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Investigation of process-affected zone in ultrasonic embossing of microchannels on thermoplastic substrates
Journal of Manufacturing Processes 50 (2020) 394–402
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Investigation of process-affected zone in ultrasonic embossing of microchannels on thermoplastic substrates
T
Ferah Sucularlia,b, M.A. Sahir Arikana, Ender Yildirima,* a b
Mechanical Engineering Department, Middle East Technical University, Ankara, Turkey Aselsan A.Ş., Radar, Electronic Warfare Systems Business Sector, Ankara, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasonic embossing Thermoplastic Microchannel Process-affected zone
In this paper, the process-affected zone in ultrasonically embossed thermoplastic substrates is investigated both numerically and experimentally. Commercialization of microfluidic devices challenges the need for high-speed manufacturing of plastic chips. Ultrasonic embossing is considered as an alternative method since the cycle time can be as low as a few seconds per chip while keeping the cost relatively low. To examine the ultrasonic embossing process, experiments were carried out to replicate 200 μm wide and 150 μm high straight channels on 3 mm thick polymethylmethacrylate (PMMA) substrates. The mold was fabricated by milling on aluminum. The features could be embossed by applying an 85 N static force at 28 kHz ultrasonic vibration and 10 μm amplitude for 5 s at room temperature with replication rates of 99.5 % and 100 % for the width and the depth, respectively. During the experiments, a clearly visible process-affected zone typically bounded by a half-circle with the center at the channel axis was observed. It was proven that the process-affected zone was bounded by the isothermal surface at the glass transition temperature of the substrate material (107 °C), both numerically and experimentally. It was also shown that the composition of the substrate material remains unaffected within the processaffected zone.
1. Introduction
promising alternative since it eliminates the need for preprocessing such as mold preparation and can be used to process a wide range of materials. However, surface damage due to the thermal nature of the process limits its use [9]. For commercial products, injection molding is a well-established mass production process, although the mold design and fabrication costs are quite high. Thus, injection molding cannot be considered as an economically feasible process for low-volume fabrication. In hot embossing, a polymer substrate is heated above its glass transition temperature and is pressed by means of a heated pattern (mold). Hot embossing molds and equipment are simpler and cheaper compared to those in injection molding due to lower temperature and pressure requirements. Therefore, hot embossing can be suitable for lower volumes. However, since the manufacturing cycle time of hot embossing is longer than that of injection molding, injection molding is more likely to be adopted for mass production. Efforts are ongoing to reduce the hot embossing cycle time and the cost of automatic embossing equipment [10]. An alternative method is ultrasonic embossing. In ultrasonic embossing, cycle time can be dramatically reduced to a few seconds per chip, which makes the technique a promising alternative for medium- to high-volume production. Moreover, the initial investment cost is low since only an ultrasonic welding
As microfluidics develops as a technology enabling especially analyses and assays in chemistry and life sciences, various techniques have been developed for the fabrication of microfluidic devices. The early examples of silicon and glass microfluidic devices dating back to the late 1970s [1] were manufactured by using well-known and readily available clean-room microfabrication techniques. However, as microfluidic devices started to commercialize, medium to high-volume manufacturing at low costs became a necessity, which could not always be met by clean-room techniques [2], since costs of the material, process, and initial infrastructure are often too high. Alternatively, cheaper polymer-based techniques utilizing elastomers or thermoplastics have been adopted for the fabrication of microfluidic devices. Micromilling [3], injection molding [4], laser photo-ablation [5], polydimethylsiloxane (PDMS) molding [6], and hot embossing [7] are some of the known polymer-based fabrication techniques of microfluidic devices. PDMS molding is commonly used for prototyping of microfluidic devices, especially for research purposes since the material is often not suitable for commercial products due to its high gas permeability and uncontrolled wettability [8]. Laser photo-ablation is a
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Corresponding author. E-mail address: [email protected] (E. Yildirim).
https://doi.org/10.1016/j.jmapro.2019.12.055 Received 26 April 2019; Received in revised form 25 December 2019; Accepted 26 December 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 50 (2020) 394–402
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temperature inside the substrate by using thermocouples. However, this provided only the local temperature variation during the process. Kosloh et al. proposed to utilize pyro-electric foils placed in a stack of polymer sheets to measure the heat distribution across the sheets [23]. However, this method was limited by the damage of the pyro-electric foils as the polymer sheets deform. Tan et al. [24] numerically investigated the temperature distribution and the material flow pattern during the process. They numerically showed that the material flow was coincident with the temperature distribution but did not report an experimental proof. In this study, a high-resolution temperature distribution in the local deformation region– process-affected zone – was observed and visualized by using thermal imaging for the first time in the literature. Also, a numerical model was developed to describe the underlying physics in the formation of the process-affected zone. Finally, this paper aims to investigate the basic principles of the ultrasonic embossing process for replicating microfluidic features on thermoplastic substrates. For this purpose, a specific feature machined on an aluminum mold was embossed on polymethylmethacrylate (PMMA) substrate under constant static load and ultrasonic vibration frequency. Deformation and material properties were investigated through optical imaging and materials testing, respectively. The temperature variation in the substrate was monitored by using a thermal camera. Thermal and optical images were compared to reveal the process-affected zone in ultrasonic embossing.
Table 1 Comparison of common polymer-based fabrication techniques and ultrasonic embossing in terms of equipment cost and cycle time. Equipment cost and cycle time information for micromilling, hot embossing, and injection molding were extracted from [13].
Equipment cost Cycle time per chip
Micromilling
Hot Embossing
Injection Molding
Ultrasonic Embossing
∼30000 $ 5−30 min
∼ 20000 $ 10−30 min
> 40000 $ 10−30 s
∼ 4000 $ * ≤10 s
* Based on the price taken from a local supplier.
machine is required to realize the process [11]. Another advantage of ultrasonic embossing is that the process reduces thermal distortion compared to hot embossing since the latter requires complete heating of the substrate, whereas there is a locally restricted heating in ultrasonic embossing [12]. The common thermoplastic-based fabrication techniques and ultrasonic embossing are compared in Table 1 in terms of cost, time, and material. Ultrasonic embossing related works published so far mostly focused on the effects of various ultrasonic embossing parameters on replicated features, and optimization of these parameters to achieve better replication rates. In one of the early studies, Mekaru et al. compared hot embossing with ultrasonic-assisted hot embossing by replicating a nickel electroformed mold on polymer substrates and reported that increasing the embossing force suppressed the effect of ultrasonic vibration [14]. Zhu et al. considered holding time, during which the mold is still in contact with the substrate after the ultrasonic vibration is stopped, as one of the process parameters and investigated its effect along with the ultrasonic embossing time and the pressure [15]. They reported that increasing the holding time improved the replication rate. Luo et al. tested thermally assisted ultrasonic embossing to replicate microfeatures on both sides of polymer substrates [16]. They investigated the effect of vibration amplitude, force, embossing time, and temperature on replication quality and reported that ultrasonic force is the most influencing factor in terms of uniformity of the replicated features. Qi et al. proposed a new approach – local thermally assisted ultrasonic embossing – in which the substrate is only locally heated by conduction through a heated mold [17]. They reported that amplitude and duration of the ultrasonic vibration and the heating temperature were significant factors. Šakalys et al. proposed locating vibroactive pads between the substrate and the hot plate in a hot embossing setup to supply the ultrasonic vibrations [18]. The method was investigated to test the effect of vibration frequency of the vibroactive pad, process time, pressure, and temperature [19]. Other than the typical process parameters such as the embossing force or pressure, time, frequency, and amplitude, the effect of the pattern geometry was also examined. Yu et al. tested the effect of the feature geometry by embossing two different molds, one consisting of square-shaped micropillars and the other being the negative of the former, on polymer substrates [20]. They reported that the replication was improved with increasing embossing time and replication rates close to 100 % can be achieved in 2.5 s. of embossing. Similarly, Lee et al. tested imprinting nickel electroplated nano protrusions by ultrasonic embossing and reported improved replication with increasing embossing time [21]. The aforementioned studies put emphasis on the effect of different parameters on the quality of the embossed features. However, it is intended to characterize the process-affected-zone in ultrasonic embossing with a bottom-up approach in this paper. Noting that the ultrasonic embossing process is distinguished by rapid localized deformation and heating of the substrate at the vicinity of the protrusions on the mold, researchers also examined the action mechanism of ultrasonic embossing to have insight. Jung et al. investigated local heating of the substrate during ultrasonic embossing both numerically and experimentally [22]. They have measured the
