STUDY OF THIN-WALL INJECTION MOLDING DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Guojun Xu, M.E. ***** The Ohio State University 2004 Dissertation Committee: Professor Kurt W. Koelling, Adviser Professor L. James Lee Professor Jose M. Castro Approved by Adviser Department of Chemical Engineering
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STUDY OF THIN-WALL INJECTION MOLDING
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate School of
The Ohio State University
By
Guojun Xu, M.E.
*****
The Ohio State University
2004
Dissertation Committee:
Professor Kurt W. Koelling, Adviser
Professor L. James Lee
Professor Jose M. Castro
Approved by
Adviser Department of Chemical Engineering
ii
ABSTRACT
Thin-wall injection molding has received increasing attention over the past few
years due to economic and environmental concerns. However, due to the difficulties
encountered in the thin-wall molding process, systematic investigation is lacking in
computer aided engineering (CAE) simulation, part quality and part design criteria.
Furthermore, the combination of viscoelastic materials, complex molding geometry and
cyclic processing conditions has generated some problems, such as flow marks, polymer
degradation, sink marks and warpage, under high-speed and high-pressure injection
molding. So it is very important to design, operate, and control thin-wall molding
optimally to guarantee part quality as well as reduce cost.
In this study, alternate and synchronous dull and glossy flow marks, two surface
quality problems, were studied. For the alternate flow marks, the effect of polymer
rheology, mold geometry, operating variables, and mold surface coatings on the
appearance of the flow marks was studied. The flow marks occurred above a critical
wall shear stress, but disappeared at high injection speeds. Mold geometry and mold
temperature were found to affect the wavelength and the width of the flow marks, while
melt temperature did not have much effect. Slip was not the cause of the generation of
iii
the alternate flow marks. For synchronous dull and glossy flow marks, the effect of
operating parameters, mold geometry, and mold surface coatings on the flow marks was
studied. The flow marks occurred above a certain flow front velocity, but were less
visible as the mold temperature was increased. It was also found that mold surface
coatings did not eliminate the flow marks. The generation of these flow marks was
explained by an entry viscoelastic flow instability.
Furthermore, thin-wall injection molding with micro-features was investigated.
The filling length in microchannels was measured and compared with simulation. The
heat transfer coefficient was found to be very sensitive to the filling length prediction. In
order to investigate the effect of input properties on the simulation output, mold cavity
pressure was studied. The goal was to understand the effect of pressure-dependent
viscosity, heat capacity, heat transfer coefficient, juncture pressure loss and pvT-data on
cavity pressure and pressure drop prediction, and evaluate the importance of each
parameter. The cavity pressure and pressure drop were measured experimentally and
compared. Furthermore, the method to improve the prediction accuracy was discussed
to help design and predict.
As the increasing use of plastics, the plastics waste has become a main concern.
The final part of the research focuses on the mechanical and rheological properties of
virgin and recycled high impact polystyrene materials. In this study, we describe our
progress in evaluating the viability of reusing post-consumer and virgin polymer blends
of HIPS from electronics equipment housings. Plastics reuse challenges are briefly
reviewed, and experimental results, such as rheological properties, mechanical
iv
properties, molecular weight and morphology of different blends, are summarized and
discussed for reuse of HIPS. Finally, the study introduces a new approach to determine
initial processing parameters for thin-wall injection molding of post-consumer resin.
v
This dissertation is dedicated to my family.
vi
ACKNOWLEDGMENTS
I would like to express sincere gratitude to my adviser, Dr. Kurt W. Koelling, for
his priceless guidance, encouragement, and support throughout this work. I also would
like to thank Dr. Julie Ann Stuart and Dr. Blaine Lilly for their instruction,
encouragement and support. Special thanks go to Dr. L. James Lee for his considerable
advice and help. I wish to thank Dr. David Tomasko, Dr. Jose Castro, and Dr. Robert
Brodkey for their valuable suggestions and comments. I would like to thank Dr. Paula
Stevenson for her proofreading and many helps during the past five years. Thanks also
go to everyone who helped me in various ways, Paul Green, Leigh Evrard and Carl
Scott. I would like to thank previous and current colleagues in the polymer research
group.
In addition, Micro Metallics Corporation and Nova Chemical, Inc. donated post-
consumer and virgin polymers, Eastman Kodak Company loaned two molds, Dow
Chemical donated polypropylene, 3M Company donated Dynamar 9613, and GenCorp
Research donated a blender. The authors thank Professor Terry Gustafson and research
assistants Tony Frost and Kristin Frost of the Chemistry Department at The Ohio State
University for measuring the infrared and Raman vibrational spectra. The author thanks
Dr. John Clay for the measurement of the molecular weight, and Michael Ferry, Tu Tran,
vii
Sadu Prabowo, Andy Divine and Eric Mosser for help in measuring some physical
properties.
Finally, I would like to thank my parents for their continuing support through the
years of my study and my wife, Xia Cao, for her understanding, support, and
encouragement.
viii
VITA
September 25, 1967…………………………..………Born - Cixi, Zhejiang, P. R. China September 1985 - July 1989………………………….B.S. Chemical Engineering Zhejiang University Hangzhou, Zhejiang, P. R. China September 1989 - March 1992……………………….M.S. Chemical Engineering Zhejiang University Hangzhou, Zhejiang, P. R. China September 1998 – present….………………………...Graduate Research Associate
The Ohio State University Columbus, OH
PUBLICATIONS
1. Guojun Xu and Kurt Koelling, "Flow Marks/Tiger striping during Thin-Wall Injection Molding of Polypropylene", J. Injection Molding Technology (Submitted).
2. Jose L. Garcia, Kurt W. Koelling, Guojun Xu, and James W. Summers, “PVC Degradation During Injection Molding: Experimental Evaluation”, Journal of Vinyl & Additive Technology (In press).
3. Christiana Kuswanti, Guojun Xu, Jianhong Qiao, Julie Ann Stuart, Kurt Koelling, and Blaine Lilly, "An Engineering Approach to Plastics Reuse", Journal of Industrial Ecology, 6, 125-35, 2003.
4. Guojun Xu and Kurt Koelling, "Flow Marks during Injection Molding", ANTEC, Nashville, TN, 566-70, 2003.
5. Guojun Xu, Jianhong Qiao, Christiana Kuswanti, Kurt Koelling, Julie Ann Stuart, and Blaine Lilly, "Characterization of Virgin/Post-consumer Blended High Impact
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Polystyrene Resins for Injection Molding", J. of Applied Polymer Science, 84, 1-8, 2002.
6. Guojun Xu and Kurt Koelling, "Flow Marks during Injection Molding", ANTEC, San Francisco, CA, 521-5, 2002.
7. Guojun Xu and Kurt Koelling, "Study of Flow Marks during Thin-Wall Injection Molding", ANTEC, Dallas, TX, 604-7, 2001.
8. Guojun Xu, Jianhong Qiao, Christiana Kuswanti, Molly Simenz, Kurt Koelling, Julie Ann Stuart, and Blaine Lilly, "Insight into Reuse of High Impact Polystyrene from Post-Consumer Electronics Equipment Housing", IEEE International Symposium on Electronics and the Environment, San Francisco, CA, 348-53, 2000.
9. G. J. Xu, Y. M. Li, Z. Z. Hou, L. F. Feng and K. Wang, "Gas-Liquid Dispersion, Mixing and Heat Transfer in a Stirred Vessel", Can. J. of Chem. Eng., 75, 299-306, 1997.
10. Y. Li, G. Xu, M. Chen and K. Wang, "Slow Pelleting Coagulation of MBS Latex", Chem. Eng. Res. & Des., 75, 81-6, 1997.
11. Xu Guojun, Lianfang Feng, Yunming Li and Wang Kai, 'Pressure Drop of Pseuo-plastic Fluids in Static Mixers', Chinese J. of Chem. Eng. (English), 5(1), 93, 1997.
12. Y. M. Li, M. W. Chen, G. J. Xu, and K. Wang, "Continuous Slow Coagulation of Polymer Latex in Series Agitated Vessels", 36th IUPAC International Symposium on Macromolecules, IUPAC MACRO SEOUL'1996, Korea, 6-p01-01, 597, 1996.
