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Shear Modification Mapping within Injection Moulded Product through Mould Filling Simulation and Molecular Degradation Assessment
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 17 (2019) 1024–1032
www.materialstoday.com/proceedings
RAMM 2018
Shear Modification Mapping within Injection Moulded Product through Mould Filling Simulation and Molecular Degradation Assessment H. W. Maya and Z. M. Ariffa* a
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia
Abstract This study gave an insight on the effect of shear modification on molecular degradation of polyamide (Nylon) 6. It also studied the feasibility of utilizing mould filling simulation package to predict shear modification and degradation in injection molded product without the need for tedious molecular weight assessment. Injection molding procedure and mould filling simulations were implemented by varying mould configuration, injection pressure and melt temperature. Viscosity number test was performed to check the molecular weight changes of the samples with respect to the moulding parameters. Results showed that shear modifications caused some level of molecular degradation within the samples by lowering the molecular weight if compared with that of obtained from the original pellets. Shear modification was found to significantly affected by the mould configuration, melt temperature and injection pressure as displayed by the mould filling simulation results. The simulation results have also successfully shown useful correlation between shear modification mapping and molecular degradation measurement obtained via viscosity number test. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: Shear Modification; Mould Filling Simulation; Molecular Degradation
* Corresponding author. Tel.: +604-5996173; fax: +604-5996907. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
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1. Introduction Shear modification also known as shear deformation is always available in most plastics processing procedures such as extrusion and injection molding. When shear modification is subjected to long chain branched molecules like polymers, it will imparts physical changes in macromolecular structure of the polymer [1-3]. In injection molding, high shear modification are often cause by the complex geometries of the mold cavity and most prevalent in the areas such as the gate and nozzle of the injection molding machine where the flow at that particular area are restricted or limited. As a result, regions of high and low shear exist within a product and these regions often cause warpage, shrinkage and cracking due to degradation and high moulded-in stresses [4,5]. Degradation of a polymer can manifest itself as a reduction of molecular weight, separation of polymer and additives, or it may result in a chemical reaction [6]. As polymer are built from many small building blocks, molecular weight acts as a determining factor to many of the plastic’s physical properties. Evaluation of molecular weight of the injection molded product is projected to be a good approach to monitor extent of degradation of a product due to shear modification during injection molding. This study aims to correlate results from shear modification mapping using mould filling simulation with experimental degradation assessment of injection molded product through viscosity number evaluation (VN) of polyamide polymer. The investigation was strategized so that the correlation can be established to predict shear modification and degradation within the injection moulded component. The outcome of the study is hoped to be able to potentially replace the tedious experimental molecular degradation assessment when a quick and reliable technique to gauge molecular degradation is needed. 2. Experimental 2.1 Materials In this study, polyamide (Nylon 6) with the grade of Amilan CM 1017 manufactured Toray Plastics (Malaysia) Sdn. Bhd. was used. This grade of polyamide 6 was an easy flowing, injection molding grade resin that has selfextinguishing properties. The material was supplied in pellet form and known to be hygroscopic. Therefore, the pellets were dried in a hot oven at 80˚C for 4 hours prior to injection molding and viscosity number testing. 2.2 Processing via Injection Molding Processing the polyamide (nylon) 6 via injection molding was conducted using Dr. Boy injection moulding machine (model BOY 22M). There three processing variables were investigated namely, gate configuration (single gated and two gated), melt temperature and injection pressure (Table 1). Table 1. Processing variable for the injection moulding procedure
Gate Configuration
Melt Temperature (˚C)
Single gated 250, 260 and 270 Two gated
Injection Pressure (bar) (Initial – Final) 15-5 30-10 50-30
A total of 5 samples were obtained in every set of injection molding with variation in injection pressure, temperature and mould wall temperature. Samples were clearly labeled indicating which samples were obtained from which injection molding parameter.