2. Materials and methods In ultrasonic embossing, microfluidic features patterned on mold are replicated on a plastic substrate by ultrasonic vibration-assisted embossing. Embossing action is created by pressing a sonotrode (also called horn) vibrating at an ultrasonic frequency on the mold, which is in contact with the substrate at relatively low static forces. This causes localized heat generation, which softens the substrate at the vicinity of the mold pattern, and high-rate plastic deformation of the substrate. After replicating the pattern, ultrasonic vibration is stopped. Then, the substrate cools down (in the order of 100 ms [25]), and the sonotrode is moved up to remove the substrate. The entire process is completed in the order of a few seconds. The process is illustrated in Fig. 1. To analyze the ultrasonic embossing process, a 500 W portable ultrasonic welding equipment with 28 kHz output frequency (Ever Green Ultrasonic EGW-2805 obtained from a local supplier) was used in the experiments. Ultrasonic head of the equipment was assembled on a vertically movable plate of a custom-made stand. The movable plate can be loaded with dead weights to provide static loading during the operation. At the base of the stand, a brass holder was placed both to mount the substrate and the mold. The holder was equipped with balltype setscrews to constrain both the mold and the substrate while allowing only vertical motion of the mold. In addition, a load cell was placed underneath the holder to monitor the embossing force during the process. Additionally, a heater plate with 400 W resistive heater and a thermocouple were included to allow ultrasonic embossing at elevated temperatures (up to 200 °C). This custom-made setup is shown in Fig. 2. The sonotrode providing a vibration amplitude of 10 μm at 28 kHz frequency was fabricated from AA7075 aluminum due to the high strength and favorable fatigue characteristics of the material. The reader may refer to various sources for sonotrode design for ultrasonic embossing [26,27]. The mold (23 × 23 × 3 mm) used in the experiments involved a single protrusion (150 μm depth, 200 μm width) to define a straight channel. The protrusion extended to the edge of the mold to allow visual inspection during the experiments. The mold was fabricated from aluminum (AA7075) by milling on a desktop machining center (Light Machines Corporation, ProLIGHT 1000 Machining Center). PMMA blocks of size 23 × 23 × 3 mm were used as substrates. The mold and 395
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Fig. 1. Illustration of ultrasonic embossing process (a) The sonotrode is placed on top of the mold and the substrate. (b) Vibration is turned on, and the sonotrode is lowered to contact the mold. (c) While vibrating, a compressive force is exerted on the sonotrode, which deforms the substrate by replicating the mold pattern. (d) After the process, vibration is turned off, and the sonotrode is raised. (e) Mold is removed to release the embossed substrate.
until ultrasonic vibration is turned off. The section view shows that minor cracks are formed at the bottom edges of the channel during the process. This indicates that the stress extensively increases at the bottom edges of the channel. It was observed that the channel could be embossed in 2 s, implying that the mold fully penetrated the substrate within this period. It was also observed that continuing the process beyond this period improves the replication rates, which was defined as the ratio of a dimension on the replicated feature to that on the mold. The average replication rates after 5 s of ultrasonic embossing of four samples were measured as 99.5 % and 100.0 % for the width and the depth of the channel, respectively. Besides the geometry of the embossed channel, the section view indicates a distinct half-circular region with its center located at the channel axis. This region, which is the process-affected zone, is bounded by an elevated surface over the top surface of the substrate and a visible half circle at the periphery of the zone. The process-affected zone for one of the embossed channels is shown in Fig. 3(a). To characterize the process-affected zone, the distance between the axis of the channel and the farthest end of the elevated surface, which is
the substrate are also shown in Fig. 2. Experiments were conducted for different embossing times (te = 2, 3, 4, and 5 s) at room temperature (25 °C) while keeping static component of the force, vibration amplitude, and frequency constant at 85 N, 10 μm, and 28 kHz, respectively. After the process, the side surface of the substrates, to which the embossed microchannel extends, were ground to reveal a clear section view of the channel. Temperature distribution at the vicinity of the embossed pattern was observed by using a thermal camera (FLIR E5) during the process. For this purpose, the emissivity of PMMA (0.95), reflected apparent temperature (28 °C), and the distance between the camera and the substrate were defined in the camera settings for its self-calibration [28]. 3. Results and discussion The section view of an embossed channel and the variation of the embossing force during the process are shown in Fig. 3(a) and (b). The force exceeds the static component of 85 N in 0.5 s. Then the force reaches its maximum at 100 N in 1 s and remains constant thereafter
Fig. 2. Details of the custom-made ultrasonic embossing test set-up. 396
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Fig. 3. (a) Section view of the embossed channel showing the process-affected zone. The close-up view of the channel indicates the minor cracks at the bottom edges of the channel. (b) Variation of embossing force with respect to embossing time. The solid line indicates the static force applied by the dead weights. (c) Change of characteristic dimension (CD) of the process-affected zone with respect to embossing time. Error bars indicate the standard error of four measurements. Embossing experiments were carried out at room temperature (25 °C). Static force, vibration amplitude, and frequency were kept constant at 85 N, 10 μm, and 28 kHz, respectively.
was expected. In addition, re-meshing was done in the same regions at every 100,000 increments in time to prevent distortion of the elements during the solution. Mesh dependency tests showed that using more than 300 elements (total of the mold and PMMA substrate) has not led to significant changes in temperature and deformation. The resulting mesh is shown in Fig. 4(b). Considering the ultrasonic source and the sonotrode used in the experiments, the vibration frequency and the amplitude of the sonotrode were taken as 28 kHz and 10 μm, respectively. The vibration of the sonotrode hammers the mold on the substrate. This hammering effect causes an intermittent downward motion of the mold during the process. This intermittent downward motion was approximated by superposing a vibrating motion with an amplitude and frequency equal to that of the sonotrode and a constant down-feed. To define the vibrating motion of the mold, a custom subroutine was coded in FORTRAN by using VUAMP available in ABAQUS/Explicit. Constant down-feed speed was defined as 75 μm/s since it was observed in preliminary tests that 150 μm deep protrusions could be replicated in 2 s. Typically, PMMA is modeled as a viscoelastic material, especially at elevated temperatures. However, the relaxation modulus – time-dependent elastic modulus – of PMMA remains almost constant for a relaxation period of as long as 101 s. for a range of working temperatures [29], which implies that the material behaves more like an elasticplastic material if the relaxation period is relatively low. Since the vibration frequency of the ultrasonic horn used in the experiments was 28 kHz, one period of straining was approximately 0.04 ms. This indicates that the relaxation period was in the order of 10−2 ms., which is far shorter than 101 s. Therefore, PMMA was modeled as an elasticplastic material. Material properties, as presented in Tables 2 and 3, were introduced in the model. In addition to these properties, true stress-true strain relations at different temperatures were needed for PMMA. This relation depends on the strain rates involved in the process. Since the strain rate is typically high in ultrasonic embossing due to high vibration frequency, true stress-true strain data between 15 °C
basically the radius of the half-circular region, was defined as the characteristic dimension. Characteristic dimension values were measured by processing section views of the process-affected zone at different embossing times by using the open-source image processing tool ImageJ. The change of characteristic dimension with respect to embossing time is shown in Fig. 3(c). To understand the physics of the process-affected zone formation, a numerical model simulating ultrasonic embossing of the same channel (200 μm-wide, 150 μm-high straight channel) was created by using ABAQUS. Noting that the channel length is much larger than the crosssectional dimensions of the microchannel, the deformations along the channel length were ignored, which resulted in a 2D model. In addition, by defining a symmetry plane passing through the half-width of the channel, only half of the mold and the substrate were modeled to reduce the solution time. In the preliminary numerical analyses, full-size models of the mold and the substrate were simulated. The results indicated that at the end of the embossing process (t = 5 s), there was no significant difference between the temperature at the outer boundaries and the temperatures at 2 mm distance from the center of the embossed feature along the width and 1.5 mm distance from the center of the mold along the height, both for the mold and the substrate. Furthermore, no material flow was observed within the region approximately 1.5 mm outside the center of the embossed feature in the substrate since the temperature was below Tg. Accordingly, the width and the height of both the substrate and the mold was selected as 2 mm and 1.5 mm, respectively. The geometry of the mold and the substrate in the model are shown in Fig. 4(a). Lateral displacement of the substrate was set to zero since the holder allows the motion of the substrate only along the axis of the sonotrode. All rotational deformations in the substrate body were allowed. Both the mold and the substrate were modeled as deformable bodies. To observe the temperature of the mold and the substrate, fournode plane stress thermally coupled quadrilateral elements were used. Mesh was locally refined at the regions where excessive deformation 397
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Fig. 4. Schematic view of (a) mold and PMMA and (b) their meshing. (c) True stress-plastic strain of PMMA (at 1 s−1 strain rate) with varying temperature [32].
