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Effect of mold temperature on the structures and mechanical properties of micro-injection molded polypropylene
Materials and Design 88 (2015) 245–251
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Effect of mold temperature on the structures and mechanical properties of micro-injection molded polypropylene Jing Jiang a, Shiwei Wang a,⁎, Bo Sun a, Shuaijiang Ma a, Jianming Zhang a, Qian Li a,⁎, Guo-Hua Hu b a b
National Center For International Joint Research of Micro-Nano Molding Technology, School of Mechanics & Engineering Science, Zhengzhou University, 450001 Zhengzhou, Henan, China Laboratory of Reactions and Process Engineering (LRGP), CNRS-University of Lorraine, 1 rue Grandville, BP 20451, 54001 Nancy, France
a r t i c l e
i n f o
Article history: Received 29 March 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 5 September 2015 Keywords: Micro-injection molding Mold temperature Micro-mechanical properties Morphology Microstructure
a b s t r a c t The effects of the mold temperature of micro-injection molding on the microstructures and mechanical properties of isotactic polypropylene (PP) gears were studied. The micro-injection molded PP gears present a skin–core type of morphology. The core layer is much thicker than the core and shear layers. The generation of β polymorph in the test samples is easily promoted during the micro-molding process with high shear rate at low temperature, or low shear rate at high temperature. Nanoindentation tests show that the modulus decreases along the flow direction and increases with increasing mold temperature. The highly oriented shear layer has the highest nanoindentation modulus compared with the skin and core layers. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Micro-injection molding (MIM) technology is expected to hold great potential in the mass production of good quality polymer micro-parts [1]. One of the main challenges associated with micro-molding resides in difficulties of controlling structures or mechanical properties of the parts [2–3]. MIM parts often exhibit a “skin–core” structure which is similar to that of conventional injection molding parts, expect that the oriented layer represents a large fraction [4]. A method was developed to explore the morphology evolution of micro-injection molded parts [5]. However, more detailed works remain to be done to explore the relationship between microstructure and mechanical properties of MIM parts. Nanoindentation could be a good mechanical test, especially for micro- or nano-composites [6–8]. Shokrieh found that the mechanical properties of polymers were improved significantly with the addition of low amounts of graphene nano-platelets [9]. During a nanoindentation test, mechanical properties are measured by the displacement that occurs when a small load is applied to the surface of the specimen using a diamond-tipped indenter. The region of the indenter tip during the test is typically less than 1 mm in diameter, allowing mechanical properties to be measured precisely in an extremely small area of the specimen. So nanoindentation test is a good means to measure the
⁎ Corresponding authors. E-mail addresses: [email protected] (S. Wang), [email protected] (Q. Li).
http://dx.doi.org/10.1016/j.matdes.2015.09.003 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
mechanical properties of different layers of MIM parts which cannot be measured easily otherwise [10]. The mold temperature is a critical factor in an injection molding process, and can have a notable effect on the morphology and properties of injection molded parts [11]. It is found that mechanical strength of semicrystalline polymers usually varies inversely with the crystallinity [12,13], which depends on mold temperature. A higher mold temperature, normally above Tg temperature for semicrystalline polymers, tends to decrease the thickness of the skin layer and increase the relative crystallinity [14,15]. For this reason, this work aims at investigating the effect of mold temperature on the micro-injection molded isotactic polypropylene (PP) by the nanoindentation test. PP micro-gears were molded at different mold temperatures. Their morphology was characterized by a polarization optical microscope. The mechanical properties of different layers and/or different positions of the micro-gears were measured by nanoindentation. The relationship between microstructure and mechanical properties provides new insights into MIM polymer parts. 2. Experimental 2.1. Materials The polypropylene used in this study was isotactic PP (F401), a homopolymer of Lanzhou Petroleum Chemical Co, Ltd., PR China, with a melt flow rate of 2.3 g/10 min (ASTM D1238, 230 °C, and 2.16 kg) from Lanzhou Petrobleum Chemical Co., Ltd. (PR China).
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Fig. 1. An isotactic polypropylene microgear. (a) Geometry of a microgear; (b) positions for wide angle X-ray diffraction tests.
2.2. Sample preparation Fig. 1a shows an MIM PP gear with a thickness of 450 μm, a gear teeth length of 1000 μm, a half gear tooth width of 350 μm, and a
gear tooth length of 1000 μm. Four positions with 800 μm gaps were selected along flow direction for testing. The micro-gear was molded using an MIM machine (Cornoplast, Babyplast 6/10P), with a temperature profile of 190 to 200 °C from the hopper to the
Fig. 2. Images observed by the polarized optical microscope of the micro-injection molded isotactic polypropylene gear slices at different mold temperatures (a) 30 (b) 40 (c) 50 (d) 60 (e) 70 and (f) 80 °C.
