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PROCESSABILITY OF WASTE PLASTIC BAG AS A NOVEL BINDER SYSTEM IN METAL INJECTION MOLDING
NUR HAFIZAH BINTI KAMARUDIN
UNIVERSITI TUN HUSSEIN ONN MALAYSIA
PROCESSABILITY OF WASTE PLASTIC BAG AS A NOVEL BINDER SYSTEM IN METAL INJECTION MOLDING PROCESS
NUR HAFIZAH BINTI KAMARUDIN
A thesis submitted in fulfillment of the requirement for the award of the Degree of Master of Mechanical Engineering
Faculty of Mechanical and Manufacturing Engineering Universiti Tun Hussein Onn Malaysia
AUGUST, 2017
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Special dedicated to my family who give me support:
Kamarudin Bin Yusof
Noraini Binti Sulaiman
Khairu Bin Kamarudin
Khartini Binti Kamarudin
Khairi Bin Kamarudin
Noor Hasliza Binti Kamarudin
Mohd Haizol Bin Kamarudin
Norfadilah Binti Kamarudin
DEDICATION
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ACKNOWLEDGEMENT
First of all, alhamdulillah and thanks to the Almighty God, Allah S.W.T for blessing me with enough time and courage to finish this project successfully. This project would not have been possible without the support and contributions of many people especially the community of UTHM. I would like to thank each and everyone for their assistance and guidance. Much appreciation goes to my supervisor, Assoc. Prof. Dr. Mohd Halim Irwan Bin Ibrahim for giving me the opportunity to execute this project. His guidance motivates me more every time and provides encouragement to make this project become reality and successful. I would like to express my highly appreciation to Mr. Shahrul and Mr. Fazlanuddin (Ceramic and Polymer laboratory), Mr. Anuar and Mr. Tarmizi (Material Science laboratory) for their kindest assistances and giving technical advises toward accomplishing this research. My honourable appreciation also goes to my beloved parent, Kamarudin Bin Yusof and Noraini Binti Sulaiman for their support and encouragement throughout my life. They have instilled me the value of hard work and dedication which helped me to become what I am today. Last, but certainly not least, I would like gratefully appreciate the support given by beloved research mates who also my siblings, Khairu Bin Kamarudin and Noor Hasliza Binti Kamarudin and those who helped making this project successful, directly or indirectly. Your contributions and personal sacrifices are truly appreciated and will be well remembered. v
ABSTRACT
Metal injection molding is the most cost effective processes in powder metallurgy to produce small and intricate part for mass production. Many researchers are mainly focused on the commercial binders, while only few of them are conducting the MIM using recycle binders. Therefore, this work focuses on the characterization of waste plastic bag as a novel binder system with combination of palm kernel (PK) in “Green MIM” of 316L Stainless Steel. Significant with this, this effort was able to reduce MIM cost, reduce disposal problem and thus, create awareness among community that waste product can also be considered as valuable things instead of disposing them. However, this work was not covering the cleaning process of the waste plastic bag (WPB). The feedstock was prepared by three formulation of 50/50 (WPB/PK), 40/60 (WPB/PK), 30/70 (WPB/PK), while keeping 60 vol. % of powder loading. Homogeneity and rheology test prove that the all formulations were qualified to proceed in molding process. The screening test of molding process was conducted via Analysis of variance. It prove that the interaction factor was not significant to the 4 responses. The combination of orthogonal array Taguchi L9 (3 ) and Grey Taguchi show that the optimal condition for conducting this process was; injection temperature of 180 °C, mold temperature of 50 °C, injection pressure of 40 % and injection speed of 50 %. Whereby, extraction in heptane solution (solvent to feed ratio of 12:1) at 70 °C within 7 hours was recognized as the optimal condition for solvent debinding. Else, a thorough study was conducted to investigate thermal debinding and sintering. The free defect of brown and sintered parts were produced. It was produced at 1360 °C by 1 °C/min within 240 minutes of soaking time in argon tube furnace. It was recorded the high strength of 782.48 MPa and 154.96 hv. Overall, this work revealed that the potential of using the waste plastic bag as a green binder for 316L Stainless Steel of MIM was achievable. vi
ABSTRAK
Pengacuan suntikan logam adalah proses yang efektif dalam aspek kos dalam metalurgi serbuk untuk menghasilkan sebahagian kecil dan rumit untuk pengeluaran besar-besaran. Ramai penyelidik memberi tumpuan kepada pengikat komersial, manakala hanya beberapa daripada mereka menggunakan pengikat kitar semula. Oleh demikian, kerja penyelidikan ini memberi tumpuan kepada pencirian beg plastik sampah sebagai sistem pengikat novel dengan gabungan kernel sawit (PK) dalam pengacuan suntikan 316L Stainless Steel. Signifikan dengan ini, usaha ini dapat mengurangkan kos, masalah pelupusan sampah dan memberi kesedaran kepada masyarakat bahawa bahan buangan juga boleh dijadikan sebagai perkara yang berharga dan bukan hanya untuk dilupuskan sahaja. Walau bagaimanapun, kerja-kerja ini tidak meliputi proses pembersihan beg plastik sampah (WPB). Bahan mentah telah disediakan kepada tiga formulasi iaitu, 50/50 (WPB/PK), 40/60 (WPB/PK), 30/70 (WPB/PK), dengan mengekalkan 60 vol. % sebagai bebanan serbuk. Kehomogenan dan ujian reologi membuktikan bahawa semua formulasi telah layak untuk digunakan di dalam proses pengacuan. Ujian saringan proses pengacuan telah dijalankan melalui Analisis Varians. Ia membuktikan bahawa faktor interaksi tidak ketara terhadap output 4 tersebut. Gabungan Taguchi L9 (3 ) dan Grey Taguchi menunjukkan bahawa keadaan yang optimum untuk menjalankan proses ini adalah; suhu penyuntikan 180 °C, suhu acuan 50 °C, tekanan suntikan sebanyak 40 % dan kelajuan suntikan sebanyak 50 %. Selain itu, pengekstrakan dalam larutan heptana (nisbah pelarut terhadap berat sampel adalah 12:1) pada 70 °C selama 7 jam telah diiktiraf sebagai keadaan yang optimum bagi penyahikatan pelarut. Selepas itu, satu kajian menyeluruh telah dijalankan untuk menyiasat penyahikatan terma dan pensinteran. Tiada sebarang kecacatan terjadi pada komponen yang dihasilkan. Ia telah dihasilkan pada 1360 °C dengan menggunakan kadar kenaikan sebanyak 1 °C/min selama 240 minit di dalam relau argon.Ia telah mencatatkan kekuatan yang tinggi iaitu 782.48 MPa dan dan kekerasan sebanyak 154.96 hv. Secara keseluruhan, kerja-kerja ini membuktikan bahawa potensi penggunaan beg plastik sampah sebagai pengikat hijau untuk 316L Stainless Steel MIM telah tercapai.
