Sintering Characteristics Of Injection Moulded Components Using Waste Rubber Biopolymer.

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

SINTERING CHARACTERISTICS OF INJECTION MOULDED

COMPONENTS USING WASTE RUBBER BIOPOLYMER

This report submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials) with Honors.

NURHASHIMA BINTI SHAFIEE

FACULTY OF MANUFACTURING ENGINEERING 2010

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SINTERING CHARACTERISTICS OF INJECTION MOULDED COMPONENTS USING WASTE RUBBER BIOPOLYMER

NURHASHIMA BINTI SHAFIEE

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DECLARATION

I hereby, declared this report entitled “Sintering Characteristics of Injection Molded Components using Waste Rubber Biopolymer” is the results of my own research except

as cited in references.

Signature : ……….

Author’s Name : NURHASHIMA BINTI SHAFIEE

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APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the degree of Bachelor of Manufacturing Engineering (Engineering Materials) with Honours. The member of the supervisory

committee is as follow:

……… (EN. JEEFFERIE B ABD RAZAK)

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ABSTRACT

This research aims to study and investigate the characteristic of injection molded gas atomized stainless steel powder by using the new developed innovative binder system. The optimum heating rate and soaking period used for the sintering process are studied, extensively in this research. The main raw materials used in this research are 17-4 PH stainless steel powder with the new biopolymer binder system which consists of paraffin wax (PW), stearic acid (SA), waste rubber (WR) and polyethylene (PE) mixture. Typically, the mean particles size distribution of 17-4 PH stainless steel powder is less than 22µm at D50. The feedstock that having 65 vol. % of metal powder were injection molded into the tensile test bar. The rapid two stage debinding process that involves the solvent extraction and thermal pyrolisis were conducted to remove the binder. Nine groups of the specimens were then sintered under the vacuum atmosphere in the temperature of 1360 0C at various combinations of heating rates and soaking periods. The specimens primarily were tested for the Critical Properties Analysis – Tensile Test. The highest tensile strength for each group of specimens was selected for further analyses of the morphological effects. The fractures surface microstructure of tested specimens were observed as to understand the fracture behavior, the grain and porosity distribution and the surface morphology and its significant correlation to the resulted mechanical properties. The results showed that the combination of 5 0C/min heating rate and 60 minutes of soaking period gives the higher density value, higher tensile strength, less porosity and homogenous grains distribution of the sintered specimens. Through this research, the development of the injection molding parts could be enhanced for the automotive applications through the establishment of innovative waste rubber new binder system for the metal injection molding parts fabrication.

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ABSTRAK

Matlamat penyelidikan ini adalah untuk mengkaji dan menyiasat ciri-ciri serbuk keluli tahan karat dibentuk melalui pengatoman gas dan diacu suntik dengan menggunakan bahan pengikat inovatif dan terbaru yang telah dibangunkan. Kadar pemanasan dan masa rendaman terbaik yang telah digunakan sewaktu proses persinteran juga dikaji secara mendalam di dalam penyelidikan ini. Bahan utama yang digunakan di dalam penyelidikan ini adalah serbuk keluli tahan karat 17-4 PH dengan bahan pengikat biopolimer yang terdiri daripada campuran lilin parafin (PW), asid stearik (SA), sisa buangan getah (WR) dan polietilina (PE). Secara tipikal, taburan saiz partikel bagi serbuk keluli tahan karat 17-4 PH adalah kurang daripada 22 µm pada D50. Bahan suapan yang mengandungi serbuk logam dengan 65 % isipadu disuntik ke dalam acuan yang berbentuk bar ujian tegangan. Dua peringkat pantas bagi proses penyahikatan yang melibatkan proses pengekstrakan larutan dan pirolisis terma dijalankan untuk menyingkirkan bahan ikatan. Kemudian, sembilan kumpulan spesimen-spesimen ini disinterkan di bawah atmosfera vakum pada suhu 1360 0C, menggunakan pelbagai kombinasi kadar pemanasan dan masa rendaman. Pada permulaannya, spesimen-spesimen diuji pada Analisa Sifat Kritikal iaitu ujian tegangan. Nilai kekuatan tegangan yang paling tinggi akan dipilih bagi setiap kumpulan spesimen untuk analisa lanjutan terhadap kesan morfologi. Struktur mikro pada permukaan spesimen yang telah retak dikaji dalam memahami sifat rekahan, taburan ira dan keliangan, dan morfologi permukaan serta hubungannya terhadap keputusan kajian sifat mekanik. Keputusan menunjukkan kombinasi antara 50C/min kadar pemanasan dan 60 minutes masa rendaman memberi nilai ketumpatan dan kekuatan tegangan yang tinggi, keliangan yang sedikit dan taburan ira yang homogen dalam spesimen yang disinter. Melalui kajian ini, pembangunan penyediaan produk logam teracu suntikan ditambahbaik bagi aplikasi kejuruteraan struktur melalui pemantapan bahan pengikat sisa buangan getah yang terbaru dan inovatif bagi fabrikasi produk logam teracu suntikan.