J. Jiang et al. / Materials and Design 88 (2015) 245–251
die. The injection pressure and injection speed were set at 40 MPa and 42 mm/s, respectively. The effect of mold temperature (30, 40, 50, 60, 70, 80 °C) on the structure and properties of MIM PP sample was investigated. 2.3. Characterization A polarization optical microscope (Olympus, BX51) was used to characterize morphology of the gear samples. One tooth was cut out of the micro-gear along a plane parallel to the flow direction from the mid region of the gear to the teeth top (Fig. 1b). The slice was about 13 μm thick to observe crystal evolution. Wide angle X-ray diffraction (WAXD) patterns of gear samples were recorded on a Bruker NanoStar system. Monochromatized CuKa radiation with a λ of 1.54 Å was used. The sample-to-detector distance was 108 mm. Four positions from the gate to the gear tooth were selected for testing, as shown in Fig. 1. Nanoindentation tests were conducted using an Agilent G200 test system (Agilent Technologies). A triangular pyramid Berkovich diamond indenter was employed. Nanoindentation tests were carried out with continuous stiffness measurements (CSM). All tests were performed at 23 °C on the slice used for polarization optical test. The deformation rate was 0.05 s−1. The nanoindentation depth and the loading time were 2000 nm and 10 s, respectively. Seven positions were indented along the flow direction from the gate to the tooth of the gear.
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corresponds to the region of the gate and the top of the gear tooth. It is seen that all the samples show a multi-layer structure. It is known that injection molded isotactic polypropylene typically shows three layers, namely the skin layer, shear layer and core layer [16]. For the MIM PP micro-gears, there exists a layer between the shear and skin layers which is rich in small crystals. The formation of this layer may be due to a lower cooling rate and accrued polymer chain mobility by shear induced heat (viscous dissipation) between the shear layer and skin layer. Along the flow direction, the thickness of the shear layer decreases and the crystal size of the core layer increases, indicating that the orientation decreases along the flow direction. Fig. 3 shows the effect of mold temperature on the evolution of the thickness of three layers of the MIM PP micro-gear along its central line from the bottom to the tooth. Whatever the mold temperature used, the thickness of the skin layer does not change much, as shown in Fig. 3a. That of the shear layer decreases from the bottom to the tooth (Fig. 3b), and that of the core layer follows an opposite trend (Fig. 3c). The effect of mold temperature is small for the thickness of the shear layer along the flow direction at the gear tooth. However, it is very important at 1 mm away from the gate, especially when the mold temperature was 50 °C. Fig. 3d shows the average thickness at all the locations for the skin, shear and core layers vs temperature. It is worth noting that the thickness of the core layer is much higher than that of the other layers. This is because the core is cooled down more slowly and therefore has more time to crystallize.
3. Results and discussion 3.1. Mult-layer structure of MIM PP gears
3.2. Effect of mold temperature on the micro-structure of MIM iPP gears
Fig. 2 shows the morphology of the gears molded at different mold temperatures. The flow direction from the bottom to the top
When the mold temperature is above 50 °C, the thickness of the skin layer and shear layer tend to increase. It is most obvious at
Fig. 3. Thickness of different layers for micro-injection molded isotactic polypropylene gears at different mold temperatures. (a) Skin layer, (b) shear layer, (c) core layer, and (d) average thickness at all the locations for the skin, shear and core layers vs temperature.
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60 °C (Fig. 3b). On the other hand, the thickness of the core layer tends to decrease. This could be attributed to the lower cooling rate it was subjected to. The thickness of the shear layer reaches a maximum at the mold temperature of 60 °C and then decreases with a further increase in the mold temperature. That of the core layer follows an opposite trend. The increase of the core thickness is due to the wall slip effect induced by the increase of mold temperature and shear rate [17]. WAXD was performed on different parts of the gear along the flow direction as shown in Fig. 1 in order to provide further insights into morphological changes. Fig. 4 shows the scatter patterns of MIM samples obtained at different mold temperatures. The brightness of the uniform Debye–Scherrer rings decreases along the flow direction, indicating that the orientation of polymer chain decreases [18]. On the other hand, it increases with increasing mold temperature, indicating that the higher the mold temperature, the higher the orientation of polymer chains.
Fig. 5 shows the WAXD curves of the micro-gears at different positions. They all show diffraction peaks at 14.1°, 17.0° and 18.5°. An additional diffraction peak appears at 16.1°, which corresponds to the PP β polymorph [19,20]. This fact indicates that both α and β crystals are formed in MIM PP micro-gears. It is known that β crystals are metastable polymorphs and that they are formed only under certain melt temperature gradient, cooling rate as well as shear rate [21]. In the microinjection molding process, PP chains can be subjected to a higher shear rate along the flow direction. A highly oriented α nuclei could be formed from the regularly arranged molecular chains and could then generate β nuclei [22]. Fig. 6 shows the crystallinity and relative content of β crystals for MIM PP micro-gears under different mold temperatures. They are calculated from the curves in Fig. 5. The crystallinity decreases significantly along the flow direction from P1 to P4 as shown in Fig. 6a. This is consistent with the morphology of Fig. 2. After 60 °C, a further increase in mold temperature increases the crystallinity.
