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Micro injection molding of a micro-fluidic platform
International Communications in Heat and Mass Transfer 37 (2010) 1290–1294
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Micro injection molding of a micro-fluidic platform☆ Chun-Sheng Chen a, Shia-Chung Chen b,c, Won-Hsion Liao d, Rean-Der Chien d,⁎, Su-Hsia Lin e a
Department of Mechanical Engineering, Lunghwa University of Science and Technology, Guishan Shiang 33306, Taiwan, ROC Department of Mechanical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC c R&D Center for Membrane Technology, Chung Yuan University, Chung-Li 32023, Taiwan, ROC d Department of Mechanical Engineering, Nanya Institute of Technology, Chung-Li 32024, Taiwan, ROC e Department of Chemical and Materials Engineering, Nanya Institute of Technology, Chung-Li 32024, Taiwan, ROC b
a r t i c l e
i n f o
Available online 5 August 2010 Keywords: Replication accuracy Micro-channel Micro injection molding
a b s t r a c t Micro fabrication of polymers is becoming increasingly important and is considered a low-cost alternative to silicon- or glass-based MEMS technologies. However, very little work has been done to study the influence of polymer resin on the replication accuracy of the micro features in micro injection molding. In this study, micro injection molding was applied to a micro-featured fluidic platform used for DNA/RNA testing. LIGAlike processes were used to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based stamp. The micro features in the stamp consisted of a micro-channel array 50 μm in pitch size. COC, PC, PMMA and PS were used as the injection molding materials. The effect of various polymer resins and molding conditions on the replication accuracy of the micro features was investigated. The width and depth of the micro-channels within the molded devices were measured and analyzed. For the micro-injection-molded devices, the accuracy of the width and depth of the micro-channels increased with increasing mold temperature, melt temperature, injection velocity and packing pressure within the regular processing window. The molded parts showed excellent replication accuracy for the COC polymer resin due to its low viscosity and low, isotropic shrinkage. The PS resin also achieved acceptable micro-channel replication accuracy under specific molding conditions. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The commercialization of the Micro-Electro-Mechanical System (MEMS) technology requires low-cost fabrication and high-volume production. MEMS applications in the life sciences (such as DNA sequencing and clinical diagnostics devices) are currently in great demand. Silicon-based MEMS products have a good surface quality but are usually expensive and are not suitable for low-cost and mass production. In addition, a Si-based material often induces problems such as lack of optical clarity, low impact strength and poor compatibility, thus limiting its widespread usage in MEMS products. Hence, it is commendable to establish MEMS products using materials other than silicon. In recent years, many polymer-based micro fabrication techniques [1] such as micro injection molding [2,3], casting [4,5] and micro hot embossing [6,7] have been developed for applications in bioand chemo-MEMS. Polymer-based materials offer a wide range of physical and chemical properties (such as low electrical conductivity and high chemical stability) and also have the advantages of low-cost and easy processing for mass production.
☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (R.-D. Chien). 0735-1933/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.06.032
Among the micro molding processes, micro injection molding of micro and micro-featured plastic products has shown a great commercial potential in recent years. The relatively expensive step of micro fabrication is only done once on a single master, or stamp, and then identical structures can be reproduced in mass quantity. The mold insert can be a wafer, glass, electroplated nickel mold, or other stamp with micro features. In micro injection molding, a mold cavity equipped with a micro-structured mold insert is first injected with a polymer melt heated above the glass transition temperature (Tg). This is followed by a packing process (an additional melt squeeze) to compensate for subsequent melt shrinkage. Then the part is cooled down below its Tg and de-molded from the mold. Because of its short cycle time, micro injection molding has been chosen as the highest preference for mass production. Many micro and micro-featured devices such as micro sampling cells, micro heat exchangers, optical grating elements, and labon-a-chip technologies [8] have been successfully injection molded. However, the low mold insert temperature often results in a fast polymer melt solidification within the micro-channels and limits the application of micro injection molding for devices with micro-channels of a high aspect ratio (the ratio of depth to width). In addition, when the size of the micro-featured part becomes large, or the micro features become geometrically more complicated, micro-injection-molded devices may lose dimensional accuracy. Therefore, micro injection molding faces a challenge in terms of process feasibility because it is
C.-S. Chen et al. / International Communications in Heat and Mass Transfer 37 (2010) 1290–1294
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difficult to make the polymer completely fill a micro-featured geometry of a high aspect ratio. The correlation of the replication accuracy with all contributing parameters is a complicated issue. A number of studies have been conducted in recent years to investigate micro injection molding of micro-structured devices. For micro injection molding, Zhao [9] reported that metering size and holding pressure time are the process parameters that affect the part quality most significantly. A more recent study by Chen et al. [10], found that mold temperature and packing pressure affect molding accuracy significantly. The influence of molding conditions on the replication accuracy of micro features using the PMMA polymer resin was investigated by Chien et al. [11]. The results showed that higher mold temperature, melt temperature, injection velocity and packing pressure can lead to better replication accuracy. It was also found that micro injection molding shows a slight advantage in replicating the depth of micro-channels but a disadvantage in replicating width when compared with hot embossing. Although micro injection molding has been used in micro fabrication for several years, the influence of the molding characteristics on the replication accuracy of micro-fluidic channels has not yet been investigated systematically. This replication and the replication quality depend on the plastic material properties and the process conditions. In this study, LIGA-like processes using a UV light aligner were used to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based biochip stamp. The PMMA, PC, COC, and PS resins were injection molded to generate biochip devices with micro-channel arrays. The dimensions of the micro-channels in the devices were measured using a 3D laser microscope. A set of systematic experiments was conducted to determine the effect of process parameters, including mold temperature, melt temperature, injection velocity and packing pressure, on the replication accuracy of the microchannels, including the replication accuracy of the depth and width. The effect of polymer resin on the replication accuracy of the micro-channels was also determined. Hopefully, this study will lead to a better understanding of the molding characteristics of micro injection molding for the fabrication of micro-fluidic channels using different polymer resins. 2. Experimental work For the micro injection molding of the biochip devices, LIGA-like processes using a UV light aligner were first implemented to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based biochip mold insert. The stamp used in the micro injection molding was 80 mm in length, 40 mm in width and 0.2 mm in thickness (Fig. 1(a)). The micro-channels in the stamp were approximately 30 μm in depth, 100 μm in width and 50 μm in pitch size. The draft angle of the side wall was approximately 8.1 to 8.4o to eliminate structural deformation due to frictional or shear forces between the mold and the biochip devices during the mold release step. A Sodick-TR30EH injection molding machine (Sodick Plustech Co., Ltd., Japan) with a maximum clamping force of 30 tons, an injection speed of 500 mm/s and a maximum injection pressure of 262 MPa was used in the micro injection molding experiments. The machine used a reciprocal screw for melt plasticization; a plunger with a V-line design provided the injection accuracy. Fig. 1(a) shows the injection mold with the stamp inserted into the center of the mold. The cavity size was 60 mm by 25 mm by 1 mm and could be vacuumed by the mold vacuum system. COC (grade 6013, TOPAS), PC (grade PC175, Chi-Mei), PMMA (grade CM205, Chi-Mei), and PS (grade PG33, Chi-Mei) were utilized as the materials of micro injection molding. Because this type of device is applied in the biomedical field, a mold release agent was not added to avoid contamination. The processing conditions of the injection molding of the polymer resins are listed in Table 1. Each processing condition was set to three different levels. Using the COC resin as an example, the melt temperature, mold temperature, injection velocity and packing pressure
Fig. 1. (a) The stamp insert used in micro injection molding. (b) Biochip devices designed with micro-channels. (c) Schematic of measurement positions of the micro-channel.