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TABLE OF CONTENTS
TITLE i DECLARATION ii DEDICATION iii ACKNOWLEDGEMENT iv ABSTRACT v ABSTRAK vi TABLE OF CONTENTS vii LIST OF TABLES xi LIST OF FIGURES xvi LIST OF SYMBOLS AND ABBREVIATIONS xxi LIST OF APPENDICES xxiii LIST OF PUBLICATIONS xxiv
CHAPTER 1 INTRODUCTION 1 1.1 Introduction 1 1.2 Background of research 1 1.3 Problem statement 3 1.4 Objectives 4 1.5 Scopes 4 1.6 Significant of study 5
CHAPTER 2 LITERATURE REVIEW 7 2.1 Introduction 7 2.2 Powder and binder system 7 2.3 Mixing 15 2.4 Rheological characterization 23 2.5 Injection molding 24 2.6 Debinding 27 viii
2.7 Sintering 36 2.8 Design of experiment 43 2.9 Summary of literature 45
CHAPTER 3 METHODOLOGY 47 3.1 Introduction 47 3.2 Flowchart 48 3.3 Material selection 48 3.3.1 Stainless steel 316L 49 3.3.2 Waste plastic bag 51 3.3.3 Palm kernel 53 3.4 Mixing 54 3.5 Injection molding 58 3.6 Debinding 60 3.6.1 Solvent 60 3.6.2 Thermal 61 3.7 Sintering 62 3.8 Material characterization 63 3.8.1 Fourier Transform Infrared Spectroscopy 63 3.8.2 Density 64 3.8.3 Flexural strength 65 3.8.4 Thermal 67 3.8.4.1 Thermo Gravimetric 67 Analysis 3.8.4.2 Differential Thermal 69 Analysis 3.8.5 Scanning Electron Micrograph 69 3.8.6 Rheometer 71 3.8.7 X-Ray Diffraction Analysis 73 3.8.8 Hardness 74 3.8.9 Tensile test 75 3.9 Design of experiment 77 3.9.1 Taguchi Method 77 3.9.2 Grey Taguchi Method 77 ix
3.9.3 Analysis of Varience 79
CHAPTER 4 RESULT AND DISCUSSION 81 4.1 Introduction 81 4.2 Binders characterization 81 4.2.1 Fourier Transform Spectroscopy 82 4.2.2 Density determination 83 4.2.3 Determination of degradation temperature 84 4.2.4 Determination of melting temperature 86 4.2.5 Summary of binder's characterization 87 4.3 Characterization of feedstock 88 4.3.1 Binder burnt-out test 88 4.3.2 Density test 89 4.3.3 Morphology structure of feedstock 90 4.3.4 Summary of feedstock characterization 91 4.4 Rheological investigation 91 4.5 Screening test of injection molding by Analysis 99 of Variance 4.5.1 Green density characteristic 103 4.5.1.1 Interaction of A and B 103 4.5.1.2 Interaction of A and C 106 4.5.1.3 Interaction of B and C 108 4.5.2 Green strength characteristic 110 4.5.2.1 Interaction of A and B 110 4.5.2.2 Interaction of A and C 113 4.5.2.3 Interaction of B and C 115 4.5.3 Summary of the screening test of 116 injection molding 4.6 Optimization of injection molding 117 4.6.1 Optimization of injection molding 117 processing condition for obtaining optimal green density 4.6.2 Optimization of injection molding 122 processing condition for obtaining x
optimal green strength 4.6.3 Multiple responses 126 4.6.4 Summary of injection molding 129 optimization 4.7 Screening process of solvent debinding 129 4.8 Optimization of solvent debinding 137 4.8.1 Palm kernel loss 139 4.8.2 Solvent part's density 145 4.8.3 Multiple responses 149 4.8.4 Summary of solvent debinding 152 4.9 Investigation of thermal debinding 153 4.10 Investigation of sintering 157 4.10.1 Morphology observation of final part 159 4.10.2 Chemical composition 162 4.10.3 Phase of sintered part 162 4.10.4 Physical and mechanical properties 164 of sintered part 4.10.5 Summary of sintering 165
CHAPTER 5 CONCLUSION AND RECOMMENDATION 167 5.1 Introduction 167 5.2 Summary and work's contribution 167 5.2.1 Binders characterization 167 5.2.2 Rheological characterization 168 5.2.3 Optimization of injection molding and 168 solvent debinding 5.2.4 Investigation of thermal debinding and 169 sintering 5.3 Recommendations 170 REFERENCES 171 APPENDICES 183 VITAE 217 xi
LIST OF TABLES
2.1 Descriptions of water and gas atomized SS316L 8 2.2 Details of the solid loading, critical solids loading 16 and optimal solids loading 2.3 Trend of mixing temperature applied in MIM 19 2.4 Summary of rheological characteristic 25 2.5 Categories of injection molding parameters 24 2.6 Description of injection molding parameters 25 2.7 Trend of the optimization parameters of the 26 injection molding 2.8 Solvent parameter used by Jamaludin et al. 32 2.9 Debinding parameter of previous study 34 2.9 Sintering parameter optimization applied by 38 Jamaludin et al. 2.10 Binder composition of Omar and Subuki 41 2.11 Sintering performances of SS316L according 41 to previous works 2.12 List of DOE in MIM 43 3.1 Stainless steel 316L powder characteristics 50 3.2 Chemical composition of Stainless steel 316L 50 3.3 Feedstock calculation for 60 vol. % powder loading 55 of three binder composition 4 3.4 Taguchi design for L9 (3 ) 77 4.1 Searched score of the reference spectrum for FTIR 82 analysis of WPB 4.2 Density of binders 84 4.3 Summary of the binder's characterization 88 4.4 The properties of commercial High Density 88 xii
Polyethylene -1 4.5 Comparison of n, E, η and αstv at shear rate 1000 s 98 4 4.6 Injection parameters for L9 (3 ) Taguchi Method 103 4.7 ANOVA table for A x B interaction of 50/50 105 (WPB/PK) for green density 4.8 ANOVA table for A x B interaction of 40/60 105 (WPB/PK) for green density 4.9 ANOVA table for A x B interaction of 30/70 106 (WPB/PK) for green density 4.10 ANOVA table for A x C interaction of 50/50 107 (WPB/PK) for green density 4.11 ANOVA table for A x C interaction of 40/60 107 (WPB/PK) for green density 4.12 ANOVA table for A x C interaction of 30/70 108 (WPB/PK) for green density 4.13 ANOVA table for B x C interaction of 50/50 108 (WPB/PK) for green density 4.14 ANOVA table for B x C interaction of 40/60 109 (WPB/PK) for green density 4.15 ANOVA table for B x C interaction of 30/70 109 (WPB/PK) for green density 4.16 ANOVA table for A x B interaction of 50/50 111 (WPB/PK) for strength 4.17 ANOVA table for A x B interaction of 40/60 112 (WPB/PK) for strength 4.18 ANOVA table for A x B interaction of 30/70 112 (WPB/PK) for strength 4.19 ANOVA table for A x C interaction of 50/50 113 (WPB/PK) for strength 4.20 ANOVA table for A x C interaction of 40/60 114 (WPB/PK) for strength 4.21 ANOVA table for A x C interaction of 30/70 114 (WPB/PK) for strength 4.22 ANOVA table for B x C interaction of 50/50 115 xiii
(WPB/PK) for strength 4.23 ANOVA table for B x C interaction of40/60 116 (WPB/PK) for strength 4.24 ANOVA table for B x C interaction of 30/70 116 (WPB/PK) for strength 4 4.25 Taguchi L9 (3 ) expresses the experimental results of 118 injection molding for optimizing green density 4.26 Response table for S/N ratio for inejction molding 118 when optimizing green density 4.27 ANOVA table of injection molding when optimizing 120 green density 4.28 Estimate of performance (characteristic: larger is 121 better) of injection molding for green density 4.29 Confirmation experiment of injection molding for 121 optimizing the green density 4 4.30 Taguchi L9 (3 ) expresses the experimental results of 122 injection molding process for optimizing green strength 4.31 Response table of S/N ration for injection molding 123 when optimizing green strength 4.32 ANOVA table of injection molding process when 124 optimizing green strength 4.33 Estimate of performance (characteristic: larger 126 is better) of injection molding process for green strength 4.34 Confirmation experiment of injection molding for 126 optimizing the green strength 4.35 Design of experiment of multiple performance for 127 injection molding 4.36 Response table for Grey Relational Grade for 127 injection molding 4.37 ANOVA table of Grey Relational Grade for 128 injection molding 4.38 Data of palm kernel loss at extraction temperature 131 xiv
of 40, 50, 60, 70 and 80 °C when immersed in heptane, hexane and isooctane for 2, 4, 6 and 8 hours of extraction while keeping the S/F ratio of 12:1 as a constant 4.39 Optimization parameters of solvent debinding 139 4.40 Taguchi method expresses the experimental results 139 of solvent debinding for optimizing PK loss 4.41 Response table of S/N ratio for solvent debinding 140 when optimizing PK loss 4.42 ANOVA table of solvent debinding when optimizing 144 the PK loss 4.43 Estimate of performance (characteristic: larger is 144 better) of solvent debinding for PK loss 4.44 Confirmation experiment of solvent debinding for 144 optimizing the PK loss 4.45 Taguchi method expresses the experimental 145 results of solvent debinding for optimizing part's density 4.46 Response table of S/N ratio for solvent debinding 146 when optimizing part's density 4.47 ANOVA table of solvent debinding when 148 optimizing the part's density 4.48 Estimate of performance (characteristic: larger 148 is better) of solvent debinding for part's density 4.49 Confirmation experiment of solvent debinding for 149 optimizing the part's density 4.50 Design of experiment of multiple performances 149 for solvent debinding 4.51 Response table of Grey Relational Grade for 150 solvent debinding 4.52 ANOVA table of Grey Relational Grade for 152 solvent debinding optimization 4.53 Optimum condition of solvent debinding when 152 optimizing multiple performances of PK loss xv