13. Y. M. Li, G. J. Xu, M. W. Chen, S. H. Ou and K. Wang, "Slow Pelleting Coagulation of Polymer Latex Emulsion", 36th IUPAC International Symposium on Macromolecules, IUPAC MACRO SEOUL'1996, Korea, 6-p01-02, 598, 1996.
14. G. J. Xu, Y. M. Li and K. Wang, "Particle Growth Kinetics for Seed Coagulation of Polymer Latex", 36th IUPAC International Symposium on Macromolecules, IUPAC MACRO SEOUL'1996, Korea, 6-p01-03, 599, 1996.
15. Hou Zhizhong, Feng Lianfang, Li Yunming, Xu Guojun, Wang Kai and Pan Zuren, "Power Consumption of Agitation in a Gas-liquid System" (Chinese), 7th National Conference on Chemical Engineering, Beijing, China, B54, 424, 1994.
16. Hou zhizhong, Li Yunming, Feng Lianfang, Xu Guojun, Wang Kai and Pan Zuren, "Study on Heat Transfer of Gas-liquid System in an Agitated Vessel" (Chinese), 7th National Conference on Chemical Engineering, Beijing, China, B53, 420, 1994.
17. Hou Zhizhong, Wang Kai, Feng Lianfang, Li Yunming, Xu Guojun and Pan Zuren, "Fluid/Wall Heat Transfer in an Agitated Gas-Liquid Reactor" (English), International Workshop on the Advances in Chemical Engineering, Hangzhou, China, 1994.
x
18. Guojun Xu, Lianfang Feng and Kai Wang, "Pressure Drop and Friction Factor for non-Newtonian Fluids in Static Mixers" (English), International Workshop on the Advances in Chemical Engineering, Hangzhou, China, 1994.
19. Hou Zhizhong, Feng Lianfang, Li Yunming, Xu Guojun and Wang Kai, "Gas-liquid Dispersion and Mixing Properties of Different Impellers in an Agitated Vessel", China Synthetic Rubber Industry (Chinese), 18(3), 147-50, 1995.
20. Hou Zhizhong, Li Yunming, Feng Lianfang, Xu Guojun and Wang Kai, "Properties of Gas-liquid Dispersion in a Baffle-gassed Multistage Agitated Vessel", China Synthetic Rubber Industry (Chinese), 18(4), 218-20, 1995.
21. Guojun Xu, Zhangmao Wang and Gantang Chen, "Study of Axial Diffusion Coefficients and Distinguish of Particulate/Aggregative Fluidization", Chemical Reaction Engineering and Technology (Chinese), 10(3), 306-10, 1994.
22. Guojun Xu, Zhangmao Wang and Gantang Chen, "A Model of Fluid Flow and Particle Circulation in a L/S Fluidized Bed", Chemical Reaction Engineering and Technology (Chinese), 11(3), 277-83, 1995.
23. Guojun Xu, "Fluidized Polymerization Reactors", China Synthetic Rubber Industry (Chinese), 18(1), 40-2, 1995.
24. Li Yunming, Xu Guojun, Ou Shuhui, Chen Miwen and Wang Kai, "Slow Coagulation of Polymer Latex" (Chinese), Annual Conference on Polymers, Guangzhou, 1179-80, 1995.
25. Zhizhong Hou, Lianfang Feng, Yunming Li, Guojun Xu and Kai Wang, "Heat Transfer Properties in Aerated Agitated Reactor", China Synthetic Rubber Industry (Chinese), 18(6), 338-40, 1995.
26. Yunming Li, Guojun Xu and Jingjing Xu, "A Study of Particle Growth in Seed Coagulation of Polymer Latex" (Chinese), Annual Conference on Polymer, Guangzhou, 1175-6, 1995.
27. Guojun Xu, Yunming Li and Jingjing Xu, "Methods of Seed Coagulation of Polymer Latex" (Chinese), Annual Conference on Polymer, Guangzhou, 1177-8, 1995.
28. Zhangmao Wang and Xu Guojun, "A Study Expansion and Axial Diffusion in a Liquid/Solid Spouted Fluidization Bed", Chemical Reaction Engineering and Technology (Chinese), 12(2), 184-8, 1996.
29. Guojun Xu, Lianfang Feng, Yuming Li, and Kai Wang, "A Study of Pressure drop for Pseudo-plastics Fluids in Kenics Mixers", China Synthetic Rubber Industry (Chinese), 19(2), 97-9, 1996.
xi
30. Deming Mao, Lianfang Feng, Guojun Xu and Kai Wang, "Effect on Control Volume and measured Points When the Beams Pass through Circular Media", Journal of Experimental Mechanics (Chinese), 11(1), 13-7, 1996.
31. Deming Mao, Lianfang Feng, Guojun Xu and Kai Wang, "Experimental Study on Agitator by LDA", Chem. Eng. J of Chinese University (Chinese), 10(3), 258-63, 1996.
32. Deming Mao, Lianfang Feng, Guojun Xu and Kai Wang, "Study of Spectral Analyses and Scales of Turbulence in Rushton Turbine", Chem. Eng. J of Chinese University (Chinese), 1996.
33. Yuming Li, Miwen Chen, Guojun Xu and Kai Wang, "Slow Pelleting Coagulation of Polymer Latex Emulsion", Chinese Chemical Letter (English), 7(3), 297-8, 1996.
FIELDS OF STUDY Major Field: Chemical Engineering Minor Field: Polymer Processing
Rheology
Chemical Reaction Engineering
xii
TABLE OF CONTENTS
Page
Abstract…………………………………………………………………………………..ii
Dedication………………………………………………………………………………...v
Acknowledgments……………………………………………………………………….vi
Vita……………………………………………………………………………………..viii
List of Tables……………….……………………………………………………….…..xv
List of Figures……………………………………………..…………………………..xvii
Chapters
1. Introduction...……………………………………………………………….…………1
2. Literature review.……………………………………………………………….……12
3.44 Frequency of flow marks vs. Flow front velocity………………………………125
4.1 The long rectangular mold base with a disk-like insert………………………...155
4.2 The rectangular mold bases with a disk-like insert……………………………156
4.3 The disk-like mold insert which contains microchannels……………………..157
4.4 SEM picture of the a microchannel……………………………………………158
4.5 Dynamic viscosity of polypropylene…………………………………………..159
4.6 Dynamic viscosity of PMMA………………………………………………….160
4.7 SEM of a micro-channel……………………………………………………….161
4.8 Measured filling lengths in microchannels for PMMA in the long mold……..162
4.9 Measured filling lengths in microchannels for PP in the long mold…………..163
4.10 Measured filling lengths in microchannels for PMMA in the long mold……..164
4.11 Measured filling lengths in microchannels for PP in the long mold…………..165
xx
4.12 Measured filling lengths in microchannels for PP in the short mold…………..166
4.13 The cavity pressure profile in the long mold and the short mold……………...167
4.14 The filling length vs. Fourier number……………………….………………...168
4.15 The effect of packing stage on filling lengths..………………...……………...169
4.16 The effect of holding pressure on filling lengths..……………...……………...170
4.17 Comparison of the filling lengths between the simulation and experiment with constant heat transfer coefficients. Main flow heat transfer coefficient=25000 W/m2.K………………………………………………………………………...171
4.18 Comparison of the filling lengths between the simulation and experiment with constant heat transfer coefficients. Main flow heat transfer coefficient=2000 W/m2.K………………………………………………………………………...172
4.19 Comparison of the filling lengths between the simulation and experiment with variable heat transfer coefficient………………………………………………173
4.20 Schematic of the mold with thickness of 1 mm……………………………….174
4.21 Heat capacity of HDPE and PS………………………………………………..175
4.22 Specific volume of HDPE……………………………………………………..176
4.23 Specific volume of PS…………………………………………………………177
4.24 Experimental and fit viscosity vs. shear rate/ frequency for PS……………….178
4.25 Experimental and fit viscosity vs. shear rate/ frequency for HDPE…………...179
4.26 Comparison of cavity pressure with/without the effect of pressure on specific volume…………………………………………………………………………180
4.27 Comparison of cavity pressure with/without the effect of pressure on viscosity………………………………………………………………………..181
4.28 Comparison of cavity pressure with different heat transfer coefficients……....182
4.29 Comparison of cavity pressure with constant Cp and temperature-dependent Cp…………………………………………………………………………..….183
4.30 Comparison of cavity pressure with/without juncture loss………………….…184
xxi
4.31 Pressure profiles right after the gate and at the end of the cavity at the injection speed of 76.2 mm/s and the melt temperature of 230 and 250°C…………...…185
4.32 Pressure profiles right after the gate at the melt temperature of 230°C with different injection speeds…………………………………………………...….186
4.33 Pressure profiles at the end of the cavity at the melt temperature of 230°C with different injection speeds……………………………………………………....187
4.34 Comparison of experimental and predicted pressure drop at the injection speed of 12.7 mm/s…………………………………………………………………..….188
4.35 Comparison of experimental and predicted pressure drop at the injection speed of 508 mm/s………………………………………………………………………..189
5.1 Film canister………………………………………...…………………..……...212
5.2 Comparison of the viscosity curves for post-consumer HIPS and virgin HIPS at 220°C……………………….…………………………….……………………213
5.3 Viscosity of Huntsman PS 702 blends with different percentages of post-consumer resin at about 200°C………………………….……………………..214
5.4 Viscosity of Nova PS 3350 blends with different percentages of post-consumer resin at about 200°C…………………………………………….……………..215
5.5 The images of different blends from ESEM (The length of the scales in the figures are 2 µm)………………………………………………………..…...….216
5.6 Raman spectroscopy of injection-molded post-consumer and Huntsman PS 702………………………………………………………………………...…...217
5.7 Infrared vibrational spectra of injection-molded post-consumer and Huntsman PS 702………………………………………………………………………….….218
5.8 Average Ra for six blends of Huntsman PS 702……………………………….219
5.9 Average Wa for six blends of Huntsman PS 702……………………………....220
5.10 Tensile strength and tensile modulus vs. weight percentage of virgin resin...…221
5.11 Flexural strength and flexural modulus vs. weight percentage of virgin resin...222
5.12 Impact strength and tensile modulus vs. weight percentage of virgin resin…...223
5.13 Meshing model of the film canister……………………………………………224
1
CHAPTER 1
INTRODUCTION
Among the large number of polymer processing operations, injection molding
has found the widest application for making articles which could be put to direct use.