Fig. 4. 2D wide angle X-ray diffraction patterns of different positions of micro-injection molded isotactic polypropylene gears at different mold temperatures.
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Fig. 5. 1D wide angle X-ray diffraction patterns of micro-injection molded isotactic polypropylene gears at different mold temperatures.
Fig. 6b shows that the content of β crystals first increases with increasing mold temperature from 30 to 40 °C. It then decreases with further increasing mold temperature till 80 °C. As mentioned above, the orientation of polymer chains decreases along the flow direction. Therefore, the formation of β polymorph can be easily promoted during the molding process by a high shear rate at low temperature, or a low shear rate at high temperature. The fact that the β crystal content is maximum at a mold temperature of 40 °C indicates that there are optimal mold temperature and flow field for the formation of β polymorph during MIM. Fig. 7 shows the effect of mold temperature on the orientation degree of different positions of the MIM gear. From the P1 to P4, the maximum deviation percentage of orientation degree within different mold temperature is 1.5%, 15.8%, 9.8% and 44.5%. It
means the sensitivity of mold temperature along flow direction is increased. As the gear tooth is away from the gate and is located at the terminal position of the flow, it is not very sensitive to the shearing action. Near the gate, the shear rate is higher and not very sensitive to the temperature. As shown in Fig. 7, the chain orientation of the most sensitive position to the temperature is located at the gear tooth shows an increase then decrease trends. This is due to the fact the polymer chains flow more easily as the mold temperature increases. When the mold temperature is higher (more than 60 °C), the temperature difference between mold and melt became smaller, effect of microscale on melt flow, such as wall slip, is more obvious, which can improve ability of molecular chain. That can enhance the random motion of molecular chain.
Fig. 6. (a) Crystallinity (b) β-crystal content of different positions of micro-injection molded isotactic polypropylene gears as a function of the mold temperature.
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of the micro-injection molded isotactic polypropylene gear a mold temperature of 50 °C. The shear layer has the highest modulus. The other three layers (fine grain, skin and core layers) exhibit a similar modulus. In order to further study the effect of the mold temperature on the mechanical properties of the shear layer of MIM PP gears, the nanoindentation test was carried out at seven different positions along flow direction. The results are shown in Fig. 8d. The modulus increases with increasing mold temperature, due to larger spherulites. This fact is accordance with the WAXD results. 4. Conclusions
Fig. 7. Effect of mold temperature on the orientation degree of different positions of microinjection molded isotactic polypropylene gear.
3.3. Effect of mold temperature on the nanoindentation properties of MIM PP gears The nanoindentation test was performed to find the relationship between structure and properties of iPP micro-gears [23,24]. Fig. 8a shows the load–displacement curves of four different layers of MIM PP gear molded at a mold temperature of 50 °C. The load force of the shear layer is higher than those of the skin and core layers, the latter being very close. On the decompression curve, the residual depth of the shear layer is the smallest. Fig. 8b shows that the average modulus of the shear layer increases with increasing mold temperature. This can be attributed to the fact that spherulites are bigger at a higher mold temperature. Fig. 8c shows the modulus-displacement curve
An obvious skin–core structure is formed during micro-injection molding of isotactic polypropylene (PP) gears, high shear and fast cooling. The core layer represents a large portion of the gears, followed by the shear and skin layers. The orientation induced by the wall slip effect decreases especially at the MIM PP gears tooth. The modulus of the shear layer is higher than those of the skin and core layers. Moreover, they are affected by shear to a great extent, compared to those of the skin and core layers. Acknowledgements The authors acknowledge the National Science Fund (No. 11372286), Basic and Advanced Technology Research Project of Henan Province (No. 152300410033), International Technological Cooperation Project (2015DFA30550) and the Key Project of Science and Technology of the Education Department of Henan Province (No. 14A430003) for their financial support of this project. The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 8. (a) Load–displacement curve; (b) average modulus values of shear layer at four locations vs. mold temperature; (c) modulus-displacement curve of micro-injection molded isotactic polypropylene gears at a mold temperature of 50 °C; (d) nanoindentation modulus of the shear layer of micro-injection molded isotactic polypropylene gears at different temperatures.