the rapid dissipation of heat, especially through the mold, again due to its relatively high thermal conductivity. However, after a certain time, heat generation due to wall friction becomes dominant, and the temperature of both the mold and the substrate increases. For the first 2 s, temperature variation at the substrate is validated by means of equivalent plastic strain and von Mises stress (Fig. 5(b) and (c)). The equivalent plastic strain and von Mises stress increases during the first 2 s (due to high deformation rate), then decreases slightly for a while (since hammering stops) and remains constant throughout the remaining process cycle. Moreover, the softening of the substrate results in homogenous stress distribution after the first 2 s of the process (Fig. 5(c)). It is also worthwhile to mention that von Mises stress increases up to about 70 MPa, which is quite higher than the fracture strength of PMMA [33], locally at the bottom edge of the replicated channels (Fig. 5(c)). This is identified as the primary reason for the cracks at the bottom of the channels shown in Fig. 3(a). The crack formation is attributed secondarily to the high temperature gradient between the bottom and the wall of the substrate (Fig. 5(a)), causing stress concentrations at the bottom edges. It can be clearly seen from Fig. 5 that the temperature of the substrate is highest in the vicinity of the mold pattern and decreases radially outwards. On the other hand, one of the most important observations that can be drawn from this result is that half-circular boundary of the process-affected zone and change of its characteristic dimension with embossing time (Fig. 3(a) and (c)) resembles the isothermal lines in numerical temperature distribution results in Fig. 5(a). It can also be seen from the numerical results in Fig. 5(a) that, the substrate temperature could have exceeded its glass transition temperature (Tg), which is expected to be about 105 °C for PMMA, in a certain region within the substrate at the vicinity of the protrusion on the mold. As a result, it was hypothesized that the temperature within the process-affected zone increases beyond the glass transition temperature of the substrate, and the half-circular boundary of the processaffected zone is the isothermal line at Tg. To test this hypothesis, the temperature distribution in the vicinity of the embossed pattern was recorded by using a thermal camera during the ultrasonic embossing process. Then, thermal images were used to generate maps of isothermal lines. The isothermal lines were then overlaid with cross-sectional microscope images of the embossed channels. For this purpose, the thermal images were previously scaled to the same magnification with the microscope images. The thermal images taken during the process were shown in Fig. 6(a)–(e), and the cross-sectional microscope image of the embossed channel and the isothermal lines overlaid with the microscope image are shown in Fig. 6(f) and (g). It is shown in Fig. 6(g) that the isothermal line at 107 °C coincides with the boundary of the process-affected zone visible on the microscope image. The temperature of the isothermal line
Table 2 Material properties of PMMA. Density and Young’s modulus values were extracted from ref. [30]. Specific heat values were extracted from [31]. Thermal conductivity was taken to be 0.19 W/m.K [31] irrespective of the temperature. Temperature (oC)
Density (kg/m3)
Young’s Modulus (MPa)
Specific Heat (kJ/kg.K)
15 30 60 90 100
1190 1185 1170 1155 1155
6000 5750 5000 4000 3500
1.33 1.40 1.64 1.88 1.98
Table 3 Material properties of aluminum (AA7075). Density (kg/m3)
Young’s Modulus (MPa)
Poisson's Ratio
Thermal Conductivity (W/mK)
Specific Heat (J/kg.K)
2800
8000
0.33
130
960
and 115 °C for PMMA at 1 s-1 (Fig. 4(c)), which is the highest strain rate data available in the literature for varying temperatures to the authors’ best knowledge, was used in the model. Simulations were carried out for 5 s of embossing time at 25 °C initial temperature of the mold and the substrate. Solutions were obtained in 51.5 h of CPU time by using Intel Core i3-6100/3.7 GHz microprocessor and 12 GB RAM. Simulation results show that the highest temperature is observed at the tip of the protrusion on the mold (Fig. 5(a)). However, the temperature distribution varies in time, which is attributed to the behavior of the process. Based on the observation that 150 μm deep protrusions could be replicated in 2 s, it can be deduced that within the first 2 s of the process, the mold hammers the substrate at ultrasonic frequency causing excessive plastic deformation. Excessive plastic work done at the tip of the protrusion causes heat generation. The generated heat is conducted through both the mold and the substrate. However, since the thermal conductivity of the substrate material (0.19 W/mK for PMMA) is less than that of the mold (130 W/mK for aluminum), the average temperature of the mold comes out to be considerably higher. Although plastic work caused by hammering is supposed to be the main cause of heat generation, the friction between the vibrating mold and substrate is also effective during this period. However, after the protrusion fully penetrates the substrate (after 2 s), only friction between the vibrating mold and channel wall is effective since there is no down movement of the mold. Right after 2 s, it was observed that local temperature at the tip of the protrusion decreases slightly for a while. This is attributed to 398
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Fig. 5. (a) Temperature, (b) equivalent plastic strain, and (c) von Mises stress distributions computed for 5 s of ultrasonic embossing at 25 °C.
microstructure evolution during ultrasonic welding of metal substrates [34]. It is foreseen that similar work on ultrasonic embossing of thermoplastic substrates would reveal the behavior of the polymer chains during the process. Another observation was related to the mechanism of channel formation. The preliminary experimental results indicated that the mold fully penetrated the substrate in the first 2 s of the process. As mentioned above in the discussion related to the numerical results, hammering and friction between the mold and the substrate were effective in this period. However, although the mold fully penetrated, it was observed that the channel cross-section could not be replicated well within this period. The channel was observed to be considerably wider on its top, reducing the replication rate in width. When the embossing time was extended to 5 s., it was observed that the channel cross-section could be better replicated yielding a 99.5 % replication rate in width. Since the protrusion on the mold has already fully penetrated the substrate in the first 2 s, hammering action would be expected to cease at this instant and only the friction between the mold and the substrate wall would be effective beyond then. Therefore, it can be deduced that extending the ultrasonic embossing time improved the replication rate by the effect of the friction between the mold and the substrate. Results of the numerical simulation and the experiments generally agree well in explaining the process-affected zone in ultrasonic embossing. However, there is a slight difference in temperature distribution obtained from the numerical model (Fig. 5(a)) and the
coincident with the boundary of the process-affected zone was then compared with the glass transition temperature of the PMMA substrates used in the experiments. To determine the glass transition temperature of the particular PMMA, differential scanning calorimeter (DSC) analysis was conducted. DSC revealed that the glass transition temperature of the particular PMMA was 107 °C (Fig. 7). Thus, it was proven that the boundary of the process-affected zone in ultrasonic embossing is the isothermal line at the glass transition temperature of the substrate being processed. On the other hand, it was also observed that there was a slight deviation between the isothermal line at glass transition temperature and the boundary of the process-affected zone (Fig. 6(g)). This deviation possibly arises from the distortions due to the orientation of the thermal camera with respect to the substrate and the mold. Another observation related to the process-affected zone was that the substrate in the zone became slightly opaquer than the original substrate. This raised doubt over whether there was any change in the composition of the substrate material. To check this, sample materials taken from the process-affected zone and the original substrate were analyzed by Fourier transform infrared spectroscopy (FTIR). The results showed that the fingerprints of the samples matched well, implying that there is no change in the structure of the material (Fig. 8). The change in the appearance of the material after ultrasonic embossing can be attributed to possible changes in the orientation of the polymer chains during the process. Shen et al. have carried out a study on 399
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Fig. 6. (a)–(e) Temperature distribution across the mold and the substrate in the embossing region during the process. The range of the temperature scale on the thermal camera was fixed as 100 °C–145 °C to filter the background signal. The temperature reading written on the bottom left of each frame shows the average temperature of the region of interest indicated by the white rectangle. (f) Section view of the resulting channel. The half-circular boundary of the process-affected zone is visible. (g) Isothermal lines overlaid with the section view of the channel. The boundary of the process-affected zone aligns with the isothermal line at 107 °C. The white dashed line was drawn to clarify the process-affected zone boundary. Isothermal lines were extracted from the thermal camera snapshot at t = 5 s.