were chosen to be 235 °C, 60 °C, 40 mm/s, and 40 MPa, respectively. Beginning with these processing parameters as standard molding criteria, each parameter was set to three different levels to investigate the influence of individual processing conditions on the replication accuracy of the micro-channels. Five samples were molded under each set of molding conditions and used for replication accuracy testing. To evaluate the replication accuracy of the molded part, the dimensions at specified positions (designated by the symbol (C), as shown in Fig. 1(b)) on the stamp were pre-measured using a 3D laser microscope (maximum magnification 16,000, KEYENCE Corp., Japan). Fig. 1(b) shows the 3D laser microscope images of the micro-channel cross sections in the mold insert at positions (A), (B) and (C),
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Table 1 Molding conditions of various polymer resins. Material
COC PC PS PMMA
Molding conditions Melt temperature (°C)
Mold temperature (°C)
Injection velocity (mm/s)
Packing pressure (MPa)
(220) (290) (190) (220)
(50) (60) (50) (60)
(20) (40) (40) (40)
(20) (80) (40) (80)
235 305 205 235
(250) (320) (220) (250)
60 (70) 70 (80) 60 (70) 70 (80)
respectively. After part ejection from the mold, the replication accuracy of the micro-channel dimensions at position (C) was examined with the 3D laser microscope and compared to the dimensions of the stamp. Fig. 1(c) shows the measurement positions of the micro-channel. The measured dimensions of the width and depth of the micro-channels from the molded devices were compared with those from the stamp, and the differences were used to characterize the replication accuracy. Five measurements at each position were carried out and the average values from these five measurements were used for analysis and correlation. 3. Results and discussions 3.1. The effect of molding conditions on replication accuracy
40 60 60 60
(60) (80) (80) (80)
40 (60) 100 (120) 60 (80) 100 (120)
the mold temperature. The replication accuracy of the depth increases at higher melt and mold temperatures. This is because higher melt and mold temperatures lower the viscosity of the melt, resulting in an easier flow of the melt into the micro-channels. Figs. 6 and 7 depict the effects of injection velocity on the micro-channel width and depth, respectively. For each polymer, a higher injection velocity leads to better replication accuracy. This can be attributed to two causes. First, a higher injection velocity may decrease the melt viscosity due to viscous heating, especially when the melt flows through a thin-walled geometry. Second, a high-speed injection reduces the melt-mold contact time and the melt temperature drop. The effect of packing pressure on the replication accuracy of the width and depth of the micro-channels is illustrated in Figs. 8 and 9. A higher packing pressure improves the width and depth replication accuracy significantly.
From the experimental observations and measurements, it can be seen that the micro-channel replication accuracy is very sensitive to the viscosity of the polymer melt and the pressure in the cavity. The higher the polymer melt temperature, the lower the melt viscosity. The low melt viscosity results in better replication accuracy because the melt can fill the micro-channels more easily. High molding pressure also improves replication accuracy, by assisting the polymer melt flow within the micro-channels and reducing the subsequent shrinkage during the cooling process. For the PMMA, PC, PS, and COC resins, the variations of the microchannel width at position (C) in the molded parts with melt temperature and mold temperature are illustrated in Figs. 2 and 3, respectively. It can be seen that the width of the micro-channels decreases with increasing melt temperature and mold temperature. Because of the molecular forces among the polymer chains, the molded device tends to be flat in shape; this leads to a larger width and smaller depth of the microchannel compared with those of the stamp. The higher the melt and mold temperatures, the closer the width to the true dimension of the stamp. The influences of melt temperature and mold temperature on the replication depth of the micro-channels are shown in Figs. 4 and 5, respectively. Examination of these figures reveals that the replication accuracy of the micro-channel depth 2 (D2) is significantly affected by
Although the PS, PC, PMMA, and COC parts with the microchannels were molded under different processing conditions in this experiment, it is worthwhile to compare the replication accuracy of the micro-channels for these four polymer resins. Comparison of the micro-channel cross section dimensions width 1 and width 2 (i.e., W1 and W2) in Figs. 2, 3, 6, and 8 reveals that for micro-channels about 30 μm in depth, 100 μm in width and 50 μm in pitch size the COC resin exhibits an advantage over the PC and PS resins, while the PMMA resin shows the worst results. Regarding the replication accuracy of the micro-channel depth, Figs. 4, 5, 7, and 9 clearly show that depth 1 (i.e., D1) is only slightly influenced by polymer resin, the deviations from the stamp depth value (30.77 μm) are less than 2.5% for the four different polymer resins. However, depth 2 (i.e., D2) is significantly influenced by polymer resin, the deviations from the stamp depth value (30.77 μm) are in the range of approximately 2.6–33.4% for the four different polymer resins. For the COC resin, it can be clearly seen from Figs. 2–9 that width 1, width 2, depth 1, and depth 2 (i.e., W1, W2, D1, and D2) deviate only in the ranges of approximately 3.34–4.59%, 0.29–2.96%, 0.27–1.99% and 1.46–
Fig. 2. Effect of melt temperature on the width of the micro-channel.