Because of the superior manufacturability and the high degree of freedom of the form of
plastics products, injection molding is one of the most widely used processes for
processing plastics. In injection molding process, the polymer melt flows through a
runner system and gates to fill the mold cavity. When the filling is completed, more
melt is packed into the mold to compensate for volume shrinkage. The cooling stage
follows until the melt solidifies. Finally the part is ejected from the mold. Thin-wall
injection molding (TWIM) is conventionally defined as molding parts that have a
nominal wall thickness of 1 mm or less and a surface area of at least 50 cm2 [Whetten
and Belcher, 1994; Fasset, 1995]. Thin wall is relative, however. It also can be named
“thin-wall” as the flow length/thickness ratio is above 100 or 150 [Mahishi, 1998;
Maloney and Poslinski, 1998]. TWIM has been paid more and more attention,
2
especially in computer, communication and consumer electronic (3C) industries, due to
economic and environmental concerns. The reason is that thin-wall molded parts could
be made lighter, more compact, less expensive, and quicker because of fast cooling
[Smialek and Simpson, 1998]. New environmental regulations require less plastic to be
used at the source or in the initial stage of manufacturing [Miller, 1995]. Thus, TWIM is
a viable option for reducing the weight and size of plastic components.
The difference between conventional injection molding and TWIM is shown in
Fig. 1.1. The solidified “skin” layers are about 0.25 mm regardless of part thickness
[Fasset, 1995]. It means that the flow channel is very narrow and thus flow resistance is
very high in TWIM. Reducing flow resistance can be reached by increasing the melt or
Material τ∗ (Pa) N (-) D1 (Pa.s) D2 (K) D3(K/Pa) A1 (-) 2A (K)
PS 38264 0.177 2.72E13 368 1.51E-7 31.0 51.6
PE 2791 0.542 8.86E13 256 9.21E-7 26.0 51.6
149
Peak P (MPa) No. Cp ηp ∆P h v
0.5”/s 3”/s 20”/s 1 1 1 1 1 1 84.07 72.28 72.95
2 1 1 1 2 2 56.64 48.66 51.2
3 1 2 2 1 1 91.78 75.98 75.21
4 1 2 2 2 2 60.35 51.11 51.52
5 2 1 2 1 2 66.9 54.71 54.09
6 2 1 2 2 1 74.78 68.65 73.44
7 2 2 1 1 2 72.91 55.51 54.35
8 2 2 1 2 1 78.8 70.61 74.2
Percent Influence (%)
Injection speed Cp ηp ∆P h v
Significant
Factors
0.5”/s 0 5.64 0 25.77 67.28 η, h, v
3”/s 0 1.18 0.13 5.79 92.60 η, h, v
20”/s 0.32 0.12 0 0.48 98.71 v
Table 4.3 Relative influence of each factor on peak cavity pressure
at different injection speeds at 230°C
150
Percent Influence (%) Injection speed Cp ηp ∆P h v
Significant
Factors
0.5”/s 0 4.27 0 24.73 70.09 η, h, v
3”/s 0 0.89 0.03 6.32 92.59 η, h, v
20”/s 0.20 0.13 0.03 0.66 98.69 v
Table 4.4 Relative influence of each factor on peak cavity pressure
at different injection speeds at 250°C
151
Percent Influence (%) Temperatur
e Cp ηp ∆P h v
Significant
Factors
300°C 0 5.81 0 27.14 61.72
v, h
320°C 0 6.39 0 28.44 58.98
v, h
Table 4.5 Relative influence of each factor on peak cavity pressure
at different melt temperatures for HDPE at 0.5”/s
152
Percent Influence (%) Injection speed Cp ηp ∆P h v
Significant
Factors
0.5”/s 0 11.49 0 82.98 2.82 η, h
3”/s 0.07 7.96 0 88.94 0 h
20”/s 21.56 2.22 0.10 69.64 6.20 Cp, η, h, v
Table 4.6 Relative influence of each factor on maximum pressure drop
at different injection speeds at 230°C
153
Percent Influence (%) Injection speed Cp ηp ∆P h v
Significant
Factors
0.5”/s 0.06 8.99 0 86.78 2.55 η, h
3”/s 1.14 5.35 0 93.27 0 Cp, η, h
20”/s 21.56 1.74 0.32 67.54 7.40 Cp, h, v
Table 4.7 Relative influence of each factor on maximum pressure drop
at different injection speeds at 250°C
154
Percent Influence (%) Temperature
Cp ηp ∆P h v
Significant
Factors
300°C 0 8.11 0 88.23 0 h
320°C 0 7.48 0 87.94 0 h
Table 4.8 Relative influence of each factor on maximum cavity pressure drop
at different melt temperatures for HDPE at 0.5”/s
155
Fig. 4.1. The long rectangular mold base with a disk-like insert.
156
Fig. 4.2. The rectangular mold bases with a disk-like insert.
4 mm
135 mm A B
Long mold
Short mold
A B
157
Fig. 4.3. The disk-like mold insert which contains microchannels.
A A
View A-A
158
Fig. 4.4. SEM picture of the a microchannel.
159
Fig. 4.5. Dynamic viscosity of polypropylene.
1.E+02
1.E+03
1.E+04
1.E-01 1.E+00 1.E+01 1.E+02Frequency (1/s)
Vis
cosi
ty (
Pa.
s)
180°C
200°C
220°C
160
Fig. 4.6. Dynamic viscosity of polypropylene
1.0E+02
1.0E+03
1.0E+04
0.10 1.00 10.00 100.00
Frequency (1/s)
Com
plex
vis
cosi
ty (P
a.s)
210°C
220°C
230°C
161
a. Top view
b. Side view
Fig. 4.7. SEM of a micro-channel.