Fig. 7. The result of DSC analysis of PMMA. Glass transition temperature was measured approximately as 107 °C.
Fig. 8. FTIR analysis result showing that the chemical structure of the substrate does not change during the process.
experiments (Fig. 6(a)–(g)). According to the numerical model, temperature increases until t = 2 s of the process. After then, both the mold and the substrate cool down between t = 2 s and t = 3 s due to the cease of hammering effect at t = 2 s, which in turn decreases heat generation due to high-rate deformation. After 3 s, both the mold and the substrate heat up again due to friction between the mold and the substrate until the end of the process. The highest temperature was observed at t = 5 s. Experimental results also showed that maximum temperature occurred at t = 5 s. However, it was observed that the temperature monotonically increased during the process, which was not predicted by the numerical model. The difference between the numerical model and the experimental result is basically attributed to
the unavailability of actual stress-strain data and viscoelastic material behavior for PMMA at high strain rates and process temperatures. This indicates the need for material testing at high strain rates and process temperatures for the materials used in ultrasonic embossing. The numerical analysis also yielded isothermal lines with a discontinuity at the interface between the mold and substrate (Fig. 5(a)), which were not observed in the thermal camera results (Fig. 6(g)). This indicates that thermal conductivity at the mold-PMMA interface could not be represented well by the numerical model, which can be considered as a possible reason for the difference between the temperature distributions in the numerical and experimental results. As shown in Fig. 3, minor cracks have formed at the bottom edges of 400
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the embossed channels during the experiments, which was also indicated by excessive equivalent stresses yielded by the numerical analysis. Preheating the substrate before the process was proposed to solve this problem. To test the proposed method, PMMA substrates were preheated to 100 °C (by using the heater plate shown in Fig. 2), which is very close to the glass transition temperature of the material (107 °C). Results showed that preheating considerably reduced the cracks since the overall substrate was softened. On the other hand, preheating resulted in undesired deformation (generally warping). This also adversely affected the replication rates, which were measured to be 103 % and 93 % for the width and the depth, respectively. Also, considering the time and power wasted for preheating, it was concluded that preheating should not be considered unless the cracks are needed to be strictly avoided depending on the application. As an alternative, the minor cracks could be cured after the process by a suitable solvent vapor treatment [35]. The results clearly show that the process-affected zone in ultrasonic embossing is bounded by the isotherm at the glass transition temperature of the substrate material as hypothesized. This isotherm also resembles the shear surface presented in [24], which is the interface between the softened material in the vicinity of the protrusion and the solid substrate bounding it. Tan et al. have numerically shown that material flow occurs inside this shear surface [24]. Therefore, the experimental results in this study also accords with the numerical findings presented in [24]. In addition to numerical and experimental investigation of the process affected zone in ultrasonic embossing process, to demonstrate the use of the process, a simple microfluidic reactor pattern composed of two inlet channels and a serpentine reaction channel was milled on an aluminum plate as the mold. This pattern was replicated on a 3 mm thick PMMA substrate by ultrasonic embossing at room temperature. Replicated microchannels were sealed by solvent assisted thermocompressive bonding of a 3 mm thick blank PMMA sheet. Before the bonding process, the embossed PMMA substrate and the blank PMMA were exposed to chloroform vapor at room temperature for 6 min. Bonding was achieved by applying a 5 kN compressive force at 85 °C for 1 min. The bonded microreactor chip with dyed water injected for visual clarity is shown in Fig. 9. As an alternative, bonding could also be achieved by ultrasonic welding. In this case, it is anticipated that elevated top surface of the embossed substrate within the process-affected zone (Fig. 3(a)) could be confined in a gutter machined in the mold to intrinsically form the energy directors [36] needed in ultrasonic welding process. The authors of this paper are currently working on the optimization of ultrasonic bonding of microfluidic chips fabricated by ultrasonic embossing. It is foreseen that combining ultrasonic welding with ultrasonic embossing would be a convenient way of rapid plastic microfluidic fabrication.
Fig. 9. (a) Machined aluminum mold for fabrication of the microreactor chip. (b) Microreactor pattern replicated on a PMMA substrate by ultrasonic embossing. (c) Bonded microreactor chip with dyed water injected.
particular PMMA). The composition of the material within and out of the process-affected zone was investigated by Fourier transform infrared spectroscopy (FTIR), which showed that the material composition remained unchanged during the process. As a result, the main conclusions of the work can be highlighted as below:
• Ultrasonic embossing takes place in two stages. In the first stage, the • • •
4. Conclusion In this work, the process-affected zone in ultrasonic embossing of thermoplastic substrates for the fabrication of microfluidic chips was investigated. For this purpose, embossing of 200 μm wide and 150 μm high straight channels on 3 mm thick PMMA substrates were tested. The results showed that the patterns could be embossed by applying an 85 N static load and an ultrasonic vibration at 28 kHz, with an amplitude of 10 μm at room temperature. It was observed that the mold fully penetrates the substrate in the first 2 s of the process. Extending the embossing time to 5 s improved the replication rate, which was determined as 99.5 % and 100.0 % for the width and the depth of the channel, respectively. Investigation of the cross-section of the embossed substrates revealed that the process-affected zone typically bounded by a visible half-circle around the channel cross-section. It was both numerically and experimentally shown that the boundary of the processaffected zone was coincident with the isothermal surface at the glass transition temperature of the substrate material (107 °C for the
mold fully penetrates the substrate, when hammering and the friction between the mold and the substrate are effective. In the second stage, since the mold fully penetrates, hammering ceases, and the friction is effective in improving the replication rate. The process-affected zone in ultrasonic embossing is bounded by a visible half-circle around the cross-section of the embossed feature. The boundary of the process-affected zone is coincident with the isothermal surface at the glass transition temperature of the substrate material. The composition of the material within the process-affected zone remains unchanged. This finding is believed to be quite valuable since material composition is essential, especially in microfluidic applications where biocompatibility is an issue.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Ferah Sucularlı contributed to carrying out the experiments, analysis, and preparation of the article. M.A. Sahir Arıkan reviewed experimental and analytical results and supervised the study. Ender Yıldırım supervised the study and contributed to the preparation of the article. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 401
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Ferah Sucularlı received her MSc and PhD degrees in Mechanical Engineering at Middle East Technical University (METU), Ankara, Turkey in 2013 and 2018, respectively. From 2014–2018, she worked as a research assistant in Mechanical Engineering at Çankaya University, Ankara, Turkey. She currently works as an R&D engineer at Radar, Electronic Warfare and Intelligence Systems Business Sector of Aselsan A.Ş. in Ankara, Turkey Her research interest focuses on the microchannel fabrication methods. M.A. Sahir Arıkan received his BSc (1979), MSc (1981) and PhD (1987) degrees in mechanical engineering from Middle East Technical University (METU), Ankara, Turkey. He is currently a professor and the chair in the same department. His research interests include CAD, CAM, Robotics, Machine Elements, Machine Design, Gear Design, Gear Dynamics, MEMS, and micromanufacturing. Ender Yıldırım received his BSc, MSc, and PhD degrees in mechanical engineering at Middle East Technical University (METU), Ankara, Turkey in 2002, 2005, and 2011, respectively. From 2012–2013, he worked as a postdoctoral researcher at Leiden Academic Center for Drug Research at Leiden University, the Netherlands. He worked as a faculty member in Mechanical Engineering Department at Cankaya University, Ankara, Turkey between 2013-2019. He is currently an assistant professor in Mechanical Engineering Department at Middle East Technical University, Ankara, Turkey. His research interests include methods of plastic microfluidic chip fabrication, capillary microfluidics, droplet-based microfluidics, bio-MEMS, and biosensors.