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DEDICATION

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ACKNOWLEDGEMENTS

First and foremost, I would like to express my gratitude to Allah S.W.T for giving me a good health and blessing throughout the duration of my final year project (PSM I & II). Behind to all of this, I also gratefully acknowledge to my supervisor, Mr. Jeefferie bin Abd Razak for his knowledge, encouragement, full supports, by providing me enormous guidance and ideas for my research programme. I also would like to express my special thanks to my co-supervisor Dr. Mohd Afian bin Omar and researcher, Mrs. Nor Syakirah bt Abdullah and Mr. Muner bin Taha from Advanced Materials Research Centre (AMREC), SIRIM, Kulim Kedah by giving me a chance to carry out my final year project at AMREC and providing me the title of my final year project. Not forgetting to all my mates especially to Siti Faizah, Nikmah, Nur Farizan, Nur Rifhan and Nurul Aina, thanks for your all kindness of delivering ideas and motivation that necessary for me to complete this report. Besides those mentioned, I also would express my gratitude with highly appreciation and dedication to my family because without them, I won’t be here to complete my study in Universiti Teknikal Malaysia Melaka.

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TABLE OF CONTENT

Abstract i

Abstrak ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Tables viii

List of Figures ix

List of Abbreviations xii

List of Symbols xiii

1.0 INTRODUCTION

1.1 Background of Study 1

1.2 Problems Statements 3

1.3 Objectives 4

1.4 Scope of Study 5

2.0 LITERATURE REVIEW

2.1 Powder Metallurgy 6

2.1.1 Metal Injection Molding 7

2.2 Feedstock 8

2.3 Metal Powder 10

2.3.1 17-4 PH Stainless Steel 12

2.4 Binder System 13

2.4.1 Paraffin Wax 15

2.4.2 Thermoplastic Waste Rubber 15

2.4.3 Stearic Acid 16

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2.6 Sintering Practice 18

2.6.1 Effect of Sintering Temperature and Atmosphere 19

2.7 Density Measurements 20

2.8 Mechanical Properties Analysis 21

2.9 Morphological Study 22

3.0 METHODOLOGY

3.1 Introduction 24

3.2 Materials 25

3.2.1 17-4 PH Stainless Steel Powder 25

3.2.2 Binder 27

3.3 Specimens Preparation 27

3.3.1 Mixing 28

3.3.2 Injection Molding 29

3.3.3 Debinding 30

3.3.3.1Solvent Extraction 30

3.3.3.2Thermal Pyrolisis 31

3.3.4 Sintering 32

3.4 Physical Properties Evaluation 34

3.4.1 Shrinkage Test 34

3.4.2 Density Test 35

3.5 Mechanical Properties Evaluation 36

3.5.1 Tensile Test 37

3.5.2 Hardness Test 37

3.6 Morphological Observation 38

3.6.1 Scanning Electron Microscope (SEM) 39

3.6.2 Optical Microscope 39

4.0 RESULTS AND DISCUSSION

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vii

4.2 17-4 PH Stainless Steel Powder Characterization 41

4.3 Observation on Sample Preparation Process 42

4.3.1 Injection Molding 43

4.3.1.1SEM Observation of Green Body 45

4.3.2 Debinding 47

4.3.2.1Study on Kinetic Solvent 48

4.3.3 Sintering 51

4.4 Shrinkage Observation 51

4.5 Critical Property Analysis - Tensile Test 55