162
Fig. 4.8. Measured filling lengths in microchannels for PMMA in the long mold.
0
50
100
150
200
250
300
0.001 0.01 0.1 1 10
Main Flow Velocity (m/s)
Cha
nnel
Hei
ght (
Mic
orm
eter
)
80°C; HP=500 PSI
80°C; HP=0 PSI
163
Fig. 4.9. Measured filling lengths in microchannels for PP in the long mold.
0
100
200
300
400
500
600
0.001 0.01 0.1 1 10
Main Flow Velocity (m/s)
Cha
nnel
Hei
ght
(Mic
orm
eter
)
80°C; HP=500 PSI
80°C; HP=0 PSI
25°C; HP=0 PSI
164
Fig. 4.10. Measured filling lengths in microchannels for PMMA in the long mold.
0
5
10
15
20
25
30
35
40
45
50
0.01 0.1 1 10Main flow velocity (m/s)
Fill
ing
leng
th (
µ
m)
80°C; HP=500 PSI
80°C; HP=0 PSI
165
Fig. 4.11. Measured filling lengths in microchannels for PP in the long mold.
0
50
100
150
200
250
300
0.001 0.01 0.1 1 10Main flow velocity (m/s)
Fill
ing
leng
th (
µ
m)
80°C; HP=0 PSI
25°C; HP=0 PSI
166
Fig. 4.12. Measured filling lengths in microchannels for PP in the short mold.
0
100
200
300
400
500
600
700
0.001 0.01 0.1 1 10
Main Flow Velocity (m/s)
Cha
nne
l Hei
ght
(Mic
orm
eter
)
Short Mold, Channel A
Short Mold, Channel B
Long Mold, Channel B
167
Fig. 4.13. The cavity pressure profile in the long mold and the short mold.
168
Fig. 4.14. The filling length vs. Fourier number.
0
0.2
0.4
0.6
0.8
1
1.2
0.01 0.1 1 10 100 1000Fo
Fill
ing
leng
th /
Dep
th
100 microns, Short B
100 microns, Short A
100 microns, Long A, B
169
Fig. 4.15 The effect of packing stage on filling lengths.
(Melt temperature 240°C, mold temperature 25°C, main flow velocity 0.2 m/s)
0
10
20
30
40
50
60
70
80
0 4 8 12 16 20 24
Cav
ity
pres
sure
(M
Pa)
Start of packing
0
50
100
150
200
250
300
350
400
25.40 27.94 30.48 30.73 30.99 34.29 50.80
Shot size (mm)
Fill
ing
leng
th (µ
m)
201 mm/s
Start of packing
25.40 27.94 30.48 30.73 30.99 34.29 50.800
10
20
30
40
50
60
70
80
0 4 8 12 16 20 24
Cav
ity
pres
sure
(M
Pa)
Start of packing
0
50
100
150
200
250
300
350
400
25.40 27.94 30.48 30.73 30.99 34.29 50.80
Shot size (mm)
Fill
ing
leng
th (µ
m)
201 mm/s
Start of packing
25.40 27.94 30.48 30.73 30.99 34.29 50.80
170
Fig. 4.16 The effect of holding pressure on filling lengths.
(Melt temperature 240°C, mold temperature 25°C, main flow velocity 0.2 m/s)
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 1900
Hold pressure (psi)
Fill
ing
leng
th (µ
m)
Main flow velocity 201 mm/s
171
Fig. 4.17. Comparison of the filling lengths between the simulation and experiment with
constant heat transfer coefficients. Main flow heat transfer coefficient=25000 W/m2.K.
0
100
200
300
400
500
600
1 10 100 1000 10000
Main flow velocity (mm/s)
Fill
ing
leng
th (
mic
ron)
Expr.
h=500
h=2000
h=2500
172
Fig. 4.18. Comparison of the filling lengths between the simulation and experiment with constant heat transfer coefficients. Main flow heat transfer coefficient=2000 W/m2.K.
0
100
200
300
400
500
600
1 10 100 1000 10000
Main flow velocity
Fill
ing
leng
th (
mic
ron)
Expr.
h=500
h=2000
h=25000
173
Fig. 4.19. Comparison of the filling lengths between the simulation and experiment with
variable heat transfer coefficient.
0
100
200
300
400
500
600
1 10 100 1000 10000
Main flow velocity (mm/s)
Fill
ing
len
gth
(m
icro
n)
Expr. Tm=25CSimu. Tm=25C
Expr. Tm=80CSimu. Tm=80C
174
Fig. 4.20. Schematic of the mold with thickness of 1 mm.
50.8
mm
152.4 mm76.2 mm
P transducer P transducer P transducer
50.8
mm
152.4 mm76.2 mm
P transducer P transducer P transducer
175
Fig. 4.21. Heat capacity of HDPE and PS.
0
2000
4000
6000
8000
10000
12000
0 50 100 150 200 250 300 350
T (oC)
Cp
(J/k
g.o C
)
PS
HDPE
176
Fig. 4.22. Specific volume of HDPE.
0.9
1.0
1.1
1.2
1.3
1.4
1.5
273 323 373 423 473 523
Temperature (K)
Spec
ific
vol
ume
(cm3 /g
)
0 MPa
50 MPa
100 MPa
150 MPa
177
Fig. 4.23. Specific volume of PS.
0.90
0.95
1.00
1.05
1.10
273 323 373 423 473 523
Temperature (K)
Spec
ific
vol
ume
(cm3 /g
)
0 MPa
50 MPa
100 MPa
150 MPa
178
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04
Shear Rate/Frequency (1/s)
Vis
cosi
ty (
Pa.
s)
180°C
200°C
220°C
180C Fit
200C Fit
220C Fit
Fig. 4.24. Experimental and fit viscosity vs. shear rate/ frequency for PS.
179
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04
Shear Rate/Frequency (1/s)
Vis
cosi
ty (
Pa.
s)
160°C180°C200°C160°C Fit180°C Fit200°C Fit
Fig. 4.25. Experimental and fit viscosity vs. shear rate/ frequency for HDPE.
180
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8
Time (s)
Cav
ity
Pre
ssur
e (M
Pa)
Effect of pressure not consideredPressure-dependent v
Fig. 4.26. Comparison of cavity pressure with/without the effect of pressure on
specific volume.
181
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5 6 7
Time (s)
Cav
ity
Pre
ssur
e (M
Pa)
Effect of pressure not consideredPressure-dependent viscosity
Fig. 4.27. Comparison of cavity pressure with/without the effect of pressure on
viscosity.
182
0
10
20
30
40
50
60
70
80
0 2 4 6 8
Time (s)
Cav
ity
Pre
ssur
e (M
Pa)
h=25000h=1500
Fig. 4.28. Comparison of cavity pressure with different heat transfer coefficients.
183
0
10
20
30
40
50
60
70
80
0 2 4 6 8
Time (s)
Cav
ity
Pre
ssur
e (M
Pa)
Constant CpTemperature-dependent Cp
Fig. 4.29. Comparison of cavity pressure with constant Cp and temperature-
dependent Cp.
184
0
10
20
30
40
50
60
70
80
0 2 4 6 8
Time (s)
Cav
ity
Pre
ssur
e (M
Pa)
No juncture lossJuncture loss included
Fig. 4.30. Comparison of cavity pressure with/without juncture loss.
185
Fig. 4.31. Pressure profiles right after the gate and at the end of the cavity at the
injection speed of 76.2 mm/s and the melt temperature of 230 and 250°C.
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18Time (s)
Pre
ssu
re (
MPa
)
Pressure after Gate, 250°CPressure at End, 250°CPressure after Gate (230°C)Pressure at End (230°C)
186
Fig. 4.32. Pressure profiles right after the gate at the melt temperature of 230°C with
different injection speeds.
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14
Time (s)
Pre
ssur
e (M
Pa)
12.7 mm/s76.2 mm/s508 mm/s
187
Fig. 4.33. Pressure profiles at the end of the cavity at the melt temperature of 230°C
with different injection speeds.
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14
Time (s)
Pre
ssur
e (M
Pa)
12.7 mm/s76.2 mm/s508 mm/s
188
Fig. 4.34. Comparison of experimental and predicted pressure drop at the injection
speed of 12.7 mm/s.