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Investigation of process-affected zone in ultrasonic embossing of microchannels on thermoplastic substrates
T
Ferah Sucularlia,b, M.A. Sahir Arikana, Ender Yildirima,* a b
Mechanical Engineering Department, Middle East Technical University, Ankara, Turkey Aselsan A.Ş., Radar, Electronic Warfare Systems Business Sector, Ankara, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasonic embossing Thermoplastic Microchannel Process-affected zone
In this paper, the process-affected zone in ultrasonically embossed thermoplastic substrates is investigated both numerically and experimentally. Commercialization of microfluidic devices challenges the need for high-speed manufacturing of plastic chips. Ultrasonic embossing is considered as an alternative method since the cycle time can be as low as a few seconds per chip while keeping the cost relatively low. To examine the ultrasonic embossing process, experiments were carried out to replicate 200 μm wide and 150 μm high straight channels on 3 mm thick polymethylmethacrylate (PMMA) substrates. The mold was fabricated by milling on aluminum. The features could be embossed by applying an 85 N static force at 28 kHz ultrasonic vibration and 10 μm amplitude for 5 s at room temperature with replication rates of 99.5 % and 100 % for the width and the depth, respectively. During the experiments, a clearly visible process-affected zone typically bounded by a half-circle with the center at the channel axis was observed. It was proven that the process-affected zone was bounded by the isothermal surface at the glass transition temperature of the substrate material (107 °C), both numerically and experimentally. It was also shown that the composition of the substrate material remains unaffected within the processaffected zone.
1. Introduction
promising alternative since it eliminates the need for preprocessing such as mold preparation and can be used to process a wide range of materials. However, surface damage due to the thermal nature of the process limits its use [9]. For commercial products, injection molding is a well-established mass production process, although the mold design and fabrication costs are quite high. Thus, injection molding cannot be considered as an economically feasible process for low-volume fabrication. In hot embossing, a polymer substrate is heated above its glass transition temperature and is pressed by means of a heated pattern (mold). Hot embossing molds and equipment are simpler and cheaper compared to those in injection molding due to lower temperature and pressure requirements. Therefore, hot embossing can be suitable for lower volumes. However, since the manufacturing cycle time of hot embossing is longer than that of injection molding, injection molding is more likely to be adopted for mass production. Efforts are ongoing to reduce the hot embossing cycle time and the cost of automatic embossing equipment [10]. An alternative method is ultrasonic embossing. In ultrasonic embossing, cycle time can be dramatically reduced to a few seconds per chip, which makes the technique a promising alternative for medium- to high-volume production. Moreover, the initial investment cost is low since only an ultrasonic welding
As microfluidics develops as a technology enabling especially analyses and assays in chemistry and life sciences, various techniques have been developed for the fabrication of microfluidic devices. The early examples of silicon and glass microfluidic devices dating back to the late 1970s [1] were manufactured by using well-known and readily available clean-room microfabrication techniques. However, as microfluidic devices started to commercialize, medium to high-volume manufacturing at low costs became a necessity, which could not always be met by clean-room techniques [2], since costs of the material, process, and initial infrastructure are often too high. Alternatively, cheaper polymer-based techniques utilizing elastomers or thermoplastics have been adopted for the fabrication of microfluidic devices. Micromilling [3], injection molding [4], laser photo-ablation [5], polydimethylsiloxane (PDMS) molding [6], and hot embossing [7] are some of the known polymer-based fabrication techniques of microfluidic devices. PDMS molding is commonly used for prototyping of microfluidic devices, especially for research purposes since the material is often not suitable for commercial products due to its high gas permeability and uncontrolled wettability [8]. Laser photo-ablation is a
⁎
Corresponding author. E-mail address: [email protected] (E. Yildirim).
https://doi.org/10.1016/j.jmapro.2019.12.055 Received 26 April 2019; Received in revised form 25 December 2019; Accepted 26 December 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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temperature inside the substrate by using thermocouples. However, this provided only the local temperature variation during the process. Kosloh et al. proposed to utilize pyro-electric foils placed in a stack of polymer sheets to measure the heat distribution across the sheets [23]. However, this method was limited by the damage of the pyro-electric foils as the polymer sheets deform. Tan et al. [24] numerically investigated the temperature distribution and the material flow pattern during the process. They numerically showed that the material flow was coincident with the temperature distribution but did not report an experimental proof. In this study, a high-resolution temperature distribution in the local deformation region– process-affected zone – was observed and visualized by using thermal imaging for the first time in the literature. Also, a numerical model was developed to describe the underlying physics in the formation of the process-affected zone. Finally, this paper aims to investigate the basic principles of the ultrasonic embossing process for replicating microfluidic features on thermoplastic substrates. For this purpose, a specific feature machined on an aluminum mold was embossed on polymethylmethacrylate (PMMA) substrate under constant static load and ultrasonic vibration frequency. Deformation and material properties were investigated through optical imaging and materials testing, respectively. The temperature variation in the substrate was monitored by using a thermal camera. Thermal and optical images were compared to reveal the process-affected zone in ultrasonic embossing.
Table 1 Comparison of common polymer-based fabrication techniques and ultrasonic embossing in terms of equipment cost and cycle time. Equipment cost and cycle time information for micromilling, hot embossing, and injection molding were extracted from [13].
Equipment cost Cycle time per chip
Micromilling
Hot Embossing
Injection Molding
Ultrasonic Embossing
∼30000 $ 5−30 min
∼ 20000 $ 10−30 min
> 40000 $ 10−30 s
∼ 4000 $ * ≤10 s
* Based on the price taken from a local supplier.
machine is required to realize the process [11]. Another advantage of ultrasonic embossing is that the process reduces thermal distortion compared to hot embossing since the latter requires complete heating of the substrate, whereas there is a locally restricted heating in ultrasonic embossing [12]. The common thermoplastic-based fabrication techniques and ultrasonic embossing are compared in Table 1 in terms of cost, time, and material. Ultrasonic embossing related works published so far mostly focused on the effects of various ultrasonic embossing parameters on replicated features, and optimization of these parameters to achieve better replication rates. In one of the early studies, Mekaru et al. compared hot embossing with ultrasonic-assisted hot embossing by replicating a nickel electroformed mold on polymer substrates and reported that increasing the embossing force suppressed the effect of ultrasonic vibration [14]. Zhu et al. considered holding time, during which the mold is still in contact with the substrate after the ultrasonic vibration is stopped, as one of the process parameters and investigated its effect along with the ultrasonic embossing time and the pressure [15]. They reported that increasing the holding time improved the replication rate. Luo et al. tested thermally assisted ultrasonic embossing to replicate microfeatures on both sides of polymer substrates [16]. They investigated the effect of vibration amplitude, force, embossing time, and temperature on replication quality and reported that ultrasonic force is the most influencing factor in terms of uniformity of the replicated features. Qi et al. proposed a new approach – local thermally assisted ultrasonic embossing – in which the substrate is only locally heated by conduction through a heated mold [17]. They reported that amplitude and duration of the ultrasonic vibration and the heating temperature were significant factors. Šakalys et al. proposed locating vibroactive pads between the substrate and the hot plate in a hot embossing setup to supply the ultrasonic vibrations [18]. The method was investigated to test the effect of vibration frequency of the vibroactive pad, process time, pressure, and temperature [19]. Other than the typical process parameters such as the embossing force or pressure, time, frequency, and amplitude, the effect of the pattern geometry was also examined. Yu et al. tested the effect of the feature geometry by embossing two different molds, one consisting of square-shaped micropillars and the other being the negative of the former, on polymer substrates [20]. They reported that the replication was improved with increasing embossing time and replication rates close to 100 % can be achieved in 2.5 s. of embossing. Similarly, Lee et al. tested imprinting nickel electroplated nano protrusions by ultrasonic embossing and reported improved replication with increasing embossing time [21]. The aforementioned studies put emphasis on the effect of different parameters on the quality of the embossed features. However, it is intended to characterize the process-affected-zone in ultrasonic embossing with a bottom-up approach in this paper. Noting that the ultrasonic embossing process is distinguished by rapid localized deformation and heating of the substrate at the vicinity of the protrusions on the mold, researchers also examined the action mechanism of ultrasonic embossing to have insight. Jung et al. investigated local heating of the substrate during ultrasonic embossing both numerically and experimentally [22]. They have measured the