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2.3 Molecular Degradation Assessment (Viscosity Number Test – ISO 307) Viscosity number test was conducted in accordance to ISO 307:2007 (E) for dilute solutions of polyamides in formic acid with concentration of 90.00% ± 0.15% adjusted through dilution process. The analysis was done on both pellet (raw material) and moulded samples. About 25 mg of sample was dissolved in a predetermined formic acid amount to form a dilute solution of the polyamide at a concentration of 0.005 g/ml. The principle of ISO 307 was based on measurement flow time of the solvent used and the prepared dilute solution with a concentration of 0.005 g/ml at 25˚C, using Ubbelohde viscometer. The viscosity number was calculated from the flow time measurements using the following equation; =
− 1 ×
(1)
where, t t c VN
= flow time of solution of the dissolved polymer samples, in seconds; = flow time of the solvent, expressed in seconds; = concentration, in grams per millitre, of the polymer in the solution; = viscosity number, expressed in millitres per gram.
Determination of VN of a polyamide provides a value that can be connected indirectly to the molecular mass of the polymer. Hence, viscosity number would give indication of changes in molecular mass which in turn acts as an assessment of the degree of molecular degradation within the tested sample [7]. 2.4 Shear Mapping Using Mould Filling Simulation Mould filling simulation investigation was implemented using Cadmould software package (Simcon GMBH). The package was utilized to perform mould filling simulations with preset conditions that emulate the actual injection molding procedure. In this study, Cadmould 3D-F Version 2.0 was used to perform all the related simulations. To conduct such simulation procedure, a 3D CAD model was constructed using Solidworks software following the exact dimension of the evaluated dumbbell mould cavity. The .prt (part) file was then converted into .stl (stereolitography) file before being input into the Cadmould software. After selecting the materials from the Cadmould database and setting all the simulated processing parameters, the simulation was initiated and the simulated flow pattern of the plastic was mapped out. Three types of variables namely; gate configuration, melt temperature and injection pressure, were used to determine effects of processing variation on shear modification experienced by polymer molecules within the injection molded product. Results obtained via simulation were evaluated on melt flow velocity and shear stress in order to relate them with shear modification indication given by the experimental molecular degradation assessment (i.e. viscosity number determination) 3. Results and Discussion 3.1 Effect of mould configuration Viscosity Number (VN) result shows that shear modification is significantly dependent to processing history. As seen in Fig. 1, pellet sample which did not experience any molding history gives the highest viscosity number reading as compared to the both injection moulded samples. Pellets that are not subjected to shear modification in processing will definitely have higher viscosity number which corresponds to higher molecular weight. Whereas, the injection moulded samples recorded reductions in VN values relative to the pellet sample. Furthermore, it can be seen that sample produced with a single gated configuration experienced the highest shear modification as reflected by its low VN value.
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
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Fig. 1. Comparison of viscosity number obtained for injection molded samples of default parameter and the original pellets.
Single gated configuration has more restricted flow path where all molten polymers are forced to flow in one direction into the cavity (refer to Fig. 2). Combined with high speed and pressure in the injection molding pressure, restricted flow and longer flow path, the molding procedure imparted a higher level of shear modification on the product. It took longer time (0.143 s) for the polymer to completely fill the mould cavity if the single gated configuration was used and hence, caused higher extent of shear modification to polymer molecules in comparison to the two gated configuration. In the case of two gated dumbbell, the symmetrical flow path make the flow much easier and faster (0.033 s). Pressure and shear stress will be distributed equally to the two gates during filling, thus, lowering the extent of thermomechanical degradation. Shear modifications cause thermomechanical degradation to occur through macromolecular chain scissions to both injection molded samples, thus, decreasing the molecular weight of these samples giving them relatively lower viscosity numbers.
Fig. 2. Simulated mould filling time of the single gated (left) and two gated (right) mould configurations.