4.5.1 Fracture Surface Morphology of Tensile Specimens 58 4.5.2 Optical Microstructure of Tensile Specimens 63

4.6 Density 66

4.7 Hardness 67

5.0 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 69

5.2 Recommendation 71

REFERENCES 72

APPENDIX

A Gantt chart PSM I B Gantt chart PSM II

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viii

LIST OF TABLES

2.1 Chemical composition of alloy 17-4pH stainless steel 12 2.2 Typical mechanical properties of 17-4pH stainless steel at high

temperature [tempering temperature = 6000C] 13

3.1 Chemical composition of 17-4 pH stainless steel powder 28

3.2 Varied heating rate and soaking period 33

4.1 Dimensions and shrinkage percentage of samples 53

4.2 Tensile properties of each specimen groups 56

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LIST OF FIGURES

2.1 Flow chart illustrating major processing steps in MIM process 7

2.2a Feedstock pellets for molding 9

2.2b Granulate feedstock for molding 9

2.3 Classification scheme for the various ferrous alloys 10 2.4 Conceptual visualization of interplay between the chemistry,

properties, microstructure and processing for PIM operation 18

2.5 Density measurement apparatus 21

2.6a The SEM micrograph of a sintered specimen’s fractured surface; the mid-part of the specimen made with the optimal sintering and molding

conditions 23

2.6b The SEM micrograph of a sintered specimen’s fractured surface; the mid-part of the specimen made with the worst sintering and molding

conditions (magnification 400x) 23

2.7a SEM micrograph of feedstock without extrusion 23

2.7b SEM micrograph of feedstock with extrusion 23

3.1 17-4 PH stainless steel metal powders 25

3.2 Flow chart of methodology 26

3.3 Binder materials 27

3.3a Polyethylene 27

3.3b Waste rubber 27

3.3c Strearic acid 27

3.3d Paraffin wax 27

3.4 Z-blade mixer 29

3.5 Compounding process 29

3.5a Feedstock paste at 100 minutes 29

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3.6 MCP HEK-GMBH vertical injection molding 30

3.7 Solvent extraction process 31

3.7a Water bath model Memmert 31

3.7b Immersion of green samples in heptanes using water bath 31

3.8 Lynn furnace 32

3.9 The heating profile of thermal pyrolisis process 32

3.10 Multi-atmosphere sintering furnace 33

3.11 The heating profile of sintering operation 34

3.12 Measuring of sintered specimens 35

3.13 Density measurement 36

3.13a Specific gravity meter 36

3.13b Specimen weighted in the air 36

3.13c Specimen weighted in water 36

3.14 Universal Testing Machine (UTM) 37

3.15 Microvickers hardness tester 38

3.16 Scanning Electron Microscope (SEM) 39

3.17 Optical Microscope (OM) 40

4.1 SEM micrograph of the 17-4PH stainless steel used in the feedstock

preparation 42

4.2 Granulated feedstock 43

4.3 Green body 44

4.4 Dimension of green body 45

4.5 The scanning electron micrograph of the green body after injection

molding; (a) fracture and (b) outer surface 46

4.6 Graph of main binder removal percentage versus time with the

morphological changes at fracture surface of sample at; (a) 10 minutes, (b) 60 minutes, (c) 120 minutes and (d) 300 minutes 50

