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Title: Building of the Residual Wall Thickness and Formation of Part Defects in Water-assisted Injection Molding (WAIM): The Combined Influence of Process and Material Parameters (Opbouw van de wanddikte en vormen van defecten in water ondersteund spuitgieten: de gecombineerde invloed van procesparameters en materiaaleigenschappen)
Other Titles: Building of the Residual Wall Thickness and Formation of Part Defects in Water-assisted Injection Molding (WAIM): The Combined Influence of Process and Material Parameters
Authors: Sannen, Sofie
Issue Date: 20-Dec-2013
Abstract: Water-assisted injection molding (WAIM) produces hollow or partially hollow products, potentially having a higher quality and a lower process cost when compared to products produced with other and earlier plastic processing techniques. Since the development of WAIM is relatively recent (1998) [1, 2], the knowledge of the process is restricted. A critical assessment of available literature (Chapter 2 of this thesis) points out that there is a lack of fundamental understanding of the water and polymer behavior during the process, leading to an unclear and unexplainable influence of process and material parameters on product quality. This thesis studies both the building of the residual wall thickness and the formation of part defects, because the thickness of the wall as well as the possible presence of defects in the final product are important quality characteristics. The accompanying insights into the water and polymer behavior reveal the principle mechanisms behind the process so that the influence of process and material parameters on product quality is elucidated. The building of the residual wall thickness is investigated with experimental observations: in addition to the residual wall thickness measurements, the water volume flow rate, water and melt pressures are monitored during the process. The measurement and corresponding interpretation of these signal progressions lead to renewing insights regarding the water and polymer behavior. Based on this understanding, a physical model is derived with which the stress between the polymer chains can be determined. This stress induces a water channel with a certain radius and consequently influences the behavior during water penetration. In Chapter 4 of this thesis, the model is elucidated and linked to experimental data. The model is further used to predict the influence of important process and material parameters on the building of the residual wall. The proposed prediction is subsequently related to the observed variations in volume flow rate, pressure and residual wall thickness. Both the qualitative and quantitative application of the model on the signal variations points out that the model is able to explain the experimental observations physically and accordingly to describe as well as predict the building of the residual wall thickness in a qualitative way. The formation of part defects is studied in experiments in which a process and/or material parameter variation is applied. With a qualitative defect analysis on the produced products, a clear definition of the occurring defect types in the current experimental setup is formulated: irregular residual wall, void, double wall and no residual wall. To these defects, responsible formation mechanisms are further unambiguously designated. The occurrence of the defects is herewith related to water and polymer properties/conditions existing during water penetration. The new and more complete insights into the different defect types and their accompanying formation mechanisms are presented in Chapter 5 of this thesis. The derived definitions and mechanisms are further used to explain observed variations in defect occurrence for altering process and material parameters in a pre-defined reference experiment. It is found that the proposed definitions and mechanisms are able to explain the experimental observations and accordingly to describe the formation of part defects. Numerical simulations on WAIM are performed with the only commercially available software package which is able to simulate the process: Moldex3D. The simulation results are validated and assessed with experimental data to support and supplement the insights into the building of the residual wall thickness and the formation of part defects of Chapter 4 and 5 respectively. The comparison between both results, as presented in Chapter 6 of this thesis, indicates similarities as well as dissimilarities, whereby the latter can be related to the presence of model and/or numerical errors in the simulation software.
Table of Contents: Voorwoord i

Abstract iii

Samenvatting v

List of symbols vii

Contents xi

Chapter 1 Introduction 1
1.1 Motivation 1
1.2 Objectives 1
1.3 Outline 2

Chapter 2 State of the art of water-assisted injection molding 5
2.1 Process 5
2.1.1 Cycle 6
2.1.2 Variants 7
2.1.3 Setup 9
2.1.4 Advantages and disadvantages 12
2.1.5 Products 14
2.2 Residual wall thickness 15
2.2.1 Influence of process parameters 16
2.2.2 Influence of material parameters 24
2.3 Part defects 33
2.3.1 Defect types 34
2.3.2 Principle mechanisms 35
2.3.3 Influence of process and material parameters 41
2.4 Conclusion 42

Chapter 3 Materials and methods 45
3.1 Materials 45
3.2 Methods 48
3.2.1 Equipment 48
3.2.2 Experiment 53
3.2.3 Analysis 55

Chapter 4 Building of the residual wall thickness 61
4.1 Exploration of process and material parameters 61
4.2 Generalization of the building process 64
4.2.1 Physical model 64
4.2.2 Experimental approach 71
4.3 Influence of process and material parameters 80
4.3.1 Material parameters 80
4.3.2 Process parameters 88
4.3.3 Process variation 99
4.3.4 Evaluation 107
4.4 Conclusion 108

Chapter 5 Formation of part defects 111
5.1 Exploration of process and material parameters 111
5.2 Current insights into the occurrence of part defects 114
5.2.1 Definition of occurring defect types 114
5.2.2 Important water and polymer properties/conditions 116
5.2.3 Designation of responsible formation mechanisms 118
5.2.4 Evaluation of the defect analysis 126
5.3 Influence of process and material parameters 129
5.3.1 Reference experiment 129
5.3.2 Process parameters 130
5.3.3 Material parameters 133
5.3.4 Evaluation 135
5.4 Conclusion 136

Chapter 6 Numerical simulation 139
6.1 State of the art in modeling water-assisted injection molding 139
6.1.1 Governing equations 140
6.1.2 Numerical methods 146
6.2 Simulating water-assisted injection molding 147
6.2.1 Description of Moldex3D 147
6.2.2 Experimental approach 148
6.2.3 Validation of simulation results 149
6.2.4 Simulation results on the building of the residual wall thickness 156
6.2.5 Simulation results on the formation of part defects 158
6.2.6 Evaluation 162
6.3 Conclusion 163

Chapter 7 Conclusions and outlook 165
7.1 Conclusions 165
7.2 Outlook 167

Bibliography 169

List of publications 175

Curriculum vitae 177
ISBN: 978-94-6018-764-3
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Soft Matter, Rheology and Technology Section
Sustainable Chemical Process Technology TC, Technology Campus Diepenbeek
Sustainable Chemical Process Technology TC

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