and part's density 4.54 Thermal debinding experimental 154 4.55 Sintering parameter 157 4.56 Sintered part performances 163 xvi
LIST OF FIGURES
2.1 Ideal binder attributes that concluded by German 11 and Bose 2.2 Mixing technique used by Ibrahim et al. 16 2.3 Type of defects: (a) wrinkles, (b) powder binder 27 separation, (c) short shot 2.4 Thermal profile used by Omar et al. 29 2.5 Hexane; (a) before, (b) after, extraction of RWFO 32 2.6 Defect of; (a) crack, (b) warping 34 2.7 Crack formed at high temperature of 60°C 35 2.9 Crack defect experienced by Omar et al. 36 2.10 XRD pattern by Rosip et al. 43 2.11 Material addition sequential in mixing 45 3.1 Flowchart of research 48 3.2 SEM image of SS316L powder 50 3.3 CPVC curve of water atomized SS316L 51 3.4 Palm kernel section cut 54 3.5 Materials that consist of waste plastic bag, 55 Stainless Steel 316L and palm kernel 3.6 Process flow of mixing technique for waste plastic 56 bag, stainless steel 316L and palm kernel 3.7 (a) Addition of waste plastic bag, (b) Addition of 57 metal powder, (c) Addition palm kernel, (d) Feedstock texture after adding up 3 materials 3.8 (a) Plastic Granulator SLM 50FY, (b) Pallet form 58 of feedstock produced after crushing 3.9 (a) Nissei 21 Horizontal Screw Injection Molding 59 xvii
machine, (b) Green part produced 3.10 Change of the solvent after green part underwent 61 solvent debinding 3.11 Argon tube furnace (Model of PTF 14/75/610) 62 3.12 (a) Fourier Transform Infrared Spectroscopy (FTIR) 64 (b) Waste plastic bag sample for FTIR 3.13 Density analytical balance 65 3.14 (a) Universal testing machine, (b) Sample location 67 that subjected to three point bending test 3.15 TGA and DTA machine, Model Linseis 68 3.16 (a) Coating machine, (b) Spark light when 70 coating process completed 3.17 Scanning Electron Micrograph 70 3.18 Capillary rheometer 71 3.19 X-Ray Diffractometer 74 3.20 Micro Hardness Tester Vickers Machine 75 3.21 Tensile machine 76 4.1 Spectra of waste plastic bag/High Density PE 83 and library search (POLYETHYLENE HIGH DENSITY/AP0049) 4.2 TGA curves of graph of palm kernel and waste 85 plastic bag 4.3 Optimum degradation curve of palm kernel and 85 waste plastic bag 4.4 Melting graph of palm kernel 86 4.5 Melting graph of waste plastic bag 86 4.6 TGA curve for 3 pieces of same batch of mixing 89 feedstock 4.7 Density distribution of the feedstock sampling 90 4.8 Scanning Electron Micrograph (SEM) 91 structure of the feedstock 4.9 Correlation of viscosity and shear rate for 93 50/50 (WPB/PK) at; (a) 160 °C, (b) 170 °C and (c) 180 °C xviii
4.10 Correlation of viscosity and shear rate for 95 40/60 (WPB/PK) at; (a) 160 °C, (b) 170 °C and (c) 180 °C 4.11 Correlation of viscosity and shear rate for 96 30/70 (WPB/PK) at; (a) 160 °C, (b) 170 °C and (c) 180 °C 4.12 Correlation of viscosity and temperature for feedstock 97 with different binder compositions 4.13 Green produced when using injection temperature of 100 130 °C, injection pressure of 20 % and injection speed of 20 % 4.14 Green produced when using injection temperature of 100 130 °C, injection pressure of 30 % and injection speed of 30 % 4.15 Green produced when using injection temperature 101 of 140 °C, injection pressure of 30 % and injection speed of 30 % 4.16 Green produced when using injection temperature 101 of 150 °C, injection pressure of 30 % and injection speed of 30 % 4.17 Green produced when using injection temperature 102 of 160 °C, injection pressure of 30 % and injection speed of 30 % 4.18 Green produced when using injection temperature 103 of 160 °C, injection pressure of 40 % and injection speed of 40 % 4.19 A x B interaction of 50/50 (WPB/PK) 104 for green density 4.20 A x B interaction of 50/50 (WPB/PK) 111 for green strength 4.21 Main effects plot (data means) for S/N ratio 119 for injection molding process when optimizing the green density 4.22 Main effects plot (data means) of S/N ratio for 124 xix
injection molding process when optimizing green strength 4.23 Response graph of Grey Relational Grade, 128
ξ(xo, xi) for injection molding 4.24 Morphology (magnification of 5000) of green part; 129 (a) surface and (b) cross section 4.25 Open-pore evolution (magnification of 5000) of 134 green part when immerse in heptane solution within 8 hours of extraction time at temperature of; (a) 40 °C, (b) 50 °C, (c) 60 °C and (d) 70 °C 4.26 Open-pore evolution as a function of 80 °C of 136 extraction temperature when immersed in heptane solution for; (a) 2 hours, (b) 4 hours, (c) 6 hours, and (d) 8 hours 4.27 Mass loss of the binder from green part when 137 immersed in heptane solution at 80 °C for different extraction time 4.28 Main effects plot (data means) of S/N ratio for 141 solvent debinding when optimizing the palm kernel loss 4.29 Main effects plot (data means) of S/N ratio for 147 solvent debinding process when optimizing the part's density
4.30 Response graph of Grey Relational Grade, ξ (xo, xi) 151 for solvent debinding 4.31 Morphology (magnification of 5000) of green part 152 when immerse in heptane solution using S/F ratio of 12:1 within 7 hours at 70 °C; (a) surface and (b) cross section 4.32 Stages in thermal debinding 154 4.33 Brown density and relative density of brown part 155 at various temperature of 500 °C to 600 °C 4.34 Brown part with free-defect 156 4.35 Morphology comparison of SEM image when 156 xx
giving magnification of 2000x for (a) green part, (b) brown part 4.36 Sintered part A 157 4.37 Sintered part B 159 4.38 Sintered part C 159 4.39 Optical microscope observation of sintered part 160 at: (a) sample A, (b) sample B , (c) sample C 4.40 Sample C of; (a) morphology, (b) EDS spectra 162 4.41 XRD diffractograms of; (a) SS316L, (b) SS316L 163 sintered part 4.42 Comparison of part dimension 165
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LIST OF SYMBOLS AND ABBREVIATIONS
ANOVA Analysis of Varience Au Gold BSE Backscattered electron C Carbon ºC Degree Celsius Ca Calcium CCBs Coal combustion by-products CPVC Critical Powder Volume Concentrations CPVP Critical Powder Volume Percentage Cr Chromium Cu Copper DOE Design of experiment DSC Differential scanning calorimetry DTA Differential Thermal Analysis EDX Energy Disperse X-ray EVA Ethylene Vinyl Acetate Fe Iron FOG Fat oil grease FTIR Fourier Transform Infrared Spectroscopy GRC Grey Relational Coefficient GRG Grey Relational Grade HAP Hydroxyapatite HDPE High Density Polyethylene LDPE Low Density Polyethylene MIM Metal injection molding Mn Manganese Mo Molybdenum NHAP Natural Hydroxyapatite Ni Nickel O Oxygen PE Polyethylene PEG Polyethlene Glycol PIM Powder injection molding PK Palm kernel xxii
PL Polystyrene PKO Palm kernel oil PMMA Polymethylene Methacrylate PP Polypropylene PS Palm stearin PW Paraffin wax RSM Response Surface Methodology SA Stearic acid SE Secondary electrons SEM Scanning Electron Microscope SF Sewage fat S/F Solvent to feed ratio Si Silicon S/N Signal to noise SS316L Stainless steel 316L Ti-6Al-4V Titanium alloy TPNR Thermoplastic natural rubber WPB Waste plastic bag TGA Thermal Gravimetric Analyzer XRD X-Ray Diffraction
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LIST OF APPENDICES
APPENDIX TITLE PAGE
A Feedstock calculation 184 B Density of distilled water 186 C Rheology calculation 188 D Mold specification and gatting system 192 E Interaction graph of screening test for injection 194 molding process F Standard in determining the green strength and 203 tensile strength G Tensile graph for sintered part 215
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LIST OF PUBLICATIONS
Journals:
(i) Mohd Halim Irwan Ibrahim and Nur Hafizah Kamarudin. (2016) “Characterization of Waste Plastic Bag as a Novel Binder System and Homogeneity Test for Stainless Steel 316L Metal Injection Molding” International Journal of Engineering and Technology. Vol. 8, pp 2523-2529. (ii) Nur Hafizah Kamarudin and Mohd Halim Irwan Ibrahim. (2017) “Effect of Immerse Temperature and Time on Solvent Debinding Process of Stainless Steel 316L Metal Injection Molding” Materials Science and Engineering. Vol. 165, pp 1-10.
