 2. 29 Corresponding experimental measurement area in numerical simulation for flat

specimen ................................................................................................................................ 67

Fig. 2. 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. 2. 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. 3. 1 Design of the multi-well cell culture dish................................................................ 73

Fig. 3. 2 Original PS foil for thickness measurement ............................................................ 74

Fig. 3. 3 Design of the thermoforming apparatus .................................................................. 79

Fig. 3. 4 Photograph of the developed thermoforming apparatus.......................................... 82

Fig. 3. 5 Metallic mold core for preliminary tests.................................................................. 82

Fig. 3. 6 Evaluation process for thermoformed sample ......................................................... 83

Fig. 3. 7 Cross-sectioning principle for PDMS mounted thermoformed sample................... 84

Fig. 3. 8 The fitted Cross-WLF curves in the range of forming temperature of PS............... 88

Fig. 3. 9 A typical simulation result for 190 µm PS film....................................................... 91

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Fig. 3. 10 Law of Laplace for cylindrical and spherical models............................................ 92

Fig. 3. 11 Cross section of a flat polymer film after inflation using grooved shape used for

Trouton ratio calculation [139] .............................................................................................. 92

Fig. 3. 12 Simulation results for extensional viscosity calculation for 190 µm PS film: a) at

simulation time t1, b) at simulation time t2............................................................................. 95

Fig. 3. 13 Simulation results for extensional viscosity calculation for 50 µm PS film: a) at

simulation time t1, b) at simulation time t2............................................................................. 98

Fig. 3. 14 Comparison between simulation and experimental results for metallic mold core:

a) Depth-to-width forming ratio of no. #1, b) difference values of no. #1, c) depth-to-width

forming ratio of no. #2, d) difference values of no. #2 ........................................................ 100

Fig. 3. 15 Infrared transmission spectrum for PS film [136] ............................................... 101

Fig. 3. 16 Spectral characteristics of radiant heaters [144].................................................. 102

Fig. 3. 17 Comparison between simulation and experimental results for FDM mold core: a)

Depth-to-width forming ratio of no. #1, b) difference values of no. #1, c) depth-to-width

forming ratio of no. #2, d) difference values of no. #2 ........................................................ 103

Fig. 4. 1 Experimental results for formed film using FDM mold core: a) Thickness of formed

film for no. #1, b) for no. #2, c) for no. #3, d) photograph for no. #4.................................. 107

Fig. 4. 2 Using FDM mold core: a) Areal draw ratio, b) normalized depth-to-width forming

ratio ...................................................................................................................................... 108

Fig. 4. 3 Experimental results for formed film using FDM mold core: a) Thickness of formed

film for no. #5, b) for no. #6, c) for no. #7, d) photograph for no. #8.................................. 110

Fig. 4. 4 Free heating experiment to study free thickness variation of 190 µm BOPS film 111

Fig. 4. 5 Experimental results using 50 µm PS film with metallic mold core: a) Thickness of

formed film for no. #1, b) for no. #2, c) for no. #3, d) for no. #4 ........................................ 113

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Fig. 4. 6 Normalized depth-to-width forming ratio for 50 µm PS film using metallic mold

core....................................................................................................................................... 114

Fig. 4. 7 Multi-response Taguchi method applied for 50 µm PS film: a) Effect of the

parameters on the replication quality and (b) contributions of each parameter................... 116

Fig. 4. 8 S/N ratio analysis for every groove using 50 µm PS film: a) 0.4 mm, b) 0.6 mm, c)

0.8 mm, d) 1.0 mm, e) 1.2 mm, and f) cross section of 0.4 mm groove of sample no. #3... 117

Fig. 4. 9 Experimental results using 190 µm PS film with metallic mold core: a) Thickness of

formed film for no. #1, b) for no. #2, c) for no. #3, d) for no. #4 ........................................ 118

Fig. 4. 10 Normalized depth-to-width forming ratio for 190 µm PS film using metallic mold

core....................................................................................................................................... 119

Fig. 4. 11 Multi-response Taguchi method applied for 190 µm PS film: a) Effect of the

parameters on the replication quality and (b) contributions of each parameter................... 122

Fig. 4. 12 S/N ratio analysis for every groove using 190 µm PS film: a) 0.4 mm, b) 0.6 mm,

c) 0.8 mm, d) 1.0 mm, e) 1.2 mm, and f) cross section of 0.4 mm groove of sample no. #3

.............................................................................................................................................. 123

Fig. 4. 13 Combined mold core for multi-scale thermoforming preliminary testing........... 125

Fig. 4. 14 Round type FDM mold core for multi-scale thermoforming preliminary testing: a)

design drawing, b) after printing, c) after acetone treatment, and d) after aluminum coating

.............................................................................................................................................. 126

Fig. 4. 15 Grooved type FDM mold core for multi-scale thermoforming preliminary testing:

a) design drawing, b) after printing, c) after acetone treatment, and d) after aluminum coating