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References [1] Z.Y. Liu, N.H. Loh, S.B. Tor, K.A. Khor, Y. Murakoshi, et al., Binder system for micropowder injection molding, Mater. Lett. 48 (1) (2001) 31–38. [2] N.I. Zulkifli, N. Samat, H. Anuar, N. Zainuddin, Mechanical properties and failure modes of recycled polypropylene/microcrystalline cellulose composites, Mater. Des. 69 (2015) 114–123. [3] G. Lucchetta, M. Sorgato, S. Carmignato, E. Savio, Investigating the technological limits of micro-injection molding in replicating high aspect ratio micro-structured surfaces, CIRP Ann. Manuf. Technol. 63 (1) (2014) 521–524. [4] C. Guo, F.H. Liu, X. Wu, H. Liu, J. Zhang, Morphological evolution of HDPE parts in the microinjection molding: comparison with conventional injection molding, J. Appl. Polym. Sci. 126 (2) (2012) 452–462. [5] S.W. Wang, Z.W. Wang, N. Zhao, J. Jiang, Q. Li, A novel morphology development of micro-injection molded isotactic polypropylene, RSC Adv. 5 (2015) 41608–41610. [6] A.M. Díez-Pascual, M.A. Gómez-Fatou, F. Ania, A. Flores, Nanoindentation in polymer nanocomposites, Prog. Mater. Sci. 67 (2015) 1–94. [7] S. Naderi, M.A. Hassan, A.R. Bushroa, Alternative methods to determine the elastoplastic properties of sintered hydroxyapatite from nanoindentation testing, Mater. Des. 67 (2015) 360–368. [8] D. Tapobrata, D. Arjun, C.G. Prakash, B. Manaswita, K.M. Anoop, N.B. Rajendra, Influence of microstructure on nano-mechanical properties of single planar solid oxide fuel cell in pre- and post-reduced conditions, Mater. Des. 53 (2014) 182–191. [9] M.M. Shokrieh, M.R. Hosseinkhani, M.R. Naimi-Jamal, H. Tourani, Nanoindentation and nanoscratch investigations on graphene-based nanocomposites, Polym. Test. 32 (1) (2013) 45–51. [10] A.A. Zadpoor, Nanomechanical characterization of heterogeneous and hierarchical biomaterials and tissues using nanoindentation: the role of finite mixture models, Mater. Sci. Eng. C 48 (2015) 150–157. [11] F. Gu, P. Hall, N.J. Miles, Q.W. Ding, T. Wu, Improvement of mechanical properties of recycled plastic blends via optimizing processing parameters using the Taguchi method and principal component analysis, Mater. Des. 62 (2014) 189–198.
251
[12] G. Kfoury, J. Raquez, F. Hassouna, J. Odent, V. Toniazzo, D. Ruch, P. Dubois, Recent advances in high performance poly(lactide): from “green” plasticization to supertough materials via (reactive) compounding, Front. Chem. 1 (2013) 1–46. [13] M. Pralay, H.N. Pham, O. Masami, Influence of crystallization on intercalation, morphology, and mechanical properties of polypropylene/clay nanocomposites, Macromolecules 35 (6) (2002) 2042–2049. [14] J.W. Housmans, M. Gahleitner, G.W.M. Peters, Structure–property relations in molded,nucleated isotactic polypropylene, Polymer 50 (10) (2009) 2304–2319. [15] V. Nagarajan, K. Zhang, M. Manjusri, A.K. Mohanty, Overcoming the fundamental challenges in improving the impact strength and crystallinity of PLA biocomposites: influence of nucleating agent and mold temperature, ACS Appl. Mater. Interfaces 7 (2015) 11203–11214. [16] B.A.G. Schrauwen, L.C.A. Von Breemen, A.B. Spoelstra, Structure, deformation, and failure of flow-oriented semicrystalline polymers, Macromolecules 37 (23) (2004) 8618–8633. [17] S.G. Hatzikiriakos, Wall slip of molten polymers, Prog. Polym. Sci. 37 (4) (2012) 624–643. [18] Y.M. Wang, H.H. Zhang, M. Li, W. Cao, C.T. Liu, et al., Orientation and structural development of semicrystalline poly(lactic acid) under uniaxial drawing assessed by infrared spectroscopy and X-ray diffraction, Polym. Test. 41 (2015) 163–171. [19] J. Varga, Beta-modification of isotactic polypropylene:preparation, structure, processing, properties, and application, J. Macromol. Sci. Phys. 41 (4) (2002) 1121–1171. [20] O. Policianová, J. Hodan, J. Brus, J. Kotek, Origin of toughness in β-polypropylene: the effect of molecular mobility in the amorphous phaseO, Polymer 60 (2015) 107–114. [21] J. Varga, J. Karger-Kocsis, Rules of supermolecular structure formation in sheared isotactic polypropylene melts, J. Polym. Sci. Part B: Polym. Phys. 34 (1996) 657–670. [22] J. Varga, G.W. Ehrenstein, Formation of β-modification of isotactic polypropylene in its late stage of crystallization, Polymer 37 (26) (1996) 5959–5963. [23] L. Shen, I.Y. Phang, T.X. Liu, Nanoindentation studies on polymorphism of nylon 6, Polym. Test. 25 (2) (2006) 249–253. [24] W.D. Zhang, I.Y. Phang, T.X. Liu, Growth of carbon nanotubes on clay: unique nanofiller for high performance nanocomposites, Adv. Mater. 18 (2006) 73–77.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Effect of mold temperature on the structures and mechanical properties of micro-injection molded polypropylene Jing Jiang a, Shiwei Wang a,⁎, Bo Sun a, Shuaijiang Ma a, Jianming Zhang a, Qian Li a,⁎, Guo-Hua Hu b a b