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8Time (s)
Cav
ity
Pre
ssu
re (
MP
a)
Experimental pressureEffect of pressure on v not consideredP-independent viscosityPressure-dependent viscosityh=1500
189
Fig. 4.35. Comparison of experimental and predicted pressure drop at the injection
speed of 508 mm/s.
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10 12 14Time (s)
Cav
ity
Pre
ssur
e (M
Pa)
Experimental pressure
Effect of P on v not considered
P-independent viscosity
P-dependent viscosity
h=1500
190
CHAPTER 5
CHARACTERIZATION OF VIRGIN/POST-CONSUMER BLENDED
HIGH IMPACT POLYSTYRENE RESINS FOR INJECTION MOLDING
5.1 INTRODUCTION
Plastics have become a common materials choice in many new products and
millions of kilograms of plastics are used annually [Society of the Plastics Industry,
2001]. The attention paid to polymer recycling has increased in the past decade because
more efficient re-use of materials will reduce the quantities of plastics sent to landfills as
well as reduce raw material extraction. Furthermore, the advent of “take-back”
legislation accelerates waste prevention practices [Gamalski, 1996; Meffert and
Kirchgeorg, 1997; Hubschman, et al., 1995]. However, only a small amount of plastics
is reused as introduced in Section 2.1. Reducing virgin resin consumption can be
achieved by reduction of material requirement or resin recycling. One strategy is to use
thinner wall molding to reduce the quantity of material required. However, thin-wall
molding requires high injection speed, high injection pressure with polymers that could
191
withstand high shear rates and possible molecular degradation. Another strategy is to
recycle resin. In this study, we focus on resin cycling. However, how to characterize the
post-consumer resin (PCR) and how to increase the percentage of the post-consumer
resin are two of the problems in recycling plastic.
Currently, only less than one percent of HIPS is recovered from the total 19%
market share [Dillon and Aqua, 2000]. Two big challenges to reuse post-consumer resin
are material contamination and degradation. Post-consumer polymers may be
contaminated from other materials [Langerak, 1997]; post-consumer products may
contain polymer blends as well as additives such as reinforcements, paint, or flame
retardants [Dillon, 1999]. Another challenge is the material degradation because
returned polymers have been exposed to various thermal and mechanical conditions.
Thus, molders are reluctant to use recycled plastics because extensive experimental
testing is required to identify plausible use and determine molding parameters.
Recyclers currently select between options such as incineration or downcycling.
The major problem to reuse PCR is that polymer databases do not contain
information about PCR. Beside the material selection assistance, polymer databases are
used in mold filling simulation to design, reduce experimental time to decide processing
parameters, and predict possible problems. If molders must use trial and error to
determine PCR molding parameters, then a higher setup time is required for PCR than
for a virgin resin that is included in the database. Manufacturers usually use virgin resin
192
databases to decide processing parameters to reduce time because molders cannot easily
decide them without the material characterization of PCR.
Therefore, our initial investigation began with characterization of the post-
consumer resin. The viscosity is one of the basic properties for the reuse of the post-
consumer resin. The melt viscosity of the post-consumer resin was measured and the
virgin resins were identified with the same melt viscosity as the PCR. Next, the melt
viscosities of post-consumer and virgin resin blends were measured. Then the
mechanical properties of blends were measured and the effect of different virgin resins
and weight percentage of virgin resins were discussed. This investigation helped us
evaluate the viability of reusing the PCR in new injection molded products. Our goal is
to characterize the relationship between the ratio of recycled content to virgin content
and the mechanical properties. The mechanical properties, including tensile properties,
flexural properties, and impact properties, of the blends with different percentages of
reuse resin were analyzed through experiments. Furthermore, we investigated the
molecular weight and morphology of molded parts to help explain and predict the
properties of recycled blends for injection molding. Understanding the relationship
between rheological and design characteristics will provide both suppliers (recyclers)
and customers (molders) with valuable insights regarding viable uses for post-consumer
resins. Meanwhile, we introduce a sequence of steps to obtain PCR input for mold
filling simulation. The purpose is to reduce the amount of experimental time to
determine molding parameters. The method is tested by molding ASTM specimens and
a thin-wall application in film canisters.
193
5.2 EXPERIMENTAL
5.2.1 Characterization of Material
It is important to identify the post-consumer polymer properties. In general, it is
nearly impossible to identify the original resin manufacturer for post-consumer polymers
in electronics equipment. In our case, we only knew the polymer was labeled HIPS (high
impact polystyrene); we did not know the original resin manufacturer or product code.
Therefore, we tested the rheological properties of the PCR in order to identify the most
suitable "virgin resin” for the blends. The PCR material we used consisted of ground
pieces of printer and monitor housings.
The size of the fragments was greater than 100 mm. The incoming fragments
were inspected manually for metal contamination and then were shredded again to reduce
their size before mixing with virgin resins. The maximum dimension of the shredded
fragments was 1-10 mm, which was close to the size of the virgin resins.
The rheometer used was a Rheometrics Mechanical Spectrometer (RMS 800).
The rheological properties of the blends which consisted of different percentages of post-
consumer HIPS and virgin resins were also studied at three temperatures: 180ºC, 200ºC,
and 220ºC. Molded discs were used for the measurement of viscosity for the blends.
5.2.2 Measurement of Molecular Weight
The molecular weight was measured by GPC (Gel Permeation Chromotography).
The samples used were molded blends with 0%, 50%, and 100% Huntsman PS 702,
194
molded blends with 50% and 100% Nova PS 3350, and never molded, 100% virgin Nova
PS 3350. The solution was prepared by dissolving blends in THF (Tetrahydrofuran).
Each sample was analyzed twice with a running time of 45 minutes and an injection
volume of 200 µl. We report the average of the two runs in Table 5.1.
5.2.3 Microscopy and Spectroscopy
For the morphological measurement, the aim is to observe the dispersion of
rubber phase in polystyrene and the size of rubber domains because the rubber particles
can affect the mechanical properties. A Philip XL-30 FEG environmental scanning
electron microscope (ESEM) was used. The sample was stained by 1% OsO4 aqueous
solution for 15 days and carbon-coated for morphological measurements. The fracture
surfaces were observed with 15 KV power. The magnification in this study varied from
200x to 10,000x.
The purpose of the Raman spectroscopy tests is to determine if there is a
detectable difference in the absorption spectra for the PCR and the virgin high impact
polystyrene. The virgin resin Huntsman PS 702 was used in the experiment. The
infrared vibrational spectra were obtained using a Bruker Equinox 55 with IR Scope 1.
The instrument was operated in reflectance mode using the 15x microscope objective and
4 cm-1 resolution. OPUS software, version 2.2, was used for instrument control and data
handling.
The Raman vibrational spectra were obtained using a Chromex Raman 2000
spectrometer upon illumination by a 785 nm diode laser DSL and imaged on a
195
Photometrics 1024 X 256 pixel red enhanced CCD detector. The spectra were taken at a
180° collection angle with a depth of focus of several mm. The laser power was typically
50 mW with a spot size of 80 µm.
5.2.4 Processing Parameters for ASTM Specimens
To determine initial processing parameters, a mold filling simulation was run on
C-MOLD 97.7. C-MOLD is a set of integrated computer aided engineering (CAE)
simulations for plastics molding processes, including injection mold filling, post-filling,
cooling, part shrinkage and warpage. C-MOLD provides recommendations for
processing parameters such as fill time, inlet and melt temperature. CAE provides an
easy-to-use data visualizer for viewing mesh information and analysis results.
C-MOLD 97.7 was used to simulate filling our mold with one of the virgin resins,
Huntsman PS 702, which had the same viscosity versus shear rate as our PCR. The
ASTM mold consisted of six cavities, including two tensile bars, two sheets and two
discs. From the results of the mold simulation and several experimental trials, the
operating parameters, such as inlet melt temperature, melt temperature, and cooling time,
were selected to injection mold ASTM specimens.