4.7 Sintered sample 51

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4.9 Shrinkage percentage for different group of sample 54 4.10 Tensile strength of sample at different sample set 56 4.11 Tensile modulus of sample at different sample set 57 4.12 The SEM images of fractured surface of the samples that used 50C/min

of heating rate for three different soaking period; (a) 30 minutes,

(b) 60 minutes and (c) 120 minutes 60

4.13 The SEM images of fractured surface of the samples that used 100C/min of heating rate for three different soaking period; (a) 30 minutes,

(b) 60 minutes and (c) 120 minutes 61

4.14 The SEM images of fractured surface of the samples that used 150C/min of heating rate for three different soaking period; (a) 30 minutes,

(b) 60 minutes and (c) 120 minutes 62

4.15 Actual density of 17-4PH sintered samples for different heating rates and

soaking periods 66

4.16 Actual hardness of 17-4PH sintered samples for different heating rates

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LIST OF ABBREVIATIONS

HDPE - High Density Polyethylene

LDPE - Low Density Polyethylene

Min - minute

MIM - Metal Injection Molding

MPa - Mega Pascal

NiB - Nickel Boride

NR - Natural Rubber

OM - Optical Microscope

PE - Polyethylene

PIM - Powder Injection Molding

PM - Powder Metallurgy

PP - Polypropylene

PR - Powdered Rubber

PW - Paraffin Wax

SA - Stearic Acid

SBR - Styrene Butadiene Rubber

SEM - Scanning Electron Microscope

T - Temperature

Tg - Glass Transition Temperature

TPWR - Thermoplastic Waste Rubber

UTM - Universal Testing Machine

WR - Waste Rubber

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xiii

LIST OF SYMBOLS

g/cm3 - gram per centimeter cube

mm - millimeter

rpm - revolution per minutes

Vol. % - percentage of volume

Wt. % - percentage of weight

µm - micron meter

µPIM - Micro Powder Injection Molding 0

C - Degree Celsius

C/min - Degree Celsius per minute

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CHAPTER 1 INTRODUCTION

1.1 Background of Study

Nowadays, major challenges in Powder Metallurgy (PM) research field are design complexity and flexibility of the fabrication process, with a low cost consideration. Researchers have tried to propose the best solution to fulfill the extensive industrial requirement. There are many improvisation and development in the field of powder metallurgy. One of the new invented methods is to introduce the very recent and innovative powder metallurgy route by combining the concept of plastic injection molding together with the utilization of the metal powder feedstock. Such a unique method mentioned is the Metal Injection Molding (MIM). Basically, this process is more complicated compared to the plastic injection molding process. The process is based on the use of fine powder particles mixed with the small quantity of wax binders and/or thermoplastic polymer to form the feedstock that can be molded (German and Bose, 1997).

MIM involves four main sequential processes, which are mixing, injection molding, debinding and sintering. Powder and binder mixed together to form the feedstock during the mixing process. Shape of the feedstock was developed through the injection molding process and followed by the debinding step. Debinding process involves the removal of the binder system. During the first stage of injection molding process, debinded product was heated up to the temperature great enough to form small, complex, precision parts and cost effective in high volumes, comparable to the material properties of conventional PM parts (German, 1990). Every process involved in MIM has significant impacts to the

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characteristics of the starting materials; the powder and the binder are critically important to the overall success of this process.

The optimum powder loading should be applied as it will affect the whole process while the feedstock has to be homogenous as possible made by intensive mixing of metal powder and the binder. The optimum powder loading applied in this study is in accordance to the previous research done by Gulsoy et al., (2007) and Ye et al., (2008).