2. Materials and methods In ultrasonic embossing, microfluidic features patterned on mold are replicated on a plastic substrate by ultrasonic vibration-assisted embossing. Embossing action is created by pressing a sonotrode (also called horn) vibrating at an ultrasonic frequency on the mold, which is in contact with the substrate at relatively low static forces. This causes localized heat generation, which softens the substrate at the vicinity of the mold pattern, and high-rate plastic deformation of the substrate. After replicating the pattern, ultrasonic vibration is stopped. Then, the substrate cools down (in the order of 100 ms [25]), and the sonotrode is moved up to remove the substrate. The entire process is completed in the order of a few seconds. The process is illustrated in Fig. 1. To analyze the ultrasonic embossing process, a 500 W portable ultrasonic welding equipment with 28 kHz output frequency (Ever Green Ultrasonic EGW-2805 obtained from a local supplier) was used in the experiments. Ultrasonic head of the equipment was assembled on a vertically movable plate of a custom-made stand. The movable plate can be loaded with dead weights to provide static loading during the operation. At the base of the stand, a brass holder was placed both to mount the substrate and the mold. The holder was equipped with balltype setscrews to constrain both the mold and the substrate while allowing only vertical motion of the mold. In addition, a load cell was placed underneath the holder to monitor the embossing force during the process. Additionally, a heater plate with 400 W resistive heater and a thermocouple were included to allow ultrasonic embossing at elevated temperatures (up to 200 °C). This custom-made setup is shown in Fig. 2. The sonotrode providing a vibration amplitude of 10 μm at 28 kHz frequency was fabricated from AA7075 aluminum due to the high strength and favorable fatigue characteristics of the material. The reader may refer to various sources for sonotrode design for ultrasonic embossing [26,27]. The mold (23 × 23 × 3 mm) used in the experiments involved a single protrusion (150 μm depth, 200 μm width) to define a straight channel. The protrusion extended to the edge of the mold to allow visual inspection during the experiments. The mold was fabricated from aluminum (AA7075) by milling on a desktop machining center (Light Machines Corporation, ProLIGHT 1000 Machining Center). PMMA blocks of size 23 × 23 × 3 mm were used as substrates. The mold and 395
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Fig. 1. Illustration of ultrasonic embossing process (a) The sonotrode is placed on top of the mold and the substrate. (b) Vibration is turned on, and the sonotrode is lowered to contact the mold. (c) While vibrating, a compressive force is exerted on the sonotrode, which deforms the substrate by replicating the mold pattern. (d) After the process, vibration is turned off, and the sonotrode is raised. (e) Mold is removed to release the embossed substrate.
until ultrasonic vibration is turned off. The section view shows that minor cracks are formed at the bottom edges of the channel during the process. This indicates that the stress extensively increases at the bottom edges of the channel. It was observed that the channel could be embossed in 2 s, implying that the mold fully penetrated the substrate within this period. It was also observed that continuing the process beyond this period improves the replication rates, which was defined as the ratio of a dimension on the replicated feature to that on the mold. The average replication rates after 5 s of ultrasonic embossing of four samples were measured as 99.5 % and 100.0 % for the width and the depth of the channel, respectively. Besides the geometry of the embossed channel, the section view indicates a distinct half-circular region with its center located at the channel axis. This region, which is the process-affected zone, is bounded by an elevated surface over the top surface of the substrate and a visible half circle at the periphery of the zone. The process-affected zone for one of the embossed channels is shown in Fig. 3(a). To characterize the process-affected zone, the distance between the axis of the channel and the farthest end of the elevated surface, which is
the substrate are also shown in Fig. 2. Experiments were conducted for different embossing times (te = 2, 3, 4, and 5 s) at room temperature (25 °C) while keeping static component of the force, vibration amplitude, and frequency constant at 85 N, 10 μm, and 28 kHz, respectively. After the process, the side surface of the substrates, to which the embossed microchannel extends, were ground to reveal a clear section view of the channel. Temperature distribution at the vicinity of the embossed pattern was observed by using a thermal camera (FLIR E5) during the process. For this purpose, the emissivity of PMMA (0.95), reflected apparent temperature (28 °C), and the distance between the camera and the substrate were defined in the camera settings for its self-calibration [28]. 3. Results and discussion The section view of an embossed channel and the variation of the embossing force during the process are shown in Fig. 3(a) and (b). The force exceeds the static component of 85 N in 0.5 s. Then the force reaches its maximum at 100 N in 1 s and remains constant thereafter
Fig. 2. Details of the custom-made ultrasonic embossing test set-up. 396
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Fig. 3. (a) Section view of the embossed channel showing the process-affected zone. The close-up view of the channel indicates the minor cracks at the bottom edges of the channel. (b) Variation of embossing force with respect to embossing time. The solid line indicates the static force applied by the dead weights. (c) Change of characteristic dimension (CD) of the process-affected zone with respect to embossing time. Error bars indicate the standard error of four measurements. Embossing experiments were carried out at room temperature (25 °C). Static force, vibration amplitude, and frequency were kept constant at 85 N, 10 μm, and 28 kHz, respectively.
was expected. In addition, re-meshing was done in the same regions at every 100,000 increments in time to prevent distortion of the elements during the solution. Mesh dependency tests showed that using more than 300 elements (total of the mold and PMMA substrate) has not led to significant changes in temperature and deformation. The resulting mesh is shown in Fig. 4(b). Considering the ultrasonic source and the sonotrode used in the experiments, the vibration frequency and the amplitude of the sonotrode were taken as 28 kHz and 10 μm, respectively. The vibration of the sonotrode hammers the mold on the substrate. This hammering effect causes an intermittent downward motion of the mold during the process. This intermittent downward motion was approximated by superposing a vibrating motion with an amplitude and frequency equal to that of the sonotrode and a constant down-feed. To define the vibrating motion of the mold, a custom subroutine was coded in FORTRAN by using VUAMP available in ABAQUS/Explicit. Constant down-feed speed was defined as 75 μm/s since it was observed in preliminary tests that 150 μm deep protrusions could be replicated in 2 s. Typically, PMMA is modeled as a viscoelastic material, especially at elevated temperatures. However, the relaxation modulus – time-dependent elastic modulus – of PMMA remains almost constant for a relaxation period of as long as 101 s. for a range of working temperatures [29], which implies that the material behaves more like an elasticplastic material if the relaxation period is relatively low. Since the vibration frequency of the ultrasonic horn used in the experiments was 28 kHz, one period of straining was approximately 0.04 ms. This indicates that the relaxation period was in the order of 10−2 ms., which is far shorter than 101 s. Therefore, PMMA was modeled as an elasticplastic material. Material properties, as presented in Tables 2 and 3, were introduced in the model. In addition to these properties, true stress-true strain relations at different temperatures were needed for PMMA. This relation depends on the strain rates involved in the process. Since the strain rate is typically high in ultrasonic embossing due to high vibration frequency, true stress-true strain data between 15 °C
basically the radius of the half-circular region, was defined as the characteristic dimension. Characteristic dimension values were measured by processing section views of the process-affected zone at different embossing times by using the open-source image processing tool ImageJ. The change of characteristic dimension with respect to embossing time is shown in Fig. 3(c). To understand the physics of the process-affected zone formation, a numerical model simulating ultrasonic embossing of the same channel (200 μm-wide, 150 μm-high straight channel) was created by using ABAQUS. Noting that the channel length is much larger than the crosssectional dimensions of the microchannel, the deformations along the channel length were ignored, which resulted in a 2D model. In addition, by defining a symmetry plane passing through the half-width of the channel, only half of the mold and the substrate were modeled to reduce the solution time. In the preliminary numerical analyses, full-size models of the mold and the substrate were simulated. The results indicated that at the end of the embossing process (t = 5 s), there was no significant difference between the temperature at the outer boundaries and the temperatures at 2 mm distance from the center of the embossed feature along the width and 1.5 mm distance from the center of the mold along the height, both for the mold and the substrate. Furthermore, no material flow was observed within the region approximately 1.5 mm outside the center of the embossed feature in the substrate since the temperature was below Tg. Accordingly, the width and the height of both the substrate and the mold was selected as 2 mm and 1.5 mm, respectively. The geometry of the mold and the substrate in the model are shown in Fig. 4(a). Lateral displacement of the substrate was set to zero since the holder allows the motion of the substrate only along the axis of the sonotrode. All rotational deformations in the substrate body were allowed. Both the mold and the substrate were modeled as deformable bodies. To observe the temperature of the mold and the substrate, fournode plane stress thermally coupled quadrilateral elements were used. Mesh was locally refined at the regions where excessive deformation 397
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Fig. 4. Schematic view of (a) mold and PMMA and (b) their meshing. (c) True stress-plastic strain of PMMA (at 1 s−1 strain rate) with varying temperature [32].