Another aspect of shear modification within the mould cavity was the variation of VN value with respect to location of sampling. Shear modification mapping was done by determining VN at three different locations away
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from the sprue position. Results in Fig. 3 shows that the VN values for the runner section is the highest as it corresponds to lower shear modification. The VN values for the middle position of the dumbbell for both gate configurations came in second after the runner section. However for the gate location, the single gated configuration exhibit a slight increase in VN while the two gated dumbbell shows a further decrease trend.
Fig. 3. Comparison of viscosity number obtained from different locations within the moulded samples
To understand better the observed trend of VN variation, mapping of shear level (monitored via melt flow velocity) within the mold cavity was simulated and the results are shown in Fig. 4. The sampling locations are indicated by the red marked regions (square for runner, triangle for gate and circle for middle section).
Fig. 4. Shear modification mapping monitored through the progress of melt flow velocity within the single gated (left) and double gated (right. configurations
Color gradient shows that the runner segment have the lowest melt velocity, the middle of the dumbbell shows a medium melt velocity and the gate area shows the highest velocity. This corresponds to low shear modification in runner section in comparison to medium shear modification in the middle of the dumbbell and the highest shear modification in the gating section. The gate section experienced the highest shear modification due to the build-up pressure and a sudden change in volume and thickness of the part.
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
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If the melt velocity pattern for single gated configuration is scrutinized, it can be seen that lower melt velocity can be detected in the middle section in comparison to gate section of the sample. This indicates that the polymer molecules experienced lower extent of shear modification (i.e. higher VN value) and the findings is contradicting with the one presented in Fig. 3. It is projected that the difference is due the fact that only the surface melt velocity was displayed in the simulated shear modification mapping. It is well known that when the molten polymer touches the relatively cold mould wall surface, the melt velocity tends to reduce due to increase in viscosity as a result of solidification process [4]. However, molten polymer in the core is still moving and being shear modified extensively. When the sampling was done for VN determination, the surface and core were extracted and the result was able to detect the extensively sheared molecules which is not mapped by the simulation representation of the surface melt velocity. 3.2 Effect of Melt Temperature Results from the experimental VN determination presented in Fig. 5 shows that viscosity number decreases with increasing melt temperature. The finding confirms the projection higher shear modification is available at elevated temperatures. Both single gated and two gated dumbbell experienced the same decreasing trend but steeper change in VN was observed for the single gated configuration.
Fig. 5. Viscosity number (VN) variation for injection moulded samples produced at different melt temperatures.
The actual temperature molten polymer during processing often exceeds the preset processing temperature due to combination of viscous heat dissipation from shearing of the polymeric chains and heat from external heaters. Since, melting point of the polyamide (nylon) 6 raw materials is relatively high (i.e. around 225˚C), extreme heat and shear modification are available and can cause thermal and mechanical degradation to the polymer molecules during processing. As a result, lower molecular weight (i.e. lower VN) can be detected within the moulded samples. Comparatively, VN for the two gated dumbbell is higher than the single gated dumbbell. Restriction in flow for the single gated dumbbell imposes higher shear modification on the sample. Meanwhile, the two gated dumbbell having two flow paths will ease the high pressure during moulding causing the material to flow easily into the cavity without much restriction. The above results can be supported by mould filling simulation where the variation in melt flow velocities in the samples can be observed at different melt temperatures. Fig. 6 displays the differences in velocity gradient between the three, single gated samples simulated at different melt temperatures. The highest melt temperature, 270 ˚C, exhibits the highest melt velocity value followed by melt temperature of 260 ˚C and the lowest is melt temperature
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250 ˚C. Location of the observation is concentrated in the middle of the dumbbell model where noticeable changes in velocities can be seen. High velocity will eventually cause high shear rate which subsequently imposed high extent of shear modification to the polymer molecules during high melt temperature processing. Therefore, it is established that elevated temperature imposes higher thermal shear stresses to the sample thus promoting the molecular degradation within the samples.
o
250 C
o
260 C
o
270 C
Fig. 6. Shear modification mapping monitored through the progress of melt flow velocity within the single gated at different melt temperatures
3.3 Effect Injection Pressure Effect of injection pressure on the VN values of the moulded samples is shown in Fig. 7. It can be observed that VN decreases with increasing pressure for both single gated and two gated mould configurations. In this investigation, injection pressures were set in 2 stages where the initial stage is a higher value than the final stage. The higher value will hold about 89.8% of filling time and the rest of the filling time will be at lower value pressure.
