PROCESSABILITY OF WASTE PLASTIC BAG AS A NOVEL … · Metal injection molding is the most cost...

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

iii

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

iv

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

responses. The combination of orthogonal array Taguchi L9 (34) 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.

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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

tersebut. Gabungan Taguchi L9 (34) 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.

vii

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

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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

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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

3.4 Taguchi design for L9 (34) 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

4.5 Comparison of n, E, η and αstv at shear rate 1000 s-1 98

4.6 Injection parameters for L9 (34) Taguchi Method 103

4.7 ANOVA table for A x B interaction of 50/50 105

(WPB/PK) for green density





4.10 ANOVA table for A x C interaction of 50/50 107






4.13 ANOVA table for B x C interaction of 50/50 108







(WPB/PK) for strength












xiii


4.23 ANOVA table for B x C interaction of40/60 116




4.25 Taguchi L9 (34) 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

4.30 Taguchi L9 (34) 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

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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


40/60 (WPB/PK) at; (a) 160 °C, (b) 170 °C and

(c) 180 °C


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 %



speed of 30 %



speed of 30 %



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


solvent debinding when optimizing the palm

kernel loss


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

xxi

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

xxiii

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

xxiv

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

-Lower price

-Improve shape retention during

debinding and sintering

-Brown strength is fairly high

-Higher packing density (needs less

binder)

-Spherical morphology guarantee a

more isotropic shrinkage

-Low shrinkage and distortion during

sintering

Disadvantage(s) -Finer particle sizes have a tendency

to agglomerate into effectively larger

particles

-Low tap density (resulting in high

sintering shrinkage)

-Larger surface area (contributing to a

low critical powder loading)

-Lower sintered density

-Higher price

-Low brown strength (any handling or

vibration can destroy the brown part)

-Smaller surface area (contributing to a

high critical powder loading)

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

- viscosity below 10 Pa.s at the molding temperature

- low viscosity change with temperature during molding

- rapid change in viscosity during cooling

- strong and rigid after cooling

- small molecule to fit between particles and avoid orientation

during flow

- minimum flow orientation

Powder interaction

- low contact angle

- adherence to powder

- chemically passive, even under high shear and at high

temperatures

- thermally stable during mixing and molding

Debinding- multiple components with differing characteristics- noncorrosive, nontoxic decomposition product- low ash content, low metallic content- decomposition temperature above molding and mixing temperatures- decomposition before sintering temperature- complete removal as the powder attains structural rigidity

Manufacturing- inexpensive and available- safe and environmentally acceptable- long shelf life, low water absorption, no volatile components- not degraded by cycling heating (reusable)- 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

powder to total volume of

the powder and binder

[29].

Composition where the

particles are tightly packed

together, without external

pressure and all space

between the particles is

filled with binders

[2,43,46].

Point where the feedstock

sufficiently having low

viscosity for molding but

exhibits good particle-

particle contact to ensure

shape preservation during

processing [43]. Description Often expressed on a

volume percentage basis

(value near 60% for PIM)

[29].

Crucial in determining the

range of an appropriate

powder loading of a

specific metal powder

[2,43,46].

-Less powder than the

critical solids loading.

-Estimated based on the

critical powder loading

[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

room temperature

Mix PMMA with acetone by using ratio

of; 1gm of PMMA: 4 ml of acetone

Mix both mixture for 15 minutes at room

temperature

Adding up PEG and mix for 15 minutes at

room temperature

Mix the all materials at 70 °C within 1 hour

Adding up

acetone will

exhibit low

viscosity and

high shear rate

compared to

other feedstock

Binder composition:

73 % Polyethylene

Glycol (PEG)

25 % Polymethyl

Methacrilate (PMMA)

2 % Stearic Acid (SA)

Figure 2.2: Mixing technique used by Ibrahim et al. [48]

Table 2.3: Trend of mixing temperature applied in MIM

Metal

powder

Powder

loading

(vol. %)

Binder

system

Item(s)

Melting

temperature

(°C)

Degradation

temperature

(°C)

Density

(g/cm3)

Mixing

temperature

(°C)

Machine Speed

(rpm)

Duration

(minutes) Year Ref.

Alumina

58

HDPE 129.78 Start/end

(470/550) 0.96

140.00 Haake Rheocord

252p 40 30 2008 13 PW 56.97

Start/end

(200/400) 0.91

SA 71.05 Start/end

(200/400) 0.94

SS316L 63,65,67,

69

PS 61.00 Start/end

(288/463) -

150.00 Sigma blade 50 120 2009 2

PE 127.00 Start/end

(390/502) -

SS316L -

LLDPE 130.00 - 0.92

135.00

Brabender

Plastograph EC

(rotary mixer)

25 60 2011 49 PS 61.31 - 0.89

SA 69.60 - 0.85

ZK60

Magnesium

Alloy

64

PS 52.00 Start/end

(288/463) 0.89

150.00 Sigma blade - 60 2012 50

LDPE 130.00 Start/end

(389/501) 0.91

WC-Co 61

PS 61.42 Start/end

(398.5/598.8) -

130.00 - - - 2014 35

PE 126.84 Start/end

(389.6/501.6) -

SS316L 60

PS 185.00 - 0.91

190.00

Brabender

Plastograph EC

(rotary mixer)

30 60 2014 12 PKO 30.00 - 0.91

SS316L 60

PP 170.00 - -

175.00

Brabender

Plastograph EC

(rotary mixer)

30 60 2014 1 SF 60.00 - -

20

Table 2.2 (continued)

SS316L

65

PS 61.60 470.40 -

190.00 Brabender

Plasticorder 50 120 2014 30

PE 125.50 502.60 -

PP 171.10 486.60 -

PW 59.80 498.60 -

TPNR 121.60 362.20 -

SS316L -

Palm

kernel

oil

30.00 363.40 0.91

190.00

Brabender

Plastograph EC

(rotary mixer)

30 60 2016 51 Waste

polysty

rene

185.40 324.30 1.05

SS316L 63 PS 70.00

- 0.89

150.00

Brabender

Plastograph EC

(rotary mixer)

- - 2017 52 LDPE 111.50 0.92

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.

Flow

behaviour

exponent

n - Lower

-Indicates the degree of shear

sensitivity

- Lower n, faster the viscosity

changes with shear rate

-less than 1 (desired for MIM)

[2,43,49,

54]

Flow

activation

energy

E kJ/mole Lower

-Expresses effect of temperature

on feedstock viscosity

-Lower E indicates that the

viscosity is not so sensitive to

temperature variation

[43,54]

Rheological

index stv - Higher

-Demonstrate the quality of the

feedstock for injection molding in

terms of flowability

[43,54]

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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