National Center For International Joint Research of Micro-Nano Molding Technology, School of Mechanics & Engineering Science, Zhengzhou University, 450001 Zhengzhou, Henan, China Laboratory of Reactions and Process Engineering (LRGP), CNRS-University of Lorraine, 1 rue Grandville, BP 20451, 54001 Nancy, France
a r t i c l e
i n f o
Article history: Received 29 March 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 5 September 2015 Keywords: Micro-injection molding Mold temperature Micro-mechanical properties Morphology Microstructure
a b s t r a c t The effects of the mold temperature of micro-injection molding on the microstructures and mechanical properties of isotactic polypropylene (PP) gears were studied. The micro-injection molded PP gears present a skin–core type of morphology. The core layer is much thicker than the core and shear layers. The generation of β polymorph in the test samples is easily promoted during the micro-molding process with high shear rate at low temperature, or low shear rate at high temperature. Nanoindentation tests show that the modulus decreases along the flow direction and increases with increasing mold temperature. The highly oriented shear layer has the highest nanoindentation modulus compared with the skin and core layers. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Micro-injection molding (MIM) technology is expected to hold great potential in the mass production of good quality polymer micro-parts [1]. One of the main challenges associated with micro-molding resides in difficulties of controlling structures or mechanical properties of the parts [2–3]. MIM parts often exhibit a “skin–core” structure which is similar to that of conventional injection molding parts, expect that the oriented layer represents a large fraction [4]. A method was developed to explore the morphology evolution of micro-injection molded parts [5]. However, more detailed works remain to be done to explore the relationship between microstructure and mechanical properties of MIM parts. Nanoindentation could be a good mechanical test, especially for micro- or nano-composites [6–8]. Shokrieh found that the mechanical properties of polymers were improved significantly with the addition of low amounts of graphene nano-platelets [9]. During a nanoindentation test, mechanical properties are measured by the displacement that occurs when a small load is applied to the surface of the specimen using a diamond-tipped indenter. The region of the indenter tip during the test is typically less than 1 mm in diameter, allowing mechanical properties to be measured precisely in an extremely small area of the specimen. So nanoindentation test is a good means to measure the
⁎ Corresponding authors. E-mail addresses: [email protected] (S. Wang), [email protected] (Q. Li).
http://dx.doi.org/10.1016/j.matdes.2015.09.003 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
mechanical properties of different layers of MIM parts which cannot be measured easily otherwise [10]. The mold temperature is a critical factor in an injection molding process, and can have a notable effect on the morphology and properties of injection molded parts [11]. It is found that mechanical strength of semicrystalline polymers usually varies inversely with the crystallinity [12,13], which depends on mold temperature. A higher mold temperature, normally above Tg temperature for semicrystalline polymers, tends to decrease the thickness of the skin layer and increase the relative crystallinity [14,15]. For this reason, this work aims at investigating the effect of mold temperature on the micro-injection molded isotactic polypropylene (PP) by the nanoindentation test. PP micro-gears were molded at different mold temperatures. Their morphology was characterized by a polarization optical microscope. The mechanical properties of different layers and/or different positions of the micro-gears were measured by nanoindentation. The relationship between microstructure and mechanical properties provides new insights into MIM polymer parts. 2. Experimental 2.1. Materials The polypropylene used in this study was isotactic PP (F401), a homopolymer of Lanzhou Petroleum Chemical Co, Ltd., PR China, with a melt flow rate of 2.3 g/10 min (ASTM D1238, 230 °C, and 2.16 kg) from Lanzhou Petrobleum Chemical Co., Ltd. (PR China).
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Fig. 1. An isotactic polypropylene microgear. (a) Geometry of a microgear; (b) positions for wide angle X-ray diffraction tests.
2.2. Sample preparation Fig. 1a shows an MIM PP gear with a thickness of 450 μm, a gear teeth length of 1000 μm, a half gear tooth width of 350 μm, and a
gear tooth length of 1000 μm. Four positions with 800 μm gaps were selected along flow direction for testing. The micro-gear was molded using an MIM machine (Cornoplast, Babyplast 6/10P), with a temperature profile of 190 to 200 °C from the hopper to the
Fig. 2. Images observed by the polarized optical microscope of the micro-injection molded isotactic polypropylene gear slices at different mold temperatures (a) 30 (b) 40 (c) 50 (d) 60 (e) 70 and (f) 80 °C.