Proceedings:
(i) Nur Hafizah Kamarudin. “Effect of Immerse Temperature and Time on Solvent Debinding Process of Stainless Steel 316L Metal Injection Molding”. 18 December 2016. International Conference on Applied Science (ICAS2016).
1CHAPTER 1
INTRODUCTION
1.1 Introduction
Metal injection molding (MIM) has been recognized to produce small and intricate part in high volume mass production [1-3]. It was being a versatile technology that can be used for a variety of alloys and ferrous metals such as carbonyl iron, iron-nickel, various stainless steels, titanium, tungsten and tungsten heavy alloys, as well as, intermetallic compounds [1,4]. Also, it was becomes a promising technology since it can produce part with a high anti corrosion, having excellent mechanical properties such as strength, hardness and such on [5-6]. In MIM technology, fine metal powders are mixed with suitable thermoplastic binder and formed into the desired shapes [7-8]. The binder provides the flow ability and formability for the fine metal powder during injection molding [5]. Then, both are removed in the next processes that called debinding. The debinding process was purpose to obtain a higher density part. Often, it was conducted by two stages of removal, which are solvent extraction and thermal pyrolysis. After that, the components are sintered in order to form strong metallic components which can be produced by ensuring the pore-free structures is prepared. This process is able to produce the components that approaching closely to its theoretical density value.
1.2 Background of research
Previous research has been done by Agote et al. used the recycled powder in ceramic injection molding [9]. The recycle porcelain was used as a powders with combination
2 of the commercial binders for injection moulding. The waste porcelain application has a remarkable economic and environmental benefit. Regarding to Critical Powder Volume Concentrations (CPVC) and rheological tests, Agote et al. believed that it was possible to used recycled porcelain in the ceramic injection molding instead of using the commercial porcelain that is expensive. Also, Ibrahim et al. worked on the MIM of Stainless Steel 316L that applying the Natural Hydroxyapatite (NHAP) as a minor powder. The bimodal system was well blended with binder system that comprised of commercial Polyethylene and palm stearin [10]. The NHAP was made by synthesizing the waste Tilapia fish bones. As known, the Hydroxyapatite (HAP) is a high biocompatibility materials due to its composition was nearly closed to the human bone. It is a valuable material that have a high cost to effort it. So that, this application of NHAP can reduce the manufacturing cost instead of using the commercial hydroxyapatite which is costly. Thus, they were investigated the process ability of natural HAP and SS316L mixture with commercial binders as a feedstock in MIM. It was claimed that the mixture (feedstock) was successfully injected and it was established as an appropriate feedstock in MIM. While, previous study by Omar et al. applied waste rubber (from rejected gloves) as a binder for MIM technology for 65 vol. % powder loading of Stainless Steel 316L [11]. This previous study found that the feedstock was injectable and the final density was achieving the 99 % of the theoretical values. It was highlighted that the best sintering temperature for obtaining highest mechanical and physical properties was at 1360 °C. Beside that’s, Amin et al. used the environmental substances which is the sewage fat (SF) as one of the binders system with combination of the Polypropylene (PP) and Stainless Steel 316L as a metal powder in metal injection molding [1]. SF has shown a great potential of being used as the major binder component for MIM process after undergo several major test such as mixing homogeneity and rheological properties. Also, Asmawi et al. applied the recycle binder which is waste polystyrene blended with palm kernel oil (PKO) and SS316L [12]. The core idea of previous study is to apply the “Green MIM” which is environmental friendly. As known, the waste polystyrene was greatly used as food packaging. In this regards, this effort was able to reduce disposal problems. 3
Thus, author decided to use waste plastic bag as a binder source of commercial High Density Polyethylene. Author believed that such waste was able to be a more beneficial industrial products. Consequently, this research is focusing on the “Green MIM” in order to reduce the manufacturing cost while decreasing the residue materials and to widen the recycled product. In that sense, the overall aims of this research is to produce the perfect final part by using the waste plastic bag as a binder without any defects and thus contributes to the sustainability of the earth.
1.3 Problem statement
For many years, developing of newly binder system often being the major attention among the researchers in order to enhance the MIM technology while keeping the sustainability of the earth. This efforts has headed to several enhancements which are cost reduction, reduce environmental problems and much more. So far, broad exploration has been present by using natural resources binder, nevertheless only few of them are focusing to the waste resources binder. Nowadays, tons of waste plastic bag are disposed daily and it was getting worse when there are no alternative ways in recycling the waste systematically. Hence, due to this fact had motivated the author to reuse the waste plastic bag and transformed it into a more valuable industrial products such as a backbone binder system in MIM, instead of using the commercial High Density Polyethylene (HDPE). Even though this type of binder is cheap and having high dimensional stability due to the crystallinity structure [13], but it is non- biodegradable material which is difficult to dispose them. As reported by Nemade et al., the waste plastic bag is known as a source of HDPE, therefore in this study the waste plastic bag also used as the backbone binder system in “Green MIM” [14]. Although, the cleaning process of waste plastic bag involve a larger cost compared to use the commercial HDPE, beside can creates awareness among community that waste product can also be considered as valuable things instead of disposing them. Also, author classified this as a delicate effort since the fact shows that HDPE was a non- biodegradable materials, and they might cause air pollution by burning it, yet introducing a disposal problem since it was daily produced in a tons amount and may affect the human health. Indeed, author have limited this research work by excluding 4 the cleaning process of the waste plastic bag. So that, the new and clean waste plastic bag are used. In fact, by implementing of this waste in MIM is not sacrificing its function as a backbone binder system which acts as a shape retention of metal part. Therefore, in this study the waste plastic bag was characterized to analyze the properties of the backbone binder system in MIM which practically having the similarity to the commercial HDPE. Also, in order to determine the flow ability of molten feedstock at a range of temperature, the rheological behavior of the feedstock has been conducted. Practically, Li et al., Adames, Vielma et al., Huang and Hsu, Raza et al., Patil et al., Ibrahim and Kamarudin claimed the effectiveness of commercial HDPE as a backbone binder system in MIM [15-20]. So, how about the waste plastic bag performance?
1.4 Objectives
The objectives of this research are: a. To characterize the binder system via Fourier Infrared Spectroscopy, density and thermal test. b. To evaluate the feedstock performance via rheological test, density, binder burnt-out test and scanning electron micrograph. c. To optimize injection molding process (by evaluating the green density and strength) and solvent debinding process (by evaluating the palm kernel loss and solvent part’s density) via Taguchi Method. d. To measure value of thermal debinding (by considering brown density and morphology) and sintering (by considering final part’s density, tensile strength, hardness, morphology and shrinkage).
1.5 Scopes
The scopes of this study were focused on: a. Palm kernel (PK) and waste plastic bag (WPB) were selected as a binder system and Stainless Steel 316L (SS316L) act as metal powder. 5
b. The new clean WPB is used. Thus, the cleaning process of WPB was not covered in this research. c. The binder composition of 50/50 (WPB/PK), 40/60 (WPB/PK) and 30/70 (WPB/PK) and the constant powder loading of 60 vol. % were used. d. Investigating the mixing procedure of SS316L, PK and WPB. e. Measuring the acceptable volume ratio between binder system and metal powder via rheological test. f. Optimization of green part (by considering green density and green strength) via density test (Archimedes’ Principle) and three-point bending test (according to ASTM Standard SI 10 as being referred in Metal Powder Industries Federation 15). g. Investigating the solvent debinding in order to assess the palm kernel diffusion through Scanning Electron Microscopic (SEM). h. Optimization of solvent debinding (by considering the palm kernel loss and solvent part’s density) via percentage of PK loss and density test (Archimedes’ Principle). i. Evaluation of brown part (part called after thermal debinding) by examining the density (Archimedes’ Principle). Besides that, the microscopic structure of the part is observe by SEM. j. Evaluation of final part (part called after sintering) by determining the parts density (Archimedes’ Principle), tensile test (according to ASTM Standard E8 as being referred in Metal Powder Industries Federation 10), hardness test and shrinkage. Also, the microscopic structure of the part is observe by SEM and optical microscope.