Fig. 7. Viscosity number (VN) variation for injection moulded samples produced with different injection pressure
At low injection pressure (15 – 5 bar), viscosity number for the single gated configuration (VN = 130.9 ml/g) and the two gated configuration (VN = 127.9 ml/g) show values just slightly lower from the VN value of the unprocessed original pellets (VN = 132.4 ml/g). This proves the lesser impact of shear modification on the polymer when the
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
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processing is done at low pressure and low injection speed. However, despite the lesser extent of shear modification, processing at low pressure would risk short molding of the product. To strengthen the VN evaluation, mould filling simulation result was consulted. Effect of injection pressure on shear stress is displayed in Fig. 8 for the single gated configuration. The simulation representations confirm the VN trend changes when high injection pressure was used.
30-10 bar 30-10 bar
15-5 bar
50-30 bar
Fig. 8. Simulated shear stresses mapping for different injection pressures in the single gated configuration
From the result, it can be projected that more shear stress will be experienced for higher injection pressure. Higher pressure provides more force per unit area thus making the injection speed faster. Faster injection molding speed increases the shear modification within the polymer melt inducing more macromolecular chain scission and as a result, it gives way for more molecular degradation to occur. Apart from that, higher injection speed will cause high shearing in the barrel of the injection molding machine and this will cause more viscous heat dissipation to occur generating additional thermal degradation on the polymeric melt. 4. Conclusion Based on viscosity number and mould filling simulation results, it can be concluded that the extent of injection molding activities imparts shear modifications which causes molecular degradation on both single gated dumbbell and two gated dumbbell resulting in lower molecular weight as compared to the original pellet. Single gated dumbbell experiments displayed higher extent of shear modification due to its restricted flow in comparison to the symmetrical flow path provided by the two gated configuration. Results from both viscosity number testing and mould filling simulation showed that shear modification increases with increasing melt temperature. Effect of injection pressure on shear modification is the most significant as high pressures caused higher extent shear modification and subsequently gave higher molecular degradation. Mould filling simulations provided useful relationship between shear stresses and melt flow velocities to the shear modification that occurred during the moulding process. 5. Acknowledgments The authors gratefully acknowledge the Ministry of Higher Education for the financial support given through Fundamental Research Grant Scheme (Ref. No.: FRGS/1/2014/TK04/USM/02/1). 6. References [1] G. Breuer & A. Schausberger (2011). Recovery of Shear Modification of Polypropylene Melts. Rheologica Acta, 50(5-6), 461-468. [2] Y.C. Kim, Y. C., K.S. Yang, & C.H. Choi (1998). Study of the relationship between shear modification and melt fracture in extrusion of LDPE. Journal of Applied Polymer Science, 70(11), 2187-2195. [3] M. Van Prooyen, T. Bremner, & A. Rudin (1994). Mechanism of shear modification of low density polyethylene. Polymer Engineering and Science, 34(7), 570-579 [4] G. Pötsch, & W. Michaeli (2008). Injection Molding: An Introduction (2nd edn.). Munich: Carl Hanser Verlag.
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[5] T Yizong, Z.M. Ariff and K.G. Liang (2017), Evaluation of Weld Line Strength in Low Density Polyethylene Specimens by Optical Microscopy and Simulation”, Journal of Engineering Science, 13, 53-62. [6] J.P. Beaumont, (2004). Runner and Gating Design Handbook: Tools for Successful Injection Molding. Munich: Hanser Gardner Publications. [7] T. Kilp, T., & J.E. Guillet (1977). A Rapid Procedure for the Determination of Viscosity-Molecular Weight Relations. Macromolecules, 10(1), 90 - 94.