J. Jiang et al. / Materials and Design 88 (2015) 245–251
die. The injection pressure and injection speed were set at 40 MPa and 42 mm/s, respectively. The effect of mold temperature (30, 40, 50, 60, 70, 80 °C) on the structure and properties of MIM PP sample was investigated. 2.3. Characterization A polarization optical microscope (Olympus, BX51) was used to characterize morphology of the gear samples. One tooth was cut out of the micro-gear along a plane parallel to the flow direction from the mid region of the gear to the teeth top (Fig. 1b). The slice was about 13 μm thick to observe crystal evolution. Wide angle X-ray diffraction (WAXD) patterns of gear samples were recorded on a Bruker NanoStar system. Monochromatized CuKa radiation with a λ of 1.54 Å was used. The sample-to-detector distance was 108 mm. Four positions from the gate to the gear tooth were selected for testing, as shown in Fig. 1. Nanoindentation tests were conducted using an Agilent G200 test system (Agilent Technologies). A triangular pyramid Berkovich diamond indenter was employed. Nanoindentation tests were carried out with continuous stiffness measurements (CSM). All tests were performed at 23 °C on the slice used for polarization optical test. The deformation rate was 0.05 s−1. The nanoindentation depth and the loading time were 2000 nm and 10 s, respectively. Seven positions were indented along the flow direction from the gate to the tooth of the gear.
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corresponds to the region of the gate and the top of the gear tooth. It is seen that all the samples show a multi-layer structure. It is known that injection molded isotactic polypropylene typically shows three layers, namely the skin layer, shear layer and core layer [16]. For the MIM PP micro-gears, there exists a layer between the shear and skin layers which is rich in small crystals. The formation of this layer may be due to a lower cooling rate and accrued polymer chain mobility by shear induced heat (viscous dissipation) between the shear layer and skin layer. Along the flow direction, the thickness of the shear layer decreases and the crystal size of the core layer increases, indicating that the orientation decreases along the flow direction. Fig. 3 shows the effect of mold temperature on the evolution of the thickness of three layers of the MIM PP micro-gear along its central line from the bottom to the tooth. Whatever the mold temperature used, the thickness of the skin layer does not change much, as shown in Fig. 3a. That of the shear layer decreases from the bottom to the tooth (Fig. 3b), and that of the core layer follows an opposite trend (Fig. 3c). The effect of mold temperature is small for the thickness of the shear layer along the flow direction at the gear tooth. However, it is very important at 1 mm away from the gate, especially when the mold temperature was 50 °C. Fig. 3d shows the average thickness at all the locations for the skin, shear and core layers vs temperature. It is worth noting that the thickness of the core layer is much higher than that of the other layers. This is because the core is cooled down more slowly and therefore has more time to crystallize.
3. Results and discussion 3.1. Mult-layer structure of MIM PP gears
3.2. Effect of mold temperature on the micro-structure of MIM iPP gears
Fig. 2 shows the morphology of the gears molded at different mold temperatures. The flow direction from the bottom to the top
When the mold temperature is above 50 °C, the thickness of the skin layer and shear layer tend to increase. It is most obvious at
Fig. 3. Thickness of different layers for micro-injection molded isotactic polypropylene gears at different mold temperatures. (a) Skin layer, (b) shear layer, (c) core layer, and (d) average thickness at all the locations for the skin, shear and core layers vs temperature.
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60 °C (Fig. 3b). On the other hand, the thickness of the core layer tends to decrease. This could be attributed to the lower cooling rate it was subjected to. The thickness of the shear layer reaches a maximum at the mold temperature of 60 °C and then decreases with a further increase in the mold temperature. That of the core layer follows an opposite trend. The increase of the core thickness is due to the wall slip effect induced by the increase of mold temperature and shear rate [17]. WAXD was performed on different parts of the gear along the flow direction as shown in Fig. 1 in order to provide further insights into morphological changes. Fig. 4 shows the scatter patterns of MIM samples obtained at different mold temperatures. The brightness of the uniform Debye–Scherrer rings decreases along the flow direction, indicating that the orientation of polymer chain decreases [18]. On the other hand, it increases with increasing mold temperature, indicating that the higher the mold temperature, the higher the orientation of polymer chains.
Fig. 5 shows the WAXD curves of the micro-gears at different positions. They all show diffraction peaks at 14.1°, 17.0° and 18.5°. An additional diffraction peak appears at 16.1°, which corresponds to the PP β polymorph [19,20]. This fact indicates that both α and β crystals are formed in MIM PP micro-gears. It is known that β crystals are metastable polymorphs and that they are formed only under certain melt temperature gradient, cooling rate as well as shear rate [21]. In the microinjection molding process, PP chains can be subjected to a higher shear rate along the flow direction. A highly oriented α nuclei could be formed from the regularly arranged molecular chains and could then generate β nuclei [22]. Fig. 6 shows the crystallinity and relative content of β crystals for MIM PP micro-gears under different mold temperatures. They are calculated from the curves in Fig. 5. The crystallinity decreases significantly along the flow direction from P1 to P4 as shown in Fig. 6a. This is consistent with the morphology of Fig. 2. After 60 °C, a further increase in mold temperature increases the crystallinity.