1.6 Significant of study
Nowadays, there are tons of waste plastic bag produced every day. Even, many countries are having a critical disposal problem. It will turn worsen when no alternative ways are conducted for managing the disposal problems. Moreover, it is become a crucial problems because of its great interconnection with the healthy level of humans, animals and plants. 6
Instead of burning (which able to produce damage gases such as carbon monoxide, nitrogen oxide, sulphur oxide, etc.) and having disposal problem, the effort of reuse the waste plastic bag as a binder system in metal injection molding was claimed as a great effort in order to overcome this several problems. In addition, the MIM is relatively new in Malaysia and it gives a great opportunity for those researchers that conducting this research. Perhaps this technology can be exploited in Malaysia in future for the benefits of all industrial sectors. To date, developing of a new locally binder system can be consider as a great concern where it was beneficial to the local industry. Beside that’s, many attempts have been made in developing a new binder system that focused on the cost reduction and shorten production process. Significant with this, the potential uses of palm kernel and waste plastic bag as a binder system for producing a better quality feedstock was investigated. Also, this research demonstrates the great effort for a better finding in the future research purpose to explore the “Green MIM”.
2CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
This chapter details focus on presenting the metal injection molding (MIM) technology specifically on the metal powder properties, binders system, binders and feedstock characterization, optimizing injection molding and solvent debinding processes, investigating thermal debinding and sintering. Overall, this chapter discussed the possible parameters and ways that need to be includes in all the processes in order to fulfil the objectives of this research. The specific requirement and technique of these were being reviewed in this section. Regarding to this, the valuable previous studies related to metal, binders, feedstock characterization and much more, are used as a guideline to conduct the present research successfully.
2.2 Powder and binder system
In MIM, there are various types of metal powder were used by researchers; i.e. Stainless Steel 316L, titanium alloy, tungsten carbide cobalt, high speed steels and such on. The appropriate metal powder selection was done by considering their main interest research either for producing the manufacturing products, biomedical applications and such on. Fundamentally, the powder can be classified into two categories; i.e., water and gas atomized [2,4]. Table 2.1 represents the advantages and disadvantages of both categories. It was stated that the water atomized was produced in irregular shape and the gas atomized was existed in spherical shape. To be highlighted that MIM parts
8 produced by water atomized particles was able to attain high brown strength that desirable in handling it’s sintering. In fact, it was due to the irregular particles cannot flow past each other’s. However, water atomized powder has the low tap density. This was resulting in high sintering shrinkage as well as the tendency of the irregular particles to slightly align during injection molding. This phenomenon tend to cause a high potential for anisotropic shrinkage to occur. Thus, this is a main reason of why the vendors that using water atomized powder was firstly conducting a milling process which purpose to produce a less irregular and slightly rounded powder. Whereas, the gas atomized have the higher packing density compared to water atomized particle. This will lead to requirement of less binder for injection molding and encouranging the low shrinkage and avoiding distortion during sintering. Also, the spherical morphology of this powder guarantees a more isotropic shrinkage as the particles cannot take a preferred direction during injection molding. Beyond, the disadvantages of this kind of powder are its higher price and lower brown strength.
Table 2.1: Descriptions of water and gas atomized SS316L
Types of powder Water atomised Gas atomised Microstructure
Particle shape Irregular Spherical Advantage(s) -Promoting excellent moldability -Higher packing density (needs less -Lower price binder) -Improve shape retention during -Spherical morphology guarantee a debinding and sintering more isotropic shrinkage -Brown strength is fairly high -Low shrinkage and distortion during sintering Disadvantage(s) -Finer particle sizes have a tendency -Higher price to agglomerate into effectively larger -Low brown strength (any handling or particles vibration can destroy the brown part) -Low tap density (resulting in high -Smaller surface area (contributing to a sintering shrinkage) high critical powder loading) -Larger surface area (contributing to a low critical powder loading) -Lower sintered density Ref. 2,4,13,21
9
Previous works shows that the melting temperature of water atomised SS316L which valued by 1371 °C to 1440 °C [21-26]. While, Barreiros et al. highlighted the challenges that must be overcome in Stainless Steel injection molding, especially during sintering [27]. The challenges are listed as below: a. To decrease binder/carbon content in the feedstock. b. To decrease carbon contamination during debinding and sintering. c. To avoid the formation of chromium carbide and presence of precipitation- free zones. d. To avoid grain growth during sintering.
German and Bose claimed that no binder is perfect and thus the binder selection should be crucially considered [7] . Hence, the selection of suitable binder is a prerequisite in producing a good quality of MIM parts successfully. The binder plays an important role as a temporary vehicle for homogeneously packing a powder into a desired shape and it is functioned to retain the shape until the beginning of sintering process [2]. The binder is usually designed as multi-component system [13]. Asmawi et al. was classified the binders into two groups [12]. The first group is the wax/polymer compounds, while another group is the binder that based on the polymer/polymer compounds. Each of the components of wax and polymer functioned specifically. The polymer functioned to impart rigidity to the part when cold, while the wax works in reducing feedstock viscosity. Also, Vielma et al. claimed that the binder system is usually designed as multi- component system [13]. The first component is functioned as a backbone binder. It was usually a thermoplastic that supports and maintains the part shape during all phases prior to the later stages of debinding. While the second component, being commonly a wax that improves the feedstock flow ability and it is removed in the early stages of debinding. During debinding, the open pores created allows the gaseous products that produced by remaining polymer to diffuse out from the part. Else, additive act as a surfactant serving as a bridge between binder and polymer can be added. There are several backbones binder; i.e., polyethylene (PE), polystyrene (PL), ethylene vinyl acetate (EVA), polypropylene (PP), polyethylene glycol (PEG), polymethyl methacrylate (PMMA) among others. Meanwhile, Huang and Hsu claimed that the backbone polymer strongly affects the dimensions and the mechanical 10 properties of the sintered part [17]. Also, previous study recommended a proper selection of backbone binder, such as HDPE, is required to increase the dimensional accuracy and quality of SS316L sintered parts. Then, the wetting agent or wax was functioned to improve material flow ability without sacrificing ease of removing it during debinding process. This binder is firstly remove via solvent debinding, leaving open pores that allowed the gaseous products of remaining polymer to diffuse out of the structure [28]. While, Vielma et al. reported that wax binder often exhibit the powder-binder separation, poor strength and a high potential of distortion during the debinding process [13]. This report was made as a precious guideline as not using the wax as a binder system. Sometimes, it is require to adding up additive (surfactant) which often used is stearic acid [2,13,28,30]. Details on the surfactant are summarized as below: a. Serving as a bridge between the binder and powder (act as lubricant). b. Reducing the friction between powder and machine/die walls. c. Improve dispersion of powder in binder (enhance miscibility between binder components). d. Enhance powder loading and green strength without sacrificing the flow properties of the mixtures. e. Giving a better homogeneity. f. Reduces the contact angle by lowering the surface energy of binder- powder interface minimizing the separation of the binder from the powder- binder mixtures during injection molding. g. Normally, it is present in a low melting temperature and affinity to preferentially adsorb onto powder surfaces, forming a densely thin outer layer on a particle surface which leads to a more homogeneous packing structure.