ScienceDirect Materials Today: Proceedings 17 (2019) 1024–1032
www.materialstoday.com/proceedings
RAMM 2018
Shear Modification Mapping within Injection Moulded Product through Mould Filling Simulation and Molecular Degradation Assessment H. W. Maya and Z. M. Ariffa* a
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia
Abstract This study gave an insight on the effect of shear modification on molecular degradation of polyamide (Nylon) 6. It also studied the feasibility of utilizing mould filling simulation package to predict shear modification and degradation in injection molded product without the need for tedious molecular weight assessment. Injection molding procedure and mould filling simulations were implemented by varying mould configuration, injection pressure and melt temperature. Viscosity number test was performed to check the molecular weight changes of the samples with respect to the moulding parameters. Results showed that shear modifications caused some level of molecular degradation within the samples by lowering the molecular weight if compared with that of obtained from the original pellets. Shear modification was found to significantly affected by the mould configuration, melt temperature and injection pressure as displayed by the mould filling simulation results. The simulation results have also successfully shown useful correlation between shear modification mapping and molecular degradation measurement obtained via viscosity number test. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: Shear Modification; Mould Filling Simulation; Molecular Degradation
* Corresponding author. Tel.: +604-5996173; fax: +604-5996907. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
1025
1. Introduction Shear modification also known as shear deformation is always available in most plastics processing procedures such as extrusion and injection molding. When shear modification is subjected to long chain branched molecules like polymers, it will imparts physical changes in macromolecular structure of the polymer [1-3]. In injection molding, high shear modification are often cause by the complex geometries of the mold cavity and most prevalent in the areas such as the gate and nozzle of the injection molding machine where the flow at that particular area are restricted or limited. As a result, regions of high and low shear exist within a product and these regions often cause warpage, shrinkage and cracking due to degradation and high moulded-in stresses [4,5]. Degradation of a polymer can manifest itself as a reduction of molecular weight, separation of polymer and additives, or it may result in a chemical reaction [6]. As polymer are built from many small building blocks, molecular weight acts as a determining factor to many of the plastic’s physical properties. Evaluation of molecular weight of the injection molded product is projected to be a good approach to monitor extent of degradation of a product due to shear modification during injection molding. This study aims to correlate results from shear modification mapping using mould filling simulation with experimental degradation assessment of injection molded product through viscosity number evaluation (VN) of polyamide polymer. The investigation was strategized so that the correlation can be established to predict shear modification and degradation within the injection moulded component. The outcome of the study is hoped to be able to potentially replace the tedious experimental molecular degradation assessment when a quick and reliable technique to gauge molecular degradation is needed. 2. Experimental 2.1 Materials In this study, polyamide (Nylon 6) with the grade of Amilan CM 1017 manufactured Toray Plastics (Malaysia) Sdn. Bhd. was used. This grade of polyamide 6 was an easy flowing, injection molding grade resin that has selfextinguishing properties. The material was supplied in pellet form and known to be hygroscopic. Therefore, the pellets were dried in a hot oven at 80˚C for 4 hours prior to injection molding and viscosity number testing. 2.2 Processing via Injection Molding Processing the polyamide (nylon) 6 via injection molding was conducted using Dr. Boy injection moulding machine (model BOY 22M). There three processing variables were investigated namely, gate configuration (single gated and two gated), melt temperature and injection pressure (Table 1). Table 1. Processing variable for the injection moulding procedure
Gate Configuration
Melt Temperature (˚C)
Single gated 250, 260 and 270 Two gated
Injection Pressure (bar) (Initial – Final) 15-5 30-10 50-30
A total of 5 samples were obtained in every set of injection molding with variation in injection pressure, temperature and mould wall temperature. Samples were clearly labeled indicating which samples were obtained from which injection molding parameter.