the rapid dissipation of heat, especially through the mold, again due to its relatively high thermal conductivity. However, after a certain time, heat generation due to wall friction becomes dominant, and the temperature of both the mold and the substrate increases. For the first 2 s, temperature variation at the substrate is validated by means of equivalent plastic strain and von Mises stress (Fig. 5(b) and (c)). The equivalent plastic strain and von Mises stress increases during the first 2 s (due to high deformation rate), then decreases slightly for a while (since hammering stops) and remains constant throughout the remaining process cycle. Moreover, the softening of the substrate results in homogenous stress distribution after the first 2 s of the process (Fig. 5(c)). It is also worthwhile to mention that von Mises stress increases up to about 70 MPa, which is quite higher than the fracture strength of PMMA [33], locally at the bottom edge of the replicated channels (Fig. 5(c)). This is identified as the primary reason for the cracks at the bottom of the channels shown in Fig. 3(a). The crack formation is attributed secondarily to the high temperature gradient between the bottom and the wall of the substrate (Fig. 5(a)), causing stress concentrations at the bottom edges. It can be clearly seen from Fig. 5 that the temperature of the substrate is highest in the vicinity of the mold pattern and decreases radially outwards. On the other hand, one of the most important observations that can be drawn from this result is that half-circular boundary of the process-affected zone and change of its characteristic dimension with embossing time (Fig. 3(a) and (c)) resembles the isothermal lines in numerical temperature distribution results in Fig. 5(a). It can also be seen from the numerical results in Fig. 5(a) that, the substrate temperature could have exceeded its glass transition temperature (Tg), which is expected to be about 105 °C for PMMA, in a certain region within the substrate at the vicinity of the protrusion on the mold. As a result, it was hypothesized that the temperature within the process-affected zone increases beyond the glass transition temperature of the substrate, and the half-circular boundary of the processaffected zone is the isothermal line at Tg. To test this hypothesis, the temperature distribution in the vicinity of the embossed pattern was recorded by using a thermal camera during the ultrasonic embossing process. Then, thermal images were used to generate maps of isothermal lines. The isothermal lines were then overlaid with cross-sectional microscope images of the embossed channels. For this purpose, the thermal images were previously scaled to the same magnification with the microscope images. The thermal images taken during the process were shown in Fig. 6(a)–(e), and the cross-sectional microscope image of the embossed channel and the isothermal lines overlaid with the microscope image are shown in Fig. 6(f) and (g). It is shown in Fig. 6(g) that the isothermal line at 107 °C coincides with the boundary of the process-affected zone visible on the microscope image. The temperature of the isothermal line
Table 2 Material properties of PMMA. Density and Young’s modulus values were extracted from ref. [30]. Specific heat values were extracted from [31]. Thermal conductivity was taken to be 0.19 W/m.K [31] irrespective of the temperature. Temperature (oC)
Density (kg/m3)
Young’s Modulus (MPa)
Specific Heat (kJ/kg.K)
15 30 60 90 100
1190 1185 1170 1155 1155
6000 5750 5000 4000 3500
1.33 1.40 1.64 1.88 1.98
Table 3 Material properties of aluminum (AA7075). Density (kg/m3)
Young’s Modulus (MPa)
Poisson's Ratio
Thermal Conductivity (W/mK)
Specific Heat (J/kg.K)
2800
8000
0.33
130
960
and 115 °C for PMMA at 1 s-1 (Fig. 4(c)), which is the highest strain rate data available in the literature for varying temperatures to the authors’ best knowledge, was used in the model. Simulations were carried out for 5 s of embossing time at 25 °C initial temperature of the mold and the substrate. Solutions were obtained in 51.5 h of CPU time by using Intel Core i3-6100/3.7 GHz microprocessor and 12 GB RAM. Simulation results show that the highest temperature is observed at the tip of the protrusion on the mold (Fig. 5(a)). However, the temperature distribution varies in time, which is attributed to the behavior of the process. Based on the observation that 150 μm deep protrusions could be replicated in 2 s, it can be deduced that within the first 2 s of the process, the mold hammers the substrate at ultrasonic frequency causing excessive plastic deformation. Excessive plastic work done at the tip of the protrusion causes heat generation. The generated heat is conducted through both the mold and the substrate. However, since the thermal conductivity of the substrate material (0.19 W/mK for PMMA) is less than that of the mold (130 W/mK for aluminum), the average temperature of the mold comes out to be considerably higher. Although plastic work caused by hammering is supposed to be the main cause of heat generation, the friction between the vibrating mold and substrate is also effective during this period. However, after the protrusion fully penetrates the substrate (after 2 s), only friction between the vibrating mold and channel wall is effective since there is no down movement of the mold. Right after 2 s, it was observed that local temperature at the tip of the protrusion decreases slightly for a while. This is attributed to 398
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Fig. 5. (a) Temperature, (b) equivalent plastic strain, and (c) von Mises stress distributions computed for 5 s of ultrasonic embossing at 25 °C.
microstructure evolution during ultrasonic welding of metal substrates [34]. It is foreseen that similar work on ultrasonic embossing of thermoplastic substrates would reveal the behavior of the polymer chains during the process. Another observation was related to the mechanism of channel formation. The preliminary experimental results indicated that the mold fully penetrated the substrate in the first 2 s of the process. As mentioned above in the discussion related to the numerical results, hammering and friction between the mold and the substrate were effective in this period. However, although the mold fully penetrated, it was observed that the channel cross-section could not be replicated well within this period. The channel was observed to be considerably wider on its top, reducing the replication rate in width. When the embossing time was extended to 5 s., it was observed that the channel cross-section could be better replicated yielding a 99.5 % replication rate in width. Since the protrusion on the mold has already fully penetrated the substrate in the first 2 s, hammering action would be expected to cease at this instant and only the friction between the mold and the substrate wall would be effective beyond then. Therefore, it can be deduced that extending the ultrasonic embossing time improved the replication rate by the effect of the friction between the mold and the substrate. Results of the numerical simulation and the experiments generally agree well in explaining the process-affected zone in ultrasonic embossing. However, there is a slight difference in temperature distribution obtained from the numerical model (Fig. 5(a)) and the
coincident with the boundary of the process-affected zone was then compared with the glass transition temperature of the PMMA substrates used in the experiments. To determine the glass transition temperature of the particular PMMA, differential scanning calorimeter (DSC) analysis was conducted. DSC revealed that the glass transition temperature of the particular PMMA was 107 °C (Fig. 7). Thus, it was proven that the boundary of the process-affected zone in ultrasonic embossing is the isothermal line at the glass transition temperature of the substrate being processed. On the other hand, it was also observed that there was a slight deviation between the isothermal line at glass transition temperature and the boundary of the process-affected zone (Fig. 6(g)). This deviation possibly arises from the distortions due to the orientation of the thermal camera with respect to the substrate and the mold. Another observation related to the process-affected zone was that the substrate in the zone became slightly opaquer than the original substrate. This raised doubt over whether there was any change in the composition of the substrate material. To check this, sample materials taken from the process-affected zone and the original substrate were analyzed by Fourier transform infrared spectroscopy (FTIR). The results showed that the fingerprints of the samples matched well, implying that there is no change in the structure of the material (Fig. 8). The change in the appearance of the material after ultrasonic embossing can be attributed to possible changes in the orientation of the polymer chains during the process. Shen et al. have carried out a study on 399
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Fig. 6. (a)–(e) Temperature distribution across the mold and the substrate in the embossing region during the process. The range of the temperature scale on the thermal camera was fixed as 100 °C–145 °C to filter the background signal. The temperature reading written on the bottom left of each frame shows the average temperature of the region of interest indicated by the white rectangle. (f) Section view of the resulting channel. The half-circular boundary of the process-affected zone is visible. (g) Isothermal lines overlaid with the section view of the channel. The boundary of the process-affected zone aligns with the isothermal line at 107 °C. The white dashed line was drawn to clarify the process-affected zone boundary. Isothermal lines were extracted from the thermal camera snapshot at t = 5 s.