Next, German and Bose summarized the ideal binder attributes to flow characteristics, powder interaction, debinding process and manufacturing [7]. It is well presented in Figure 2.1. Previous author affirmed that the appropriate binder selection should considered the all requirements in order to conduct a successful MIM technology. They are claiming that the well-defined facts are consistent with another previous works. 11
Flow characteristic Debinding - viscosity below 10 Pa.s at the molding temperature - multiple components with differing characteristics - low viscosity change with temperature during molding - noncorrosive, nontoxic decomposition product - rapid change in viscosity during cooling - low ash content, low metallic content - strong and rigid after cooling - decomposition temperature above molding and mixing - small molecule to fit between particles and avoid orientation temperatures during flow - decomposition before sintering temperature - minimum flow orientation - complete removal as the powder attains structural rigidity
Powder interaction Manufacturing - low contact angle - inexpensive and available - adherence to powder - safe and environmentally acceptable - chemically passive, even under high shear and at high - long shelf life, low water absorption, no volatile components temperatures - not degraded by cycling heating (reusable) - thermally stable during mixing and molding - high lubricity - high strength and stiffness - high thermal conductivity - low thermal expansion coefficient - soluble in common solvents - short chain length, no orientation
Figure 2.1: Ideal binder attributes that concluded by German and Bose [7]
Meanwhile, Arifin et al. reported that a good binder system should be easy to remove in initial removal stage which usually performed by solvent debinding [31]. Where else, the remaining binder remove using next secondary debinding, so called thermal debinding. In this stage, the backbone binder was removed through the pore channels that facilitate by the primary debinding stage. Other than that’s, the ideal binder for MIM should also have the decomposition temperature above mixing and injection molding temperature. Also, it must be lower than the sintering temperature. Vielma et al. and Patil et al. present some specific requirement for a good binder. First, the binder should facilitate the mixing as well as the injection molding [13,32]. Then, the binder should exhibit the low viscosity at injection temperature, rigid and strong after cooling. Other than that’s, the decomposition product should be non-corrosive, non-toxic and have low ash and metallic contents. Also, it required to have the high lubricity, long shelf life, high thermal conductivity and low thermal expansion. In manufacturing view, the binder must be inexpensive and environmentally friendly. Else, Wen et al. stated that the binder should [33]: a. Have a low melting temperature and quickly solidify. b. Have sufficient strength at room temperature (≥4 MPa), a low viscosity (≤10 Pa.s) and a good fluidity at injection temperature. c. Being chemically passive and having the ability to wet the particle with a low contact angle (<5 °) and ideally adhere to the particle surface. 12
d. Being easily remove after shaping and ideally not leaving any residue that potentially causes contamination. The decomposed by products should be non-corrosive and non-toxic. e. Being commercially available and practically affordable.
Basically, the binder systems constituents for injection moulding application can be divide into two categorize, which are comprised of [31]: a. Low molecular weight polymer: has low temperature decomposition. b. High molecular weight polymer: has relatively high temperature decomposition.
Previous study by Vielma et al. applying the virgin HDPE and paraffin wax as a binder system for the manufacturing of bronze and M2 high speed steels parts [13]. Previous author highlighted that HDPE was commonly used as a binder for powder and plastic injection molding. It was found that having HDPE as a binder was able to give various advantages and they are listed as below: a. Able to produce a good feedstock homogeneity. b. Able to achieve a suitable rheological behaviour which promotes the easy moldability. c. The crystalline feature of the HDPE make it perform a good mechanical strength of the part. d. In advances, the crystallinity feature of the HDPE is able to avoid defects such as swelling, bubble and such on.
Else, Patil et al. have been reported that HDPE was a good secondary binder components since that it can produce the high strength part and able to serve as a backbone polymers during debinding [32]. While, Huang and Tsu studied the effect of different backbone polymer, which are High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE), on the properties of Stainless Steel 316L MIM parts [17]. Previous authors confirmed that the better use of HDPE rather than LDPE. There are 5 conclusions drawn from this previous work: a. The use of an LDPE as backbone polymer results in a favourable flow behaviour that able to enhance green part’s surface quality. 13
b. The LDPE/HDPE backbone polymer helps to eliminate the crack defect from sintered part. c. The use of an HDPE backbone polymer was able to stabilize the spiral flow. This achievement promotes the improvement on dimensions, density, and hardness stabilities of the sintered parts. d. HDPE backbone binder provides better dimensional stability than does LDPE. e. HDPE backbone polymer improves the stability of density by 30 % and the hardness by 64 %.
Ibrahim et al. used Polyethylene Glycol (PEG), Polymethyl Methacrylate (PMMA) and stearic acid (SA) as a binder system for water atomized SS316L metal injection molding [34]. It was highlighted that the PEG and PMMA are highly preferable due to its ability to be easily removed through water leaching and thermal debinding. Moreover, the removing of PEG was only required water as the solution to extract it. This effort was considered as a delicate effort since it was promoting the low cost of debinding. While, SA was added to improve the binder properties such as surface wetting, spreading, adsorption, and binder strengthening. Often, the surfactant was often added as an additive that consists of a functional group adhering to the powder surface and an oriented molecular chain extending into the binder. Also, other researchers such as Amin et al., Jamaludin et al., Sulong et al., Chua et al., Chen et al., Rosip et al. and Rajabi et al. used this established binder system regarding the beneficial facts mentioned above [24,35-40]. It is differ with Supriadi et al. which are investigating the three types of binder system which comprised of Ethylene Vinyl Acetate (EVA) based, polypropylene (PP) based and wax-based system [41]. The wax-based are consisting of paraffin wax, bee’s wax and carnauba wax. It was found that the wax-based binder system achieved the lowest viscosity and heat capacity as well as greater pseudo plasticity than other binder systems. Also, the feedstock which was mixed with wax-based binder system performed the smallest deformation. This result inferred that wax-based binder system was appropriate as binder materials for very fine metal powder applied in MIM. Conversely, some of the researches are trying to manipulate the potential of environmental substances for being added as a binder for green MIM. For examples are palm kernel, palm stearin, carnauba wax and beeswax [1]. All of these are affirmed 14 as an environmental friendly binder components and available resource in Malaysia. Instead of that, others are trying to use water soluble binder like PEG where the first stage of debinding was performed using the water leaching. While, another work of Ibrahim et al. used recycle binder of restaurant waste fat and oil (RWFO) as a binder system of water atomized SS316L [42]. RWFO extracted from the grease traps have been long analyzed as a potential feedstock for biodiesel due to its numerous amounts of fatty acids. Asmawi et al. applied 60 wt. % waste polystyrene (PS) and 40 wt. % palm kernel oil (PKO) which well blended with 60 vol. % of water atomized SS316L to produce feedstock for MIM [14]. It was found that the feedstock shows a good homogeneity, which was proven by conducting several tests; i.e., feedstock density, binder burn-out, rheology and Scanning Electron Micrograph (SEM) observation. Omar and Subuki applied 50/60/70 wt. % of palm stearin (PS) and 50/40/30 wt. % of polypropylene (PP) as a binder system to be mixed with 65 vol. % gas atomized SS316L [3]. These authors claimed that the higher PS content provided the better sintered part’s density. Nor et al. applied palm stearin as the binder system in MIM and the study of characterization of the feedstock was conducted [2]. In previous study, Titanium alloy was mixed with 60 wt. % of palm stearin and 40 wt. % of polyethylene [35]. Palm stearin has been reported that having a good attribute as a binder system in MIM [2,43]. Since it was an available resource in Malaysia, palm stearin has been a potential binder system that can be applied in MIM. Moreover, it was consist of fatty acid which functioned as a surface active agent for many binders that are used [2]. Instead of that, the major benefit of the palm stearin is on the ability of modifications that can be done on its chemistry and rheological properties in order to meet the specific requirement of MIM. Meanwhile, Asmawi et al. produced feedstock that consist of 60 vol. % SS316L, 60 wt. % of waste polystyrene and 40 wt. % of palm kernel [12]. It has been reported that the waste polystyrene give a major threat to the environment because this typical plastics are non-biodegradable. Thus, its disposal in landfills is limited due to high cost of the incineration and space. Therefore, the major constraint of applying the waste polystyrene is about the wettability and particle bonding between metal powder and waste polystyrene. Previous author was stressed out several problem of applying the waste polystyrene which are the moldability performance, compatibility 15 issue and its diffusion during thermal degradation as it contains hydrocarbon chain with a phenyl group attached to every other carbon atom.