Fig. 4. 2D wide angle X-ray diffraction patterns of different positions of micro-injection molded isotactic polypropylene gears at different mold temperatures.
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Fig. 5. 1D wide angle X-ray diffraction patterns of micro-injection molded isotactic polypropylene gears at different mold temperatures.
Fig. 6b shows that the content of β crystals first increases with increasing mold temperature from 30 to 40 °C. It then decreases with further increasing mold temperature till 80 °C. As mentioned above, the orientation of polymer chains decreases along the flow direction. Therefore, the formation of β polymorph can be easily promoted during the molding process by a high shear rate at low temperature, or a low shear rate at high temperature. The fact that the β crystal content is maximum at a mold temperature of 40 °C indicates that there are optimal mold temperature and flow field for the formation of β polymorph during MIM. Fig. 7 shows the effect of mold temperature on the orientation degree of different positions of the MIM gear. From the P1 to P4, the maximum deviation percentage of orientation degree within different mold temperature is 1.5%, 15.8%, 9.8% and 44.5%. It
means the sensitivity of mold temperature along flow direction is increased. As the gear tooth is away from the gate and is located at the terminal position of the flow, it is not very sensitive to the shearing action. Near the gate, the shear rate is higher and not very sensitive to the temperature. As shown in Fig. 7, the chain orientation of the most sensitive position to the temperature is located at the gear tooth shows an increase then decrease trends. This is due to the fact the polymer chains flow more easily as the mold temperature increases. When the mold temperature is higher (more than 60 °C), the temperature difference between mold and melt became smaller, effect of microscale on melt flow, such as wall slip, is more obvious, which can improve ability of molecular chain. That can enhance the random motion of molecular chain.
Fig. 6. (a) Crystallinity (b) β-crystal content of different positions of micro-injection molded isotactic polypropylene gears as a function of the mold temperature.
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of the micro-injection molded isotactic polypropylene gear a mold temperature of 50 °C. The shear layer has the highest modulus. The other three layers (fine grain, skin and core layers) exhibit a similar modulus. In order to further study the effect of the mold temperature on the mechanical properties of the shear layer of MIM PP gears, the nanoindentation test was carried out at seven different positions along flow direction. The results are shown in Fig. 8d. The modulus increases with increasing mold temperature, due to larger spherulites. This fact is accordance with the WAXD results. 4. Conclusions
Fig. 7. Effect of mold temperature on the orientation degree of different positions of microinjection molded isotactic polypropylene gear.
3.3. Effect of mold temperature on the nanoindentation properties of MIM PP gears The nanoindentation test was performed to find the relationship between structure and properties of iPP micro-gears [23,24]. Fig. 8a shows the load–displacement curves of four different layers of MIM PP gear molded at a mold temperature of 50 °C. The load force of the shear layer is higher than those of the skin and core layers, the latter being very close. On the decompression curve, the residual depth of the shear layer is the smallest. Fig. 8b shows that the average modulus of the shear layer increases with increasing mold temperature. This can be attributed to the fact that spherulites are bigger at a higher mold temperature. Fig. 8c shows the modulus-displacement curve
An obvious skin–core structure is formed during micro-injection molding of isotactic polypropylene (PP) gears, high shear and fast cooling. The core layer represents a large portion of the gears, followed by the shear and skin layers. The orientation induced by the wall slip effect decreases especially at the MIM PP gears tooth. The modulus of the shear layer is higher than those of the skin and core layers. Moreover, they are affected by shear to a great extent, compared to those of the skin and core layers. Acknowledgements The authors acknowledge the National Science Fund (No. 11372286), Basic and Advanced Technology Research Project of Henan Province (No. 152300410033), International Technological Cooperation Project (2015DFA30550) and the Key Project of Science and Technology of the Education Department of Henan Province (No. 14A430003) for their financial support of this project. The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 8. (a) Load–displacement curve; (b) average modulus values of shear layer at four locations vs. mold temperature; (c) modulus-displacement curve of micro-injection molded isotactic polypropylene gears at a mold temperature of 50 °C; (d) nanoindentation modulus of the shear layer of micro-injection molded isotactic polypropylene gears at different temperatures.