Once the powder formulation has been established, further concern will be given to the ingredient mixing. The uniformity of the feedstock is crucial, since most inhomogenities condition cannot be corrected in the subsequent processing step. The best mixing occurs with high shear, but not to the point where the higher mixing shear damages the particles or overheats the binder. A properly mixed material will consist of homogenous powder dispersion in the binder with no internal porosity or agglomerates. Inhomogenities results in nonuniform viscosities, uneven molding and difficulties at the sintering stage (Klar and Samal, 2007).

Ideally, the feedstock design must be adequately considered the easiness of molding and strict control over the final dimensions. To achieve this situation, the feedstock used must have low molecular weight polymer binders as to reduce viscosity and to ease the molding process. If there are no voids or pores present in the feedstock, there are sufficient binders to fill in all interparticle spaces. Several defects that commonly occur are surface wrinkles, binder separation, weld lines, sink marks, flashing and unmature shot (Norhamidi et al., 2002).

After the molding, attentions are then given to the debinding stage. The binder use must be disposable component in the feedstock mixture. Failure to remove most of the binder before the sintering could results in component distortion, cracking and contamination. Removing the binder without disrupting the particles is a delicate process which can be successfully achieved in multiple steps. When the binder is heated, it softens and is unable to withstand the stress from the gravity, and temperature gradients or internal

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vapor pockets. Alternatively, when the pores are partially open, the capillary forces resist distortion as the binder softens. Thus, progressive stepwise removal of the binder is mandatory to hold the component shape.

Final step involved in MIM process is sintering, which take place at high temperatures in order to bond the particles together. Sintering is a thermal treatment for bonding the particles into a coherent, predominantly solid structure via mass transport events that often occur on the atomic scale (German, 1996). The binder in the molded part that produces via injection molding process is removed through the solvent extraction prior to the thermal pyrolisis. The thermally debound part or the brown part still can retain the shape due to the friction among the powder particles even though all the binder has been removed at the earlier stage. Hence, this part is very fragile and needs to be handled carefully for the next sintering process which later performs to achieve desired final mechanical, physical and chemical properties. High final density is important for the optimization of the desired attributes. The final properties of the product can be further improved with additional heat and mechanical treatments.

1.2 Problem Statements

Numerous research efforts in the metal injection molding (MIM) are related to the physical and mechanical properties investigation. Selection of powder and binder system will affect these properties significantly. Binder system in MIM processing is the important key consideration which provides powder with good flowability and formability that necessary for the molding operation (Klar and Samal, 2007). It is important and possible to develop a new binder system that could be used in the MIM processing which exhibit economical and environmental friendly characteristics for the application in automotive, tooling, medical and hardware component. The mixture of powder and binder is termed the feedstock. In the feedstock mixture, it is expected that each powder particle, should be enveloped by a very thin film surface of the binder and has a tight contact with each other. At the same time, pores between the powder particles

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are homogenously filled with the binder. However, it is very difficult to get that kind of homogenous ideal powder and binder mixture condition (Li et al., 2007). Usually, the method for removing the binder is by thermal debinding that is a major controlled step in the MIM process. However, the great quantity of gas is produced by the evaporation and degradation of the binder which later produces many defects, such as bubbles and cracks in the molded parts. Thermal debinding process also consumes quite a long duration of operation (Li et al., 2003). Besides, the other aspect that should be considered is related to the component shrinkage and warpage. They are very low during the injection molding step and increases significantly during the debinding and sintering steps. As a result, the components may be out of design tolerances or, in the worst case; it may be cracked, excessively warped or chemically incorrect with a very poor density (Heaney and Spina, 2007). In view of that, a new binder system which is thermoplastic waste rubber (TPWR) will be developed as to improve these processes and to encounter major drawbacks of this process. The uses of waste rubber as novel binder system in the MIM processing to replace natural rubber is receiving great attention, due to the advantage of renewability, thermal stability and high shear viscosity. These waste rubber outstanding properties such as very weak adhesion between the waste rubber and polymer is due to the crosslinked structure of the waste rubber (Tan et al., 2008). Therefore, in this study, waste rubber will be utilized, formulates and evaluates as a new binder system for the MIM processing due to their above mention advantageously.