Fig. 3. Effect of mold temperature on the width of the micro-channel.
3.2. Comparison of replication accuracy of micro-channels for various polymer resins
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Fig. 4. Effect of melt temperature on the depth of the micro-channel.
Fig. 7. Effect of injection velocity on the depth of the micro-channel.
7.60%, respectively, from the corresponding dimensions of the stamp under the selected molding conditions. Therefore, the COC resin has good replication accuracy. This is because COC has a good flowability, easily reproduces submicron-size surface features, and fills complex thin-walled parts. It also has low and substantially isotropic shrinkage during molding, resulting in a little warpage. Alternatively, on comparing the replication accuracy of the micro-channels for the PS, PMMA, and PC resins, it can be
seen that the PS resin under specific molding conditions (i.e., melt temperature 205 °C, mold temperature 70 °C, injection velocity 60 mm/s and packing pressure 60 MPa) achieves acceptable micro-channel replication accuracy. However, the PC and PMMA resins give poor accuracy of depth 2 and width 1, respectively. In summary, micro injection molding using the COC resin achieves the required micro-channel molding accuracy. The molded parts
Fig. 5. Effect of mold temperature on the depth of the micro-channel.
Fig. 8. Effect of packing pressure on the width of the micro-channel.
Fig. 6. Effect of injection velocity on the width of the micro-channel.
Fig. 9. Effect of packing pressure on the depth of the micro-channel.
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show excellent replication accuracy for the COC resin due to its low viscosity and low, isotropic shrinkage. The PS resin also achieves acceptable micro-channel replication accuracy under specific molding conditions.
project of the specific research fields in the Chung Yuan Christian University, Taiwan, under grant CYCU-98-CR-ME.
References 4. Conclusions In this study, micro injection molding of micro-featured devices used for DNA/RNA testing was carried out using a Ni–Co-based stamp. The effect of polymer resin and molding conditions on the replication accuracy of the micro features was investigated using a 3D laser microscope. The width and depth of the micro-channels were analyzed and correlated. Based on the measured results, it was found that for the micro-injection-molded devices, the accuracy of the width and depth of the micro-channels increased with increasing mold temperature, melt temperature, injection velocity and packing pressure within the regular processing window. Both the COC and PS polymer resins achieved the required micro-channel accuracy. The molded parts showed excellent replication for the COC polymer resin due to its low viscosity and low, isotropic shrinkage. The PS resin also achieved acceptable replication accuracy under specific molding conditions. Acknowledgments This research was supported by The Center-of-Excellence Program on Membrane Technology from the Ministry of Education and the
[1] M. Heckele, W.K. Schomburg, 2004 Review on micro molding of thermoplastic polymers, J. Micromech. Microeng. 14 (2004) R1–R14. [2] R.M. McCormick, R.J. Nelson, M.G. Alonso-Amigo, D.J. Benvegnu, H.H. Hopper, Microchannel electrophoretic separations of DNA in injection-molded plastic substrates, Ann. Chem. 69 (1997) 2626–2630. [3] T. Hanemann, M. Heckele, V. Piotter, Current status of micromolding technology, Polym. News 25 (2000) 224–229. [4] D.C. Duffy, J.C. McDonald, O.J.A. Schueller, G.M. Whitesides, Rapid prototyping of microfluidic systems in polydimethylsiloxane, Anal. Chem. 70 (2000) 4974–4984. [5] M.T. Gale, Replication techniques for diffractive optical elements, Microelectronic Eng. 34 (1997) 321–339. [6] M.U. Kopp, H.J. Crabtree, A. Manz, Developments in technology and applications of microsystems, Curr. Opin. Chem. Biol. 1 (1997) 410–419. [7] M. Freemantal, Microfluidic devices for μ-TAS applications