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2.3 Molecular Degradation Assessment (Viscosity Number Test – ISO 307) Viscosity number test was conducted in accordance to ISO 307:2007 (E) for dilute solutions of polyamides in formic acid with concentration of 90.00% ± 0.15% adjusted through dilution process. The analysis was done on both pellet (raw material) and moulded samples. About 25 mg of sample was dissolved in a predetermined formic acid amount to form a dilute solution of the polyamide at a concentration of 0.005 g/ml. The principle of ISO 307 was based on measurement flow time of the solvent used and the prepared dilute solution with a concentration of 0.005 g/ml at 25˚C, using Ubbelohde viscometer. The viscosity number was calculated from the flow time measurements using the following equation; =
− 1 ×
(1)
where, t t c VN
= flow time of solution of the dissolved polymer samples, in seconds; = flow time of the solvent, expressed in seconds; = concentration, in grams per millitre, of the polymer in the solution; = viscosity number, expressed in millitres per gram.
Determination of VN of a polyamide provides a value that can be connected indirectly to the molecular mass of the polymer. Hence, viscosity number would give indication of changes in molecular mass which in turn acts as an assessment of the degree of molecular degradation within the tested sample [7]. 2.4 Shear Mapping Using Mould Filling Simulation Mould filling simulation investigation was implemented using Cadmould software package (Simcon GMBH). The package was utilized to perform mould filling simulations with preset conditions that emulate the actual injection molding procedure. In this study, Cadmould 3D-F Version 2.0 was used to perform all the related simulations. To conduct such simulation procedure, a 3D CAD model was constructed using Solidworks software following the exact dimension of the evaluated dumbbell mould cavity. The .prt (part) file was then converted into .stl (stereolitography) file before being input into the Cadmould software. After selecting the materials from the Cadmould database and setting all the simulated processing parameters, the simulation was initiated and the simulated flow pattern of the plastic was mapped out. Three types of variables namely; gate configuration, melt temperature and injection pressure, were used to determine effects of processing variation on shear modification experienced by polymer molecules within the injection molded product. Results obtained via simulation were evaluated on melt flow velocity and shear stress in order to relate them with shear modification indication given by the experimental molecular degradation assessment (i.e. viscosity number determination) 3. Results and Discussion 3.1 Effect of mould configuration Viscosity Number (VN) result shows that shear modification is significantly dependent to processing history. As seen in Fig. 1, pellet sample which did not experience any molding history gives the highest viscosity number reading as compared to the both injection moulded samples. Pellets that are not subjected to shear modification in processing will definitely have higher viscosity number which corresponds to higher molecular weight. Whereas, the injection moulded samples recorded reductions in VN values relative to the pellet sample. Furthermore, it can be seen that sample produced with a single gated configuration experienced the highest shear modification as reflected by its low VN value.
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
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Fig. 1. Comparison of viscosity number obtained for injection molded samples of default parameter and the original pellets.
Single gated configuration has more restricted flow path where all molten polymers are forced to flow in one direction into the cavity (refer to Fig. 2). Combined with high speed and pressure in the injection molding pressure, restricted flow and longer flow path, the molding procedure imparted a higher level of shear modification on the product. It took longer time (0.143 s) for the polymer to completely fill the mould cavity if the single gated configuration was used and hence, caused higher extent of shear modification to polymer molecules in comparison to the two gated configuration. In the case of two gated dumbbell, the symmetrical flow path make the flow much easier and faster (0.033 s). Pressure and shear stress will be distributed equally to the two gates during filling, thus, lowering the extent of thermomechanical degradation. Shear modifications cause thermomechanical degradation to occur through macromolecular chain scissions to both injection molded samples, thus, decreasing the molecular weight of these samples giving them relatively lower viscosity numbers.