.............................................................................................................................................. 126

Fig. 4. 16 Dimension variation of the FDM mold cores under various processing step...... 127

Fig. 4. 17 Groove type metallic mold core for for multi-scale thermoforming preliminary

testing................................................................................................................................... 128

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Fig. 4. 18 Post type metallic mold core for for multi-scale thermoforming preliminary testing

.............................................................................................................................................. 128

Fig. 4. 19 Results for multi-scale thermoforming preliminary testing using groove type

metallic mold core: a) Round type FDM mold core and b) groove type FDM mold core... 129

Fig. 4. 20 Results for multi-scale thermoforming preliminary testing using post type metallic

mold core: a) Round type FDM mold core and b) groove type FDM mold core ................ 130

Fig. 4. 21 Thin aluminum layer on formed film .................................................................. 131

Fig. 4. 22 Measurement position and method for multi-scale thermoforming sample ........ 134

Fig. 4. 23 Cross-section view for 50 µm samples................................................................ 134

Fig. 4. 24 Cross-section view for 190 µm samples.............................................................. 135

Fig. 4. 25 Thickness results using combined mold core: a) 50 µm PS film, b) 190 µm PS film

.............................................................................................................................................. 136

Fig. 4. 26 Replication quality results using combined mold core: Normalized height-to-width

forming ratio ........................................................................................................................ 136

Fig. 4. 27 Replication quality results using combined mold core: a) Normalized ratio of

height, b) normalized ratio of width..................................................................................... 138

Fig. 4. 28 Measurement positions for each multi-scale thermoforming sample .................. 139

Fig. 4. 29 Variation of thickness in each thermoformed sample in case of using 50 µm PS

film....................................................................................................................................... 139

Fig. 4. 30 Variation of thickness in each thermoformed sample in case of using 190 µm PS

film....................................................................................................................................... 140

Fig. 4. 31 Uniformity of thickness of thermoformed film using a. 50 µm PS film, b. 190 µm

PS film ................................................................................................................................. 141

Fig. 4. 32 Positions of cavities of FDM mold core on the thermoforming apparatus.......... 142

Fig. 4. 33 Uniformity of replication quality of thermoformed film using both 50 µm and 190

µm PS film........................................................................................................................... 142

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Fig. 4. 34 Variation of thickness in center portion of 3 thermoformed samples in case of

using 50 µm PS film ............................................................................................................ 144

Fig. 4. 35 Variation of thickness in center portion of 3 thermoformed samples in case of

using 190 µm PS film .......................................................................................................... 145

Fig. 4. 36 Repeatability of thickness of thermoformed film using a. 50 µm PS film, b. 190

µm PS film........................................................................................................................... 146

Fig. 4. 37 Repeatability of replication quality of thermoformed film using both 50 µm and

190 µm PS film.................................................................................................................... 146

Fig. 5. 1 A possible application for multi-scale thermoforming .......................................... 148

Fig. 5. 2 Concept design of the multi-scale microfluidic hanging drop chip: a) With micro

semi-spherical structures, b) with micro semi-cylindrical structures................................... 149

Fig. 5. 3 A thermal bonded sample using the current apparatus.......................................... 152

Fig. 5. 4 Cross-section views of a thermal sealed cavity ..................................................... 153

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List of Tables

Table 2. 1 Processing parameters and detailed conditions for DOE...................................... 32

Table 2. 2 Nonlinear regression variables for each experiment............................................. 33

Table 2. 3 L9(34

) orthogonal array with the calculated evaluation index and S/N ratio (MSD:

mean standard deviation) ....................................................................................................... 35

Table 2. 4 S/N ratios for each DOE parameter ...................................................................... 36

Table 2. 5 Result of ANOVA................................................................................................. 37

Table 2. 6 Parameters for view factor calculation in current experimental setup .................. 52

Table 3. 1 Initial thickness of PS foils for thermoforming experiment.................................. 75

Table 3. 2 Bill of materials (BOM) for the thermoforming apparatus................................... 79

Table 3. 3 Material properties of PS film............................................................................... 89

Table 3. 4 Constant for the Cross-WLF model for PS........................................................... 90

Table 3. 5 Parameters for the Cross-WLF model .................................................................. 90

Table 3. 6 Parameters for preliminary tests using metallic mold core................................... 99

Table 3. 7 Parameters for preliminary tests using FDM mold core ..................................... 102

Table 4. 1 Processing parameters and their detailed conditions investigated in the present

study for FDM mold core..................................................................................................... 105

Table 4. 2 Parameter sets for experiments using FDM mold core....................................... 106

Table 4. 3 Processing parameters and their detailed conditions investigated in the present

study for metallic mold core ................................................................................................ 112

Table 4. 4 Parameter sets for experiments using metallic mold core................................... 112