fabricated by polymer hot embossing, Chem. Eng. News 22 (1999) 72–82. [8] X.C. Shan, R. Maeda, Y. Murakoshi, Micro hot embossing for replication of microstructures, Jpn. J. Appl. Phys. 42 (2003) 3859–3862. [9] J. Zhao, R.H. Mayes, G. Chen, H. Xie, P.S. Chan, Effects of process parameters on the molding process, Poly. Eng. Sci. 43 (2003) 1542–1554. [10] S.C. Chen, J.A. Chang, Y.J. Chang, S.W. Chau, Micro injection molding of micro fluidic platform, SPE ANTEC Tech. Papers 61 (2005) 556–560. [11] R.D. Chien, Micromolding of biochip devices designed with microchannels, Sens. Actuator A-Phys. 128 (2006) 238–247.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Micro injection molding of a micro-fluidic platform☆ Chun-Sheng Chen a, Shia-Chung Chen b,c, Won-Hsion Liao d, Rean-Der Chien d,⁎, Su-Hsia Lin e a
Department of Mechanical Engineering, Lunghwa University of Science and Technology, Guishan Shiang 33306, Taiwan, ROC Department of Mechanical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC c R&D Center for Membrane Technology, Chung Yuan University, Chung-Li 32023, Taiwan, ROC d Department of Mechanical Engineering, Nanya Institute of Technology, Chung-Li 32024, Taiwan, ROC e Department of Chemical and Materials Engineering, Nanya Institute of Technology, Chung-Li 32024, Taiwan, ROC b
a r t i c l e
i n f o
Available online 5 August 2010 Keywords: Replication accuracy Micro-channel Micro injection molding
a b s t r a c t Micro fabrication of polymers is becoming increasingly important and is considered a low-cost alternative to silicon- or glass-based MEMS technologies. However, very little work has been done to study the influence of polymer resin on the replication accuracy of the micro features in micro injection molding. In this study, micro injection molding was applied to a micro-featured fluidic platform used for DNA/RNA testing. LIGAlike processes were used to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based stamp. The micro features in the stamp consisted of a micro-channel array 50 μm in pitch size. COC, PC, PMMA and PS were used as the injection molding materials. The effect of various polymer resins and molding conditions on the replication accuracy of the micro features was investigated. The width and depth of the micro-channels within the molded devices were measured and analyzed. For the micro-injection-molded devices, the accuracy of the width and depth of the micro-channels increased with increasing mold temperature, melt temperature, injection velocity and packing pressure within the regular processing window. The molded parts showed excellent replication accuracy for the COC polymer resin due to its low viscosity and low, isotropic shrinkage. The PS resin also achieved acceptable micro-channel replication accuracy under specific molding conditions. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The commercialization of the Micro-Electro-Mechanical System (MEMS) technology requires low-cost fabrication and high-volume production. MEMS applications in the life sciences (such as DNA sequencing and clinical diagnostics devices) are currently in great demand. Silicon-based MEMS products have a good surface quality but are usually expensive and are not suitable for low-cost and mass production. In addition, a Si-based material often induces problems such as lack of optical clarity, low impact strength and poor compatibility, thus limiting its widespread usage in MEMS products. Hence, it is commendable to establish MEMS products using materials other than silicon. In recent years, many polymer-based micro fabrication techniques [1] such as micro injection molding [2,3], casting [4,5] and micro hot embossing [6,7] have been developed for applications in bioand chemo-MEMS. Polymer-based materials offer a wide range of physical and chemical properties (such as low electrical conductivity and high chemical stability) and also have the advantages of low-cost and easy processing for mass production.
☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (R.-D. Chien). 0735-1933/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.06.032
Among the micro molding processes, micro injection molding of micro and micro-featured plastic products has shown a great commercial potential in recent years. The relatively expensive step of micro fabrication is only done once on a single master, or stamp, and then identical structures can be reproduced in mass quantity. The mold insert can be a wafer, glass, electroplated nickel mold, or other stamp with micro features. In micro injection molding, a mold cavity equipped with a micro-structured mold insert is first injected with a polymer melt heated above the glass transition temperature (Tg). This is followed by a packing process (an additional melt squeeze) to compensate for subsequent melt shrinkage. Then the part is cooled down below its Tg and de-molded from the mold. Because of its short cycle time, micro injection molding has been chosen as the highest preference for mass production. Many micro and micro-featured devices such as micro sampling cells, micro heat exchangers, optical grating elements, and labon-a-chip technologies [8] have been successfully injection molded. However, the low mold insert temperature often results in a fast polymer melt solidification within the micro-channels and limits the application of micro injection molding for devices with micro-channels of a high aspect ratio (the ratio of depth to width). In addition, when the size of the micro-featured part becomes large, or the micro features become geometrically more complicated, micro-injection-molded devices may lose dimensional accuracy. Therefore, micro injection molding faces a challenge in terms of process feasibility because it is
C.-S. Chen et al. / International Communications in Heat and Mass Transfer 37 (2010) 1290–1294
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difficult to make the polymer completely fill a micro-featured geometry of a high aspect ratio. The correlation of the replication accuracy with all contributing parameters is a complicated issue. A number of studies have been conducted in recent years to investigate micro injection molding of micro-structured devices. For micro injection molding, Zhao [9] reported that metering size and holding pressure time are the process parameters that affect the part quality most significantly. A more recent study by Chen et al. [10], found that mold temperature and packing pressure affect molding accuracy significantly. The influence of molding conditions on the replication accuracy of micro features using the PMMA polymer resin was investigated by Chien et al. [11]. The results showed that higher mold temperature, melt temperature, injection velocity and packing pressure can lead to better replication accuracy. It was also found that micro injection molding shows a slight advantage in replicating the depth of micro-channels but a disadvantage in replicating width when compared with hot embossing. Although micro injection molding has been used in micro fabrication for several years, the influence of the molding characteristics on the replication accuracy of micro-fluidic channels has not yet been investigated systematically. This replication and the replication quality depend on the plastic material properties and the process conditions. In this study, LIGA-like processes using a UV light aligner were used to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based biochip stamp. The PMMA, PC, COC, and PS resins were injection molded to generate biochip devices with micro-channel arrays. The dimensions of the micro-channels in the devices were measured using a 3D laser microscope. A set of systematic experiments was conducted to determine the effect of process parameters, including mold temperature, melt temperature, injection velocity and packing pressure, on the replication accuracy of the microchannels, including the replication accuracy of the depth and width. The effect of polymer resin on the replication accuracy of the micro-channels was also determined. Hopefully, this study will lead to a better understanding of the molding characteristics of micro injection molding for the fabrication of micro-fluidic channels using different polymer resins. 2. Experimental work For the micro injection molding of the biochip devices, LIGA-like processes using a UV light aligner were first implemented to prepare a silicon-based SU-8 photoresist, followed by electroforming to make a Ni–Co-based biochip mold insert. The stamp used in the micro injection molding was 80 mm in length, 40 mm in width and 0.2 mm in thickness (Fig. 1(a)). The micro-channels in the stamp were approximately 30 μm in depth, 100 μm in width and 50 μm in pitch size. The draft angle of the side wall was approximately 8.1 to 8.4o to eliminate structural deformation due to frictional or shear forces between the mold and the biochip devices during the mold release step. A Sodick-TR30EH injection molding machine (Sodick Plustech Co., Ltd., Japan) with a maximum clamping force of 30 tons, an injection speed of 500 mm/s and a maximum injection pressure of 262 MPa was used in the micro injection molding experiments. The machine used a reciprocal screw for melt plasticization; a plunger with a V-line design provided the injection accuracy. Fig. 1(a) shows the injection mold with the stamp inserted into the center of the mold. The cavity size was 60 mm by 25 mm by 1 mm and could be vacuumed by the mold vacuum system. COC (grade 6013, TOPAS), PC (grade PC175, Chi-Mei), PMMA (grade CM205, Chi-Mei), and PS (grade PG33, Chi-Mei) were utilized as the materials of micro injection molding. Because this type of device is applied in the biomedical field, a mold release agent was not added to avoid contamination. The processing conditions of the injection molding of the polymer resins are listed in Table 1. Each processing condition was set to three different levels. Using the COC resin as an example, the melt temperature, mold temperature, injection velocity and packing pressure
Fig. 1. (a) The stamp insert used in micro injection molding. (b) Biochip devices designed with micro-channels. (c) Schematic of measurement positions of the micro-channel.