The mold filling simulation required input for the resin properties. They can be
obtained from commercial resin databases or resin suppliers. However, resin databases
only contain virgin resin. So, we identified a virgin resin with similar viscosity to use as
input. In our approach, recyclers do not need to know the original resin manufacturers or
196
product codes. We assume that the PCR has been processed and sorted by manual
disassembly [Meacham, et al., 1999] or new bulk recycling methods [Arola and Biddle,
2000] so that it is not contaminated by other materials.
5.2.5 Physical Properties of ASTM Specimens
Six different weight percentages of blends were prepared, as shown in Table 5.2.
Two selected virgin resins, Huntsman PS 702 and Nova PS 3350, were used. These
virgin resins were selected because they had the close viscosity versus shear rate curve as
the PCR. The blends were mixed for one minute in a Little Ford Lodige Precision Mixer.
The ASTM specimens were prepared with a 50 ton Sumitomo injection molding
machine. The virgin material and post-consumer resin were mixed completely and then
were dried at 160ºF for 2 hours prior to injection molding. According to the results of the
mold design, the barrel temperature was set from 380ºF to 440ºF from the rear zone to
front zone. The mold temperature was kept at 77ºF.
For blends with Huntsman PS 702, the physical properties tested include: tensile
strength and modulus (ASTM D 638) at 23.3ºC and humidity 21%, flexural strength and
modulus (ASTM D 790) at 18.4ºC and humidity 12%, and notched Izod impact strength
(ASTM D 256) at 18.4ºC and humidity 21%. For blends with Nova PS 3350, all tests
were performed at 27.3ºC and humidity 30%.
Roughness and waviness measure the small-scale surface irregularities.
Roughness represents the range of groove heights of the surfaces while waviness is the
regression line (mean line) of the roughness profile. Two surface parameters, roughness
197
average (Ra) and waviness average (Wa) were measured. According to ISO, ANSI, and
DIN standards, Ra is the arithmetic average deviation of the roughness profile from the
roughness centerline, while Wa is the arithmetic average deviation of the waviness profile
from the waviness centerline [Sander, 1991]. The test was performed using the
Perthometer with a Gaussian filter type. The tests were conducted with a straight line
(entire trace) tilt correction and an evaluation range of 4.00 mm. One data point was
collected at each of five locations on each of three ASTM impact discs per blend for a
total of fifteen data points per blend.
5.2.6 Application
The ASTM test specimen mold is specially designed to minimize material
damage during molding. So, we tested the recycled material under high shear stress
condition to assess the ability of the material to withstand more realistic industrial use.
The film canister mold, loaned by Eastman Kodak Company, was used to test the post-
consumer/virgin polymer blends and is a thin-wall application compared to the original
printer and monitor housings. The film canister is shown in Fig. 5.1. The canister base
has variations in thickness as well as the recycling logo. In Table 5.3, the mold design
characteristics are listed.
C-MOLD 97.7 was used to simulate filling the film canister mold with the virgin
resin, Huntsman 702. From the results of simulation, combined with experimental trials,
the operating parameters, such as inlet melt temperature, melt temperature, cooling time,
etc., were selected for making film canister specimens, as reported in section 5.3.6. At
198
the same operating parameters, six types of blends of different percentage of PCR were
used to mold canisters. In order to compare the two virgin resins, we processed blends
under the same conditions. However, Nova PS 3350 has a lower melt index of 0.27
g/min compared to the Huntsman PS 702 melt index of 0.75 g/min [IDES, 1999]. Thus,
we used a higher injection velocity for virgin Nova PS 3350 after several experimental
runs.
One of the quality indicators tested for the canisters was the tensile strength. The
specimens consisted of strips of uniform width and thickness. According to ASTM
standards, we chose 100 mm as the width with a thickness less than 0.8 mm. Since the
thickness of the canister wall was less than 1 mm, the ASTM D-882-97 was adjusted
slightly by shortening the length of the specimens from 101.6 to 98 mm and 46 mm. To
ensure uniform width, calipers with 0.25 mm capability were used to check the specimen
width. The utmost care was exercised in cutting specimens to prevent nicks and tears that
may cause premature failure. To eliminate the anisotropic effect of the material, two sets
of test specimens were prepared having their long axes parallel with and normal to the
flow direction. The flow direction of the material in injection molding was from the
bottom to top.
5.3 RESULTS AND DISCUSSION
5.3.1 Characterization of Material
The rheological properties of the ground post-consumer HIPS were studied at
three temperatures: 180°C, 200°C, and 220°C. Fig. 5.2 is the viscosity versus frequency
199
curve of the post-consumer material at 220°C. We identified two virgin resin candidates,
Huntsman PS 702 and Nova PS 3350, in the C-MOLD resin databases by comparing the
viscosity curves.
The viscosity of blends of different percentage of the recycled resins was also
investigated at three temperatures: 180ºC, 200ºC and 220ºC. Figs. 5.3 and 5.4 show the
viscosity of blends with Huntsman PS 702 or Nova PS 3350 versus frequency at
approximately 200ºC, respectively. It is found that all blends are shear thinning. It is also
shown that the viscosity increases with the increase of the percentage of the PCR.
5.3.2 Molecular Weight
The molecular weights are listed in Table 5.1. It is shown that molecular weight
and polydispersity of blends with Huntsman PS 702 increase with the increase of the
percentage of Huntsman PS 702. However, for blends with Nova PS 3350, the molecular
weight decreases with the increase of the percentage of Nova PS 3350, though the
polydisperisity increases as the percentage of Nova PS 3350 increases. We can see that
all blends, including recycled resin, have similar molecular weight and polydispersity,
which would lead us to predict similar mechanical properties.
5.3.3 Microscopy and Spectroscopy
Fig. 5.5 shows the environmental scanning electron microscope (ESEM) images
of different blends. It is found that for virgin resins, 100% Huntsman PS 702 and Nova
PS 3350, the outer surfaces are dotted with a broad range of rubber domain with many
200
large rubber particles. The diameter is about 2 µm. However, for 50% Huntsman PS 702
and 50% Nova PS 3350, we only observed relatively smaller rubber particles. The
diameter is about 1 µm. The surface structures for the 50% blends are less regular
compared to those of virgin resins. For the PCR, we did not observe well defined rubber
domains, and the surface was seemingly covered with poorly defined dispersed rubber-
phase and some very small particles which may be contaminants.
Figs. 5.6 and 5.7 show the Raman Spectroscopy and Infrared vibrational spectra
of recycled resin and virgin resin Huntsman PS 702, respectively. It is shown that
recycled resin and virgin resin consist of almost the same components. Combined with
the results of the molecular weight measurements, we predict that it is possible to mix the
recycled resin and virgin resin for potential synergistic improvement of their properties.
5.3.4 Processing Parameters for ASTM Specimens
At first, the geometry was evaluated and then the mesh for the C-COLD
simulation was created. Processing parameters for the ASTM specimens of Huntsman PS
702 from the C-MOLD simulation are given in Table 5.4.
To compare the mechanical properties of the blends of Huntsman PS 702 to the
properties of blends of Nova PS 3350, the same injection molding parameters were used
to prepare the specimens of blends of Nova PS 3350.
5.3.5 Physical Properties of ASTM Specimens
For the six blends of Huntsman PS 702, the minimum, maximum, and average for
the Ra and the Wa are shown in Figs. 5.8 and 5.9 respectively. As shown in the figures,
201
the roughness average was fairly stable for the various blends but the waviness average
was best for the 0% virgin material. Due to our sample size of fifteen data points per
blend, further tests are being conducted with a larger sample size.
For the blends of Huntsman PS 702, the results of the physical properties tested
are shown in Figs. 5.10-5.12. For each physical property, six samples were tested. The
data shown in Figs. 5.10-5.12 are the averages of each sample. Fig. 5.10 shows the
tensile modulus and tensile strength of the blends for two different virgin resins versus
weight percentage of virgin resin. It is found that generally, both the tensile strength and
tensile modulus decrease slightly with the increase of the weight percentage of virgin
resin for the blends with virgin resin Huntsman PS 702, while the tensile strength and
tensile modulus increase slightly with the increase of the weight percentage of virgin
resin for the blends with virgin resin Nova PS 3350. The standard deviation of 12
samples at each percentage was calculated for each physical property. The average of the
standard deviations for the six blends of the tensile strength and the tensile modulus are
0.63 and 67 respectively. Fig. 5.11 illustrates the results of flexural modulus and flexural
strength. It is shown that flexural strength, like the tensile strength for the blends of
Huntsman PS 702, decreases slightly. For the blends of Nova PS 3350, flexural strength
has the same trend as tensile strength and increases slightly. However, flexural modulus
has no specific changing trend for the blends of both Huntsman PS 702 and Nova PS
3350. The average of the standard deviations over the six blends of the flexural strength
and the flexural modulus are 0.60 and 46 respectively.