Fig. 2. Simulated mould filling time of the single gated (left) and two gated (right) mould configurations.
Another aspect of shear modification within the mould cavity was the variation of VN value with respect to location of sampling. Shear modification mapping was done by determining VN at three different locations away
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from the sprue position. Results in Fig. 3 shows that the VN values for the runner section is the highest as it corresponds to lower shear modification. The VN values for the middle position of the dumbbell for both gate configurations came in second after the runner section. However for the gate location, the single gated configuration exhibit a slight increase in VN while the two gated dumbbell shows a further decrease trend.
Fig. 3. Comparison of viscosity number obtained from different locations within the moulded samples
To understand better the observed trend of VN variation, mapping of shear level (monitored via melt flow velocity) within the mold cavity was simulated and the results are shown in Fig. 4. The sampling locations are indicated by the red marked regions (square for runner, triangle for gate and circle for middle section).
Fig. 4. Shear modification mapping monitored through the progress of melt flow velocity within the single gated (left) and double gated (right. configurations
Color gradient shows that the runner segment have the lowest melt velocity, the middle of the dumbbell shows a medium melt velocity and the gate area shows the highest velocity. This corresponds to low shear modification in runner section in comparison to medium shear modification in the middle of the dumbbell and the highest shear modification in the gating section. The gate section experienced the highest shear modification due to the build-up pressure and a sudden change in volume and thickness of the part.
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
1029
If the melt velocity pattern for single gated configuration is scrutinized, it can be seen that lower melt velocity can be detected in the middle section in comparison to gate section of the sample. This indicates that the polymer molecules experienced lower extent of shear modification (i.e. higher VN value) and the findings is contradicting with the one presented in Fig. 3. It is projected that the difference is due the fact that only the surface melt velocity was displayed in the simulated shear modification mapping. It is well known that when the molten polymer touches the relatively cold mould wall surface, the melt velocity tends to reduce due to increase in viscosity as a result of solidification process [4]. However, molten polymer in the core is still moving and being shear modified extensively. When the sampling was done for VN determination, the surface and core were extracted and the result was able to detect the extensively sheared molecules which is not mapped by the simulation representation of the surface melt velocity. 3.2 Effect of Melt Temperature Results from the experimental VN determination presented in Fig. 5 shows that viscosity number decreases with increasing melt temperature. The finding confirms the projection higher shear modification is available at elevated temperatures. Both single gated and two gated dumbbell experienced the same decreasing trend but steeper change in VN was observed for the single gated configuration.
Fig. 5. Viscosity number (VN) variation for injection moulded samples produced at different melt temperatures.
The actual temperature molten polymer during processing often exceeds the preset processing temperature due to combination of viscous heat dissipation from shearing of the polymeric chains and heat from external heaters. Since, melting point of the polyamide (nylon) 6 raw materials is relatively high (i.e. around 225˚C), extreme heat and shear modification are available and can cause thermal and mechanical degradation to the polymer molecules during processing. As a result, lower molecular weight (i.e. lower VN) can be detected within the moulded samples. Comparatively, VN for the two gated dumbbell is higher than the single gated dumbbell. Restriction in flow for the single gated dumbbell imposes higher shear modification on the sample. Meanwhile, the two gated dumbbell having two flow paths will ease the high pressure during moulding causing the material to flow easily into the cavity without much restriction. The above results can be supported by mould filling simulation where the variation in melt flow velocities in the samples can be observed at different melt temperatures. Fig. 6 displays the differences in velocity gradient between the three, single gated samples simulated at different melt temperatures. The highest melt temperature, 270 ˚C, exhibits the highest melt velocity value followed by melt temperature of 260 ˚C and the lowest is melt temperature
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250 ˚C. Location of the observation is concentrated in the middle of the dumbbell model where noticeable changes in velocities can be seen. High velocity will eventually cause high shear rate which subsequently imposed high extent of shear modification to the polymer molecules during high melt temperature processing. Therefore, it is established that elevated temperature imposes higher thermal shear stresses to the sample thus promoting the molecular degradation within the samples.