were chosen to be 235 °C, 60 °C, 40 mm/s, and 40 MPa, respectively. Beginning with these processing parameters as standard molding criteria, each parameter was set to three different levels to investigate the influence of individual processing conditions on the replication accuracy of the micro-channels. Five samples were molded under each set of molding conditions and used for replication accuracy testing. To evaluate the replication accuracy of the molded part, the dimensions at specified positions (designated by the symbol (C), as shown in Fig. 1(b)) on the stamp were pre-measured using a 3D laser microscope (maximum magnification 16,000, KEYENCE Corp., Japan). Fig. 1(b) shows the 3D laser microscope images of the micro-channel cross sections in the mold insert at positions (A), (B) and (C),
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Table 1 Molding conditions of various polymer resins. Material
COC PC PS PMMA
Molding conditions Melt temperature (°C)
Mold temperature (°C)
Injection velocity (mm/s)
Packing pressure (MPa)
(220) (290) (190) (220)
(50) (60) (50) (60)
(20) (40) (40) (40)
(20) (80) (40) (80)
235 305 205 235
(250) (320) (220) (250)
60 (70) 70 (80) 60 (70) 70 (80)
respectively. After part ejection from the mold, the replication accuracy of the micro-channel dimensions at position (C) was examined with the 3D laser microscope and compared to the dimensions of the stamp. Fig. 1(c) shows the measurement positions of the micro-channel. The measured dimensions of the width and depth of the micro-channels from the molded devices were compared with those from the stamp, and the differences were used to characterize the replication accuracy. Five measurements at each position were carried out and the average values from these five measurements were used for analysis and correlation. 3. Results and discussions 3.1. The effect of molding conditions on replication accuracy
40 60 60 60
(60) (80) (80) (80)
40 (60) 100 (120) 60 (80) 100 (120)
the mold temperature. The replication accuracy of the depth increases at higher melt and mold temperatures. This is because higher melt and mold temperatures lower the viscosity of the melt, resulting in an easier flow of the melt into the micro-channels. Figs. 6 and 7 depict the effects of injection velocity on the micro-channel width and depth, respectively. For each polymer, a higher injection velocity leads to better replication accuracy. This can be attributed to two causes. First, a higher injection velocity may decrease the melt viscosity due to viscous heating, especially when the melt flows through a thin-walled geometry. Second, a high-speed injection reduces the melt-mold contact time and the melt temperature drop. The effect of packing pressure on the replication accuracy of the width and depth of the micro-channels is illustrated in Figs. 8 and 9. A higher packing pressure improves the width and depth replication accuracy significantly.