2.3 Mixing
Mixing is a process where all materials are mixed in purpose to produce homogeneous feedstock. The materials include are consist of the metal powder, primary binder and secondary binder or else the surfactant if necessary, which is depend on the binder system that applied. Knowing the deficiencies in quality of the feedstock cannot be corrected by subsequent processing modification, the feedstock preparation was being the most crucial process in MIM technology. Consequently, it is essential to mix the selected powders and binder in correct composition and technique [44]. So that, the homogeneous feedstock with free of powder-binder separation or particle segregation was able to be obtained. However, failure in dispersing powder uniformly in the binder and unsuitable rheological behaviour of feedstock will cause molding defects such as distortion, cracks, or voids that will lead to non-uniform shrinkage or warping in the sintered parts. Prior study by Supati et al. discovered that there are several parameters such as time, temperature, sequence of material addition, powder size and shape, formulation of binder, shear rate, and powder loading that must be considered for producing good homogeneity of feedstock [44]. In that sense, they was establish the suitable mixing parameters for the research which are consisting of mixing temperature, speed and powder loading, while the other parameters are kept constantly. It was to be highlighted that the appropriate volumetric powder loading and best mixing conditions are essential to determine the former in order to obtain a moldable feedstock since powder loading controls the net dimensional shrinkage and influences the final part quality. Else, German and Bose mentioned that it was need to focus on three things in evaluating the feedstock [7]. The terms are solids loading, critical solid loading and optimal solid loading. Displayed in Table 2.2 was the description of all terms. Noted that the best powder loading could minimised the distortion, defects and shrinkage of the injected part [45].
16
Table 2.2: Details of solid loading, critical solids loading and optimal solids loading
Item Solids loading Critical solid loading Optimal solid loading Definition Volumetric ratio of solid Composition where the Point where the feedstock powder to total volume of particles are tightly packed sufficiently having low the powder and binder together, without external viscosity for molding but [29]. pressure and all space exhibits good particle- between the particles is particle contact to ensure filled with binders shape preservation during [2,43,46]. processing [43]. Description Often expressed on a Crucial in determining the -Less powder than the volume percentage basis range of an appropriate critical solids loading. (value near 60% for PIM) powder loading of a -Estimated based on the [29]. specific metal powder critical powder loading [2,43,46]. [43].
Moreover, German and Bose highlighted that the optimum powder loading should be kept approximately 2 % to 5 % lower than critical loading [7]. This was also supported by Nor et al. and Amin et al. which applying this principle while fixing optimum powder loading in their studies [2,43]. Previous work by Benson et al. reported that the higher powder loading was resulting in a better shape retention. Also, this type of powder loading tend to enhance sintering part, specifically in minimizing sintered part’s shrinkage [47]. However, beyond on the certain powder volume percent, the possibility for producing inhomogeneous feedstock was higher compared to lower powder loading due to the difficulties in mixing process. Other than that’s, the high feedstock viscosity would make it unsuitable for injection molding. However, for the low powder loading, it may result in powder-binder separation under high pressure during molding, and may cause difficulties in obtaining the near full of theoretical density. Thus, it was recommended to keep maximum powder loading while keeping the feedstock viscosity as low as possible. Whereby, previous work by Amin et al. found that the critical powder loading was a vital factor in order to determine rheological properties and inter-particle distance [43]. Also, it was mentioned that the critical powder loading may influences by several powder characteristics; i.e., mean size (coarse or fine), particle shape (spherical or irregular), and particle size distribution (wide, narrow, monomodal or bimodal) and by the binder system. Else, a thorough study by Patil et al. claimed that the fineness was the essential requirement of the powder [32]. It was mentioned that the mean particle size should be less than 30 microns for the MIM technology. Consequent of the achievable 17 fineness, the size distribution should encourage the packing of the powder necessarily. The particle surfaces also should be clean in order to prevent powder agglomeration. Also, Jamaludin et al. stated that the fine powder performed a better densification than the coarse powder, and it was due to the larger surface exhibit in the powder itself [21]. Another concern was related to the molecular weight of the binders. Subuki et al. applied a greater molecular weight of polypropylene (PP), polyethylene (PE), thermoplastic natural rubber (TPNR) as the backbone polymer due to the high contribution towards strength characteristic especially during thermal debinding [30]. Meanwhile, Paraffin wax (PW) and palm stearin (PS) were used as the primary binder system due to their low melting point and viscosity. Next, the mixing technique is able to affect feedstock’s homogeneity [28]. Thus, Table 2.3 shows the mixing technique that was applied by previous researchers. It was built according to several properties; i.e., type of metal powder, powder loading, feedstock composition, type of mixer, mixing speed, mixing temperature and the mixing duration. Details in this was discussed as below. There are several types of machine that are commonly used; i.e., roll mills, high-shear mixers, shear rolls and screw extruders [16,28]. The first two are examples of batch operation’s machine, while the last two are suitable for continuous operation. The machine selection was depending on the details application and materials to be used for feedstock preparation. For examples, a very fine particles which highly probability of agglomeration is requiring the planetary or z-blade mixers. This type of particles need a longer mixing duration instead of not using the preferable mixing machine. While, it was recommended the use of the twin-screw extruders or shear rolls for high volume productions. Thus, it was beneficial for researchers to know the capability and capacity of their mixer to ensure the successful of producing the good feedstock homogeneity. Here was some valuable information that can be summarized accordingly to Table 2.3: a. The mixing temperature was fixed in range temperature of the highest melting temperature and the lowest degradation temperature [7]. b. By referring the highest melting temperature of the binders, the mixing temperature was fixed by adding up about 3.16 °C to 38.50 °C. c. So, it was suggested to the researchers to adding up the aforementioned range in purpose to conduct a successfully mixing. 18
Supati et al. claimed that the shear rate was controlled by speed of the mixing blades [44]. Increase of mixing speed promotes increase of feedstock homogeneity. The temperature rise due to the amount of energy input by mixer blades was converted to heat during shearing of the mixer increase with increase of speed. This was particularly detrimental to low melting point binder constituents, if any. Again highlighted, the mixing temperature should be in the range of lowest degradation temperature and the highest melting temperature of the binders [2,13,43]. A higher mixing temperature is required to enhance the deagglomeration process that cause by the higher binder viscosity [44]. Nevertheless, too high mixing temperature will cause the powder-binder separation and heterogeneous feedstock are produce. Ibrahim et al. works on the water atomized SS316L powder for micro metal injection molding which focusing on the rheological optimization [48]. As known a good mixing technique was too important for ensuring the good homogeneity and flow ability of the molten feedstock during mixing and injection molding process. Thus, previous study highlighted the mixing technique that was applied. It was presented in Figure 2.2 below.