202
As shown in Fig. 5.12, the impact strength of the blends of Huntsman PS 702
increases with the increase of weight percentage of recycled HIPS when the percentage is
small. At 75% and greater recycled HIPS, the strength reaches a stable value. For impact
strength of the blends of Nova PS 3350, it decreases with the increase of weight
percentage of virgin resin when the percentage is small. At 75% and greater virgin resin,
the strength reaches a stable value. The average of the standard deviation over the six
blends of the impact strength is 2.6.
Though Raman Spectroscopy and Infrared vibrational spectra show that recycled
resin and virgin resin consist of almost the same components, and the blends have similar
molecular weight and polydispersity, ESEM shows that the different blends have very
different microstructure and different rubber domain sizes. Thus, it is not surprising that
the different blends have different mechanical properties because the mechanical
properties of HIPS can be affected by the amount of rubber added, the type of rubber,
rubber size distribution, phase volume, the degree of crosslinking, or the level of adhesion
[Hobbs, 1986; Cook, et al., 1993]. The reason for the higher tensile modulus, tensile
strength, flexural strength, and impact strength of PCR compared to Huntsman PS 702
probably results from the higher tensile modulus and tensile strength of the original
material or the addition of reinforcements in pure resin when the printers and monitors
were made. Also, the experiments demonstrated that the mechanical properties of
recycled HIPS were slightly lower than those of Nova PS 3350. It is interesting to note
that the mechanical properties of blends with Huntsman PS 702 and recycled resin are
slightly better than the properties of the selected virgin material Huntsman PS 702. Our
203
experiments demonstrate that it is possible to reuse the post-consumer resin. Relative to
the selected virgin materials with the same viscosities as the post-consumer resin, reuse of
the post-consumer resin is an attractive option.
We compared our 100% PCR tensile and flexural properties with those published
in a study comparing disassembled versus shredded HIPS from post-consumer television
sets [Langerak, 1997]. It is found that the tensile modulus of our blends is lower than that
of the disassembled or shredded HIPS in the published study [Langerak, 1997]; however
the tensile strength at yield of our blends is larger. It is also shown that the flexural
modulus of our blend is lower than that of disassembled or shredded HIPS in the other
study [Langerak, 1997], but the flexural strength is almost the same. The differences in
mechanical properties of the PCR in the two studies may result from the different brands
of the original materials.
5.3.6 Application
Before making the real film cans, the injection molding process was simulated by
C-MOLD 97.7. The mesh is shown in Fig. 5.13. The simulation results are listed in
Table 5.5.
Film canisters were made using post-consumer Huntsman HI/PS 702 virgin resin
blends. To obtain initial machine settings, we used the simulation results from C-MOLD
97.7 and IDES's handbook of injection molding specs [IDEAS, 1999]. The main
difference between these resources is the processing temperature. The handbook
recommended a lower temperature, 221°C, while C-MOLD recommended a higher
204
temperature, 243°C. After experimental trials, we selected the machine settings as shown
in Table 5.5.
The tensile tests for the film canisters were performed on the Instron machine.
The results are listed in Table 5.6. It is shown that the tensile strength of the blends with
Huntsman PS 702 increases with the increase of the weight percentage of recycled resin.
The reason is that Huntsman PS 702 has lower tensile strength than the PCR and thus the
PCR increases the tensile strength of the blend. If the PCR is cheaper and has a higher
tensile strength than a virgin resin with similar rheology, then the PCR can be selected to
increase the mechanical property or properties.
5.4 CONCLUSIONS
To determine the initial processing conditions for injection molding virgin/post-
consumer resin blends, a precharacterized resin must be designated for a C-MOLD
simulation. To select a precharacterized resin for the C-MOLD simulation, virgin resin
viscosity curves were matched with the PCR viscosity curve. Then the recommended C-
MOLD simulation processing parameters were further refined for the blends for the
ASTM test standard specimens by running several experimental runs. In our proposed
approach, we can characterize and represent the PCR in a mold filling simulation by the
virgin resin in the database. Experimental testing to determine injection molding
parameters for various blends is greatly reduced by this approach.
All blends have similar molecular weight and polydispersity. Furthermore, the
recycled resin and virgin resin consist of almost the same components, as shown in their
205
Raman and infrared spectra. For the ASTM specimens molded with either set of blends,
the mechanical properties are similar. The tensile modulus, tensile strength, and flexural
strength increase slightly with the increase of the weight percentage of PCR for the
blends of Huntsman PS 702. The impact strength increases with the increase of weight
percentage of PCR when the percentage is small and finally the strength reaches a stable
value. It is found that the physical properties of blends having recycled resin are better
than the properties of virgin resin Huntsman PS 702. On the other hand, the mechanical
properties of PCR with Nova PS 3350 are slightly lower when compared to the pure
virgin Nova PS 3350 resin.
206
Materials Mn Mw Polydispersity
100% Huntsman 702 58198 180875 3.06
50% Huntsman 702 56730 171486 3.03
0% Huntsman 702 55262 162099 2.93
100% Nova 3350 54129 196963 3.64
50% Nova 3350 55724 181306 3.26
Virgin Nova 3350 57577 183095 3.18
Table 5.1 Molecular weight (Number average Mn and weight average Mw)
and polydispersity
207
No Weight percentage Of virgin resin (%)
Weight percentage of Recycled material (%)
1 100 0
2 85 15
3 75 25
4 50 50
5 25 75
6 0 100
Table 5.2 Weight percentage blends
208
Description Film canister
Maximum dimension 49.40 mm
Maximum flow length 65 mm
Volume 560 mm 3
Thickness 0.76 mm
Gate geometry Rectangular,
0.130 in wide
0.075 in deep
Projected area 730 mm 2
Table 5.3 Mold design characteristics
209
Max machine clamp force 4.90E+007 N
Max machine injection volume 0.02 m3
Max machine injection pressure 1.8E+008 Pa
Max machine injection rate 0.006667 m3/s
Fill time 2.00 s
Post-fill time 12.08 s
Mold-open time 2 s
Ambient temperature 298 K
Min/max melt temperature 449.15/533.15 K
Transition temperature 365.15 K
Inlet melt temperature 522.09 K
Average coolant temperature 298 K
Table 5.4 Processing parameters from C-MOLD
210
Resin Huntsman HI/PS 702 Coolant Pure water The maximum flow length 65 mm Thickness 0.76 mm Projected area 7.3 cm 2 Volume 0.56 cm 3 Coolant channel diameter 7 mm Clamp force 50 ton(m) Mold open time 2 s Mold temperature 34.5°C Min. Processing temperature 176°C Max. Processing temperature 260°C Max. machine inj. Press. 180 MPa Melt temperature 242.9°C Fill time 0.49 s
Table 5.5 CMOLD parameters for film canister
211
wt% of virgin resin Tensile strength (MPa) of Hunstman PS 702
injection pressure, and mold surface coatings on the appearance of the flow marks was
studied. It was found that the most important factor affecting the flow marks was
injection speed. The flow marks occurred above a critical wall shear stress, but
disappeared at high injection speeds. Mold geometry had an effect on the flow marks,
but mold temperature and melt temperature did not have much effect on the flow marks.
No difference was observed between the crystallinity of dull regions and shiny regions.