1.3 Objectives

The main objectives of this study are:

i. To investigate the sintering characteristics by determining the physical, mechanical and microstructural properties of the injection molded 17-4 PH stainless steel using new developed binder system that is waste rubber.

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1.4 Scope of Study

This research was started with the selection of raw material and early characterization of its properties. The dimensional analysis of the powder particle will be carried out by utilizing the Scanning Electron Micrograph (SEM).

Next stage is the material processing which consists of four sequential processing steps of MIM processes. The first step is material formulation and mixing process to produce the granulate feedstock. Then, the next step will proceed into the injection molding process of the specimens. The feedstock after the mixing will be injection molded by using a vertical injection molding. After that, the third step will be followed by debinding of the molded specimens. At this stage, the binder system of these specimens will be removed from the green specimens. The process will be done in two separate stages which are solvent extraction method using water bath whereby the specimens will be immersed in heptanes, followed by the thermal pyrolisis that will be carried out by using the debinding furnace. Finally, the injection molded specimens will be sintered at varying heating rate by using the vacuum atmosphere sintering furnace. It is anticipated that the sintering process will gives strong interparticles bonds and removes or reduces the void spaces by densification mechanism. Hence, sintering causes substantial shrinkage in the MIM parts (German, 1996).

In line with the objectives of this research, the heating rate and soaking time of sintering process will be studied as to understand the physical and mechanical properties of the sintering molded parts, fabricated by utilizing the 17-4 PH stainless steel raw material. In this research, the parameters of mixing process and injection molding process are fixed in order to produce good quality of green specimens in accordance to the previous research done by Gulsoy et al. (2007) and Omar and Subuki, (2006).

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CHAPTER 2 LITERATURE REVIEW

This chapter reviews related study done by the previous researchers on powder injection molding through the metal injection molding method. This literature reviews are focused on the usage of the different binder system and its significant effects to the physical, mechanical and morphological structures of the metal injected samples that sintered at the various sintering parameters. The technical roles of the polymeric based binder system to assist the efficacy of the powder metallurgy operation will be further reviewed.

2.1 Powder Metallurgy

Powder Metallurgy (PM) is a highly evolved method in the manufacturing of reliable net shaped components. Principally, it starts by simultaneously blending the elemental or pre-alloyed powders, and then followed by compacting this blend in a die. Finally, the sintering or heating process is done to the pressed part in a controlled furnace atmosphere to bond the particles metallurgically (Beddoes and Bibby, 1990). The PM process is a unique part fabrication method. This method is highly cost effective in producing simple or complex parts at or close to the final dimensions (Kalpakjian and Schmid, 2006). There are four common process categories in the initial stage of components shaping. It can be classified into rigid die compaction, powder injection molding, extrusion and hot isostatic pressing (Klar and Samal, 2007). In this research, powder metallurgy which utilizing the injection molding method will be further investigated.

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7 2.1.1 Metal Injection Molding

Technically, metal injection molding (MIM) that also can be referred as powder injection molding is using a feedstock that is high in their particle content. Figure 2.1 shows the major various steps involved in MIM; which are mixing, injection molding, debinding and sintering process (Beddoes and Bibby, 1990; German, 2007). The process begins with the mixing of the selected metal powder with the binders to produce the granulate feedstock. Then, the feedstocks are injection molded into the tensile bar through the injection molding machine that later produced green part of specimens. Next, the binder is removed from the green parts by a solvent extraction and thermal pyrolisis process. The remaining porous powder structures are sintered to near full density. The product may then be further densified, heat treated, or machined into the final shape (German, 2007).

Figure 2.1: Flow chart illustrating major processing steps in MIM process (Beddoes and Bibby, 1990; German, 2007)

Metal Powder Binder System

Injection Molding Mixing

Debinding

Thermal Pyrolisis Solvent Extraction

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