From the experimental observations and measurements, it can be seen that the micro-channel replication accuracy is very sensitive to the viscosity of the polymer melt and the pressure in the cavity. The higher the polymer melt temperature, the lower the melt viscosity. The low melt viscosity results in better replication accuracy because the melt can fill the micro-channels more easily. High molding pressure also improves replication accuracy, by assisting the polymer melt flow within the micro-channels and reducing the subsequent shrinkage during the cooling process. For the PMMA, PC, PS, and COC resins, the variations of the microchannel width at position (C) in the molded parts with melt temperature and mold temperature are illustrated in Figs. 2 and 3, respectively. It can be seen that the width of the micro-channels decreases with increasing melt temperature and mold temperature. Because of the molecular forces among the polymer chains, the molded device tends to be flat in shape; this leads to a larger width and smaller depth of the microchannel compared with those of the stamp. The higher the melt and mold temperatures, the closer the width to the true dimension of the stamp. The influences of melt temperature and mold temperature on the replication depth of the micro-channels are shown in Figs. 4 and 5, respectively. Examination of these figures reveals that the replication accuracy of the micro-channel depth 2 (D2) is significantly affected by
Although the PS, PC, PMMA, and COC parts with the microchannels were molded under different processing conditions in this experiment, it is worthwhile to compare the replication accuracy of the micro-channels for these four polymer resins. Comparison of the micro-channel cross section dimensions width 1 and width 2 (i.e., W1 and W2) in Figs. 2, 3, 6, and 8 reveals that for micro-channels about 30 μm in depth, 100 μm in width and 50 μm in pitch size the COC resin exhibits an advantage over the PC and PS resins, while the PMMA resin shows the worst results. Regarding the replication accuracy of the micro-channel depth, Figs. 4, 5, 7, and 9 clearly show that depth 1 (i.e., D1) is only slightly influenced by polymer resin, the deviations from the stamp depth value (30.77 μm) are less than 2.5% for the four different polymer resins. However, depth 2 (i.e., D2) is significantly influenced by polymer resin, the deviations from the stamp depth value (30.77 μm) are in the range of approximately 2.6–33.4% for the four different polymer resins. For the COC resin, it can be clearly seen from Figs. 2–9 that width 1, width 2, depth 1, and depth 2 (i.e., W1, W2, D1, and D2) deviate only in the ranges of approximately 3.34–4.59%, 0.29–2.96%, 0.27–1.99% and 1.46–
Fig. 2. Effect of melt temperature on the width of the micro-channel.
Fig. 3. Effect of mold temperature on the width of the micro-channel.
3.2. Comparison of replication accuracy of micro-channels for various polymer resins
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Fig. 4. Effect of melt temperature on the depth of the micro-channel.
Fig. 7. Effect of injection velocity on the depth of the micro-channel.
7.60%, respectively, from the corresponding dimensions of the stamp under the selected molding conditions. Therefore, the COC resin has good replication accuracy. This is because COC has a good flowability, easily reproduces submicron-size surface features, and fills complex thin-walled parts. It also has low and substantially isotropic shrinkage during molding, resulting in a little warpage. Alternatively, on comparing the replication accuracy of the micro-channels for the PS, PMMA, and PC resins, it can be
seen that the PS resin under specific molding conditions (i.e., melt temperature 205 °C, mold temperature 70 °C, injection velocity 60 mm/s and packing pressure 60 MPa) achieves acceptable micro-channel replication accuracy. However, the PC and PMMA resins give poor accuracy of depth 2 and width 1, respectively. In summary, micro injection molding using the COC resin achieves the required micro-channel molding accuracy. The molded parts
Fig. 5. Effect of mold temperature on the depth of the micro-channel.
Fig. 8. Effect of packing pressure on the width of the micro-channel.
Fig. 6. Effect of injection velocity on the width of the micro-channel.
Fig. 9. Effect of packing pressure on the depth of the micro-channel.
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show excellent replication accuracy for the COC resin due to its low viscosity and low, isotropic shrinkage. The PS resin also achieves acceptable micro-channel replication accuracy under specific molding conditions.
project of the specific research fields in the Chung Yuan Christian University, Taiwan, under grant CYCU-98-CR-ME.
References 4. Conclusions In this study, micro injection molding of micro-featured devices used for DNA/RNA testing was carried out using a Ni–Co-based stamp. The effect of polymer resin and molding conditions on the replication accuracy of the micro features was investigated using a 3D laser microscope. The width and depth of the micro-channels were analyzed and correlated. Based on the measured results, it was found that for the micro-injection-molded devices, the accuracy of the width and depth of the micro-channels increased with increasing mold temperature, melt temperature, injection velocity and packing pressure within the regular processing window. Both the COC and PS polymer resins achieved the required micro-channel accuracy. The molded parts showed excellent replication for the COC polymer resin due to its low viscosity and low, isotropic shrinkage. The PS resin also achieved acceptable replication accuracy under specific molding conditions. Acknowledgments This research was supported by The Center-of-Excellence Program on Membrane Technology from the Ministry of Education and the
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