Mix SS316L with SA for 5 min at Mix PMMA with acetone by using ratio room temperature of; 1gm of PMMA: 4 ml of acetone
Adding up Mix both mixture for 15 minutes at room acetone will temperature Binder composition: exhibit low 73 % Polyethylene viscosity and Glycol (PEG) high shear rate 25 % Polymethyl compared to Methacrilate (PMMA) other feedstock Adding up PEG and mix for 15 minutes at 2 % Stearic Acid (SA) room temperature
Mix the all materials at 70 °C within 1 hour Figure 2.2: Mixing technique used by Ibrahim et al. [48]
Table 2.3: Trend of mixing temperature applied in MIM
Item(s) Powder Metal Binder Melting Degradation Mixing loading Density Speed Duration powder system temperature temperature temperature Machine Year Ref. (vol. %) (g/cm3) (rpm) (minutes) (°C) (°C) (°C) Start/end HDPE 129.78 0.96 (470/550) 58 Start/end Haake Rheocord Alumina PW 56.97 0.91 140.00 40 30 2008 13 (200/400) 252p
Start/end SA 71.05 0.94 (200/400) Start/end PS 61.00 - 63,65,67, (288/463) SS316L 150.00 Sigma blade 50 120 2009 2 69 Start/end PE 127.00 - (390/502) LLDPE 130.00 - 0.92 Brabender SS316L - PS 61.31 - 0.89 135.00 Plastograph EC 25 60 2011 49 SA 69.60 - 0.85 (rotary mixer) Start/end ZK60 PS 52.00 0.89 (288/463) Magnesium 64 150.00 Sigma blade - 60 2012 50 Start/end Alloy LDPE 130.00 0.91 (389/501) Start/end PS 61.42 - (398.5/598.8) WC-Co 61 130.00 - - - 2014 35 Start/end PE 126.84 - (389.6/501.6) PS 185.00 - 0.91 Brabender SS316L 60 190.00 Plastograph EC 30 60 2014 12 PKO 30.00 - 0.91 (rotary mixer) PP 170.00 - - Brabender SS316L 60 175.00 Plastograph EC 30 60 2014 1 SF 60.00 - - (rotary mixer)
20
Table 2.2 (continued)
PS 61.60 470.40 - 65 PE 125.50 502.60 - Brabender SS316L PP 171.10 486.60 - 190.00 50 120 2014 30 Plasticorder PW 59.80 498.60 - TPNR 121.60 362.20 - Palm kernel 30.00 363.40 0.91 Brabender oil SS316L - 190.00 Plastograph EC 30 60 2016 51 Waste (rotary mixer) polysty 185.40 324.30 1.05 rene PS 70.00 0.89 Brabender SS316L 63 - 150.00 Plastograph EC - - 2017 52 LDPE 111.50 0.92 (rotary mixer)
Whereas, Asmawi et al. mixed 60 vol. % Stainless Steel 316L with 60 wt. % of waste polystyrene and 40 wt. % of palm kernel oil [12]. The materials are mixed at 190°C with the rotational speed of 30 rpm using Brabender Plastograph EC within 1 hour. It is worth to know that the mixing temperature of 190 °C was selected in order to prevent the binder constituent from degrade since it was within the highest melting temperature (185 °C) and the lowest degradation temperature of the binder system (324 °C). The waste polystyrene and palm kernel oil are fully melted at this temperature. The mixing was started by adding up the waste polystyrene and mix it for 10 minutes. Next, the metal powder was added and followed by palm kernel oil, and they are mixed for 60 minutes. Later, the blended feedstock was taken out from the mixer and leave to cool at room temperature before being crushed into small pallet using Granulator machine. Then, previous study by Subuki et al. mixed 65 vol. % SS316L with various type of binder and composition [30]. Three different binder systems are prepared; i.e., feedstock A (70 % PS/30 % PE), feedstock B (70 % PS/30 % PP), feedstock C (40 % TPNR/60 % PW). Owing to greater molecular weight, PP, PE and TPNR are selected as the backbone polymer. It was expected that this binder system was able to attain a high strength part especially during thermal debinding. While the PW and PS are used as the primary binder and the major fraction in binder system due to their low viscosity and melting point. The mixing was conducted by using Brabender Plasticorder at 190 °C for about 2 hours with a rotational speed of 50 rpm. First, a quarter amount of polymer is loaded into the mixer bowl. Then, the powder was loaded gradually together with the remaining binder. The homogenous feedstock was assumed to be produced regarding to the mixing torque yielded a stable and consistent value. Also, a thorough knowledge of the material properties of the developed binder system and feedstock are essential for successful of the powder injection molding (PIM) [8]. In view of the above, characterization of a developed binder system and feedstock has been reported as a compulsory process. It is supported by Subuki et al. which claimed that the powder, binders and feedstock characterization was one of the most important steps in metal injection molding technology because the subsequent processes (molding, debinding, and sintering) were depend on the properties of the binders and feedstock [30]. It have been too crucial because the part defects cannot be controlled in the subsequent processing steps [43]. So, the all steps involved in
22 characterisation of feedstock is needed to be monitored since they will affect the final product [2,53]. Nor et al. stated that the metal powder and binder’s characterization are being the crucial step in purpose to understand the whole process of MIM [2]. Aforementioned, the initial properties of the feedstock will dictate the final properties of the sintered part. Then, there are some characterization tests that have been conducted in this study. There are density test, scanning electron microscopy (SEM), critical powder volume percentage (CPVP), energy dispersive X-ray diffraction (XRD), thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC). While, Amin et al. studied the potential used of sewage fat as binder in MIM technology [1]. The sewage fat (SF) is also called as fat oil grease (FOG). The previous study found that the binder formulation of 40/60 (SF/PP) was the best binder composition according to homogeneity and rheological test. According to Amin et al., the cemented carbide (WC-Co) was mixed at an optimal powder loading (within 2 % to 5 % lower than CPVP value which is 65 %) with 60 % palm stearin and 40 % polyethylene [43]. It has been reported that the optimal powder loading of the powder was valued by 61 %. The result was based on rheological characterization that has been conducted. It was consisted of several properties, i.e., pseudoplastic behaviour, powder law index (n) lower than 1, low activation energy and highly moldability index. Whereas, Amin et al. also conducted the study of mixing homogeneity and rheological characterization for optimal binder formulation for metal injection molding of Stainless Steel 316L with two binder formulation of 70/30 and 60/40 between Polypropylene (PP) and Sewage Fat (SF) [1]. The best rheological behaviour are studied according to viscosity, shear rate and powder law index. Thus, the test shows that the both formulations are exhibited pseudoplastic behaviour. After homogeneity test and rheological test, it show that the sewage fat has shown a great potential for being used as the major binder component for MIM process. Previous author found that 60/40 (PP/SF) was exhibit the ideal binder composition according on rheological analysis and mixing homogeneity. Other than that’s, Vielma et al. conducted the homogeneity test using three methods [13]. First, the torque value was applied to measure the resistance of rotor blades. Probably, when a steady state of the torque reached, it can be said that the 23 uniform mixing of the feedstock was achieved and leads in producing the good feedstock homogeneity. Next, the density for three crushed feedstock of the same batch of mixing session are measured. The small deviation from the mean density indicates the good feedstock’s homogeneity. Third, the capillary rheometer was used. The pressure fluctuation was being studied through a small capillary while keeping the shear rate as a constant. They stated that the small variation of testing time and capillary pressure indicates the homogeneous feedstock, whereby the high fluctuations indicates heterogeneous distribution of power in the binder. Details stated are when the minimum and maximum pressure represent binder rich and solid-rich feedstock portions, respectively.
2.4 Rheological characterization
Generally, the rheological characterization was conducted to investigate the rheological behavior existed in a flow material. Previous work by Agote et al., Nor et al., Ibrahim et al. and others claimed that the pseudoplastic/shear thinning behavior was required in MIM technology [2,9,34]. Technically, the pseudoplastic exhibit when the viscosity was decrease as the increase of shear rate. At the meantime, the viscosity must be in range of 10 Pa.s to 1000 Pa.s [13,54-56]. Else, the rheological behavior of dilatant was extremely avoided in MIM [45,54]. This undesired behavior could be indicate when the viscosity is increase with the shear rate. Noted that the existing of this undesirable behavior was indicating the powder binder separation occurred during the flow of molten feedstock. While, Ibrahim et al. claimed that it was desired in MIM for the viscosity should decreased fast with the increase of shear rate during injection molding [54]. The high shear sensitivity is important to produce a complex, delicate and miniaturized parts. Also, they found that the lower value of E stated that any small fluctuation of temperature during molding will not result in sudden viscosity change. Noted that a sudden change could cause undue stress concentrations in molded parts, resulting in cracking and distortion. Table 2.4 summarized the rheological characteristic which desired in MIM.
24
Table 2.4: Summary of rheological characteristic
Item(s) Symbol Unit Requirement Description(s) Ref. -Indicates the degree of shear Flow sensitivity [2,43,49, behaviour n - Lower - Lower n, faster the viscosity 54] exponent changes with shear rate -less than 1 (desired for MIM) -Expresses effect of temperature [43,54] Flow on feedstock viscosity activation E kJ/mole Lower -Lower E indicates that the energy viscosity is not so sensitive to temperature variation -Demonstrate the quality of the [43,54] Rheological - Higher feedstock for injection molding in index stv terms of flowability
2.5 Injection molding
At the injection molding stage, the part is formed into a desired shape, so called by green part, which was produced by applying the specific range of heat and pressure. The feedstock is heated at a temperature above the melting points of binders but below the highest degradation temperature of binders [32]. Noted that, the too high in feedstock viscosity resulting in parts defect during injection molding. Previous works by Kamaruddin et al. and Chua et al. applied Taguchi Method in order to optimize the injection molding [37,57]. The listed molding parameters that have been included in their studies are nozzle temperature, front temperature, injection pressure, speed, cooling time, holding time, injection time and much more . Berginc et al. had classified the molding parameters into two categories which are comprised of controlled and uncontrolled parameters [58]. Table 2.5 shows both types of parameter, while displayed in Table 2.6 is the injection molding parameters indication that prepared for briefly elaborates some injection parameters.
Table 2.5: Categories of injection molding parameters [58]
Controlled parameters Uncontrolled parameters Material temperature Die shape Injection speed Pressure drop Duration of holding pressure Mold -temperature variation Holding pressure Mixture homogeneity Cycle time Material heating due to friction Mold temperature Moisture in the material, etc.
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