o
250 C
o
260 C
o
270 C
Fig. 6. Shear modification mapping monitored through the progress of melt flow velocity within the single gated at different melt temperatures
3.3 Effect Injection Pressure Effect of injection pressure on the VN values of the moulded samples is shown in Fig. 7. It can be observed that VN decreases with increasing pressure for both single gated and two gated mould configurations. In this investigation, injection pressures were set in 2 stages where the initial stage is a higher value than the final stage. The higher value will hold about 89.8% of filling time and the rest of the filling time will be at lower value pressure.
Fig. 7. Viscosity number (VN) variation for injection moulded samples produced with different injection pressure
At low injection pressure (15 – 5 bar), viscosity number for the single gated configuration (VN = 130.9 ml/g) and the two gated configuration (VN = 127.9 ml/g) show values just slightly lower from the VN value of the unprocessed original pellets (VN = 132.4 ml/g). This proves the lesser impact of shear modification on the polymer when the
H.W. May and Z.M. Ariff / Materials Today: Proceedings 17 (2019) 1024–1032
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processing is done at low pressure and low injection speed. However, despite the lesser extent of shear modification, processing at low pressure would risk short molding of the product. To strengthen the VN evaluation, mould filling simulation result was consulted. Effect of injection pressure on shear stress is displayed in Fig. 8 for the single gated configuration. The simulation representations confirm the VN trend changes when high injection pressure was used.
30-10 bar 30-10 bar
15-5 bar
50-30 bar
Fig. 8. Simulated shear stresses mapping for different injection pressures in the single gated configuration
From the result, it can be projected that more shear stress will be experienced for higher injection pressure. Higher pressure provides more force per unit area thus making the injection speed faster. Faster injection molding speed increases the shear modification within the polymer melt inducing more macromolecular chain scission and as a result, it gives way for more molecular degradation to occur. Apart from that, higher injection speed will cause high shearing in the barrel of the injection molding machine and this will cause more viscous heat dissipation to occur generating additional thermal degradation on the polymeric melt. 4. Conclusion Based on viscosity number and mould filling simulation results, it can be concluded that the extent of injection molding activities imparts shear modifications which causes molecular degradation on both single gated dumbbell and two gated dumbbell resulting in lower molecular weight as compared to the original pellet. Single gated dumbbell experiments displayed higher extent of shear modification due to its restricted flow in comparison to the symmetrical flow path provided by the two gated configuration. Results from both viscosity number testing and mould filling simulation showed that shear modification increases with increasing melt temperature. Effect of injection pressure on shear modification is the most significant as high pressures caused higher extent shear modification and subsequently gave higher molecular degradation. Mould filling simulations provided useful relationship between shear stresses and melt flow velocities to the shear modification that occurred during the moulding process. 5. Acknowledgments The authors gratefully acknowledge the Ministry of Higher Education for the financial support given through Fundamental Research Grant Scheme (Ref. No.: FRGS/1/2014/TK04/USM/02/1). 6. References [1] G. Breuer & A. Schausberger (2011). Recovery of Shear Modification of Polypropylene Melts. Rheologica Acta, 50(5-6), 461-468. [2] Y.C. Kim, Y. C., K.S. Yang, & C.H. Choi (1998). Study of the relationship between shear modification and melt fracture in extrusion of LDPE. Journal of Applied Polymer Science, 70(11), 2187-2195. [3] M. Van Prooyen, T. Bremner, & A. Rudin (1994). Mechanism of shear modification of low density polyethylene. Polymer Engineering and Science, 34(7), 570-579 [4] G. Pötsch, & W. Michaeli (2008). Injection Molding: An Introduction (2nd edn.). Munich: Carl Hanser Verlag.
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