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References [1] Z.Y. Liu, N.H. Loh, S.B. Tor, K.A. Khor, Y. Murakoshi, et al., Binder system for micropowder injection molding, Mater. Lett. 48 (1) (2001) 31–38. [2] N.I. Zulkifli, N. Samat, H. Anuar, N. Zainuddin, Mechanical properties and failure modes of recycled polypropylene/microcrystalline cellulose composites, Mater. Des. 69 (2015) 114–123. [3] G. Lucchetta, M. Sorgato, S. Carmignato, E. Savio, Investigating the technological limits of micro-injection molding in replicating high aspect ratio micro-structured surfaces, CIRP Ann. Manuf. Technol. 63 (1) (2014) 521–524. [4] C. Guo, F.H. Liu, X. Wu, H. Liu, J. Zhang, Morphological evolution of HDPE parts in the microinjection molding: comparison with conventional injection molding, J. Appl. Polym. Sci. 126 (2) (2012) 452–462. [5] S.W. Wang, Z.W. Wang, N. Zhao, J. Jiang, Q. Li, A novel morphology development of micro-injection molded isotactic polypropylene, RSC Adv. 5 (2015) 41608–41610. [6] A.M. Díez-Pascual, M.A. Gómez-Fatou, F. Ania, A. Flores, Nanoindentation in polymer nanocomposites, Prog. Mater. Sci. 67 (2015) 1–94. [7] S. Naderi, M.A. Hassan, A.R. Bushroa, Alternative methods to determine the elastoplastic properties of sintered hydroxyapatite from nanoindentation testing, Mater. Des. 67 (2015) 360–368. [8] D. Tapobrata, D. Arjun, C.G. Prakash, B. Manaswita, K.M. Anoop, N.B. Rajendra, Influence of microstructure on nano-mechanical properties of single planar solid oxide fuel cell in pre- and post-reduced conditions, Mater. Des. 53 (2014) 182–191. [9] M.M. Shokrieh, M.R. Hosseinkhani, M.R. Naimi-Jamal, H. Tourani, Nanoindentation and nanoscratch investigations on graphene-based nanocomposites, Polym. Test. 32 (1) (2013) 45–51. [10] A.A. Zadpoor, Nanomechanical characterization of heterogeneous and hierarchical biomaterials and tissues using nanoindentation: the role of finite mixture models, Mater. Sci. Eng. C 48 (2015) 150–157. [11] F. Gu, P. Hall, N.J. Miles, Q.W. Ding, T. Wu, Improvement of mechanical properties of recycled plastic blends via optimizing processing parameters using the Taguchi method and principal component analysis, Mater. Des. 62 (2014) 189–198.
251
[12] G. Kfoury, J. Raquez, F. Hassouna, J. Odent, V. Toniazzo, D. Ruch, P. Dubois, Recent advances in high performance poly(lactide): from “green” plasticization to supertough materials via (reactive) compounding, Front. Chem. 1 (2013) 1–46. [13] M. Pralay, H.N. Pham, O. Masami, Influence of crystallization on intercalation, morphology, and mechanical properties of polypropylene/clay nanocomposites, Macromolecules 35 (6) (2002) 2042–2049. [14] J.W. Housmans, M. Gahleitner, G.W.M. Peters, Structure–property relations in molded,nucleated isotactic polypropylene, Polymer 50 (10) (2009) 2304–2319. [15] V. Nagarajan, K. Zhang, M. Manjusri, A.K. Mohanty, Overcoming the fundamental challenges in improving the impact strength and crystallinity of PLA biocomposites: influence of nucleating agent and mold temperature, ACS Appl. Mater. Interfaces 7 (2015) 11203–11214. [16] B.A.G. Schrauwen, L.C.A. Von Breemen, A.B. Spoelstra, Structure, deformation, and failure of flow-oriented semicrystalline polymers, Macromolecules 37 (23) (2004) 8618–8633. [17] S.G. Hatzikiriakos, Wall slip of molten polymers, Prog. Polym. Sci. 37 (4) (2012) 624–643. [18] Y.M. Wang, H.H. Zhang, M. Li, W. Cao, C.T. Liu, et al., Orientation and structural development of semicrystalline poly(lactic acid) under uniaxial drawing assessed by infrared spectroscopy and X-ray diffraction, Polym. Test. 41 (2015) 163–171. [19] J. Varga, Beta-modification of isotactic polypropylene:preparation, structure, processing, properties, and application, J. Macromol. Sci. Phys. 41 (4) (2002) 1121–1171. [20] O. Policianová, J. Hodan, J. Brus, J. Kotek, Origin of toughness in β-polypropylene: the effect of molecular mobility in the amorphous phaseO, Polymer 60 (2015) 107–114. [21] J. Varga, J. Karger-Kocsis, Rules of supermolecular structure formation in sheared isotactic polypropylene melts, J. Polym. Sci. Part B: Polym. Phys. 34 (1996) 657–670. [22] J. Varga, G.W. Ehrenstein, Formation of β-modification of isotactic polypropylene in its late stage of crystallization, Polymer 37 (26) (1996) 5959–5963. [23] L. Shen, I.Y. Phang, T.X. Liu, Nanoindentation studies on polymorphism of nylon 6, Polym. Test. 25 (2) (2006) 249–253. [24] W.D. Zhang, I.Y. Phang, T.X. Liu, Growth of carbon nanotubes on clay: unique nanofiller for high performance nanocomposites, Adv. Mater. 18 (2006) 73–77.