However, it was found from Scanning Electron Microscopy that the melt in dull regions
was only slightly oriented while the melt in shiny regions was highly oriented. It was
also found that coating these surfaces did not prevent the occurrence of the flow marks,
although it could alleviate them. It was also found that the polymer with the highest
dynamic viscosity, elastic modulus, first normal stress difference, transient extensional
viscosity, and longest relaxation time exhibited flow marks over a wide range of
processing conditions. Slip was not the cause of the generation of the alternate flow
marks. The generation of the flow marks was explained by an entry viscoelastic flow
instability.
226
Synchronous dull and glossy flow marks were also studied. The effect of
operating parameters, mold geometry, and mold surface coatings on the flow marks was
investigated. The flow marks occurred above a certain flow front velocity. It was also
found in the experiment that the flow marks were dimmer as the mold temperature was
increased. No difference was observed between the crystallinity of dull and shiny
regions. However, polymer was highly oriented in shiny region while it was slightly
oriented in dull regions. It was also found that mold surface coatings did not eliminate
the flow marks. Extrusion experiments showed that helical gross melt fracture occurred
for both HDPEs. Finally, it was proposed that an entry viscoelastic instability was the
reason for the generation of the synchronous flow marks.
For the future work, we will prove the mechanism of the entry flow instability.
More evidence is favored for the proposed mechanism. For example, the possible
pressure fluctuation relating to melt fracture (flow marks) will be monitored. Using the
glass window mold in our lab, the flow before the gate and the flow front will be
visualized and recorded by high-speed camcorder. Then the flow will be analyzed.
Moreover, the possibility of slip will be analyzed. The extensional viscosity will be
measured to describe the fountain flow more accurately, and its effect on the formation
of vortices will be analyzed. Furthermore, fundamental mechanism for the formation of
the flow marks will be studied. The detailed morphology, crystallinity and structure of
crystalloids, and the effective thickness of the flow marks will be investigated.
6.2 EXPERIMENTS WITH MICRO-FEATURES AND SIMULATION
ACCURACY IMPROVEMENT
Thin-wall injection molding with micro-features was studied experimentally and
numerically. The filling lengths in microchannels are affected by injection speed, mold
temperature and channel location. It was found that high injection speed or high mold
temperature results in longer filling length. Moreover, the filling lengths in
227
microchannels increase with the decrease in the filling time flowing from the
microchannels to the main flow end. Furthermore, the filling lengths in microchannels
are simulated by a hybrid simulation code with a combination of the momentum
equation and the Hele-Shaw model, and compared with experimental results. The code
has fewer elements and requires less computation time. The simulation shows that the
filling lengths in microchannels are sensitive to the heat transfer coefficients in the main
flow cavity and in the microchannel and extra attention is needed to determine proper
heat transfer coefficient. Using the variable heat transfer coefficient, the filling length in
the long mold is predicted accurately.
Our future work will study the thin-wall injection molding of smaller
microchannels with the width of 50 µm and the depth of 250 µm. The morphology of
the microchannels, demolding problem, filling, freezing pattern, repeatability, durability,
and the deformation of the wall of the microchannels will be studied. Moreover, the
filling lengths in microchannels with different main flow thicknesses will be compared
to study which main flow thickness is beneficial to long filling lengths. The argument is
that in the thick mold the melt temperature is high but the pressure drop is low; in the
thin mold the temperature is low but the pressure drop is high. So it is difficult to decide
which mold thickness is favorable to long filling lengths. Furthermore, the filling length
will be measured and it will be compared with simulation results. The effect of heat
transfer coefficients both in the main flow and in the microchannels will be paid full
attention.
For the cavity pressure, the simulation showed that the effect of pressure on the
specific volume is the most important factor to predict the peak cavity pressure. The
effect of pressure on the viscosity and the heat transfer coefficient are also significant.
The heat capacity and the juncture loss are relatively less important compared to other
factors considered here. Therefore, it is very important to use proper material property
228
models when running simulation of thin-wall injection molding. It was also shown that
the significant factors are somewhat different to predict maximum cavity pressure drop.
The effect of the pressure-dependent viscosity, the heat capacity, the heat transfer
coefficient, the juncture pressure loss and the pvT-data on the cavity pressure and
pressure drop were studied. Another important thermal property, thermal conductivity,
would be included for future work. Furthermore, future work could study the effect of
these properties on the filling length in main flow and even in microchannels.
When the injection speed was high, the discrepancy between the simulation
results and experimental data was large and no good agreement could be achieved no
matter what property models were used. So, the reason for the discrepancy might not be
included within the factors we considered. The possible reason may be the difference
between the set operating values and the actual conditions the machine reached. For
example, the actual injection speed is intrinsically slower than the speed one sets,
especially at high injection speeds, as the machine needs response time to reach the
desired constant injection speed. The actual temperature in the barrel may be different
from the set temperature. The effect of these differences should be checked.
Material property measurement and models will affect the simulation results and
proper conclusions. The pressure dependent viscosity was measured under relatively
low pressure and then extrapolated to high pressure. Future work should measure the
viscosity under very high pressure to get a more accurate pressure dependent viscosity
model. The heat capacity was measured at a low heating rate of 3.33ºC/s. It is very
useful to get the “true” value because the cooling rate is very fast in thin-wall injection
processes. The heat transfer coefficient has a large effect on the cavity pressure and the
default value 25,000 W/m2⋅K must be re-evaluated to obtain good simulation results
because other researchers’ work and the current work showed that the default is too
large.
229
Finally, the software itself may affect the final pressure prediction due to its
simplification, such as the assumption of Hele-Shaw flow. Hele-Shaw flow neglects
flow in the gapwise direction and gives the average information in the gapwise direction.
It cannot accurately predict the fluid behavior at the flow front and the flow near or at
solid walls, the phenomenon occurring at the merging of two or more streams (weld
lines), and the kinematics in ribs, gates, or sudden contractions/enlargement. Moldflow
is a 22
1D software and uses mid-plane mesh. So, developing the code with less
assumptions or 3-D mesh based on our group’s previous work may provide more
accurate pressure prediction.
6.3 REUSE OF HIPS
This part focuses on the mechanical and rheological properties of virgin and
recycled high impact polystyrene materials. The study shows that all blends have similar
molecular weight and polydispersity. Furthermore, the recycled resin and virgin resin
consist of almost the same components, as shown in their Raman and infrared spectra.
For the ASTM specimens molded with either set of blends, the mechanical properties are
similar. The tensile modulus, tensile strength, and flexural strength increase slightly
with the increase of the weight percentage of PCR for the blends of Huntsman PS 702.
The impact strength increases with the increase of weight percentage of PCR when the
percentage is small and finally the strength reaches a stable value. It is found that the
physical properties of blends having recycled resin are better than the properties of virgin
resin Huntsman PS 702. On the other hand, the mechanical properties of PCR with
Nova PS 3350 are slightly lower when compared to the pure virgin Nova PS 3350 resin.
Our experiments demonstrate that the PCR may have good material properties and may
even be used in a more challenging application.
230
Moreover, the study introduces a new approach to determine initial processing
parameters for injection molding of post-consumer resin. To determine the initial
processing conditions for injection molding virgin/post-consumer resin blends, we can
characterize and represent the PCR in a mold filling simulation by the virgin resin in the
database. This approach greatly reduces experimental testing to determine injection
molding parameters for various blends. This approach for plastics recycling is novel
because we started with an initial rheological investigation of PCR characteristics rather
than tracking the original virgin resin. We also tested our new approach by molding the
PCR in a thinner wall design application.
There are several areas of this research requiring more study in future. In our
study, two different virgin resins were identified by PCR characterization. Both
candidates had similar viscosity versus shear rate curves, but different melt flow indices.
Because plastics have complex properties, further study is needed to identify the
properties of the unknown PCR, and then find virgin resins that match additional
characteristics, such as mechanical properties. Then, the mechanical properties of a
specific design with different percentage of PCR will be predicted. It will further
improve the decision tool to decide the threshold of recycling.
Our study used HIPS from computer and monitor housing. More cases are
needed to get more general conclusions. To investigate the sensitivity of our approach to
grade mixtures is an interesting extension of this work. Because shredding different
plastic parts may generate a reground mixture of HIPS PCR grades, it will be useful to
determine whether the viscosity versus shear rates of the resin grade mixture could be
used to identify a proxy virgin resin.
231
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