Fig. 7. The result of DSC analysis of PMMA. Glass transition temperature was measured approximately as 107 °C.
Fig. 8. FTIR analysis result showing that the chemical structure of the substrate does not change during the process.
experiments (Fig. 6(a)–(g)). According to the numerical model, temperature increases until t = 2 s of the process. After then, both the mold and the substrate cool down between t = 2 s and t = 3 s due to the cease of hammering effect at t = 2 s, which in turn decreases heat generation due to high-rate deformation. After 3 s, both the mold and the substrate heat up again due to friction between the mold and the substrate until the end of the process. The highest temperature was observed at t = 5 s. Experimental results also showed that maximum temperature occurred at t = 5 s. However, it was observed that the temperature monotonically increased during the process, which was not predicted by the numerical model. The difference between the numerical model and the experimental result is basically attributed to
the unavailability of actual stress-strain data and viscoelastic material behavior for PMMA at high strain rates and process temperatures. This indicates the need for material testing at high strain rates and process temperatures for the materials used in ultrasonic embossing. The numerical analysis also yielded isothermal lines with a discontinuity at the interface between the mold and substrate (Fig. 5(a)), which were not observed in the thermal camera results (Fig. 6(g)). This indicates that thermal conductivity at the mold-PMMA interface could not be represented well by the numerical model, which can be considered as a possible reason for the difference between the temperature distributions in the numerical and experimental results. As shown in Fig. 3, minor cracks have formed at the bottom edges of 400
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the embossed channels during the experiments, which was also indicated by excessive equivalent stresses yielded by the numerical analysis. Preheating the substrate before the process was proposed to solve this problem. To test the proposed method, PMMA substrates were preheated to 100 °C (by using the heater plate shown in Fig. 2), which is very close to the glass transition temperature of the material (107 °C). Results showed that preheating considerably reduced the cracks since the overall substrate was softened. On the other hand, preheating resulted in undesired deformation (generally warping). This also adversely affected the replication rates, which were measured to be 103 % and 93 % for the width and the depth, respectively. Also, considering the time and power wasted for preheating, it was concluded that preheating should not be considered unless the cracks are needed to be strictly avoided depending on the application. As an alternative, the minor cracks could be cured after the process by a suitable solvent vapor treatment [35]. The results clearly show that the process-affected zone in ultrasonic embossing is bounded by the isotherm at the glass transition temperature of the substrate material as hypothesized. This isotherm also resembles the shear surface presented in [24], which is the interface between the softened material in the vicinity of the protrusion and the solid substrate bounding it. Tan et al. have numerically shown that material flow occurs inside this shear surface [24]. Therefore, the experimental results in this study also accords with the numerical findings presented in [24]. In addition to numerical and experimental investigation of the process affected zone in ultrasonic embossing process, to demonstrate the use of the process, a simple microfluidic reactor pattern composed of two inlet channels and a serpentine reaction channel was milled on an aluminum plate as the mold. This pattern was replicated on a 3 mm thick PMMA substrate by ultrasonic embossing at room temperature. Replicated microchannels were sealed by solvent assisted thermocompressive bonding of a 3 mm thick blank PMMA sheet. Before the bonding process, the embossed PMMA substrate and the blank PMMA were exposed to chloroform vapor at room temperature for 6 min. Bonding was achieved by applying a 5 kN compressive force at 85 °C for 1 min. The bonded microreactor chip with dyed water injected for visual clarity is shown in Fig. 9. As an alternative, bonding could also be achieved by ultrasonic welding. In this case, it is anticipated that elevated top surface of the embossed substrate within the process-affected zone (Fig. 3(a)) could be confined in a gutter machined in the mold to intrinsically form the energy directors [36] needed in ultrasonic welding process. The authors of this paper are currently working on the optimization of ultrasonic bonding of microfluidic chips fabricated by ultrasonic embossing. It is foreseen that combining ultrasonic welding with ultrasonic embossing would be a convenient way of rapid plastic microfluidic fabrication.
Fig. 9. (a) Machined aluminum mold for fabrication of the microreactor chip. (b) Microreactor pattern replicated on a PMMA substrate by ultrasonic embossing. (c) Bonded microreactor chip with dyed water injected.
particular PMMA). The composition of the material within and out of the process-affected zone was investigated by Fourier transform infrared spectroscopy (FTIR), which showed that the material composition remained unchanged during the process. As a result, the main conclusions of the work can be highlighted as below:
• Ultrasonic embossing takes place in two stages. In the first stage, the • • •
4. Conclusion In this work, the process-affected zone in ultrasonic embossing of thermoplastic substrates for the fabrication of microfluidic chips was investigated. For this purpose, embossing of 200 μm wide and 150 μm high straight channels on 3 mm thick PMMA substrates were tested. The results showed that the patterns could be embossed by applying an 85 N static load and an ultrasonic vibration at 28 kHz, with an amplitude of 10 μm at room temperature. It was observed that the mold fully penetrates the substrate in the first 2 s of the process. Extending the embossing time to 5 s improved the replication rate, which was determined as 99.5 % and 100.0 % for the width and the depth of the channel, respectively. Investigation of the cross-section of the embossed substrates revealed that the process-affected zone typically bounded by a visible half-circle around the channel cross-section. It was both numerically and experimentally shown that the boundary of the processaffected zone was coincident with the isothermal surface at the glass transition temperature of the substrate material (107 °C for the
mold fully penetrates the substrate, when hammering and the friction between the mold and the substrate are effective. In the second stage, since the mold fully penetrates, hammering ceases, and the friction is effective in improving the replication rate. The process-affected zone in ultrasonic embossing is bounded by a visible half-circle around the cross-section of the embossed feature. The boundary of the process-affected zone is coincident with the isothermal surface at the glass transition temperature of the substrate material. The composition of the material within the process-affected zone remains unchanged. This finding is believed to be quite valuable since material composition is essential, especially in microfluidic applications where biocompatibility is an issue.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Ferah Sucularlı contributed to carrying out the experiments, analysis, and preparation of the article. M.A. Sahir Arıkan reviewed experimental and analytical results and supervised the study. Ender Yıldırım supervised the study and contributed to the preparation of the article. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 401
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Ferah Sucularlı received her MSc and PhD degrees in Mechanical Engineering at Middle East Technical University (METU), Ankara, Turkey in 2013 and 2018, respectively. From 2014–2018, she worked as a research assistant in Mechanical Engineering at Çankaya University, Ankara, Turkey. She currently works as an R&D engineer at Radar, Electronic Warfare and Intelligence Systems Business Sector of Aselsan A.Ş. in Ankara, Turkey Her research interest focuses on the microchannel fabrication methods. M.A. Sahir Arıkan received his BSc (1979), MSc (1981) and PhD (1987) degrees in mechanical engineering from Middle East Technical University (METU), Ankara, Turkey. He is currently a professor and the chair in the same department. His research interests include CAD, CAM, Robotics, Machine Elements, Machine Design, Gear Design, Gear Dynamics, MEMS, and micromanufacturing. Ender Yıldırım received his BSc, MSc, and PhD degrees in mechanical engineering at Middle East Technical University (METU), Ankara, Turkey in 2002, 2005, and 2011, respectively. From 2012–2013, he worked as a postdoctoral researcher at Leiden Academic Center for Drug Research at Leiden University, the Netherlands. He worked as a faculty member in Mechanical Engineering Department at Cankaya University, Ankara, Turkey between 2013-2019. He is currently an assistant professor in Mechanical Engineering Department at Middle East Technical University, Ankara, Turkey. His research interests include methods of plastic microfluidic chip fabrication, capillary microfluidics, droplet-based microfluidics, bio-MEMS, and biosensors.
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